Effects of anterior vertebral grafting on the traumatized lumbar spine after pedicle screw-plate fixation

Multivariate analysis of risk factors for predicting supplementary posterior instrumentation after anterolateral decompression and instrumentation in treating thoracolumbar burst fractures

Background

Although anterolateral decompression and instrumentation has several advantages in treating thoracolumbar burst fractures, the risk factors for supplementary posterior instrumentation are still unclear.

Methods

We retrospectively reviewed 238 patients who underwent anterolateral decompression and instrumentation for single-level thoracolumbar burst fractures from January 2010 and March 2012. The influences of several potential risk factors that might affect supplementary posterior instrumentation were assessed using univariate and multivariate analyses.

Results

Twenty seven patients who developed worsening back pain without neurological deterioration after the anterolateral approach treatment need further posterior instrumentation fixation. The univariate analysis showed that age, disruption of the posterior longitudinal ligament complex (PLC), and fracture level were the risk factors for supplementary posterior instrumentation. However, age and integrity of the PLC were the independent risk factors for supplementary posterior instrumentation by multivariate analyses.

Conclusions

Supplemental posterior instrumentation was necessary in 11.3% of cases following anterolateral decompression and instrumentation in the present study. Older age and disruption of the PLC were the independent risk factors in prediction of supplementary posterior instrumentation in treating thoracolumbar burst fractures.

Risk factors for supplementary posterior instrumentation after anterolateral decompression and instrumentation in thoracolumbar burst fractures

BACKGROUND

In spite of the established benefits of anterolateral decompression and instrumentation (ALDI) for thoracolumbar burst fractures (TLBF), the indications for supplementary posterior instrumentation remain unclear.

METHODS

A retrospective review of clinical and radiographic data of a prospective cohort of 73 patients who underwent ALDI for TLBF from T12 to L4.

RESULTS

The mean age of the cohort was 42 ± 15 years, with 49 males and 24 females. Forty-six patients had neurological deficit, and 27 were intact. Owing to symptomatic settling, supplemental posterior instrumentation was performed in 7 out of 73 patients. The age of patients requiring supplemental posterior instrumentation (59 ± 14 years) exceeded that of patients who did not (41 ± 16, p=0.004). Otherwise, the patients who required posterior instrumentation were comparable to those treated with ALDI in terms of body mass index (BMI), American Spinal Injury Association (ASIA) scores on admission and follow-up, residual spinal canal, and local kyphosis on admission and follow-up. The posterior ligamentous complex (PLC) integrity was assessed in 38 patients in whom the MRI scans were retrievable, 31 successfully treated with ALDI, and all 7 undergoing supplementary posterior instrumentation. Subgroup analysis demonstrated that there was no difference in the incidence of PLC disruption between the 2 groups (p=0.257).

CONCLUSIONS

Secondary supplemental posterior instrumentation was deemed necessary in 10% of cases following ALDI. Age was the only significant risk factor predicating supplemental posterior instrumentation.

Surgical management of minimally invasive anterior lumbar interbody fusion with stand-alone interbody cage for L4-5 degenerative disorders: clinical and radiographic findings

Surgical treatment for degenerative spinal disorders is controversial, although lumbar fusion is considered an acceptable option for disabling lower back pain. Patients underwent instrumented minimally invasive anterior lumbar interbody fusion (mini-ALIF) using a retroperitoneal approach except for requiring multilevel fusions, severe spinal canal stenosis, high-grade spondylolisthesis, and a adjacent segments disorders. We retrospectively reviewed the clinical records and radiographs of 142 patients who received mini-ALIF for L4-5 degenerative lumbar disorders between 1998 and 2010. We compared preoperative and postoperative clinical data and radiographic measurements, including the modified Japanese Orthopaedic Association (JOA) score, visual analog scale (VAS) score for back and leg pain, disc height (DH), whole lumbar lordosis (WL), and vertebral wedge angle (WA). The mean follow-up period was 76 months. The solid fusion rate was 90.1% (128/142 patients). The average length of hospital stay was 6.9 days (range, 3–21 days). The mean blood loss was 63.7 ml (range, 10–456 ml). The mean operation time was 155.5 min (range, 96–280 min). The postoperative JOA and VAS scores for back and leg pain were improved compared with the preoperative scores. Radiological analysis showed significant postoperative improvements in DH, WL, and WA, and the functional and radiographical outcomes improved significantly after 2 years. The 2.8% complication rate included cases of wound infection, liquorrhea, vertebral body fractures, and a misplaced cage that required revision. Mini-ALIF was found to be associated with improved clinical results and radiographic findings for L4-5 disorders. A retroperitoneal approach might therefore be a valuable treatment option.

Reduction of the domino effect in osteoporotic vertebral compression fractures through short-segment fixation with intravertebral expandable pillars compared to percutaneous kyphoplasty: a case control study

Background

Osteoporotic vertebral compression fracture is the leading cause of disability and morbidity in elderly people. Treatment of this condition remains a challenge. Osteoporotic vertebral compression fractures can be managed with various approaches, but each has limitations. In this study, we compared the clinical outcomes obtained using short-segment fixation with intravertebral expandable pillars (I-VEP) to those obtained with percutaneous kyphoplasty in patients who had suffered vertebral compression fractures.

Methods

The study included 46 patients with single-level osteoporotic thoracolumbar fractures. Twenty-two patients in Group I underwent short-segment fixation with I-VEP and 24 patients in Group II underwent kyphoplasty. All patients were evaluated pre- and postoperatively using a visual analogue scale, anterior height of the fractured vertebra, and kyphotic angle of the fractured vertebra. The latter 2 radiological parameters were measured at the adjacent segments as well.

Results

There was no significant difference between the groups in terms of gender or fracture level, but the mean age was greater in Group II patients (p = 0.008). At the 1-year follow-up, there were no significant differences in the visual analogue scale scores, anterior height of the fractured vertebra, or the value representing anterior height above the fractured vertebra and kyphotic angle below the fractured vertebra, after adjusting for the patients’ gender, fracture level, and age. When considered separately, the anterior height below the fractured vertebra was significantly higher and the kyphotic angle above the fractured vertebra was significantly smaller in Group I than in Group II (p = 0.029 and p = 0.008, respectively). The kyphotic angle of the fractured vertebra was significantly smaller in Group II than in Group I (p < 0.001).

Conclusions

In older individuals with vertebral compression fractures, kyphoplasty restored and maintained the collapsed vertebral body with less kyphotic deformity than that induced by short-segment fixation with I-VEP. Short-segment fixation with I-VEP was more effective in maintaining the integrity of adjacent segments, which prevented the domino effect often observed in patients with osteoporotic kyphotic spines.

Biomechanics of polyaryletherketone rod composites and titanium rods for posterior lumbosacral instrumentation

Object

Interest is increasing in the development of polyaryletherketone (PAEK) implants for posterior lumbar fusion. Due to their inherent physical properties, including radiolucency and the ability to customize stiffness with carbon fiber reinforcement, they may be more advantageous than traditional instrumentation materials. Customization of these materials may allow for the development of a system that is stiff enough to promote fusion, yet flexible enough to avoid instrumentation failure. To understand the feasibility of using such materials in posterior lumbosacral instrumentation, biomechanical performances were compared in pure moment and combined loadings between two different PAEK composite rods and titanium rods.

Methods

Four human cadaver L3–S1 segments were subjected to pure moment and combined (compressionflexion and compression-extension) loadings as intact specimens, and after L-4 laminectomy with complete L4–5 facetectomy. Pedicle screw/rod fixation constructs were placed from L-4 to S-1, and retested with titanium, pure poly(aryl-ether-ether-ketone) (PEEK), and carbon fiber reinforced PEEK (CFRP) rods. Reflective markers were fixed to each spinal segment. The range of motion data for the L3–S1 column and L4–5 surgical level were obtained using a digital 6-camera system. Four prewired strain gauges were glued to each rod at the level of the L-4 screw and were placed 90° apart along the axial plane of the rod to record local strain data in the combined loading mode. Biomechanical data were analyzed using the ANOVA techniques.

Results

In pure moment, when compared with intact specimens, each rod material similarly restricted motion in each mode of bending, except axial rotation (p < 0.05). When compared with postfacetectomy specimens, each rod material similarly restricted motion (p < 0.05) in all bending modes. In combined loading, rod stiffness was similar for each material. Rod strain was the least in the titanium construct, intermediate in the CFRP construct, and maximal in the pure PEEK construct.

Conclusions

Pure PEEK and CFRP rods confer equal stiffness and resistance to motion in lumbosacral instrumentation when compared with titanium constructs in single-cycle loading. The carbon fiber reinforcement reduces strain when compared with pure PEEK in single-cycle loading. These biomechanical responses, combined with its radiolucency, suggest that the CFRP may have an advantage over both titanium and pure PEEK rods as a material for use in posterior lumbosacral instrumentation. Benchtop fatigue testing of the CFRP constructs is needed for further examination of their responses under multicycle loading.

ROD CANTILEVER TECHNIQUES

CANTILEVER BEAM FIXATION techniques have a broad application in spine surgery, including the treatment of thoracolumbar spinal deformities. There are traditionally three cantilever beam fixation types described: fixed moment arm, nonfixed moment arm, and applied moment arm. In practice, however, most constructs are applied in a hybrid fashion. The basic tenets of cantilever beam fixation are provided in this article.

A review of the management of thoracolumbar burst fractures

BACKGROUND

Burst fractures account for more than half of all thoracolumbar fractures, which often cause a neurologic deficit and present a significant economic burden to the family and society. Accepted methods of treatment of thoracolumbar burst fractures include conservative therapy, posterior reduction and instrumentation, and anterior decompression and instrumentation. However, the management of thoracolumbar burst fractures has been the subject of much controversy.

METHODS

Publications reporting clinical data relating to the thoracolumbar burst fractures were reviewed. These articles were determined via review of the results of PubMed searches and articles gathered through compilation of references from those articles.

RESULTS

There exist different criteria for the choice of the management based on the severity of kyphotic deformity, canal compromise, vertebral height loss, and neurologic status. To our knowledge, none of the existing criteria for the treatment of thoracolumbar burst fractures are generally accepted.

CONCLUSIONS

In thoracolumbar burst fractures without a neurologic deficit, there is no superiority of conservative therapy over operative therapy. When the neurologic involvement is significant, the choice of operative management is advised. Also, there is no obvious superiority of one approach over the other.

Biomechanical effects of the body augmenter for reconstruction of the vertebral body

STUDY DESIGN

An in vitro biomechanical study of the stabilizing effects of the body augmenter and posterior instrumentation on experimental thoracolumbar fractures with vertebral defects.

OBJECTIVE

To evaluate the effects of the body augmenter and instrumentation on the stability of the spine-device construct.

SUMMARY OF BACKGROUND DATA

Posterior instrumentations alone are widely used to accomplish spinal reduction and provide stability for an injured spine; however, implant failure rates have been reported to be approximately 20%. Transpedicular discectomy and bone graft has reported only 33% fusion rates. Combined anterior bony strut and posterior instrumentation was a challenge to geriatric patients with vulnerable medical conditions and possible vascular and pulmonary complications. Therefore, a new design, the body augmenter, tries to reconstruct the vertebral body through internal mechanical support and also encourage bony fusion. This study is to evaluate its initial mechanical effects.

METHODS

Twenty fresh porcine T11-L3 vertebrae were harvested. The L1 vertebra with one third or one half corpectomy was performed to simulate a fracture injury with vertebral defects. Posterior instrumentation alone (PI group), posterior instrumentation with body augmenters (BA group), and anterior instrumentation with tricortical bony strut and DCP 1 level above and 1 level below the fracture site (DCP group) were applied as treatment strategies. Load-displacement and torque-angle plots were generated and used to calculate axial stiffness and torsional rigidity for these constructs with vertebral fracture at the L1 vertebrae. Axial compression, extension, and flexion tests were performed at intact and spine-device constructs to document spinal stability.

RESULTS

The construct stability had a complex association to the device applied. In the one third corpectomy group, the BA group had significantly higher compression stiffness than the PI group. In the one half corpectomy group, the flexion and compression stiffness of the BA group became significantly greater than the PI group, and the extension stiffness is significantly higher than the DCP group.

CONCLUSIONS

The body augmenters combined with posterior instrumentation increased the spinal construct stability during compression, flexion, and extension. According to results in this study, the body augmenter could provide a better initial stability of construct and prevent the implant failure of posterior instrumentation and may be a feasible substitute for the anterior role in the future.

Late results of thoracolumbar fractures after posterior instrumentation and transpedicular bone grafting

STUDY DESIGN

A retrospective clinical study was performed.

OBJECTIVE

To study clinical and radiologic late results after posterior stabilization of thoracolumbar fractures with internal fixator and interbody fusion via transpedicular bone grafting.

SUMMARY OF BACKGROUND DATA

The posterior approach, using an internal fixator, is a standard procedure for stabilizing the injured thoracolumbar spine. Transpedicular bone grafting was invented by Daniaux in 1986 for achieving an interbody fusion. Pedicle screw fixation with additional transpedicular fusion has remained controversial because of inconsistent reports and a lack of late results.

METHODS

Between January 1989 and July 1992, 76 patients with thoracolumbar fractures were operatively treated, and after a mean of more than 3 years, 56 of 62 patients (90%) still alive who had their implants removed were examined.

RESULTS

According to the Magerl classification, 33 patients sustained Type A, 13 Type B, and 10 Type C fractures. Three patients with incomplete paraplegia returned to normal. In one case of complete paraplegia, no change occurred. The mean operative time was 3 hours. In this study, two complications (3.6%) were observed: one iatrogenic vertebral arch fracture without consequences and one deep infection. Compared with the preoperative status, follow-up examinations demonstrated permanent physical and social sequelae: The percentage of individuals able to do physical labor was reduced by half (22 to 11 patients), whereas the share of unemployed or retired patients doubled (4 to 8 patients). At the time of follow-up examination, only 21 of 42 patients continued in sports. The assessment of reported problems and functional outcome with the Hannover spine score reflected a significant difference between the status before injury (96.6/100 points) and at the time of follow-up evaluation (71. 4/100 points) (P < 0.001).The radiographic assessment in the lateral plane (Cobb technique) demonstrated a significant (P < 0.001) mean restoration from an initial angle of -15.6 degrees (kyphosis) to +0. 4 degrees (lordosis). Serial postoperative radiographic follow-up assessment showed progressive loss of correction. At follow-up examination, a mean difference from the postoperative angle of 10.1 degrees was found (P < 0.001). Compared with the preoperative deformity, a mean improvement of 6.1 degrees (average, -9.7 degrees ) at follow-up examination was noted. The addition of transpedicular cancellous bone grafting did not decrease the loss of correction. Computed tomography scans after implant removal were performed in nine cases: Only three of nine patients showed evidence of intervertebral fusion. No correlation could be found between the Magerl classification and radiographic outcome. However, the preoperative wedge angle of the vertebral body correlated significantly with the postoperative loss of reduction.

CONCLUSIONS

Because of the disappointing results from this study, the authors cannot recommend the additional transpedicular cancellous bone grafting as an interbody fusion technique after posterior stabilization in cases of complete or incomplete burst injury to the vertebral body.

Can insertional torque predict screw loosening and related failures? An in vivo study of pedicle screw fixation augmenting posterior lumbar interbody fusion

STUDY DESIGN

An investigation of the relation between intraoperative insertional torque of pedicle screws, bone mineral density of the vertebra, and development of screw loosening in vivo.

OBJECTIVES

To determine the usefulness of intraoperative measurement of the insertional torque of pedicle screws.

SUMMARY OF BACKGROUND DATA

Some biomechanical studies have demonstrated that the insertional torque is highly correlated with bone mineral density and the stability of pedicle screws in vitro.

METHODS

Pedicle screw fixation was performed with posterior lumbar interbody fusion in 62 consecutive patients. The mean age of the patients at the time of surgery was 58 years. The insertional torque of pedicle screws was measured intraoperatively in all patients. The mean follow-up period was 2.7 years.

RESULTS

The mean insertional torque was 1.28 +/- 0.37 Nm in patients with screw loosening and 1.50 +/- 0. 40 Nm in patients without the problem. The mean insertional torque in patients with compression fractures in the upper vertebra adjacent to the fixed segment was 0.83 +/- 0.23 Nm. There was no significant difference between the mean insertional torque in patients with screw loosening and those without the condition. The mean insertional torque in patients without screw loosening was significantly greater than in patients with compression fractures (P < 0.01). A high correlation was found between insertional torque and bone mineral density (P < 0.01).

CONCLUSIONS

Although a high correlation was found between the insertional torque of pedicle screws and bone mineral density in vivo, the insertional torque could not objectively predict screw loosening.

Posterior lumbar interbody fusion: a retrospective study of complications after facet joint excision and pedicle screw fixation in 148 cases

We retrospectively evaluated neurological and other complications related to posterior lumbar interbody fusion (PLIF) performed by facet joint excision and pedicle screw fixation, in 148 consecutive patients with degenerative disorders. Their mean age at surgery was 59 (19-80) years. The mean follow-up period was 3 (2-6.5) years. Overall, 91 complications in 75 cases were observed. Although no permanent neural damage was detected, transient neural palsy occurred in 8% of the cases. Dural tear, partial misplacement, loosening, breakage of the pedicle screw and loss of correction were seen in 6, 6, 4, 1 and 1 of the cases, respectively. Deep infection of the fused segment developed in 2 cases. We conclude that PLIF, performed by facet joint excision and pedicle screw fixation, demonstrated a very low incidence of osteosynthesis failure, such as screw loosening, breakage and loss of correction. However, the high incidence of other complications, particularly neurological impairment, is still a disadvantage of this technically-demanding procedure.

Characteristics of pedicle screw loading. Effect of surgical technique on intravertebral and intrapedicular bending moments

STUDY DESIGN

A static nondestructive bending analysis of pedicle screws inserted into vertebral analogues was conducted. Pedicle screw load was studied as a function of variables in insertion technique.

OBJECTIVES

To determine how the sagittal bending moment in pedicle screws is affected by changes in pedicle screw length, insertional depth, and sagittal placement.

BACKGROUND DATA

An unexpectedly high rate of clinical failure has been observed in pedicle screws used in short-segment instrumentation for unstable burst fractures. The majority of screws fail in sagittal bending within the pedicle. Little is known of the insertion technical factors that affect in situ loads incurred by pedicle screws.

METHODS

Synthetic vertebral analogues were fabricated. Pedicle screws internally instrumented with strain gauges were used as load transducers to determine screw bending moments within the pedicle and body of the analogue. Analogues were loaded in compression to simulate loading of an unstable burst fracture.

RESULTS

Screw bending moments within the pedicle increased 33% and 52% when screws were left 3 mm and 5 mm short of full insertion. Intrapedicular moments increased 20% to 29% in screws inserted superiorly or inferiorly within the pedicle. Thirty-five-millimeter screws developed intrapedicular moments 16% higher than 40-mm and 45-mm screws.

CONCLUSIONS

In situ pedicle screw loads increased significantly as a direct result of variations in surgical technique. Screws left short of full insertion, placed off center in the sagittal plane of the pedicle, or less than 40 mm long developed increased intrapedicular bending moments.

Biomechanical testing sequelae relevant to spinal fusion and instrumentation

The increasing prevalence of spinal disorders and associated treatments has produced a dramatic increase in the number of available devices. The biomechanical evaluation leading to the design, development, and implementation of spinal instrumentation has resulted in a number of in vitro and in vivo testing methods. This article reviews some of the methods and associated results obtained by various evaluation techniques of spinal fusion hardware. Current work and future considerations also are presented.

Stability potential of spinal instrumentations in tumor vertebral body replacement surgery

STUDY DESIGN

The multidirectional stability potential of anterior, posterior, and combined instrumentations applied at L1-L3 was studied after L2 corpectomy and replacement with a carbon-fiber implant.

OBJECTIVES

To evaluate the biomechanical characteristics of short-segment anterior, posterior, and combined instrumentations in lumbar spine tumor vertebral body replacement surgery.

SUMMARY OF BACKGROUND DATA

The biomechanical properties of many different spinal instrumentations have been studied in various spinal injury models. Only a few studies, however, investigate the stabilization methods in spinal tumor vertebral body replacement surgery.

METHODS

Eight fresh frozen human cadaveric thoracolumbar spine specimens (T12-L4) were prepared for biomechanical testing. Pure moments (2.5 Nm, 5 Nm, and 7.5 Nm) of flexion-extension, left-right axial torsion, and left-right lateral bending were applied to the top vertebra in a flexibility machine, and the motions of the L1 vertebra with respect to L3 were recorded with an optoelectronic motion measurement system after reconditioning. The L2 vertebral body was resected and replaced by a carbon-fiber cage. Different fixation methods were applied to the L1 and L3 vertebrae. One anterior, two posterior, and two combined instrumentations were tested. Load-displacement curves were recorded and neutral zone and range of motion parameters were determined.

RESULTS

The anterior instrumentation provided less potential stability than the posterior and combined instrumentations in all motion directions. The anterior instrumentation, after vertebral body replacement, showed greater motion than the intact spine, especially in axial torsion (range of motion, 10.3 degrees vs 5.5 degrees; neutral zone, 2.9 degrees vs. 0.7 degrees; P < 0.05). Posterior instrumentation provided greater rigidity than the anterior instrumentation, especially in flexion-extension (range of motion, 2.1 degrees vs. 12.6 degrees; neutral zone, 0.6 degrees vs. 6.1 degrees; P < 0.05). The combined instrumentation provided superior rigidity in all directions compared with all other instrumentations.

CONCLUSIONS

Posterior and combined instrumentations provided greater rigidity than anterior instrumentation. Anterior instrumentation should not be used alone in vertebral body replacement.

An experimental study of lumbar destabilization. Restabilization and bone density

STUDY DESIGN

An investigation of the effects of bone density on lumbar spine stability using destabilizing and restabilizing procedures.

OBJECTIVES

To measure cadaveric vertebral bone densities computed tomographic scans and to correlate the measured densities with lumbar spine stability in the intact and during sequential destabilization and restabilization.

SUMMARY OF BACKGROUND DATA

The stabilizing effects of lumbar pedicle screw fixation have been widely described. Numerous construct failure mechanisms have been observed, including screw loosening in osteoporosis. Although previous studies have analyzed the effect of bone density on the compression strength of bone similar to that used in interbody fusion and the relationship of pedicle screw pull-out strength to vertebral bone density, a combined study of bone density and construct stability using an interbody bone spacer with pedicle fixation has not been performed.

METHODS

Bone densities were measured in 20 human cadaveric lumbar spines using computed tomography scans and a hydroxyapatite phantom. After the specimens were mounted in a testing frame, the L4-L5 motion segments were subjected to cyclic axial compression-torsional loads, and axial and rotational intervertebral displacements were monitored. Laminectomy, facetectomy, and pedicle screw-plate fixation were performed sequentially in three specimens. Ten others underwent these procedures with an additional destabilization procedure, discectomy, after facetectomy. Seven others underwent the same sequence as the previous group, followed by the insertion of interbody bone. Cyclic testing was resumed after each procedure.

RESULTS

Average bone densities varied widely among the specimens. Average bone densities of the pedicle and of the vertebral body for individual specimens were well-correlated (r = 0.897). Displacements were recorded as a percentage of the intact state before destabilization; average percentages are reported as follows: axial displacements increased after facetectomy (145%) and subsequent discectomy (251%), and rotational displacements increased after facetectomy (295%) and discectomy (390%). Instrumentation without interbody bone resulted in specimens with decreased axial (126%) and rotational (156%) displacements. The addition of interbody bone further decreased axial (111%) and rotational (117%) displacements. The rotational stabilization provided by instrumentation was well-correlated with vertebral bone density (r = 0.804). This correlation was enhanced by the use of interbody bone (r = 0.939).

CONCLUSION

The unstable lumbar spine can be partially stabilized using fixation. Interbody bone provides additional stability. The immediate stability provided by pedicle screws is greater in lumbar vertebrae with higher bone density.

The effect of bone quality on pedicle screw loading in axial instability. A synthetic model

STUDY DESIGN

In this biomechanical analysis of pedicle screw bending moments, custom-fabricated vertebral analogues were loaded in axial compression to produce sagittal bending forces. Moments were measured directly from internally instrumented pedicle screws.

OBJECTIVES

To establish the role of cancellous vertebral modulus on pedicle screw bending moments within the vertebral body and the vertebral pedicle.

SUMMARY OF BACKGROUND DATA

Pedicle screws are often used to manage axial instability of the spine. Clinical studies report a high incidence of screw bending failure, resulting in kyphosis and pain in some patients. Factors predisposing to bending failure are not well understood, although recent studies have shown that vertebral morphometry is important.

METHODS

Axially canullated 7.0-mm pedicle screws, internally instrumented with paired strain gauges, were inserted into analogue vertebrae of uniform dimension. Cancellous modulus was varied from 25-100 MPa. Screws were rigidly mounted to a vertical testing frame, and axial loads were applied to the superior vertebral endplate, producing sagittal bending moments. Moments were recorded from gauges applied in the intrapedicular and intravertebral portions of the screw. Mean moments were compared using a Student's t test, with significance defined as P < 0.05.

RESULTS

Cancellous modulus did not affect bending moments experienced in either the intrapedicular or intravertebral portions of the pedicle screws. Gauge accuracy was excellent, and with no gauge drift.

CONCLUSIONS

Although small changes in pedicle morphometry can alter screw bending moments significantly, changes in cancellous modulus had no measurable impact on bending moments at these same loads. Bone density is likely to play a limited role in screw bending failure.

A new microsurgical technique for minimally invasive anterior lumbar interbody fusion

STUDY DESIGN

A series of patients were prospectively studied to determine the morbidity and possible complications of minimally invasive anterior lumbar interbody fusion by two new microsurgical approaches (retroperitoneal for segments L2-L3, L3-L4, and L4-L5, and transperitoneal for L5-S1).

OBJECTIVES

To investigate the feasibility of performing an anterior lumbar interbody fusion through a 4-cm skin incision and a standardized muscle-splitting approach.

SUMMARY OF BACKGROUND DATA

The utility of anterior lumbar interbody fusion with or without posterior instrumentation for the treatment of various degenerative or postoperative lesions associated with low back pain is still a matter of debate. Regardless of the indications for surgery, use of the anterior approach in the lumbar spine is known to be associated with considerable surgical trauma, a high postoperative morbidity, and, occasionally, unacceptably high complication rates. Laparoscopic anterior interbody fusion of L5-S1 to eliminate some of these problems has been recently described. However, a minimally invasive surgical concept that covers all lumbar segments from L2 to S1 has not been described before now.

METHODS

A standardized, microsurgical retroperitoneal approach to levels L2-L3, L3-L4, and L4-L5 and a microsurgical transperitoneal approach through a "minilaparotomy" to L5-S1 are described. The first 25 patients (retroperitoneal, n = 20; transperitoneal, n = 5) treated with these methods are evaluated with respect to intraoperative data such as blood loss, operating time, intraoperative and postoperative complications, as well as preliminary fusion results.

RESULTS

There were no general or technique-related complications in the first series of 25 patients. Postoperative morbidity was low in all patients, with negligible wound pain. Average blood loss was 67.8 ml for the retroperitoneal technique and 168 ml for the transperitoneal approach. No blood transfusion was necessary. All patients showed solid bony fusion.

CONCLUSIONS

The microsurgical approaches described in this article are atraumatic techniques to reach the lumbar spinal levels L2-L3, L3-L4, L4-L5, and L5-S1. They represent microsurgical modifications of the surgical approaches well known to the spine surgeon. They can be learned in a step-by-step fashion, starting with a conventional skin incision and, once the surgeon is familiar with the instruments, moving on to the microsurgical technique. The approaches are not restricted to the type of fusion (iliac crest autograft) presented in this series.

The effect of pedicle morphometry on pedicle screw loading. A synthetic model

STUDY DESIGN

Static nondestructive bending analysis of pedicle screws inserted into vertebral analogues was conducted. Pedicle screw bending load was studied as a function of pedicle morphometry.

OBJECTIVES

To determine how sagittal bending moment in pedicle screws is affected by changes in pedicle height, length, and width.

BACKGROUND DATA

An unexpectedly high rate of clinical failure has been observed in pedicle screws used in short-segment instrumentation for axially unstable fractures. The majority of screws fail in sagittal bending within the pedicle. To date, little is known of the exogenous factors that affect in situ loads incurred by pedicle screws.

METHODS

Synthetic vertebral analogues were fabricated, varying pedicle height, length, or width independently. Pedicle screws internally instrumented with strain gages were used as load transducers to determine screw bending moments within the pedicle and body of the analogue. Analogues were loaded in compression to simulate loading of an unstable burst fracture.

RESULTS

Screw bending moments within the pedicle increased incrementally with increasing pedicle length, rising 30% as length increased from 8.0 mm to 12.0 mm. Screw moment increased 20% when pedicle height dropped below 15.0 mm, consistent with a threshold effect. Changes in pedicle width did not affect screw loads within the pedicle.

CONCLUSIONS

In situ pedicle screw loads increased significantly as pedicle length increased and as pedicle height decreased. Pedicle screws instrumented internally with strain gages are an effective research instrument allowing measurement of in situ loading along the axis of the screw.

Biomechanical analysis of thoracolumbar interbody constructs. How important is the endplate?

STUDY DESIGN

A biomechanical study of human cadaveric thoracic vertebral bodies was conducted using several anterior fusion options subjected to axial loads. This study emphasized the contribution of the endplate to resistance of graft subsidence.

OBJECTIVES

To determine the importance of the vertebral endplate in resisting subsidence of various constructs into the vertebral body; the relative efficacy of potential alternative graft constructs such as iliac crest, ribs, humerus, and titanium mesh cage; and the importance of bone mineral content, vertebral level, and cross-sectional graft area on construct subsidence.

SUMMARY OF BACKGROUND DATA

As the fixation length of anterior and posterior spinal constructs is reduced, load sharing of the anterior column has become more important to reduce failure of the shorter devices. Several alternative graft constructs and surgical techniques have been used for reconstruction of the anterior column. There exist little comparative data as to whether any of these constructs are superior and whether the vertebral endplate contributes significantly to the integrity of the construct.

METHODS

Sixty-three isolated human cadaveric vertebral bodies from T3 to T12 were used to test seven different constructs in direct axial load onto prepared endplates with an electrohydraulic testing device. These constructs were: 1) titanium mesh cage (17 x 22 mm) on intact endplate, 2) C-shaped humerus on intact endplate, 3) tricorticated iliac graft in "tee configuration" on intact endplate, 4) tricorticated iliac graft in cancellous trough, 5) triple rib strut graft, 6) single rib on endplate, and 7) single rib on cancellous body. Dual X-ray absorptiometry assessment of bone mineral content was performed. A uniaxial load was applied with force and displacement data collected to determine maximal load to "failure" of the vertebral body.

RESULTS

Preservation of vertebral endplate did not significantly increase the resistance to graft subsidence. The titanium cage construct provided the greatest resistance to axial load.

CONCLUSIONS

Preservation of the vertebral endplate may not offer a significant biomechanical advantage in reconstructing the anterior column. Several alternative constructs are mechanically equivalent.