Distractive properties of a threaded interbody fusion device. An in vivo model

Is there any advantage of using stand-alone cages? A numerical approach

Background

Segment fusion using interbody cages supplemented with pedicle screw fixation is the most common surgery for the treatment of low back pain. However, there is still much controversy regarding the use of cages in a stand-alone fashion. The goal of this work is to numerically compare the influence that each surgery has on lumbar biomechanics.

Methods

A non-linear FE model of the whole lumbar spine was developed to compare between two types of cages (OLYS and NEOLIF) with and without supplementary fixation. The motion of the whole spine was analysed and the biomechanical environment of the adjacent segments to the operated one was studied. Moreover, the risk of subsidence of the cages was qualitatively evaluated.

Results

A great ROM reduction occurred when supplementary fixation was used. This stiffening increased the stresses at the adjacent levels. It might be hypothesised that the overloading of these segments could be related with the clinically observed adjacent disc degeneration. Meanwhile, the stand-alone cages allowed for a wider movement, and therefore, the influence of the surgery on adjacent discs was much lower. Regarding the risk of subsidence, the contact pressure magnitude was similar for both intervertebral cage designs and near the value of the maximum tolerable pressure of the endplates.

Conclusions

A minimally invasive posterior insertion of an intervertebral cage (OLYS or NEOLIF) was compared using a stand-alone design or adding supplementary fixation. The outcomes of these two techniques were compared, and although stand-alone cage may diminish the risk of disease progression to the adjacent discs, the spinal movement in this case could compromise the vertebral fusion and might present a higher risk of cage subsidence.

Electronic supplementary material

The online version of this article (10.1186/s12938-019-0684-8) contains supplementary material, which is available to authorized users.

Application of a novel porous tantalum implant in rabbit anterior lumbar spine fusion model: in vitro and in vivo experiments

BACKGROUND

Some porous materials have been developed to enhance biologic fusion of the implants to bone in spine fusion surgeries. However, there are several inherent limitations. In this study, a novel biomedical porous tantalum was applied to in vitro and in vivo experiments to test its biocompatibility and osteocompatibility.

METHODS

Bone marrow-derived mesenchymal stem cells (BMSCs) were cultured on porous tantalum implant. Scanning electron microscope (SEM) and Cell Counting Kit-8 assay were used to evaluate the cell toxicity and biocompatibility. Twenty-four rabbits were performed discectomy only (control group), discectomy with autologous bone implanted (autograft group), and discectomy with porous tantalum implanted (tantalum group) at 3 levels: L3-L4, L4-L5, and L5-L6 in random order. All the 24 rabbits were randomly sacrificed at the different post-operative times (2, 4, 6, and 12 months; n = 6 at each time point). Histologic examination and micro-computed tomography scans were done to evaluate the fusion process. Comparison of fusion index scores between groups was analyzed using one-way analysis of variance. Other comparisons of numerical variables between groups were made by Student t test.

RESULTS

All rabbits survived and recovered without any symptoms of nerve injury. Radiographic fusion index scores at 12 months post-operatively between autograft and tantalum groups showed no significant difference (2.89 ± 0.32 vs. 2.83 ± 0.38, F = 244.60, P = 0.709). Cell Counting Kit-8 assay showed no significant difference of absorbance values between the leaching liquor group and control group (1.25 ± 0.06 vs. 1.23 ± 0.04, t = -0.644, P = 0.545), which indicated the BMSC proliferation without toxicity. SEM images showed that these cells had irregular shapes with long spindles adhered to the surface of tantalum implant. No implant degradation, wear debris, or osteolysis was observed. Histologic results showed solid fusion in the porous tantalum and autologous bone implanted intervertebral spaces.

CONCLUSION

This novel porous tantalum implant showed a good biocompatibility and osteocompatibility, which could be a valid biomaterial for interbody fusion cages.

Subsidence following anterior lumbar interbody fusion (ALIF): a prospective study

BACKGROUND

Anterior lumbar interbody fusion (ALIF) is a widely used surgical technique for disorders of the lumbar spine. One potential complication is the subsidence of disc height in the post-operative period. Few studies have reported the rate of subsidence in ALIF surgery prospectively. We prospectively evaluated the rate of subsidence in adult patients undergoing ALIF.

METHODS

Results were obtained by reviewing scans of 147 patients. Disc heights were measured on radiographic scans taken pre-operatively in addition to post-operatively immediately, at 6 weeks and at 18 months. The anterior and posterior intervertebral disc heights were measured. Subsidence was defined as greater than or equal to 2 mm loss of height.

RESULTS

A total of 15 patients (10.2%) had subsidence, with 7 being male. Each case was of delayed cage subsidence (DCS) >6 weeks postoperatively. The mean subsidence was 4.7 mm (range, 2.4-7.8). Mean anterior disc height was 8.6±0.4 mm preoperatively, which improved to 15.1±0.5 mm at latest follow-up. Mean posterior disc height was 4.7±0.2 mm preoperatively, which improved to 8.7±0.4 mm at latest follow-up. The mean lumbar lordosis (LL) angle was 42.5°±10.8° and the mean local disc angle (LDA) was 6.7°±4.0°. The 91.2% (n=114/125) of patients with appropriate radiological follow-up demonstrated fusion by latest follow-up. There was no correlation between subsidence rate with patient reported outcomes [Visual Analog Scale (VAS), Oswestry Disability Index (ODI) and Short Form 12 Item survey (SF-12)] and fusion rates. There was a significant negative correlation between LL and extent of subsidence (Pearson correlation =-0.754, P=0.012).

CONCLUSIONS

In conclusion, we found that the subsidence rate at follow-up was generally low following standalone ALIF for this patient series. Patient clinical outcomes and bony fusion rates were not significantly influenced by subsidence.

Prospective randomized controlled trial of The Stabilis Stand Alone Cage (SAC) versus Bagby and Kuslich (BAK) implants for anterior lumbar interbody fusion

Background

Degenerative disc disease is common and debilitating for many patients. If conservative extensive care fails, anterior lumbar interbody fusion has proven to be an alternative form of surgical management. The Stabilis Stand Alone Cage(SAC) was introduced as a method to obtain stability and fusion. The purpose of this study was to determine whether the Stabilis Stand Alone Cage (SAC) is comparable in safety and efficacy to the Bagby and Kuslich (BAK) device.

Methods

As part of a prospective, randomized, controlled FDA trial, 73 patients underwent anterior interbody fusion using either the SAC(56%) or the BAK device (44%).

Results

Background characteristics were similar between the two groups. There was no significant difference between the SAC and BAK groups in mean operative time or mean blood loss during surgery. Adverse event rates did not differ between the groups. Assessment of plain radiographs could not confirm solid fusion in 63% of control and 71% of study patients. Functional scores from Owestry and SF-36 improved in both groups by the two-year follow-up. There were no significant differences between the SAC and BAK patients with respect to outcome.

Conclusions

Both the Stabilis Stand Alone Cage and the BAK Cage provided satisfactory improvement in function and pain relief, despite less than expected radiographic fusion rates. The apparent incongruency between fusion rates and functional outcomes suggests that either radiographs underestimate the true incidence of fusion, or that patients are obtaining good pain relief and improved function despite a lower rate of fusion than previously reported. This was a Level III study.

Radiographic and clinical evaluation of cage subsidence after stand-alone lateral interbody fusion

Object

Indirect decompression of the neural structures through interbody distraction and fusion in the lumbar spine is feasible, but cage subsidence may limit maintenance of the initial decompression. The influence of interbody cage size on subsidence and symptoms in minimally invasive lateral interbody fusion is heretofore unreported. The authors report the rate of cage subsidence after lateral interbody fusion, examine the clinical effects, and present a subsidence classification scale.

Methods

The study was performed as an institutional review board–approved prospective, nonrandomized, comparative, single-center radiographic and clinical evaluation. Stand-alone short-segment (1- or 2-level) lateral lumbar interbody fusion was investigated with 12 months of postoperative follow-up. Two groups were compared. Forty-six patients underwent treatment at 61 lumbar levels with standard interbody cages (18 mm anterior/posterior dimension), and 28 patients underwent treatment at 37 lumbar levels with wide cages (22 mm). Standing lateral radiographs were used to measure segmental lumbar lordosis, disc height, and rate of subsidence. Subsidence was classified using the following scale: Grade 0, 0%–24% loss of postoperative disc height; Grade I, 25%–49%; Grade II, 50%–74%; and Grade III, 75%–100%. Fusion status was assessed on CT scanning, and pain and disability were assessed using the visual analog scale and Oswestry Disability Index. Complications and reoperations were recorded.

Results

Pain and disability improved similarly in both groups. While significant gains in segmental lumbar lordosis and disc height were observed overall, the standard group experienced less improvement due to the higher rate of interbody graft subsidence. A difference in the rate of subsidence between the groups was evident at 6 weeks (p = 0.027), 3 months (p = 0.042), and 12 months (p = 0.047). At 12 months, 70% in the standard group and 89% in the wide group had Grade 0 or I subsidence, and 30% in the standard group and 11% in wide group had Grade II or III subsidence. Subsidence was detected early (6 weeks), at which point it was correlated with transient clinical worsening, although progression of subsidence was not observed after the 6-week time point. Moreover, subsidence occurred predominantly (68%) in the inferior endplate. Fusion rate was not affected by cage dimension (p > 0.999) or by incidence of subsidence (p = 0.383).

Conclusions

Wider cages avoid subsidence and better restore segmental lordosis in stand-alone lateral interbody fusion. Cage subsidence is identified early in follow-up and can be accessed using the proposed classification scale.

MRI measurement of neuroforaminal dimension at the index and supradjacent levels after anterior lumbar interbody fusion: a prospective study

Background

Anterior interbody fusion has previously been demonstrated to increase neuroforaminal height in a cadaveric model using cages. No prior study has prospectively assessed the relative change in magnetic resonance imaging (MRI) demonstrated neuroforaminal dimensions at the index and supradjacent levels, after anterior interbody fusion with a corticocancellous allograft in a series of patients without posterior decompression. The objective of this study was to determine how much foraminal dimension can be increased with indirect foraminal decompression alone via anterior interbody fusion, and to determine the effect of anterior lumbar interbody fusion on the dimensions of the supradjacent neuroforamina.

Methods

A prospective study comparing pre- and postoperative neuroforaminal dimensions on MRI scan among 26 consecutive patients undergoing anterior lumbar interbody fusion without posterior decompression was performed. We studies 26 consecutive patients (50 index levels) that had undergone anterior interbody fusion followed by posterior pedicle screw fixation without distraction or foraminotomy. We used preoperative and postoperative MRI imaging to assess the foraminal dimensions at each operated level on which the lumbar spine had been operated. The relative indirect foraminal decompression achieved was calculated. The foraminal dimension of the 26 supradjacent untreated levels was measured pre- and postoperatively to serve as a control and to determine any effects after anterior interbody fusion.

Results

In this study, 8 patients underwent 1 level fusion (L5-S1), 12 patients had 2 levels (L4-S1) and 6 patients had 3 levels (L3-S1). The average increase in foraminal dimension was 43.3% (p < 0.05)-19.2% for L3-4, 57.1% for L4-5, and 40.1% for L5-S1. Mean pre- and postoperative supradjacent neuroforaminal dimension measurements were 125.84 mm² and 124.89 mm², respectively. No significant difference was noted (p > 0.05).

Conclusions

Anterior interbody fusion with a coriticocancellous allograft can significantly increase neuroforaminal dimension even in the absence of formal posterior distraction or foraminotomy; anterior interbody fusion with a coriticocancellous allograft has little effect on supradjacent neuroforaminal dimensions.

A surgical option for multilevel anterior lumbar interbody fusion with ponte osteotomy to achieve optimal lumbar lordosis and sagittal balance

Objective

To document lumbar lordosis (LL) of the spine and its change during surgeries with the different height but the same angle setting of the anterior cage. Additionally, we attempted to determine if sufficient LL is achieved at different cage heights and to quantify the change in LL during multi-level anterior lumbar interbody fusion (ALIF).

Methods

The medical records and radiographs of 42 patients who underwent more than 2 level ALIFs between 2008 and 2009 were retrospectively reviewed. We evaluated 3 parameters seen on lateral whole spine radiographs : LL, pelvic incidence (PI), and sagittal vertical axis (SVA). The mean follow-up time was 28.1 months and the final follow-up radiographs of all patients were reviewed at least 2 years after surgery. Statistical analysis was performed using the paired t-tests.

Results

Lumbar lordosis had changed up to 30 degrees immediately and 2 years after surgery (preoperative mean LL, SVA : 22.45 degrees, 112.31 mm; immediate postoperative mean LL, SVA : 54.45 degrees, 37.36 mm; final follow-up mean LL, SVA : 49.56 degrees, 26.95 mm). Our goal of LL is to obtain as much PI as possible, preoperative mean PI value was 55.38±3.35. The pre-operative and two year post-surgery follow-up mean of the Japanese Orthopedic Association score were 9.2±0.6 and 13.2±0.6 (favorable outcome rate : 95%), respectively. In addition, we were able to obtain good clinical outcomes and sagittal balance with a subsidence rate of 22.7%.

Conclusion

We were able to achieve sufficient LL, such that it was similar to the PI, utilizing multi-level ALIF with the use of a tall cage with the same angle setting of the cage. We have found out that achieving sufficient lumbar lordosis and sagittal balance require an anterior lumbar cage with high angle and height.

In vivo experimental study of hat type cervical intervertebral fusion cage (HCIFC)

The purpose of this study was to compare the characteristics of interbody fusion achieved using the hat type cervical intervertebral fusion cage (HCIFC) with those of an autologous tricortical iliac crest graft, Harms cage and the carbon cage in a goat cervical spine model. Thirty-two goats underwent C3-4 discectomy and fusion. They were subdivided into four groups of eight goats each: group 1, autologous tricortical iliac crest bone graft; group 2, Harms cage filled with autologous iliac crest graft; group 3, carbon cage filled with autologous iliac bone; and group 4, HCIFC filled with autologous iliac graft. Radiography was performed pre- and postoperatively and after one, two, four, eight and 12 weeks. At the same time points, disc space height, intervertebral angle, and lordosis angle were measured. After 12 weeks, the goats were killed and fusion sites were harvested. Biomechanical testing was performed in flexion, extension, axial rotation, and lateral bending to determine the stiffness and range of motion. All cervical fusion specimens underwent histomorphological analyses. One week after operation, the disc space height (DSH), intervertebral angle (IVA) and lordosis angle (LA) of HCIFC and carbon cage were statistically greater than those of autologous iliac bone graft and Harms cage. Significantly higher values for DSH, IVA and LA were shown in cage-treated goats than in those that received bone graft over a 12-week period. The stiffness of Harms cage in axial rotation and lateral bending were statistically greater than that of other groups. Radiographic and histomorphological evaluation showed better fusion results in the cage groups than in the autologous bone group. HCIFC can provide a good intervertebral distractability and sufficient biomechanical stability for cervical fusion.

Cosmetic and functional outcomes following paramedian and anterolateral retroperitoneal access in anterior lumbar spine surgery

Object

In this paper, the authors review the functional and cosmetic outcomes and complications in 300 patients who underwent treatment for lumbar spine disease via either an anterior paramedian or conventional anterolateral retroperitoneal approach.

Methods

Seven surgeons performed anterior lumbar surgeries in 300 patients between August 2004 and December 2006. One hundred and eighty patients were treated with an anterior paramedian approach, and 120 patients with an anterolateral retroperitoneal approach. An access surgeon was used in 220 cases (74%). Postoperative evaluation in all patients consisted of clinic visits, assessment with the modified Scoliosis Research Society–30 instrument, as well as a specific questionnaire relating to wound appearance and patient satisfaction with the wound.

Results

At a mean follow-up of 31 months (range 12–47 months), the mean Scoliosis Research Society–30 score (out of 25) was 21.2 in the patients who had undergone the anterior paramedian approach and 19.4 in those who had undergone the anterolateral retroperitoneal approach (p = 0.005). The largest differences in quality of life measures were observed in the areas of pain control (p = 0.001), self-image (p = 0.004), and functional activity (p = 0.003), with the anterior paramedian group having higher scores in all 3 categories. Abdominal bulging in the vicinity of the surgical site was the most common wound complication observed and was reported by 22 patients in the anterolateral retroperitoneal group (18%), and 2 patients (1.1%) in the anterior paramedian group. Exposures of ≥ 3 levels with the anterolateral approach were associated with abdominal bulging (p = 0.04), while 1- or 2-level exposures were not (p > 0.05). Overall satisfaction with incisional appearance was higher in patients with an anterior paramedian incision (p = 0.001) and with approaches performed by an access surgeon (p = 0.004).

Conclusions

Patients who undergo an anterior paramedian approach to the lumbar spine have a higher quality of life and better cosmetic outcomes than patients undergoing an anterolateral retroperitoneal approach.

Tapered cages in anterior lumbar interbody fusion: biomechanics of segmental reactions

Object

The aim of this study was to determine the in vitro biomechanical responses of lumbar spinal segments after implantation of tapered cages.

Methods

Range of motion (ROM)– and stiffness-related data were determined in 10 human cadaveric T12–S1 columns subjected to flexion, extension, and lateral bending modes before and after anterior lumbar interbody fusion in which stand-alone LT-CAGE devices were used. The overall column showed no significant changes in ROM or stiffness. At the instrumented level, stiffness increased significantly (p < 0.05) in flexion and lateral bending modes. Indications of instability in extension were present, but these values were not statistically significant. There was no evidence of adjacent-level instability at any level in any mode, except for the segment superior to the fixation level in flexion; here there was a significant increase in ROM (p < 0.05) and a decrease in stiffness.

Conclusions

The anatomical conformity and bilateral placement of cages provide ample stability and rigidity at the treated level, comparable to that of other cage systems. Because hypermobility is traditionally related to early degenerative changes, the present results appear to suggest that cages do not significantly contribute to such alterations.

Does bioresorbable cage material influence segment stability in spinal interbody fusion?

To reduce long term complications associated with nonresorbable interbody fusion cages, bioresorbable cages are being developed. We investigated the influence of bioresorbable cage material on segment stability, intervertebral disc height and fusion in vivo using radiostereometric analysis comparing 70/30 poly(L-lactide-co-D,L-lactide) (PLDLLA) cages with titanium cages. Twenty-eight goats were randomized to receive PLDLLA (n = 21) or a titanium control (n = 7) cage at L3-L4. Range of motion for flexion and extension and change in intervertebral disc height were measured before and after surgery and at followup (3, 6, and 12 months). Fusion was graded with a validated radiographic score. Although the PLDLLA cage could not provide the optimal environment for a successful high fusion rate, the range of motion of the PLDLLA segments gradually decreased in time and was similar to the titanium control group at 12 months. In addition the decrease of intervertebral disc height was similar for both PLDLLA (1.4 +/- 0.8 mm) and titanium (1.3 +/- 1.0 mm) specimens. Both results showed a bioresorbable cage does not lead to less decrease of motion or more loss of intervertebral disc height in time compared to titanium. This study therefore supports further development of a bioresorbable cage concept.

Biomechanical changes at adjacent segments following anterior lumbar interbody fusion using tapered cages

STUDY DESIGN

A biomechanical evaluation of anterior cages in a calf lumbar spine model.

OBJECTIVES

To determine changes in spinal motion and intradiscal pressures at immediately adjacent lumbar motion segments following anterior insertion of tapered cages.

SUMMARY OF BACKGROUND DATA

Stand-alone anterior lumbar interbody fusion (ALIF) is an effective approach in the treatment of discogenic low back pain. A tapered lumbar (LT) cage design attempts to restore physiologic lordosis and sagittal balance. We are not aware of any previous biomechanical evaluation of the effects of LT cages on adjacent motion segments.

METHODS

Nine fresh calf spines (L2-L5) were procured for the study. Pure moments (up to 8.5 Nm) in flexion, extension, and lateral bending were applied to the L2 vertebra in five steps through a nonconstrained loading system. With each step of loading, three-dimensional rotation at three intervertebral disc levels was obtained through a three-camera motion analysis system, and intradiscal pressures within the nucleus pulposus of the two nonoperated discs were measured with miniature transducers. The spines were tested initially intact and following paired anterior LT cage insertion.

RESULTS

Following ALIF, small to moderate increase in motion was found at both adjacent segments in flexion (superior: 12.5%, P < 0.05; inferior: 11.3%, P < 0.02) and lateral bending (superior: 7.8%, P < 0.02; inferior: 6.6%, P < 0.02). An increase in intradiscal pressure was noted at the superior adjacent segment under flexion (21%, P < 0.01) and lateral bending (16%, P < 0.03). Intradiscal pressure changes at the inferior adjacent level were not significant.

CONCLUSIONS

Statistically significant changes in intradiscal pressures and motion were found at the adjacent levels following a single-level stand-alone ALIF procedure using paired LT cages.

Animal models for spinal fusion

Animal models for spinal fusion are essential for preclinical testing of new fusion methods and adjuncts. They allow for control of individual variables and quantification of outcome measures. Model characteristics are considered. Preclinical experiments to evaluate proof of concept, feasibility, and efficacy are generally studied in an orderly progression from smaller to larger animal models with an evolving cascade of evidence which has become known as the "burden of proof". Methods of fusion analysis include manual palpation, radiographs, computed tomography, histology, biomechanical testing, and molecular analysis. Models which have been developed in specific species are reviewed. This sets the stage for the interpretation of studies evaluating bone graft materials such as allograft, demineralized bone matrices, bone morphogenetic proteins, ceramics, and others with consideration of the variables affecting their success. As evidence accumulates, clinical trials and applications are defined.

Polyetheretherketone as a biomaterial for spinal applications

Threaded lumbar interbody spinal fusion devices (TIBFD) made from titanium have been reported to be 90% effective for single-level lumbar interbody fusion, although radiographic determination of fusion has been intensely debated in the literature. Using blinded radiographic, biomechanic, histologic, and statistical measures, we evaluated a radiolucent polyetheretherketone (PEEK)-threaded interbody fusion device packed with autograft or rhBMP-2 on an absorbable collagen sponge in 13 sheep at 6 months. Radiographic fusion, increased spinal level biomechanical stiffness, and histologic fusion were demonstrated for the PEEK cages filled with autograft or rhBMP-2 on a collagen sponge. No device degradation or wear debris was observed. Only mild chronic inflammation consisting of a few macrophages was observed in peri-implant tissues. Based on these results, the polymeric biomaterial PEEK may be a useful biomaterial for interbody fusion cages due to the polymer's increased radiolucency and decreased stiffness.

Treatment of traumatic cervical spine instability with interbody fusion cages: a prospective controlled study with a 2-year follow-up

INTRODUCTION

The purpose of this prospective cohort study was to define indications and analyze the clinical and radiographic results of using interbody cages to surgically treat traumatic cervical spine instability.

PATIENTS AND METHODS

53 patients were treated by monosegmental anterior discectomy and interbody fusion using either autologous tricortical iliac crest bone graft and CSLP (cervical spine locking plate) (bone graft group, n=26) or Syncage-C filled with autologous cancellous bone grafts and CSLP (cage group n=27). Indications for surgery were traumatic cervical spine instability classified according to the AO classification as B1, B2, B3, C2, or C3. Intraoperative parameters such as blood loss and operation time were assessed. Prior to surgery and at follow-up (6,12, and 24 months), evaluation included measurement of neck pain, shoulder/arm pain, muscle strength, Neck Pain Disability Index (NPDI), and Cervical Spine Functional Score (CSFS). Neurological and overall outcome was assessed using the ASIA impairment scale and Odom's criteria, respectively. In addition, radiographic evaluation, including plain x-rays, flexion-extension views, and CT scans was performed. Fusion, segmental mobility, segmental lordosis/kyphosis and disc space height were determined.

RESULTS

Operation time and hospital stay were significantly shorter (p<0.05) in the cage group than in the bone graft group. After 6,12, and 24 months there was no difference between either group in pain, muscle strength, NPDI, CSFS, neurological and overall outcome. Although the cage group showed a trend for prolonged fusion process, there was no statistically significant difference between the groups for all radiographic parameters.

CONCLUSION

Under strict indications, cages offer a valid alternative to a tricortical iliac crest bone graft in the surgical treatment of monosegmental traumatic cervical spine instability. Although there was no significant difference between the cage and the bone graft group in the functional and radiographic outcome, less donor site morbidity, a shorter operating time, and a reduced hospital stay might result in cost-effectiveness of cages.

Subsidence after anterior lumbar interbody fusion using paired stand-alone rectangular cages

The authors conducted a study to determine at what stage after surgery the subsidence occurred, and to assess the relationships of radiographic fusion and the recurrence of symptoms with the development of subsidence. Ninety patients underwent a single-level anterior lumbar interbody fusion (ALIF) using paired stand-alone rectangular cages between November 2000 and June 2002. All patients had regular clinical or imaging follow-up for a minimum of 19 months (range 19-38 months, mean = 27 months). The ratio of male to female patients was 1:3.1. The patients' ages at the time of ALIF ranged from 25 to 72 years, with a mean of 53 years. The preoperative and postoperative intervertebral disc heights were serially measured by plain radiographs. The location of cage subsidence into the vertebral body and times until the presence of subsidence were also assessed. The mean preoperative intervertebral disc height was 11.6+/-3.1 mm, which spread immediately after surgery to 16.9+/-2.0 mm. This increase was statistically significant (P = 0.001). At the last follow-up visit, the mean intervertebral disc height had been reduced to 13.2+/-2.4 mm. Sixty-nine of 90 patients (76.7%) developed cage subsidence into the surrounding vertebral body. Subsidence was more often noted in the superior endplate above the cage with regard to the location of cage subsidence [superior endplate: 27 patients (39.1%), inferior endplate: 12 patients (17.3%), both: 30 patients (43.6%)]. The onset of subsidence varied from 0.25 to 8 months after surgery (median, 2.75 months). The 8-, 12-, and 16-week actuarial rates for developing cage subsidence were 38.9, 63.4, and 70.7%, respectively, when using the Kaplan-Meier method. There was no statistical correlation between the recurrence of symptoms (P = 0.3952) and radiographic fusion (P = 0.9518) with the log-rank test in development of subsidence. This study demonstrates that cage subsidence is an expected occurrence after ALIF using stand-alone rectangular cages. The 3- and 4-month actuarial rates for developing cage subsidence were 63.4 and 70.7%, respectively, and cage subsidence had no correlation with recurrence of symptoms and radiographic fusion in our study.

Evaluation of effects of selected factors on inter-vertebral fusion—a simulation study

This study simulated the effects of inter-vertebral disc degeneration and bone density distribution on the structural stiffness and strength provided by inter-vertebral fusion. Based on the original and redistributed bone density distributions, the effects of selected factors, including contact area between device/graft and vertebral endplates, endplate conditions, and bone growth capacity were evaluated using a factorial design of experiment. The simulation results suggested that the degeneration of inter-vertebral disc significantly affected the bone density and density distribution in adjacent vertebrae. The mechanical strength immediately after instrumentation is the worst case of device/graft subsidence. After that procedure, bone will adapt itself to the changed loading conditions and therefore reduce the risk of subsidence. A deficiency in structural stiffness immediately after instrumentation could be the "worst-case scenario" depending on the combinations of selected factors. The simulation results demonstrated that the contact area and initial bone density distribution should be considered jointly while estimating the risk of device/graft subsidence. The endplate condition is a secondary factor on the subsidence risk, compared with the contact area and initial bone density distribution.

Three- to six-year follow-up of stand-alone BAK cages implanted by a single surgeon

BACKGROUND CONTEXT

Multiple studies involving the outcomes of anterior interbody cages have been published, but the majority were by authors who designed the cage. No outcome studies with Bagby and Kuslich (BAK) cages implanted by a single-surgeon have either 3 years of follow-up or at least 25 patients.

PURPOSE

To determine the 3- to 6-year clinical outcomes, including fusion rate, revision rate, complications and functional status of patients who underwent placement of anterior, stand-alone BAK cages by a single surgeon.

STUDY DESIGN

This is a retrospective cohort study of patients who underwent anterior, stand-alone BAK cage placement by a single surgeon with a minimum of 3 years of follow-up.

PATIENT SAMPLE

A total of 46 consecutive patients who underwent placement of anterior BAK cages from 1997 to 1999 were the study group.

OUTCOME MEASURES

Complications, fusion rate, revision rate, Prolo scores, Oswestry scores, pain scores, patient satisfaction.

METHODS

A retrospective review of charts of patients undergoing anterior stand-alone placement of BAK cages over the study period were reviewed. Demographic data were collected, and postoperative radiographs and clinic notes were reviewed. Patients were then contacted to complete a survey to determine Prolo and Oswestry scores, workers' compensation status and general satisfaction with the procedure. Patients were also asked to undergo repeat radiographs of their lumbar spine to determine if the operative levels had fused.

RESULTS

Follow-up was available on 33 of 46 patients (72%). At least 10 patients (22%) required revision surgery. Ten patients (22%) had 14 total complications not requiring revision surgery. Seventy percent of patients had a fair or poor outcome as assessed by the Prolo rating system, and 58% of patients had at least "severe disability" according to the Oswestry outcome scale. Fifty percent of patients were satisfied with their surgery.

CONCLUSIONS

This single-surgeon series of stand-alone BAK cages demonstrates significantly worse clinical outcomes than has been previously reported. The use of stand-alone BAK cages for degenerative disc disease should be reconsidered given the large number of patients with unacceptable outcomes.

[Bone integration and apposition of porous and non porous metallic orthopaedic biomaterials]

The objective of this study was to evaluate the functionality of two intervertebral fusion implants, a porous nickel-titanium and a conventional titanium cage system in a sheep model. Eighteen sheep each received the two-implant devices at L2-L3 and L4-L5 lumbar levels. The sheep were sacrificed at three different postsurgical periods: three, six and 12 months. Lumbar segments were harvested. Qualitative (macroscopic and microscopic) and quantitative (histomorphometric) histological analysis were carried out on histological slides. The results indicated that a porous nickel-titanium had obtained a better osseointegration than the titanium implant. The functionality of two implants seemed to be influenced by the implant structure and shape. However, biocompatibility of two implants seemed comparable.

[Biodegradable cage. Osteointegration in spondylodesis of the sheep cervical spine]

Bioabsorbable implants are commonplace in knee and shoulder surgery. Bioabsorbable poly(l-lactide-co-D,L-lactide) (PLDLLA) cage devices have potential benefits over autologous tricortical iliac crest bone graft and metallic cages for cervical spine interbody fusion. The purpose of this study was to compare interbody fusion of an autologous tricortical iliac crest bone graft with that of a bioabsorbable cage using a sheep cervical spine interbody fusion model. This study was designed to determine differences in (1) the ability to preserve postoperative distraction, (2) biomechanical stability, and (3) histological characteristics of intervertebral bone matrix formation. Sixteen full-grown Merino sheep underwent C3/4 discectomy and fusion. After 12 weeks, there was no significant difference between the results with the bioabsorbable PLDLLA cages and tricortical bone grafts. The cage also did not show advanced interbody fusion but did, however, show large osteolysis, which allows skepticism regarding the value of this bioabsorbable implant.

Anwendung eines Bandscheibenersatzimplantats aus einer neuartigen porösen TiO2/Glas-Keramik – Teil 2: Biomechanische Untersuchungen nach Implantation in die Schafs-Halswirbelsäule / Application of a Stand-Alone Interbody Fusion Cage Based on Porous TiO2/glass Composites – Part 2: Biomechanical Evaluation after Implantation in the Sheep Cervical Spine

Animals are becoming more and more common as in vivo models for the human spine. Especially the sheep cervical spine is stated to be of good comparability and usefulness in the evaluation of in vivo radiological, biomechanical and histological behaviour of new bone replacement materials, implants and cages for cervical spine interbody fusion. In preceding biomechanical in vitro examinations human cervical spine specimens were tested after fusion with either a cubical stand-alone interbody fusion cage manufactured from a new porous TiO2/glass composite (Ecopore) or polymethylmethacrylate (PMMA) after discectomy. Following our first experience with the use of the new material and its influence on the primary stability after in vitro application we carried out fusions of 20 sheep cervical spines levels with either PMMA or an Ecopore-cage, and performed radiological examinations during the following 2-4 months. In this second part of the study we intended the biomechanical evaluation of the spine segments with reference to the previously determined morphological findings, like subsidence of the implants, significant increase of the kyphosis angle and degree of the bony fusion along with the interpretation of the results. 20 sheep cervical spines segments with either PMMA- or Ecopore-fusion in the levels C2/3 and C4/5 were tested, in comparison to 10 native corresponding sheep cervical spine segments. Non-destructive biomechanical testing was performed, including flexion/extension, lateral bending and axial rotation using a spine testing apparatus. Three-dimensional range of motion (ROM) was evaluated using an ultrasound measurement system. In the native spine segments C2/3 and C4/5 the ROM increased in cranio-caudal direction particulary in flexion/extension, less pronounced in lateral flexion and axial rotation (p < 0.05). The overall ROM of both tested segments was greatest in lateral flexion, reduced to 52% in flexion/extension and to 16% in axial rotation. After 2 months C2/3- and C4/5-segments with PMMA-fusion and C2/3-segments with Ecopore-interposition showed decrease of ROM in lateral flexion in comparison to the native segments, indicating increasing stiffening. However, after 4 months all operated segments, independent from level or implanted material, were stiffer than the comparable native segments. The decrease of the ROM correlated with the radiological-morphological degree of fusion. Our evaluation of the new porous TiO2/glass composite as interbody fusion cage has shown satisfactory radiological results as well as distinct biomechanical stability and fusion of the segments after 4 months in comparison to PMMA. After histological analysis of the bone-biomaterial-interface, further examinations of this biomaterial previous to an application as alternative to other customary cages in humans are necessary.

[Vertebral body replacement in spine surgery]

Autografts and allogeneous bone grafts as well as cages are used for the reconstruction of the anterior column after corpectomy. Recently, expandable cages for vertebral body replacement have been developed. Based on our own experience, the purpose of this study was to summarize the available biomechanical and clinical data of expandable corpectomy cages and to compare it with established fixation techniques. If used correctly, expandable cages offer several surgical advantages in comparison to non-expandable cages. However there were no significant differences between the biomechanical properties of expandable and non-expandable cages. Additionally, design variations of expandable corpectomy cages did not show any significant impact on the biomechanical stability. Currently available mid-term clinical and radiological data on the treatment of fractures, metastasis and infection of the cervical, thoracic and lumbar spine demonstrated no significant difference between expandable and non-expandable cages. However, the increased stress-shielding effect of expandable cages compared to non-expandable cages might result in a deterioration of the long-term clinical outcome.

Bioabsorbable interbody cages in a sheep cervical spine fusion model

STUDY DESIGN

An experimental study using a sheep cervical spine interbody fusion model.

OBJECTIVES

To compare interbody fusion of an autologous tricortical iliac crest bone graft with two bioabsorbable cages and to determine whether there are differences between the three interbody fusion techniques in 1) the ability to preserve postoperative distraction, 2) the biomechanical stability, and 3) the histologic characteristics of intervertebral bone matrix formation.

SUMMARY AND BACKGROUND DATA

Bioabsorbable cages would be beneficial compared with metallic cages; however, currently no suitable bioabsorbable interbody fusion cage is available.

METHOD

Twenty-four sheep underwent C3/C4 discectomy and fusion. The following stabilization techniques were used: Group 1) autologous tricortical iliac crest bone graft (n = 8); Group 2) bioabsorbable cage made of 70/30 poly(l-lactide-co-d,l-lactide) (experimental) filled with autologous cancellous bone graft (n = 8); Group 3) bioabsorbable cage made of a polymer-calciumphosphate composite (Biomet Europe, Dordrecht, The Netherlands) filled with autologous cancellous bone graft (n = 8). Radiographic scans to determine disc space height were performed before and after surgery and after 1, 2, 4, 8, and 12 weeks, respectively. After 12 weeks, animals were killed, and fusion sites were evaluated using functional radiographic views in flexion and extension. Quantitative computed tomographic scans were used to assess bone mineral density, bone mineral content, and bony callus volume. Biomechanical testing was performed in flexion, extension, axial rotation, and lateral bending to determine stiffness, ROM, and neutral and elastic zone. Histomorphological and histomorphometrical analysis were performed to evaluate fusion and foreign body reactions associated with the bioabsorbable cages.

RESULTS

Over a 12-week period, the polymer-calciumphosphate composite cage showed significantly higher values for disc space height compared with the bone graft and the poly(l-lactide-co-d,l-lactide) cage. Additionally, the polymer-calciumphosphate composite cage demonstrated a significantly higher stiffness and lower ROM, neutral zone, and elastic zone in axial rotation and lateral bending than any other group. However, quantitative computed tomographic scans demonstrated cracks in six of the eight polymer-calciumphosphate composite cages after 12 weeks. Histologically, the highest bone volume/total volume ratio and the highest fusion rate were found in the polymer-calciumphosphate composite cage group. Although the poly(l-lactide-co-d,l-lactide) cage showed grade I through III foreign body reactions in all fusion areas, only two animals developed grade I foreign body reactions with the polymer-calciumphosphate composite cage.

CONCLUSION

After 12 weeks, there was no significant difference between the bioabsorbable poly(l-lactide-co-d,l-lactide) cage and the tricortical bone graft. In comparison to the tricortical bone graft, the bioabsorbable polymer-calciumphosphate composite cage showed significantly better distractive properties, a significantly higher biomechanical stiffness, and an advanced interbody fusion; however, six of eight polymer-calciumphosphate composite cages cracked. Although the fate of the foreign body reactions and the cracks is currently unclear for both bioabsorbable cages, the early appearance of large osteolysis associated with use of the poly(l-lactide-co-d,l-lactide) cage allows skepticism regarding the value of this bioabsorbable implant.

A two-cage reconstruction versus a single mega-cage reconstruction for lumbar interbody fusion: an experimental comparison

BACKGROUND

Interbody cages are used as an adjunct to anterior lumbar interbody fusion, but exposure and insertion of two cages can be difficult. A biomechanical study was performed to compare the stability and exposed surface for fusion obtained with interbody reconstruction using two traditional cylindrical cages (18-mm diameter) vs. a single expanded mega-cage (24-mm diameter). A single-cage technique could result in safer exposure, shorter operating time, and less cost.

METHODS

STUDY DESIGN

nondestructive testing of L5-S1 motion segments with cages compared the two configurations, and direct measure of the size of the fusion bed was made.

PATIENT SAMPLE

16 human cadaveric lumbar motion segments.

OUTCOME MEASURES

significant differences in motion segment stiffness and cancellous surface areas were compared using a Wilcoxon rank sum test. Motion segments were biomechanically tested intact, and then tested again after insertion of two interbody cages (n=8) or a single mega-cage (n=8). Nondestructive biomechanical loading was performed consisting of: (1) compression (maximum load 900 N); (2) Flexion, extension, left and right lateral bending (maximum moment 18 Nm); and (3) left and right torsion (maximum moment 10 Nm). From the load-deformation curves obtained, stiffness values were calculated to compare the two-cage and the single mega-cage reconstructions. After testing, the specimens were disarticulated and the surface area of the endplate bed created in the cancellous bone (of both vertebrae) was measured to compare the potential vascular surface for osteogenesis with both constructs.

RESULTS

The averages of the normalized values of stiffness were significantly greater for the two-cage group as compared to the mega-cage group in flexion only (1.08 vs. 0.74, p<0.05). For extension, torsion and lateral bending there was no significant difference in stiffness. In compression the two-cage group was stiffer, although not significantly (0.92 vs. 0.68, p<0.07). The average cancellous bed surface area was slightly greater for the single-cage reconstruction (1,208 mm(2) vs. 1,155 mm(2)), although this difference was not significant.

CONCLUSIONS

The stiffness with a single anterior mega-cage was significantly lower in flexion compared with two standard cages. However, in all other modes of testing the constructs were statistically equivalent, although neither construct was significantly stiffer than the intact specimen. Additionally, the single mega-cage provides an equivalent cancellous bed for fusion as compared to dual cages. While this study is not sufficient to recommend human application, these results and our previous experience with the successful in vivo use of a single cage in rhesus monkeys [4] suggest that the single expanded anterior cage may be an acceptable concept although subsidence risk needs further investigation. The potential advantages of a single mega-cage (safer for the foramen, safer for the vessels, more consistent decortication and possibly cheaper) further suggest that examination should be given to this method as an approach to anterior interbody reconstruction in selected patients.

Bone tissue engineering and spinal fusion: the potential of hybrid constructs by combining osteoprogenitor cells and scaffolds

In this paper, we discuss the current knowledge and achievements on bone tissue engineering with regard to spinal fusion and highlight the technique that employs hybrid constructs of porous scaffolds with bone marrow stromal cells. These hybrid constructs potentially function in a way comparable to the present golden standard, the autologous bone graft, which comprises besides many other factors, a construct of an optimal biological scaffold with osteoprogenitor cells. However, little is known about the role of the cells in autologous grafts, and especially survival of these cells is questionable. Therefore, more research will be needed to establish a level of functioning of hybrid constructs to equal the autologous bone graft. Spinal fusion models are relevant because of the increasing demand for graft material related to this procedure. Furthermore, they offer a very challenging environment to further investigate the technique. Anterior and posterolateral animal models of spinal fusion are discussed together with recommendations on design and assessment of outcome parameters.

A prospective, randomized controlled clinical trial of anterior lumbar interbody fusion using a titanium cylindrical threaded fusion device

STUDY DESIGN

A prospective, randomized, controlled clinical trial comparing a cylindrical threaded titanium cage to a femoral ring allograft control for anterior lumbar interbody fusion.

OBJECTIVE

To compare these two implants with regard to arthrodesis. Secondary outcome measures included pain relief, neurological status, and general health status.

SUMMARY OF BACKGROUND DATA

Anterior lumbar interbody fusion is a well-accepted procedure using trapezoidal femoral ring allografts or cylindrical titanium cages. Clinical and biomechanical studies evaluating these two distinct constructs are numerous; however, no prospective, randomized study comparing them has been done.

METHODS

A multicenter trial of 140 patients: 78 were randomized to the cylindrical threaded titanium cage device treatment arm and 62 patients randomized into the control group. All had autogenous iliac crest bone graft packed into the device. All patients had a single-level stand-alone anterior lumbar interbody fusion at either the L4-L5 or L5-S1 interspace for symptomatic degenerative disc disease. Radiographic fusion data were collected as well as multiple types of outcome data, including pain/disability scores, neurologic status, and overall health.

RESULTS

At 12 months, 97% of the cylindrical threaded titanium cage device group and 40% of the control group demonstrated radiographic fusion. At 24 months, 97% of the cylindrical threaded titanium cage group and 52% of the control group showed radiographic fusion. These fusion rate differences are statistically significant (P < 0.001). The Oswestry and neurologic scores were not significantly different between groups.

DISCUSSION

This is the first prospective, randomized, multicenter clinical trial that compares fusion cage results to control data.

CONCLUSION

Cylindrical threaded titanium cages have a higher fusion rate, comparable improvements in clinical outcome (Oswestry, Low Back Pain Questionnaire, SF-36), and fewer secondary supplemental fixation procedures compared to the femoral ring allograft control.

Perioperative Complications of Threaded Cylindrical Lumbar Interbody Fusion Devices

Few data are available to evaluate approach-related differences in perioperative complications with lumbar interbody fusion devices. Complications occurring in the intraoperative and immediate postoperative period were identified and categorized for 31 consecutive posterior lumbar interbody fusions (PLIFs) and 88 consecutive anterior lumbar interbody fusions (ALIFs). In this study, all lumbar interbody fusions were conducted with threaded cylindrical devices as stand-alone internal fixation devices. Multivariate analysis was used to account for potential covariates and identify factors associated with an increased complication risk. Twenty-two percent of the patients had a perioperative complication. The relative risk of having a perioperative complication was 4.75 times higher for the PLIF group. All intraoperative complications occurred in the PLIF group. The relative risk of having a major postoperative complication was 6.8 times higher in the PLIF group than the ALIF group. Anterior approached patients tended to have visceral (ileus, 6%) and vascular (deep venous thrombosis, 2%) complications. In the posterior group, complications were neurologic and dura related (pseudomeningocele, 16%; epidural hematoma, 3%) and occurred most frequently in patients that had had previous posterior lumbar surgery (31% with major complication).

Anterior lumbar fusion with paired BAK standard and paired BAK Proximity cages: subsidence incidence, subsidence factors, and clinical outcome

BACKGROUND CONTEXT

Anterior lumbar interbody fusion (ALIF) procedures have a known incidence of subsidence. The individual risk of subsidence for specific lumbar levels in ALIF procedures has not been determined.

PURPOSE

To evaluate the incidence of subsidence with two ALIF constructs. A paired Bagby and Kuslich (BAK) standard cage construct is compared with a paired BAK Proximity construct (Sulzer Spine-Tech, Minneapolis, MN). Study purpose is to evaluate lumbar intervertebral disc subsidence including the subsidence incidence for each disc level and with single- and two-level constructs. Also evaluated is the site of maximal subsidence within each end plate, risk with increased reaming depth, fusion incidence and clinical outcome.

STUDY DESIGN

A consecutive series of 70 fused levels fused with paired standard BAK cages is compared with a subsequent series of 70 fused levels using paired Proximity BAK cages.

PATIENT SAMPLE

The study population is derived from a consecutive series of ALIF procedures completed by a single surgeon. In 1998 the construct was changed from dual-standard to dual-Proximity cages. In the year 2000 there were 52 patients with a 2-year follow-up. These were matched to the previous 52 patients with dual-standard construct.

OUTCOME MEASURES

Clinical outcome was determined using pre- and postoperative Oswestry questionnaires. Postoperative questionnaires were completed at the yearly follow-up. Radiographic outcome was determined by fusion status and evaluation of subsidence. Also evaluated was reaming depth and cage size for each fused level.

METHODS

A total of 52 patients with ALIF procedures using paired BAK standard cages (the SS group) were studied with a group of 52 patients using paired BAK Proximity cages (the PP group). The study population was derived from an ongoing prospective study of consecutive ALIF fusion procedures completed by a single surgeon. Disc height measurements were used to determine subsidence. Reaming depth, fusion status and the site of maximal subsidence were all recorded. Clinical outcome was determined with a pre- and postoperative Oswestry functional questionnaire.

RESULTS

Subsidence greater than 2 mm was noted in 5 of the 70 SS fused levels and in 9 of the 70 fused PP levels. Subsidence was always at the L4-L5 level in the SS subsided levels. Subsidence was in two-level fusions in all but one of the SS constructs. Subsidence was at the L4-L5 level in eight of the nine subsided PP levels. Subsidence was associated with increased reaming depth and the use of larger cage sizes. Subsidence was usually in the posterior and superior end plate. The clinical outcome was not affected by subsidence. Subsidence incidence was not associated with the age, sex or weight of the patient.

CONCLUSIONS

There is a statistically significant increased incidence of subsidence at the L4-L5 level as opposed to other fused lumbar levels in ALIF fusions with BAK cage constructs. There is an increased incidence of subsidence with the PP constructs. Subsidence also is associated with increased reaming depth and with larger cage sizes. The lowest risk for subsidence was with single-level dual-standard cage constructs.

Reduction of disc space distraction after anterior lumbar interbody fusion with autologous iliac crest graft

STUDY DESIGN

A retrospective review with long-term clinical and radiologic assessment was conducted.

OBJECTIVE

To assess the severity and reasons for the reduction of disc space distraction after successful autograft fusion of the lumbar spine and its clinical consequences.

SUMMARY OF BACKGROUND DATA

Anterior lumbar interbody fusion is an established treatment for lumbar disc degeneration. It is not firmly established whether the grafted level narrows after surgery, and if so, what the clinical consequences are.

METHODS

This study assessed 67 patients who underwent anterior lumbar interbody fusion at L4-L5 with autologous iliac crest graft. The disc space height and angle between L4 and L5 were serially measured. Times until fusion and the presence of symptoms before and after surgery and at the latest follow-up assessment were noted.

RESULTS

The mean follow-up period was 14 years (range, 2.5-32 years). The fusion rate was 96% (64 of 67 patients), and the mean time to fusion was 9 months. In the group that had successful fusion, there was an initial increase in disc space distraction followed by a reduction in 55 patients (86%). The mean preoperative disc space height was 12.1 mm, which increased immediately after surgery to 16.2 mm, but had been reduced to 12.6 mm at the latest follow-up assessment. The reduction in distraction occurred within the first 3 months after surgery and was correlated with age, but not with recurrence of symptoms, the amount of initial distraction, or the gender of the individual. A similar trend was seen with L4-L5 segmental angulation.

CONCLUSIONS

Reduction of disc space distraction after anterior lumbar interbody fusion using tricortical iliac crest bone graft is a common finding. Despite this, the fusion rate is high, and there is no association with symptom recurrence.

In vitro and in vivo degradation of bioabsorbable PLLA spinal fusion cages

The in vitro and in vivo degradation of poly-L-lactic acid cages used as an adjunct to spinal arthrodesis was investigated. In the in vitro experiments cages were subjected to aging up to 73 weeks in phosphate-buffered solution (pH 7.4) at 37 degrees C. Inherent viscosity, crystallinity, and mechanical strength were determined at different time points. In the in vivo study, the poly-L-lactic acid cages were packed with bone graft and implanted in the L3-L4 spinal motion segment of 18 Dutch milk goats. At 12, 26, and 52 weeks, the motion segments were isolated and poly-L-lactic acid samples retrieved. On evaluation, the in vivo implanted cages showed an advanced decline in inherent viscosity compared to the cages subjected to in vitro degradation experiments. At 6 months of implantation, the geometrical shape and original height of 10 mm was maintained during 6 months of follow up. This finding fits well with the observation that mechanical strength was maintained for a period of 6 months in vitro. At 12 months, the poly-L-lactic acid cage had been disintegrated into multiple fragments with signs of absorption. Despite the high-load-bearing conditions, the poly-L-lactic acid cage allowed interbody fusion to occur without collapse of the cage.

Preliminary experience with anterior interbody titanium cage fusion for treatment of cervical disc disease

This study evaluated the efficacy and safety of titanium cage implants in cervical reconstruction to treat cervical spondylosis. Surgical data covered a 4-year period from January 1999 to December 2002 and included 34 consecutive patients, 20 men and 14 women, with ages ranging from 27 to 84 years (mean, 57 years). Patients underwent anterior cervical microdiscectomy followed by interbody fusion with a titanium cage implant (rather than an autogenous iliac crest bone graft) at a single level ranging from C3 to C7. Twenty-one patients had a herniated intervertebral disc, nine had degenerative disc disease, and four had previous failed autograft fusion surgery that required revision. At clinical presentation, 26 patients had neck pain, 23 had radiculopathy, and nine had myelopathy. Diagnostic imaging studies included spinal dynamic roentgenography, computerized tomography, and magnetic resonance imaging. Lesions were located at C3-4 in seven cases, C4-5 in 14 cases, C5-6 in nine cases, and C6-7 in four cases. The follow-up period ranged from 7 to 48 months (mean, 26 months). Results revealed that the procedure was technically feasible. There were no intra- or postoperative complications. The most commonly used cage was 9 mm high. Imaging studies showed no cage instability, migration, or pseudarthrosis. Although mild subsidence (< 5 mm) was observed in three cases, these patients preserved adequate postoperative cervical lordosis and the subsidence did not preclude a good clinical result. The advantages of this procedure over a similar operation using traditional tricorticate bone graft are: no graft morbidity; shorter operation time (mean time saved, 35 minutes); reduced blood loss (average blood loss, 75 mL); and early postoperative ambulation (mean, 4.7 hospital days). Nearly all patients rapidly lost their neck pain (92%, 24/26) and radicular symptoms (87%, 20/23) after surgery. The recovery rate from myelopathy was 44% (4/9). Progressive bony shield formation over the anterior/posterior cortex (sentinel sign) indicated fusion in five cases.

Porous titanium-nickel for intervertebral fusion in a sheep model: part 1. Histomorphometric and radiological analysis

Porous titanium-nickel (PTN) implants represent an alternative to traditional intervertebral fusion cages. Indeed, PTN materials possess interconnecting pores with cell capillarity properties that may promote bone ingrowth and intervertebral fusion without the need for bone grafting. In this study, a PTN intervertebral fusion device was compared to a conventional TiAlV cage packed with autologous bone in a sheep model. The two devices were implanted at two noncontiguous intervertebral lumbar sites for 3, 6, and 12 months. PTN osseointegration showed a time-dependent trend increasing from 21.4% to 37.6% (3-12 months), whereas TiAlV cages remained at the same level of bone ingrowth (22.7%-25.4%; 3-12 months). Furthermore, PTN bone apposition (10.9%-24.2%; 3-12 months) was significantly higher than that of TiAlV implants (1.1%-5.1%; 3-12 months; p < 0.001, ANOVA). Radiological fusion scores increased with postsurgery time regardless of material type, but were consistently superior for PTN (12.5-18.5; 3-12 months) than for TiAlV cages (2.0-15.0; 3-12 months; p < 0.001, ANOVA). Implant materials were not significantly different according to the radiological interbody index based on preoperative disc height: Interbody index began at 132.6% (PTN) and 123.5% (TiAlV) immediately after surgery, then declined to 80.8% (PTN) and 91.0% (TiAlV) after 12 months. Nevertheless, ungrafted PTN constituted an excellent substrate for osteogenic cell integration and represents a new osteoconductive biomaterial with improved fusion characteristics in comparison to conventional TiAlV cages.

Gene therapy and tissue engineering in repair of the musculoskeletal system

Historically, surgeons have sought and used different procedures in order to augment the repair of various skeletal tissues. Now, with the completion of the Human Genome Project, many researchers have turned to gene therapy as a means to aid various ailments. In the orthopedic field, many strides have been made toward using gene therapy and tissue engineering in a clinical setting. In this review, several studies are outlined in different areas that gene therapy has or will influence orthopedic surgery. Gene therapy and tissue engineering can aid in fracture healing and spinal fusions by inducing bone formation, ligamentous repairs by increasing the production of connective tissue fibers, intervertebral disc disease by creating potential replacements, and articular cartilage repairs by providing means to improve cartilage. As we continue to see great contributions, such as the few mentioned here, this field will continue to mature and develop.

Bioabsorbable poly-L-lactic acid cages for lumbar interbody fusion: three-year follow-up radiographic, histologic, and histomorphometric analysis in goats

STUDY DESIGN

Long-term evaluation was performed for bioabsorbable poly-L-lactic acid cages in a goat interbody fusion model.

OBJECTIVE

To assess the radiographic, histologic, and histomorphometric characteristics of poly-L-lactic acid cages during 3 years of follow-up evaluation.

SUMMARY OF BACKGROUND DATA

Failed cage fusions may be related to cage design and material in addition to the surgical technique used. To overcome material-related complications and to explore the potential benefits of bioabsorbable cages, poly-L-lactic acid cages have been designed.

METHODS

For this study, 36 Dutch milk goats underwent a lumbar interbody fusion procedure (L3-L4). Two types of custom-made cage devices were impacted with bone graft and implanted: poly-L-lactic acid cages (n = 30) and titanium cages (n = 6). Sequential harvesting of surgically managed motion segments (intervals: 3, 6, 12, 24, and 36 months) was performed for analysis.

RESULTS

In poly-L-lactic acid specimens, permanent interbody fusion could be achieved within 6 months after surgery with maintenance of cage height. Titanium specimens showed no interbody fusion within this period. Radiographic follow-up evaluation (6-36 months) showed interbody fusion in 86% (19/22) of poly-L-lactic acid specimens, as compared with 33% (2/6) of titanium specimens. After 36 months of implantation, in one half of the specimens, poly-L-lactic acid cages were completely absorbed. Bone histomorphometry showed complete bone remodeling after 2 years of follow-up evaluation. During the study period, no local or distant adverse histologic effects were observed.

CONCLUSIONS

The current study showed that poly-L-lactic acid cage devices are feasible for lumbar interbody fusion. New poly-L-lactic acid cages designed for clinical practice might be a viable alternative to current nonabsorbable cage devices.

Resorbable cages for spinal fusion: an experimental goat model

OBJECT

A biomechanical cadaveric study and an in vivo monosegmental spinal fusion study were performed to evaluate a novel bioresorbable poly(L-lactic acid) (PLLA) cage.

METHODS

The yield strength of a spinal segment was chosen as the main design parameter for the resorbable cages to be used in a goat model. In a 3-year in vivo study the authors found fusion to be significantly faster and more complete when using PLLA cages compared to titanium cages with the same dimensions. In the PLLA group, the intervertebral grafting height did not change and bone remodeling within the cage was completed 2 years after implantation. In terms of degradation of the PLLA, similar features were observed in vivo and in vitro.

CONCLUSIONS

Degradation was almost completed 3 years after implantation. Tissue reaction was mild during the 3-year period.

Evaluation of 70/30 poly (L-lactide-co-D,L-lactide) for use as a resorbable interbody fusion cage

OBJECT

Titanium lumbar interbody spinal fusion devices are reported to be 90% effective in cases requiring single-level lumbar interbody arthrodesis, although radiographic determination of fusion has been debated.

METHODS

Using blinded radiographic, biomechanical, histological, and statistical measures, researchers in the present study evaluated a radiolucent 70/30 poly(L-lactide-co-D,L-lactide) interbody fusion device packed with autograft or recombinant human bone morphogenetic protein-2 on a collagen sponge in 25 sheep at 3, 6, 12, 18, and 24 months. A trend of increased fusion stiffness, radiographic fusion, and histologically confirmed fusion was demonstrated at 3 months to 24 months postimplantation. Device degradation was associated with a mild to moderate chronic inflammatory response at all postoperative sacrifice times.

CONCLUSIONS

Use of this material in interbody fusion may be a viable alternative to metals.

Biomechanical evaluation of immediate stability with rectangular versus cylindrical interbody cages in stabilization of the lumbar spine

BACKGROUND

Recent cadaver studies show stability against axial rotation with a cylindrical cage is marginally superior to a rectangular cage. The purpose of this biomechanical study in cadaver spine was to evaluate the stability of a new rectangular titanium cage design, which has teeth similar to the threads of cylindrical cages to engage the endplates.

METHODS

Ten motion segments (five L2-3, five L4-5) were tested. From each cadaver spine, one motion segment was fixed with a pair of cylindrical cages (BAK, Sulzer Medica) and the other with paired rectangular cages (Rotafix, Corin Spinal). Each specimen was tested in an unconstrained state, after cage introduction and after additional posterior translaminar screw fixation. The range of motion (ROM) in flexion-extension, lateral bending, and rotation was tested in a materials testing machine, with +/- 5 Nm cyclical load over 10 sec per cycle; data from the third cycle was captured for analysis.

RESULTS

ROM in all directions was significantly reduced (p < 0.05) with both types of cages. There was no significant difference in reduction of ROM in flexion-extension (p = 0.6) and rotation (p = 0.92) between the two cage groups, but stability in lateral bending was marginally superior with the rectangular cages (p = 0.11). Additional posterior fixation further reduced the ROM significantly (p < 0.05) in most directions in both cage groups, but did not show any difference between the cage groups.

CONCLUSIONS

There was no significant difference in immediate stability in any direction between the threaded cylindrical cage and the new design of the rectangular cage with endplate teeth.

Resorbable cages for spinal fusion: an experimental goat model

A biomechanical cadaver study and an in vivo monosegmental spinal fusion study were performed to evaluate a novel bioresorbable poly-L-lactic acid (PLLA) cage. The yield strength of a spinal segment was chosen as the main design parameter for the resorbable cages to be used in a goat model. A 3-year in vivo study revealed a significantly faster and more complete fusion using PLLA cages as compared to titanium cages with the same dimensions. In the PLLA group, the intervertebral grafting height did not change and bone remodeling within the cage was completed 2 years after implantation. In terms of degradation of the PLLA, similar features were observed in vivo and in vitro. Degradation was almost completed 3 years after implantation. Tissue reaction was mild during the 3-year period.

Evaluation of 70/30 D,L-PLa for use as a resorbable interbody fusion cage

Titanium lumbar interbody spinal fusion devices are reported to be 90% effective for single-level lumbar interbody fusion, although radiographic determination of fusion has been debated. Using blinded radiographic, biomechanic, histologic, and statistical measures, researchers in the present study evaluated a radiolucent 70/30 poly(L-lactide-co-D,L-lactide) (70/30 D,L-PLa) interbody fusion device packed with autograft or rhBMP-2 on a collagen sponge in 25 sheep at 3, 6, 12, 18, and 24 months. A trend of increased fusion stiffness, radiographic fusion, and histologic fusion was demonstrated from 3 months to 24 months. Device degradation was associated with a mild to moderate chronic inflammatory response at all postoperative sacrifice times.

Load transfer mechanisms in cylindrical interbody cage constructs

STUDY DESIGN

A finite-element model was created to study parametrically the load transfer characteristics of a lower lumbar motion segment implanted with dual anteroposterior cylindrical interbody cages.

OBJECTIVES

To describe the frontal plane bone-implant interface stresses acting on a generic cylindrical interbody cage, to evaluate the effect of implant material properties on these stresses, and to determine the associated load transfer mechanisms.

SUMMARY OF BACKGROUND DATA

From a biomechanical perspective, the long-term success of an interbody cage fusion depends on effective load transfer. The cage must stress the graft sufficiently to promote fusion, while keeping bone-implant interface stresses in a range that will prevent implant subsidence or loosening. At this writing, no published study has described interface stresses or load transfer mechanisms for these devices.

METHODS

A planar finite-element model was used to simulate uniform compression loading of the implanted segment. Material properties of the interbody cage were varied to simulate cortical bone, titanium, and stainless steel implants. Normal and shear interface stresses were output along the length of the interface.

RESULTS

Magnitudes of both the normal and shear interface stresses were substantially higher at the medial and lateral sides of the interface than in the center. Interface stresses were largely independent of implant material.

CONCLUSIONS

Cylindrical interbody implants have inherent limitations, including stress concentrations at the bone-implant interface and possible stress shielding of the graft. The results from the current study suggest that implants made of cortical bone have substantially the same load transfer characteristics as metal devices of similar geometry.

Low fusion rate after L5-S1 laparoscopic anterior lumbar interbody fusion using twin stand-alone carbon fiber cages

STUDY DESIGN

Prospective study of a cohort of patients who underwent L5-S1 laparoscopic anterior lumbar interbody fusion.

OBJECTIVES

To assess the fusion rate and the clinical outcome more than 2 years after L5-S1 laparoscopic anterior lumbar interbody fusion using twin stand-alone carbon-fiber cages.

SUMMARY OF BACKGROUND DATA

The first reports on laparoscopic anterior lumbar interbody fusion using stand-alone cages appeared in 1995. Since then several articles have reported contradictory data regarding fusion rate. There are no publications describing the fusion rate of stand-alone lumbar carbon-fiber cages.

METHODS

The authors evaluated 12 patients (mean age 36.5 years) in whom endoscopic L5-S1 anterior lumbar interbody fusion was performed using twin stand-alone laparoscopic carbon-fiber cages. Clinical evaluation was carried out prospectively by the use of three self-evaluation scales. Radiologic evaluation was performed by an independent radiologist using dynamic flexion-extension films and CT scans at 6 and 12 months after surgery and subsequently every year until fusion was demonstrated.

RESULTS

After a mean follow-up of 36.6 months (range 24-63 months) the clinical condition of the patients was significantly better than their preoperative status: visual analog scale (P < 0.01), Prolo score (P < 0.05), and Waddell Disability Index (P < 0.01). L5-S1 mobility did not exceed 5 degrees in any dynamic study. However, the overall CT scan fusion rate at 2 years of follow-up was 16.6%. Three years after surgery, CT demonstrated fusion in one of five patients.

CONCLUSION

Two years after endoscopic L5-S1 anterior lumbar interbody fusion using twin stand-alone laparoscopic carbon-fiber cages, the fusion rate was unacceptably low. However, the clinical outcomes of these patients were significantly improved compared with their preoperative status.

Pullout test with three lumbar interbody fusion cages

STUDY DESIGN

In vitro biomechanical testing was performed on 12 cadaveric human lumbar spines.

OBJECTIVE

To determine the initial dislocation resistance, as quantified by the pullout force of three different cage designs.

SUMMARY OF BACKGROUND DATA

Interbody cage devices frequently are used as stand-alone cages in the surgical treatment of degenerative conditions in the lumbar spine. In contrast to the wide clinical acceptance of interbody fusion cages, there are only a few biomechanical studies of posterior pullout trials.

METHODS

Cylindrical threaded cages (Ray TFC Surgical Dynamics), bullet-shaped cages (Stryker), and newly designed rectangular titanium cages with an endplate anchorage device (Marquardt) were used for posterior interbody implants. For each device, the pullout test was performed in four specimens on both sides (L3-L4).

RESULTS

In the pullout test, the Stryker cages required a median pullout force of 130 N (minimum, 100 N; maximum, 220 N), as compared with the higher pullout force of the Marquardt cages (median, 605 N; minimum, 450 N; maximum, 680 N), and the Ray cages (median, 945 N; minimum, 125 N; maximum, 2230 N).

CONCLUSIONS

Differences in pullout resistance were noted depending on the cage design. A cage design with threads or a hook device provides superior stability, as compared with ridges. The initial pullout resistance was highest for the Ray cages and lowest for the Stryker cages.

In vitro stability of FRA spacers with integrated crossed screws for anterior lumbar interbody fusion

STUDY DESIGN

Mechanical testing of human lumbar functional spine units was carried out after instrumenting the disc space with femoral ring allografts (FRAs) with and without integrated crossed anterior screws applied into the adjacent bodies.

OBJECTIVES

To assess the stability of FRA construct with and without the integrated crossed anterior screws and to compare that with the intact specimen.

SUMMARY OF BACKGROUND DATA

Most modern methods of achieving anterior lumbar intervertebral fusion rely on the use of interbody spacers to restore and maintain intervertebral height, overall alignment, and stability while facilitating arthrodesis. The FRAs have the advantage of biologic compatibility but may not have enough stability when used as stand-alone devices. FRA spacers alone are less stiff in torsion and extension compared with other instrumented constructs. Increased motion could lead to higher failure rates because of graft migration and pseudarthrosis. This makes the use of supplementary anterior or posterior fixation necessary. The current authors hypothesized that the addition of anterior integrated crossed screws applied through the FRA spacer into the adjacent vertebral bodies would increase the stability of the FRA spacer in extension and torsion in the absence of further posterior instrumentation.

METHODS

Seven fresh-frozen human cadaveric lumbar spine functional spine units were tested applying cantilever bending moments in flexion, lateral bending, torsion, and extension. The specimens were tested in the following sequence: intact, with FRA spacer alone and with FRA spacer and integrated crossed screws. The stiffness of each construct was then compared with the intact specimen. Specimens with obvious deformity on radiographs or dual-energy radiograph absorptiometry t score values <-1.2 were excluded.

RESULTS

The addition of the integrated crossed metal screw system improved the stiffness of the construct by 53% over the intact (P = 0.02) and by 31% over the FRA alone in extension (P = 0.01), whereas it improved the stiffness by 40% over the intact (P = 0.03) and by 18% over the FRA alone in torsion (P = 0.21). The crossed screw system did not improve the stiffness compared with intact in either flexion or lateral bending modes.

CONCLUSIONS

Although there is a trend toward improved stability of the FRA spacers in torsion with the addition of the metal screws, this is not statistically significant. The integrated crossed anterior metal screw system significantly improves the stability of the FRA spacers in extension when used for anterior lumbar interbody fusion.

The effect of cage stiffness on the rate of lumbar interbody fusion: an in vivo model using poly(l-lactic Acid) and titanium cages

STUDY DESIGN

A goat interbody fusion model using poly-(L-lactic acid) and titanium cages was designed to evaluate the effect of cage stiffness on lumbar interbody fusion.

OBJECTIVE

To investigate the effect of cage stiffness on the rate of interbody fusion.

SUMMARY OF BACKGROUND DATA

Various types of cages considerably exceed the stiffness of vertebral bone, which ultimately may lead to postoperative complications. To avoid these complications, poly-(L-lactic acid) cages with limited stiffness have been designed. The mechanical integrity of the cages remains intact for at least 6 months.

METHODS

Interbody fusions were performed at L3-L4 of 15 Dutch milk goats, and one of three cages was randomly implanted: 1) a titanium cage (n = 3), 2) a stiff poly-(L-lactic acid) cage (n = 6), or 3) a flexible poly-(L-lactic acid) cage (n = 6). Interbody fusion was assessed radiographically by three independent observers 3 and 6 months after surgery.

RESULTS

At 3 months, all the poly-(L-lactic acid) specimens showed ingrowth of new bone, but with radiolucency in the fusion mass. At 6 months, solid arthrodesis was observed in four of six poly-(L-lactic acid) specimens, advanced ingrowth in one specimen, and infection in one specimen. Titanium cages showed ingrowth of bone, but with radiolucency in the fusion mass. Interbody fusion using poly-(L-lactic acid) cages showed a significantly higher rate statistically (P = 0.016) and more complete fusion than titanium cages of the same design.

CONCLUSIONS

The reduced stiffness of poly-(L-lactic acid) cages showed enhanced interbody fusion, as compared with titanium cages after 6 months. Bioabsorbable poly-(L-lactic acid) cages thus may be a viable alternative to current interbody cage devices, thereby avoiding the concomitant problems related to their excessive stiffness. However, the bioabsorbability of the poly-(L-lactic acid) cages awaits investigation in a long-term study currently underway.

Influence of cage design on interbody fusion in a sheep cervical spine model

OBJECT

The purpose of this study was to compare the characteristics of interbody fusion achieved using an autologous tricortical iliac crest bone graft with those of a cylinder- and a box-design cage in a sheep cervical spine model. This study was designed to determine whether there are differences between three interbody fusion procedures in: 1) ability to preserve postoperative distraction; 2) biomechanical stability; and 3) histological characteristics of intervertebral bone matrix formation.

METHODS

Twenty-four sheep underwent C3-4 discectomy and fusion in which the following were used: Group 1, autologous tricortical iliac crest bone graft (eight sheep); Group 2, titanium cylinder-design cage filled with autologous iliac crest bone graft (eight sheep); and Group 3, titanium box-design cage filled with autologous iliac crest graft (eight sheep). Radiography was performed pre- and postoperatively and after 1, 2, 4, 8, and 12 weeks. At the same time points, disc space height, intervertebral angle, and lordosis angle were measured. After 12 weeks, the sheep were killed, and fusion sites were evaluated by obtaining functional radiographs in flexion and extension. Quantitative computerized tomography scans were acquired to assess bone mineral density, bone mineral content, and bone callus volume. Biomechanical testing was performed in flexion, extension, axial rotation, and lateral bending. Stiffness, range of motion, neutral zone, and elastic zone were determined. Histomorphological and histomorphometric analyses were performed, and polychrome sequential labeling was used to determine the time frame of new bone formation. Over a 12-week period significantly higher values for disc space height and intervertebral angle were shown in cage-treated sheep than in those that received bone graft. Functional radiographic assessment revealed significantly lower residual flexion-extension movement in sheep with the cylinder cage-fixed spines than in those that received bone graft group. The cylinder-design cages showed significantly higher values for bone mineral content, bone callus content, and stiffness in axial rotation and lateral bending than the other cages or grafts. Histomorphometric evaluation and polychrome sequential labeling showed a more progressed bone matrix formation in the cylindrical cage group than in both other groups.

CONCLUSIONS

Compared with the tricortical bone graft, both cages showed significantly better distractive properties. The cylindrical cage demonstrated a significantly higher biomechanical stiffness and an accelerated interbody fusion compared with the box-design cage and the tricortical bone graft. The differences in bone matrix formation within both cages were the result of the significantly lower stress shielding on the bone graft by the cylinder-design cage.

Advances in posterior lumbar interbody fusion

Since the introduction of the PLIF by Cloward in the 1950s, there have been great improvements in our understanding of the pathophysiology of spinal degenerative diseases and the potential morbidities associated with their surgical management. The advent of durable interbody fusion cages and bone grafting alternatives has lessened the morbidity of bone graft harvesting and disc space collapse and has providing improved stiffness and stability to the affected spinal motion segment. Although criticized by some surgeons for the potential for spinal destabilization and nerve root injuries, newer techniques of posterior interbody access and improved implant designs have led to shortened surgical times, less neurologic injury, and improved overall outcomes. The introduction of the transforaminal lumbar interbody fusion procedure has allowed surgeons to achieve successful fusion without the risk of nerve root tethering that is seen so frequently with standard PLIF techniques. Further study and long-term follow-up regarding its efficacy are necessary before it can be recommended to supplant the more traditional PLIF techniques.

Histologic evaluation of the efficacy of rhBMP-2 compared with autograft bone in sheep spinal anterior interbody fusion

STUDY DESIGN

The sheep anterior lumbar spinal fusion model was used to study the efficacy of recombinant human bone morphogenetic protein-2 (rhBMP-2)-collagen composite in comparison with autograft to enhance spinal interbody fusion. Comparisons were drawn from temporal radiographic and end-point biomechanical and histologic data.

OBJECTIVE

To analyze histologically the ability of rhBMP-2 to achieve complete arthrodesis between vertebral bodies.

SUMMARY OF BACKGROUND DATA

Studies using rhBMP for enhancement of anterior interbody fusion have used numerous endpoints. However, systematic histologic evaluation of the fusion has not been conducted.

METHODS

Twelve sheep underwent single-level anterior lumbar interbody fusion performed with a cylindrical fenestrated titanium interbody fusion device (INTER FIX, Medtronic Sofamor Danek, Inc., Memphis, TN). The device was filled either with rhBMP-2-collagen (n = 6) or autogenous iliac crest bone graft (n = 6). Radiologic evaluation was carried out at 2-month intervals, and all sheep were killed 6 months after surgery. Nondestructive biomechanical testing for stiffness to flexion, extension, and lateral bending moments, un-decalcified histology, and qualitative and quantitative histologic evaluation were performed.

RESULTS

Radiographs revealed a bony bridge anterior to the cage in five of six rhBMP-2-treated animals, whereas it was present only in one of five in the autogenous bone graft group. Segments treated with rhBMP-2 were 20% stiffer in flexion than autograft-treated segments at 6 months. Six of six in the rhBMP-2 group and two of six in the autograft group showed complete fusion. There was a significantly higher rate of bony continuity observed at the fenestrations of the rhBMP-2 group. Three times more number of cage fenestrations in the rhBMP-2 group demonstrated "all-bone" when compared with the autograft group (P < 0.001). Further, the scar tissue in and around the autograft-treated cages was 16-fold more (P < 0.01) than that seen for rhBMP-2-treated cages.

CONCLUSIONS

The study demonstrates that rhBMP-2 can lead to earlier radiologic fusion and a more consistent increased stiffness of the segments when compared with autograft in sheep anterior lumbar interbody fusion. Furthermore, a three times higher histologic fusion rate is attainable with significantly reduced fibrous tissue around the implant when rhBMP-2 is used.

Retroperitoneal lateral lumbar interbody fusion with titanium threaded fusion cages

OBJECT

Intertransverse arthrodesis in which instrumentation is placed is associated with an excellent fusion rate; however, treatment of patients with symptomatic nonunion presents a number of difficulties. Revision posterior and traditional anterior procedures are associated with methodological problems. For example, in the latter, manipulation of the major vessels from L-2 to L-4 may be undesirable. The authors describe a method for performing retroperitoneal lumbar interbody fusion (LIF) in which a threaded cage is placed from L-2 through L-5 via a lateral trajectory, and they also detail a novel technique for implanting a cage from L-5 to S-1 via an oblique trajectory. Although they present data obtained over a 2-year period in the study of 15 patients, the focus of this report is primarily on describing the surgical procedure.

METHODS

The lateral lumbar spine was exposed via a standard retroperitoneal approach. Using the anterior longitudinal ligament as a landmark, the L2-3 through L4-5 levels were fitted with instrumentation via a true lateral trajectory; the L5-S1 level was fitted with instrumentation via an oblique trajectory. A single cage was placed at each instrumented level. Fifteen symptomatic patients in whom previous lumbar fusion had failed underwent retroperitoneal LIF. Thirty-eight levels were fitted with instrumentation. There have been no instrumentation-related failures, and fusion has occurred at 37 levels during the 2-year postoperative period.

CONCLUSIONS

The use of retroperitoneal LIF in which threaded fusion cages are used avoids the technical difficulties associated with repeated posterior procedures. In addition, it allows L2-S1 instrumentation to be placed anteriorly via a single surgical approach. This construct has been shown to be biomechanically sound in animal models, and it appears to be a useful alternative for the management of failed multilevel intertransverse arthrodesis.

Mesh cages for spinal deformity in adults

Correction of deformity including restoration of sagittal plane malalignment in patients with spinal disorders is facilitated by placement of structural allografts and cages into the anterior column. The effectiveness of these implants will be reviewed with a focus on anterior column implantation of titanium mesh cages. In a consecutive study of adult patients who had spinal fusions for sagittal plane deformities using structural titanium mesh cages, cage status, fusion status, and outcome were assessed. Followup ranged from 24 to 62 months (average, 33 months). No cage failure of extrusion was observed. The average segmental improvement in lordosis with cage implantation was 11 degrees with a loss of correction of less than 1 degree at latest followup. No radiographic gold standard for fusion assessment with metallic implants in the anterior column could be found in the literature. The use of radiographic techniques for fusion assessment and functional outcome data for patients with implanted titanium mesh cages will be discussed.