Biomechanical studies on two anterior thoracolumbar implants in cadaveric spines

Biomechanical evaluation of an integrated fixation cage during fatigue loading: a human cadaver study

OBJECTIVE

Lumbar cages with integrated fixation screws offer a low-profile alternative to a standard cage with anterior supplemental fixation. However, the mechanical stability of integrated fixation cages (IFCs) compared with a cage with anterior plate fixation under fatigue loading has not been investigated. The purpose of this study was to compare the biomechanical stability of a screw-based IFC with a standard cage coupled with that of an anterior plate under fatigue loading.

METHODS

Eighteen functional spinal units were implanted with either a 4-screw IFC or an anterior plate and cage (AP+C) without integrated fixation. Flexibility testing was conducted in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) on intact spines, immediately after device implantation, and post-fatigue up to 20,000 cycles of FE loading. Stability parameters such as range of motion (ROM) and lax zone (LZ) for each loading mode were compared between the 2 constructs at multiple stages of testing. In addition, construct loosening was quantified by subtracting post-instrumentation ROM from post-fatigue ROM.

RESULTS

IFC and AP+C configurations exhibited similar stability (ROM and LZ) at every stage of testing in FE (p ≥ 0.33) and LB (p ≥ 0.23) motions. In AR, however, IFCs had decreased ROM compared with AP+C constructs at pre-fatigue (p = 0.07) and at all post-fatigue time points (p ≤ 0.05). LZ followed a trend similar to that of ROM in AR. ROM increased toward intact motion during fatigue cycling for AP+C and IFC implants. IFC specimens remained significantly (p < 0.01) more rigid than specimens in the intact condition during fatigue for each loading mode, whereas AP+C construct motion did not differ significantly (p ≥ 0.37) in FE and LB and was significantly greater (p < 0.01) in AR motion compared with intact specimens after fatigue. Weak to moderate correlations (R2 ≤ 56%) were observed between T-scores and construct loosening, with lower T-scores leading to decreased stability after fatigue testing.

CONCLUSIONS

These data indicate that a 4-screw IFC design provides fixation similar to that provided by an AP+C construct in FE and LB during fatigue testing and better stability in AR motion.

Biomechanical evaluation of an interfacet joint decompression and stabilization system

A majority of the middle-aged population exhibit cervical spondylosis that may require decompression and fusion of the affected level. Minimally invasive cervical fusion is an attractive option for decreasing operative time, morbidity, and mortality rates. A novel interfacet joint spacer (DTRAX facet screw system, Providence Medical) promises minimally invasive deployment resulting in decompression of the neuroforamen and interfacet fusion. The present study investigates the effectiveness of the device in minimizing intervertebral motion to promote fusion, decompression of the nerve root during bending activity, and performance of the implant to adhere to anatomy during repeated bending loads. We observed flexion, extension, lateral bending, and axial rotation resonant overshoot mode (ROM) in cadaver models of c-spine treated with the interfacet joint spacer (FJ spacer) as stand-alone and supplementing anterior plating. The FJ spacer was deployed bilaterally at single levels. Specimens were placed at the limit of ROM in flexion, extension, axial bending, and lateral bending. 3D images of the foramen were taken and postprocessed to quantify changes in foraminal area. Stand-alone spacer specimens were subjected to 30,000 cycles at 2 Hz of nonsimultaneous flexion-extension and lateral bending under compressive load and X-ray imaged at regular cycle intervals for quantitative measurements of device loosening. The stand-alone FJ spacer increased specimen stiffness in all directions except extension. 86% of all deployments resulted in some level of foraminal distraction. The rate of effective distraction was maintained in flexed, extended, and axially rotated postures. Two specimens demonstrated no detectable implant loosening (<0.25 mm). Three showed unilateral subclinical loosening (0.4 mm maximum), and one had subclinical loosening bilaterally (0.5 mm maximum). Results of our study are comparable to previous investigations into the stiffness of other stand-alone minimally invasive technologies. The FJ spacer system effectively increased stiffness of the affected level comparable to predicate systems. Results of this study indicate the FJ spacer increases foraminal area in the cervical spine, and decompression is maintained during bending activities. Clinical studies will be necessary to determine whether the magnitude of decompression observed in this cadaveric study will effectively treat cervical radiculopathy; however, results of this study, taken in context of successful decompression treatments in the lumbar spine, are promising for the continued development of this product. Results of this biomechanical study are encouraging for the continued investigation of this device in animal and clinical trials, as they suggest the device is well fixated and mechanically competent.

Biomechanical analysis of anterior versus posterior instrumentation following a thoracolumbar corpectomy

Object

The objective of this study was to evaluate the biomechanical properties of lateral instrumentation compared with short- and long-segment pedicle screw constructs following an L-1 corpectomy and reconstruction with an expandable cage.

Methods

Eight human cadaveric T10–L4 spines underwent an L-1 corpectomy followed by placement of an expandable cage. The spines then underwent placement of lateral instrumentation consisting of 4 monoaxial screws and 2 rods with 2 cross-connectors, short-segment pedicle screw fixation involving 1 level above and below the corpectomy, and long-segment pedicle screw fixation (2 levels above and below). The order of instrumentation was randomized in the 8 specimens. Testing was conducted for each fixation technique. The spines were tested with a pure moment of 6 Nm in all 6 degrees of freedom (flexion, extension, right and left lateral bending, and right and left axial rotation).

Results

In flexion, extension, and left/right lateral bending, posterior long-segment instrumentation had significantly less motion compared with the intact state. Additionally, posterior long-segment instrumentation was significantly more rigid than short-segment and lateral instrumentation in flexion, extension, and left/right lateral bending. In axial rotation, the posterior long-segment construct as well as lateral instrumentation were not significantly more rigid than the intact state. The posterior long-segment construct was the most rigid in all 6 degrees of freedom.

Conclusions

In the setting of highly unstable fractures requiring anterior reconstruction, and involving all 3 columns, long-segment posterior pedicle screw constructs are the most rigid.

An in vitro biomechanical comparison of single-rod, dual-rod, and dual-rod with transverse connector in anterior thoracolumbar instrumentation

BACKGROUND

After thoracolumbar corpectomy, standard anterolateral instrumentation may consist of dual rods with cross-connectors. However, when the vertebral bodies are small or involved with disease, only 1 rod may be possible.

OBJECTIVE

To compare the biomechanics of an in vitro L1 corpectomy model using 1 rod, 2 rods, or 2 rods with 2 cross-connectors.

METHODS

Eight fresh frozen human cadaveric spines were potted from T9 to L3. Pure moments of 1.5, 3, and 4.5 Nm were applied, and the motion of the spine was measured using 3 infrared cameras. Loads were applied in flexion and extension, right and left lateral bending, and right and left axial rotation. Each spine was first tested in the intact state. After performing an L1 corpectomy and replacement with a carbon fiber reinforced polymer cage, 3 constructs were tested: single rod (1R), dual rod (2R), and dual rod with 2 transverse connectors (CC).

RESULTS

Analysis of variance suggests significant main effects of load (P < .0001), axis (P = .022), construct (P =.0019), and individual spine (P < .0001). Overall, the single-rod construct is significantly less rigid than the intact spine in axial rotation. There is no significant difference between the intact spine and either the dual-rod construct or the dual-rod cross-connector construct.

CONCLUSION

In our in vitro model of anterior spinal stabilization after corpectomy and grafting, a single-rod construct is significantly less rigid than the intact spine. Addition of a second rod returns the rigidity of the spine to the intact state. A dual-rod cross-connector construct is significantly more rigid than a single-rod construct.

Simultaneous posterior and anterior approaches with posterior vertebral wall preserved for rigid post-traumatic kyphosis in thoracolumbar spine

STUDY DESIGN

A retrospective study.

OBJECTIVE

To evaluate the radiological and clinical results of simultaneous surgery with preservation of the posterior vertebral wall for rigid post-traumatic kyphosis in the thoracolumbar spine.

SUMMARY OF BACKGROUND DATA

Management of rigid post-traumatic kyphosis has been a challenge for surgeons. Current widely used posterior osteotomy procedures have the disadvantages of significant invasiveness, spinal column shortening, and instrumentation-related complications.

METHODS

From 2004 to 2009, 21 patients with rigid post-traumatic kyphosis in the thoracolumbar spine (T11-L2) were managed in our hospital. Average kyphotic angle was 45.2° ± 11.2° (range, 31°-67°). The surgical technique used was posterior and anterior circumferential release and anterior corpectomy with posterior vertebral wall preservation and short segmental instrumentation. Preoperative and postoperative kyphotic angle was measured to assess the degree of kyphosis correction and maintenance. Changes in low back pain were assessed by Japanese Orthopaedic Association scores.

RESULTS

All patients were successfully managed with this procedure without major complications. Most patients (19 of 21) were instrumented with anterior-only fixation, while posterior interspinal wire was added in 2 patients with osteoporosis. The mean blood loss was 470 mL (range, 300-700 mL). Patients were followed for an average of 32 months (range, 6-70 mo) postoperatively. Back pain was relieved to some degree in all patients and the improvement in Japanese Orthopaedic Association scores was 76.9% ± 7.9. Average kyphotic angle was 6.0° ± 5.7° (range, -2 to 17) immediately after surgery and 7.2° ± 5.8° (range, -3 to 17) at final follow-up. Average of 1° of correction loss was documented and all patients obtained solid fusion uneventfully.

CONCLUSION

This technique is indicated for most patients with rigid post-traumatic kyphosis in the thoracolumbar spine and can yield satisfactory clinical results not only in terms of pain relief, kyphosis correction, vertebral height restoration, and spinal canal integrity preservation, but also in reducing the risk of excessive bleeding and spinal cord injury.

In vitro biomechanics of an expandable vertebral body replacement with self-adjusting end plates

BACKGROUND CONTEXT

Unstable burst fractures of the thoracolumbar spine may be treated surgically. Vertebral body replacements (VBRs) give anterior column support and, when used with supplemental fixation, impart rigidity to the injured segments. Although some VBRs are expandable, device congruity to the vertebral end plates is imprecise and may lead to stress risers and device subsidence.

PURPOSE

The objective of this study was to compare the rigidity of a VBR that self-adjusts to the adjacent vertebral end plates versus structural bone allograft and with an unsupported anterior column in a traumatic burst fracture reconstruction model.

STUDY DESIGN

Biomechanical flexibility testing with rod strain measurement.

PATIENT SAMPLE

Twelve T11-L3 human spine segments.

OUTCOME MEASURES

Range of motion, neutral zone, and posterior fixation rod stress (moments).

METHODS

Flexibility testing was performed to ± 6 Nm in flexion-extension, lateral bending, and axial rotation on 12 intact human T11-L3 specimens. Burst fractures were created in L1, and flexibility testing was repeated in three additional states: subtotal corpectomy with posterior instrumentation (PI) only from T12 to L2, reconstruction with a femoral strut allograft and PI, and reconstruction with a VBR (with self-adjusting end plates) and PI. The PI consisted of pedicle screws and strain gage instrumented rods that were calibrated to measure rod stress via flexion-extension bending moments.

RESULTS

There was no statistical difference in range of motion or neutral zone between the strut graft and VBR constructs, which both had less motion than the PI-only construct in flexion/extension and torsion and were both less than the intact values in flexion/extension and lateral bending (p < .05). Posterior rod moments were significantly greater for the PI-only construct in flexion/extension relative to the strut graft and VBR states (p = .03).

CONCLUSIONS

This study, which simulated the immediate postoperative state, suggests that a VBR with self-adjusting end plate components has rigidity similar to the standard strut graft when combined with PI. Posterior rod stress was not significantly increased with this type of VBR compared with the strut graft reconstruction. The benefits of burst fracture stabilization using a self-adjusting VBR ultimately will not be known until long-term clinical studies are performed.

Biomechanical study of anterior spinal instrumentation configurations

The biomechanical impact of the surgical instrumentation configuration for spine surgery is hard to evaluate by the surgeons in pre-operative situation. This study was performed to evaluate different configurations of the anterior instrumentation of the spine, with simulated post-operative conditions, to recommend configurations to the surgeons. Four biomechanical parameters of the anterior instrumentation with simulated post-operative conditions have been studied. They were the screw diameter (5.5-7.5 mm) and its angle (0 degrees - 22.5 degrees), the bone grip of the screw (mono-bi cortical) and the amount of instrumented levels (5-8). Eight configurations were tested using an experimental plan with instrumented synthetic spinal models. A follower load was applied and the models were loaded in flexion, torsion and lateral bending. At 5 Nm, average final stiffness was greater in flexion (0.92 Nm/degrees) than in lateral bending (0.56 Nm/degrees) and than in torsion (0.26 Nm/degrees). The screw angle was the parameter influencing the most the final stiffness and the coupling behaviors. It has a significant effect (p < or = 0.05) on increasing the final stiffness for a 22.5 degrees screw angle in flexion and for a coronal screw angle (0 degrees) in lateral bending. The bi-cortical bone grip of the screw significantly increased the initial stiffness in flexion and lateral bending. Mathematical models representing the behavior of an instrumented spinal model have been used to identify optimal instrumentation configurations. A variation of the angle of the screw from 22.5 degrees to 0 degrees gave a global final stiffness diminution of 13% and a global coupling diminution of 40%. The screw angle was the most important parameter affecting the stiffness and the coupling of the instrumented spine with simulated post-operative conditions. Information about the effect of four different biomechanical parameters will be helpful in preoperative situations to guide surgeons in their clinical choices.

Adjacent level load transfer following vertebral augmentation in the cadaveric spine

STUDY DESIGN

In vitro biomechanics.

OBJECTIVE

To determine if osteoporotic vertebral compression fracture (VCF) augmentation increases adjacent level load transfer.

SUMMARY OF BACKGROUND DATA

Osteoporotic VCF subsequent to augmentation may result from disease progression or increased adjacent level load transfer, or both.

METHODS

There were 11 T3-T7 and 10 T8-T12 divided by lumbar bone mineral density into a normal group (No. 1; n = 11) and an osteoporotic group (No. 2; n = 10). Strain and centrum stress were measured on T4 and T6 (T3-T7), and T9 and T11 (T8-T12) during tests in the intact state, following a centrum defect, during and after an augmented VCF at T5 or T10, and during a subsequent VCF. Stiffness and strength were compared: between groups 1 and 2; among intact, defect, and augmented VCF states; and between the initial and subsequent VCF.

RESULTS

Group 1 was stiffer than 2 in compression (P = 0.01) and flexion (P = 0.07), with no difference in adjacent level load transfer (strain P = 0.72, centrum stress P = 0.36) or strength (P = 0.07). The centrum defect reduced compressive stiffness from the intact (P = 0.001), which was partially restored following VCF augmentation (P = 0.006). There were no differences in flexion stiffness (P > or = 0.14). Adjacent level load transfer in flexion exceeded that in compression (strain P = 0.001, centrum stress P = 0.19). Initial and subsequent VCF occurred at similar forces (P = 0.26) with higher adjacent level load at subsequent (strain and centrum stress P = 0.04).

CONCLUSIONS

Augmentation of multilevel spinal segments with VCF produced by combined compression, flexion, and a centrum defect normalizes adjacent level load transfer at physiologic loads. In both normal and osteoporotic spinal segments, as loads approach those of the initial VCF, protection from augmentation is lost, and subsequent adjacent level VCFs occur from extreme loading, and not the augmentation process.

Biomechanical comparison of two stabilization techniques of the atlantoaxial joints: transarticular screw fixation versus screw and rod fixation

OBJECTIVE

To compare the biomechanical stability imparted to the C1 and C2 vertebrae by either transarticular screw fixation (TSF) or screw and rod fixation (SRF) techniques in a cadaver model.

METHODS

Ten fresh ligamentous human cervical spine specimens were harvested from cadavers. The specimens were tested sequentially in the intact state, after injury and stabilization (unilateral left side and bilateral), and after fatiguing to 5000 cycles (0.5 Hz) at +/-1.0 N.m of flexion and extension. The specimens were stabilized by use of TSF in 5 spines or SRF in the other 5 spines. The data were converted to angular displacements, and the stabilized cases were compared with intact states for evaluating the efficacies of the two techniques in stabilizing the C1-C2 segments.

RESULTS

In the TSF group, the unilateral fixation using one screw imparted a significant stability in only the axial rotation mode. The unilateral procedure in the SRF group was effective in stabilization in all modes except in extension. The bilateral procedure in both of the groups was effective across the C1-C2 segment. However, the SRF group afforded higher stability than the corresponding TSF group in the flexion and extension modes. The degree of stability did not change after fatigue compared with the prefatigue data.

CONCLUSION

In general, a surgeon should undertake a bilateral fixation to achieve sufficient stability across the atlantoaxial complex, and either technique will provide satisfactory results, although the SRF technique may be better in the flexion and extension modes. One should use the SRF procedure while trying to achieve stability with a unilateral system.

Effect of lumbar interbody cage geometry on construct stability: a cadaveric study

STUDY DESIGN

Biomechanical study to investigate three-dimensional motion behavior of cadaveric spines in various surgical simulations.

OBJECTIVES

To determine the effect of cage geometry on the construct stability.

SUMMARY OF BACKGROUND DATA

There is a wide variety of cage/spacer designs available for lumbar interbody fusion surgery. These range from circular, tapered, and rectangular with and without curvature. However, the effectiveness of cages with different designs and materials to stabilize a decompressed intervertebral space has not been fully studied.

METHODS

Six fresh ligamentous lumbar spine specimens (L1-S2) were subjected to pure moments in the six loading directions. The resulting spatial orientations of the vertebrae were recorded using Optotrak Motion Measurement System. Measurements were made sequentially for intact, bilateral spacer placements across L4-L5 using a posterior approach, supplemented with pedicle screw-rod system fixation, and after the cyclic loading in flexion-extension mode.

RESULTS

The stability tended to decrease after the bilateral cage placement as compared with the intact for all loading cases except flexion. In flexion, the angular displacement decreased to 80% of the intact. However, there was no significant statistical difference seen in stability between intact and after bilateral spacer placement. Following the addition of posterior fixation using pedicle screw-rod system, the stability significantly increased in all directions. Cyclic loading did not have any significant effect on the stability.

CONCLUSIONS

Stand-alone cages restore motion to near-intact levels at best, and supplement instrumentation is essential for significantly increasing the stability of the decompressed segment. The effects of cage geometry and Young's modulus of the cage material do not seem to influence the stability, as compared with the other cagedesigns, especially after supplemental fixation with a posterior system.

Comparison of two interbody fusion cages for posterior lumbar interbody fusion in a cadaveric model

Although the Brantigan cage and Bagby and Kuslich (BAK) cage have different geometrical characteristics, clinical observations suggest that they are equally effective in restoring disc height and stability across the involved spinal segments. This study was designed to compare their performance as posterior lumbar interbody fusion devices at two levels in fresh ligamentous cadaver lumbar spines (L2-S1). After mounting in a testing frame, the three-dimensional load-displacement behaviour of each vertebra was quantified using the Selspot II Motion Measurement System for; the intact state, posterior decompression, and stabilisation, using a pair of Brantigan or BAK cages across L4-S1, additional stabilisation using Isola spinal instrumentation across L4-S1, and cyclic loading in flexion/extension. In the "cage-only" state, the Brantigan cage did not restore the stability in right axial rotation, whereas the BAK cage not only restored stability in all six directions but also improved lateral bending. After implanting the posterior instrumentation, both groups exhibited similar stability, and cyclic loading did not alter this. Although the Brantigan cage appears less effective than the BAK cage, implantation of posterior instrumentation significantly improves stability and reduces the differences between them. This underscores the need to use posterior instrumentation to achieve a higher initial stability.

Comparative biomechanical testing of anterior and posterior stabilization procedures

STUDY DESIGN

This is a comparative in vitro biomechanical study in a calf lumbar spine model.

OBJECTIVES

The objective was to compare the primary stability of an anterior instrumentation, an intercorporal cage in combination with an anterior instrumentation, and a posterior instrumentation for monosegmental spondylodesis.

SUMMARY OF BACKGROUND DATA

Spondylodesis can be achieved through a posterior lumbar fusion, posterior lumbar intercorporal fusion, or an anterior lumbar intercorporal fusion. The posterior lumbar fusion is the gold standard, although the anterior approach offers some potential advantages to the transpedicular posterior techniques.

METHODS

Stability testing was performed on 30 calf lumbar spine motion segments in a physiologic state (n = 30), with either an isolated anterior (MACS) or posterior instrumentation (SOCON), and with an anterior instrumentation augmented with an intercorporal cage (MACS-Cage, n = 10, respectively). Range of motion, neutral zone, and bending stiffness were measured under pure bending to 10 Nm, and bending stiffness under axial loads of up to 1500 N.

RESULTS

The isolated posterior instrumentation was found to be more stable than the isolated or augmented anterior instrumentation in flexion/extension, although no significant differences were observed in lateral bending or axial rotation. The results of this biomechanical study suggest that an augmented anterior instrumentation provides similar stability for bony fusion as does the golden standard posterior instrumentation, with the exception of flexion/extension.

CONCLUSION

An augmented anterior instrumentation may provide similar stability for bony fusion as does the posterior instrumentation.

Biomechanical studies of an artificial disc implant in the human cadaveric spine

Object. The authors compared the biomechanical performance of the human cadaveric spine implanted with a metallic ball-and-cup artificial disc at L4–5 with the spine's intact state and after anterior discectomy.

Methods. Seven human L2—S1 cadaveric spines were mounted on a biomechanical testing frame. Pure moments of 0, 1.5, 3.0, 4.5, and 6.0 Nm were applied to the spine at L-2 in six degrees of motion (flexion, extension, right and left lateral bending, and right and left axial rotation). The spines were tested in the intact state as well as after anterior L4–5 discectomy. The Maverick disc was implanted in the discectomy defect, and load testing was repeated.

The artificial disc created greater rigidity for the spine than was present after discectomy, and the spine performed biomechanically in a manner comparable with the intact state.

Conclusions. The results indicate that in an in vitro setting, this model of artificial disc stabilizes the spine after discectomy, restoring motion comparable with that of the intact state.

Biomechanical in vitro comparison of different mono- and bisegmental anterior procedures with regard to the strategy for fracture stabilisation using minimally invasive techniques

Endoscopic minimally invasive techniques have become an established method of fracture stabilisation in the spine. In view of this fact, anterior stabilisation strategies must be reconsidered, as monosegmental A 3.1 compression fractures are increasingly being stabilised endoscopically from the anterior aspect using minimally invasive techniques. This study investigated the biomechanical necessity of anterior two-point or four-point stabilisation in the instrumentation of mono- and bisegmental fractures. In three biomechanical in vitro studies, burst fracture stabilisation was simulated, and anterior short fixation devices were tested under load with pure moments up to 3.75 Nm to evaluate the biomechanical stabilising characteristics of different kinds of instrumentations in flexion/extension, lateral bending, and axial rotation. Only anterior four-point stabilisation resulted in sufficient primary stability both in mono- and bisegmental instrumentation and therefore represents the standard procedure in open as well as in minimally invasive spinal surgery.

Anterior-Only Stabilization of Three-Column Thoracolumbar Injuries

OBJECTIVE

The optimal treatment of "unstable" thoracolumbar injuries remains controversial. Studies have shown the advantages of direct anterior decompression of thoracolumbar injuries along with supplemental posterior instrumentation as a combined or staged procedure. Others have also shown success in decompression as a single-stage anterior procedure, largely limited to two-column (anterior and middle) injuries. A retrospective review of all available clinical and radiographic data was used to classify unstable three-column thoracolumbar fractures according to the Association for the Study of Internal Fixation (AO) classification system. This was conducted to evaluate the efficacy of stand-alone anterior decompression and reconstruction of unstable three-column thoracolumbar injuries, utilizing current-generation anterior spinal instrumentation.

METHODS

Between 1992 and 1998, 40 patients underwent anterior decompression and two-segment anteriorly instrumented reconstruction for three-column thoracolumbar fractures. Retrospective review of all available clinical and radiographic data was used to classify these unstable injuries according to the AO classification system, evaluating for neurologic changes, spinal canal compromise, preoperative and postoperative segmental angulation, and arthrodesis rate.

RESULTS

According to the AO classification system, there were 24 (60%) type B1.2, 10 (25%) type B2.3, 5 (12.5%) type C1.3, and 1 (2.5%) type C2.1 three-column injuries. Preoperative canal compromise averaged 68.5% and vertebral height loss averaged 44.5%. There were no cases of neurologic deterioration, and 30 (91%) patients with incomplete neurologic deficits improved by at least one modified Frankel grade. Mean preoperative segmental kyphosis of 22.7 degrees was improved to an early mean of 7.4 degrees (P < 0.0001). At latest follow-up, angulation had increased by an average 2.1 degrees but maintained significant improvement from preoperative measurements (P < 0.0001). There was one early construct failure due to technical error. Thirty-seven of the remaining patients (95%) went on to apparently stable arthrodesis.

CONCLUSIONS

Current types of anterior spinal instrumentation and reconstruction techniques can allow some types of unstable three-column thoracolumbar injuries to be treated in an anterior stand-alone fashion. This allows direct anterior decompression of neural elements, improvement in segmental angulation, and acceptable rates of arthrodesis without the need for supplemental posterior instrumentation.

Posterior instrumentation reduces differences in spine stability as a result of different cage orientations: an in vitro study

STUDY DESIGN

A multisegmental cadaveric spine model was used to quantify the load-displacement behavior of intact spine specimens, specimens injured and stabilized using Bagby and Kuslich (BAK) cages as lumbar interbody fusion devices with or without posterior instrumentation across two levels.

OBJECTIVES

To compare the stabilities imparted by the cages placed using an oblique and conventional posterior approaches and to determine the effects of supplementary posterior instrumentation.

SUMMARY OF BACKGROUND DATA

The BAK cage as posterior lumbar interbody fusion (PLIF) has been used to restore disc height, reduce morbidity, provide immediate stability to the patients, and enhance fusion rates. The obliquely inserted BAK cage has the advantages of reducing exposure and precise implantation. The biomechanical efficacy of this procedure is sparse, especially in comparison to the PLIF with and without posterior instrumentation.

METHODS

Nine fresh human ligamentous spines (L2-S1) were affixed within a testing frame for determining their load-displacement behaviors. Load testing in clinically relevant modes was performed sequentially for the intact and the following procedures across the L4-S1 segment: posterior destabilization, stabilization using two parallel BAK cages (CBAK group) or one oblique BAK cage (OBAK group), further stabilization with posterior instrumentation, and finally cyclic loading in flexion-extension. Spatial positions of the LEDs attached to vertebral bodies were recorded using a three-dimensional motion measurement system.

RESULTS

When used alone to restore stability, the orientation of the cage affected the outcome. In flexion OBAK orientation and in extension CBAK orientation provided better stability (decrease in motion with respect to intact case), compared with the other orientation. In lateral bending, CBAK orientation was found to be better than OBAK. In axial mode, CBAK orientation was effective in both directions while OBAK was effective only in right axial rotation. With the supplementary posterior fixation, the differences in stability resulting from the orientations were not noticeable at all, both before and after cyclic tests.

CONCLUSIONS

Owing to the differences in the surgical approach and the amount of dissection, the stability for the cages when used alone as a function of cage orientation was different. These subtle differences were reduced by the use of posterior fixation device, underscoring the importance of using instrumentation when cages are used as PLIFs. However, the oblique insertion may be more favorable since it requires less exposure, enables precise implantation, and is less expensive, especially when used with supplementary instrumentation.

Influence of screw-cement enhancement on the stability of anterior thoracolumbar fracture stabilization with circumferential instability

The influence of additional dorsal structure damage on anterior stabilization of a thoracolumbar fracture is still unknown. Screw-cement enhancement can be used to reinforce the stability of anterior instrumentation. We have developed a new anchorage system for fixation of anterior stabilization devices, adapted through geometric optimization and the additional option of cementation after screw insertion. This study examines the question of whether this enhancement is strong enough to enable a single anterior procedure and still compensate for dorsal instability. Various spinal reconstruction procedures were evaluated biomechanically in an increasing ventrodorsal instability model for thoracolumbar fracture stabilization. A biomechanical in vitro study, simulating stabilized defect situations (corporectomy/vertebrectomy) with strut grafting and overbridging instrumentation, was performed on six human T10-L2 cadaveric specimens. The primary stability parameters, range of motion and neutral zone, were evaluated with or without anterior screw-cement enhancement. This was compared with a single conventional anterior stabilization without a dorsal defect (corporectomy). It was also compared with a single anterior, posterior or combined procedure in the presence of additional dorsal structure damage (vertebrectomy). The use of an additional cementable screw dowel enhanced the primary stability of the anterior instrumentation, compensating for dorsal instability. These results are warranted for the clinical use of minimally open or endoscopic techniques, creating the highest possible primary stability while performing a single anterior enhanced instrumentation with a tissue-preserving approach.

Solvent-preserved, bovine cancellous bone blocks used for reconstruction of thoracolumbar fractures in minimally invasive spinal surgery-first clinical results

We investigated the osseointegration of solvent-preserved, xenogenous cancellous bone blocks in the treatment of unstable fractures of the thoracolumbar junction. In 22 patients, the anterior repair procedure was performed by thoracoscopy or minimally invasive retroperitoneal surgery. Twenty-two patients had undergone monosegmental anterior fusion and were surveyed prospectively. Solvent-preserved, bovine cancellous bone blocks were used in 11 patients; iliac crest bone graft was used in the others. Follow-up after 12 months included CT scans, which revealed successful osseointegration in eight out of 11 patients who had received autogenous iliac crest bone grafts, while three patients showed a partial integration. There were no graft fragmentations. In patients who had received solvent-preserved, xenogenous cancellous bone blocks, complete osseointegration was achieved at the graft-bone interface in only two out of 11 cases, after 1 year. Partial integration was found in three patients. In view of these results, autogenous iliac crest bone grafts are still the unrivalled standard for defect repair in spinal surgery.

Biomechanical testing of anterior and posterior thoracolumbar instrumentation in the cadaveric spine

Object. Thoracolumbar burst fractures frequently require surgical intervention. Although the use of either anterior or posterior instrumentation has advantages and disadvantages, there have been few studies in which these two approaches have been compared biomechanically.

Methods. Ten human cadaveric spines were subjected to subtotal L-3 corpectomy. In five spines placement of L-3 wooden strut grafts with lateral L2–4 dual rod and screw instrumentation was performed. Five other spines underwent L1–5 pedicle screw fixation. The spines were fatigued between steps of the experiment. The spines were load tested with pure moments of 1.5, 3, 4.5, and 6 Nm in the intact state and after placement of instrumentation in six degrees of freedom (flexion, extension, right and left lateral bending, and right and left axial rotation).

In axial rotation posterior instrumentation significantly increased spinal rigidity compared with that of the intact state, whereas anterior instrumentation did not. Combined anterior—posterior instrumentation did not significantly increase the rigidity of the spine when compared with anterior or posterior instrumentation alone. Posterior instrumentation alone provided a greater reduction in angular rotation compared with anterior instrumentation alone in all degrees of freedom; however, statistical significance was achieved only in extension at 6 Nm.

Conclusions. The increased rigidity provided by pedicle screw instrumentation compared with the intact state or with anterior instrumentation is due to the longer construct spanning five levels and the three-column engagement of the pedicle screws. The decision to use anterior or posterior instrumentation should be based on the clinical necessity of canal decompression and correction of angulation.

Thoracolumbar fracture stabilization: comparative biomechanical evaluation of a new video-assisted implantable system

Minimally invasive techniques for spinal surgery are becoming more widespread as improved technologies are developed. Stabilization plays an important role in fracture treatment, but appropriate instrumentation systems for endoscopic circumstances are lacking. Therefore a new thoracoscopically implantable stabilization system for thoracolumbar fracture treatment was developed and its biomechanical in vitro properties were compared. In a biomechanical in vitro study, burst fracture stabilization was simulated and anterior short fixation devices were tested under load with pure moments to evaluate the biomechanical stabilizing characteristics of the new system in comparison with a currently available system. With interbody graft and fixation the new system demonstrated higher stabilizing effects in flexion/extension and lateral bending and restored axial stability beyond the intact spine, as well as having comparable or improved effects compared with the current system. Because of this biomechanical characterization a clinical trial is warranted; the usefulness of the new system has already been demonstrated in 45 patients in our department and more than 300 cases in a multicenter study which is currently under way.

Biomechanical analysis of anterior instrumentation for lumbar corpectomy

STUDY DESIGN

In vitro biomechanical assessment of spinal stability after corpectomy reconstruction. OBJECTIVES To gain a more thorough understanding of the biomechanical properties of anterior plate versus dual rod systems used for anterior lumbar corpectomy reconstruction.

SUMMARY OF BACKGROUND DATA

Vertebral corpectomy is commonly required in the treatment of several types of spinal pathology (fracture, tumor, infection). Stabilization with strut allograft and anterior instrumentation can be accomplished with one of several anterior implant systems. These include plate systems and rod-based systems with theoretically different structural properties.

METHODS

Two instrumentation systems, the ATL Z-plate and the Antares system, were each applied to 10 calf lumbar spines with a cortical strut graft reconstructing an L3 corpectomy defect. All spines were tested biomechanically to determine construct stiffness under physiologic loads in multiple planes and then tested in torsion to failure.

RESULTS

There was greater stiffness (P < 0.05) in all directions of bending (flexion, extension, lateral bending) for the Antares dual rod construct compared to the Z-plate constructs. No significant difference was noted in either torsional testing under physiologic loads or torque to failure between the groups.

CONCLUSIONS

Although there was significantly greater resistance to bending with the dual rod construct, the ultimate selection of a system will require an individual analysis of implant profile, construct demand, and ease of use. Both systems provided secure initial fixation following lumbar corpectomy; however, the Antares system may increase the likelihood of graft incorporation in cases with greater instability and higher load demands.

Thoracolumbar fracture management: anterior approach

The surgeon who treats patients with spine trauma must be able to apply a variety of management techniques to achieve optimal care of the patient. The anterior surgical approach is appropriate for some thoracolumbar burst fractures in patients with neurologic deficit and without posterior ligamentous injury. Surgery is most often indicated for patients with incomplete deficit, especially those with a large retropulsed fragment, marked canal compromise, severe anterior comminution, or kyphosis <30 degrees. This approach provides excellent visualization of the anterior aspect of the dura mater for decompression. Reconstruction of the anterior body defect can be done with autograft, allograft, or a cage. Supplementation of the graft with anterior internal fixation helps prevent kyphosis. Clinical results demonstrate improved neurologic function in most patients as well as low pseudarthrosis rates. In patients with incomplete deficit, improvement in neurologic function usually can be expected with few complications.

Enhanced primary stability through additional cementable cannulated rescue screw for anterior thoracolumbar plate application

OBJECT

The authors conducted a study to investigate the biomechanical in vitro influence of a new anchorage system for fixation of anterior stabilization devices and the possibility of using additional cement after screw insertion to compensate for poor bone quality. The incidence of osteoporosis-related fractures has increased nearly twofold in the last decade. Because of problems associated with anterior screw fixation such as loosening, mechanical failure, and the weakness of osteoporotic bone, current surgical treatments of vertebral body (VB) fractures are problematic. This is due to poor fixation strength of anterior screws in the adjacent segments. The aim of this study was to determine whether a new cemented and uncemented VB screw provides improved primary stability following placement of anterior instrumentation in cases of fracture.

METHODS

The primary stability-related parameters of a new uncemented/cemented screw were compared with those of conventional monocortical screw fixation in a burst fracture model in which strut graft and anterior overbridging instrumentation were used. The use of the new uncemented screw improved the range of motion (ROM) of the stabilized spine in flexion-extension by approximately 22%, in rotation by 20%, and in lateral bending by 15%. Additional cementation improved the ROM by approximately 41% in flexion-extension, 32% in rotation, and 30% in lateral bending compared with conventional monocortical screw fixation.

CONCLUSIONS

The new cannulated screw improves fixation strength and primary stability parameters. It is useful in the initial treatment of fractures in cases of poor bone quality and as a rescue device if previously inserted screws do not remain securely in place.

In vitro biomechanical studies of an anterior thoracolumbar implant

After L1 corpectomy in T11-L3 human cadaveric spine, anterior thoracolumbar instrumentation with strut grafting restores spinal stability. T12-L2 angular rotation was measured in response to moments of 0.0, 1.5, 3.0, 4.5, and 6.0 Nm in flexion, extension, lateral bending, and axial rotation, respectively. The spines were tested: 1) intact; 2) after partial L1 corpectomy, grafting, and instrumentation (Profile plate, DePuy-AcroMed, Raynham, MA), with the wooden dowel graft screwed to the plate; 3) without graft screw fixation; and 4) after flexion-extension cyclic fatiguing for 5000 cycles at a load of +/-3.0 Nm. Before and after fatiguing, the instrumented spine was significantly (p <or= 0.05) stiffer than the intact spine in flexion, extension, and right and left lateral bending but not in axial rotation. There were no significant differences between the constructs with or without graft-to-plate fixation before or after fatigue. The instrumented spines were more rigid in bending away from the implant than bending toward the implant. Anterior spinal instrumentation with the Profile implant augments stiffness in the sagittal and coronal planes but not in the axial plane. Although graft-to-plate fixation may prevent graft migration into the canal, it does not contribute to spinal rigidity.

A biomechanical comparison between anterior and transverse interbody fusion cages

STUDY DESIGN

Human cadaveric lumbar spines underwent placement of threaded fusion cages (TFCs) in either an anterior or transverse orientation. Spines underwent load testing and angular rotation measurement in the intact state, after diskectomy, after cage placement, and after fatiguing. Angular rotations were compared between cage orientations and interventions.

OBJECTIVE

To determine which cage orientation resulted in greater immediate stability.

SUMMARY OF BACKGROUND DATA

There has been extensive biomechanical study of interbody fusion cages. The lateral orientation has been increasingly used for intervertebral fusion, but a direct biomechanical comparison between cages implanted either anteriorly or transversely in human cadaveric spines has not been performed.

METHODS

Fourteen spines were randomized into the anterior group (anterior diskectomy and dual anterior cage placement) and the lateral group (lateral diskectomy and single transverse cage placement). Pure bending moments of 1.5, 3.0, 4.5, and 6.0 Nm were applied in flexion, extension, lateral bending, and axial rotation. Load testing was performed while intact, after diskectomy, after cage placement, and after fatiguing. Angular rotation was compared between anterior and lateral groups and, within each group, among the different interventions.

RESULTS

Segmental ranges of motion were similar between spines undergoing either anterior or lateral cage implantation.

CONCLUSIONS

These results demonstrate few differences between angular rotation after either anterior or lateral TFC implantation. These findings add to data that find few differences between orientation of implanted TFCs. Combined with a decreased risk of adjacent structure injury through a lateral approach, these data support a lateral approach for lumbar interbody fusion.

A study of stiffness protocol as exemplified by testing of a burst fracture model in sagittal plane

STUDY DESIGN

An in vitro biomechanical study of lumbar spine segments.

OBJECTIVE

To study the characteristics of the stiffness test protocol.

SUMMARY OF BACKGROUND DATA

In an in vitro study using a flexibility protocol, forces are applied and motions are measured; no center of rotation needs to be specified. In a study using a stiffness protocol, the forces are measured and the motions are applied. This does require the center of rotation to be specified. Many biomechanical studies of the spine are available, but there is lack of clarity concerning which of these two test protocols is appropriate to achieve a certain study goal.

METHODS

Five-vertebrae lumbar spine specimens with burst fractures in the middle vertebrae (L1) were used. Specially designed apparatus applied flexion and extension rotations around five centers of rotations located on anteroposterior line through the middle of L1. Maximum moment of 4 Nm was applied.

RESULTS

The authors found load-displacement curves, ranges of motion, and neutral zones obtained at the five centers of rotations to be markedly different. The center of rotation located at the posterior longitudinal ligament produced large range of motion and neutral zones in comparison to the centers of rotation located at the anterior longitudinal ligament and the spinous process tip (P<0.01).

CONCLUSIONS

The stiffness protocol requires that a center of rotation be specified. Shown here is the significant variability in the load-displacement curves, depending on the choice of the location of the center of rotation. Certain center of rotation locations may block the natural motions of the spine and may result in tissue damage.

In vitro biomechanical analysis of three anterior thoracolumbar implants

OBJECT

The goal of this study was to evaluate the comparative efficacy of three commonly used anterior thoracolumbar implants: the anterior thoracolumbar locking plate (ATLP), the smooth-rod Kaneda (SRK), and the Z-plate.

METHODS

In vitro testing was performed using the T9-L3 segments of human cadaver spines. An L-1 corpectomy was performed, and stabilization was achieved using one of three anterior devices: the ATLP in nine spines, the SRK in 10, and the Z-plate in 10. Specimens were load tested with 1.5-, 3-, 4.5-, and 6-Nm in flexion and extension, right and left lateral bending, and right and left axial rotation. Angular motion was monitored using two video cameras that tracked light-emitting diodes attached to the vertebral bodies. Testing was performed in the intact state in spines stabilized with one of the three aforementioned devices after the devices had been fatigued to 5000 cycles at +/- 3 Nm and after bilateral facetectomy. There was no difference in the stability of the intact spines with use of the three devices. There were no differences between the SRK- and Z-plate-instrumented spines in any state. In extension testing, the mean angular rotation (+/- standard deviation) of spines instrumented with the SRK (4.7 +/- 3.2 degrees) and Z-plate devices (3.3 +/- 2.3 degrees) was more rigid than that observed in the ATLP-stabilized spines (9 +/- 4.8 degrees). In flexion testing after induction of fatigue, however, only the SRK (4.2 +/- 3.2 degrees) was stiffer than the ATLP (8.9 +/- 4.9 degrees). Also, in extension postfatigue, only the SRK (2.4 +/- 3.4 degrees) provided more rigid fixation than the ATLP (6.4 +/- 2.9 degrees). All three devices were equally unstable after bilateral facetectomy. The SRK and Z-plate anterior thoracolumbar implants were both more rigid than the ATLP, and of the former two the SRK was stiffer.

CONCLUSIONS

The authors' results suggest that in cases in which profile and ease of application are not of paramount importance, the SRK has an advantage over the other two tested implants in achieving rigid fixation immediately postoperatively.

Spinal stability with anterior or posterior ray threaded fusion cages

OBJECT

The authors conducted a study to determine if the rigidity supplied to the spine by posterior placement of the Ray threaded fusion cage (TFC) is further enhanced by the placement of pedicle screws and, additionally, if bilateral anteriorly placed TFCs render the spine more rigid than a single anteriorly placed TFC.

METHODS

Ten human cadaveric spinal specimens (L2-S1) were affixed within a testing frame. Loads of 1.5, 3, 4.5, and 6 Nm were applied to the spine in six degrees of freedom: flexion-extension, right and left lateral bending, and right and left axial rotation. Motion in an x, y, and z cartesian axis system was tracked using dual video cameras following light-emitting diodes attached to the spine and base plate. Load testing of the spines was performed in the intact mode, following which the spinal segments were randomized to receive anterior or posterior instrumentation. In five spine specimens we performed posterior discectomy, posterior lumbar interbody fusion (PLIF) with placement of femoral rings and pedicle screws, PLIF with bilateral TFCs, and bilateral TFCs with pedicle screws. Five other spines underwent anterior-approach discectomy, followed by implantation of a unilateral cage and bilateral cages. Load testing was performed after each step.

CONCLUSION

Spines in which PLIF with pedicle screws and TFCs with pedicle screws were placed were more rigid than after discectomy in all directions of motion except flexion. Anterior discectomy provided significantly (p < or = 0.05) less stability in left and right axial rotation than the intact spines and following posterior discectomy. Following anterior implantation of bilateral TFCs, spines were significantly more rigid than after discectomy in all directions except extension.

Biomechanical studies of a dynamized anterior thoracolumbar implant

STUDY DESIGN

An in vitro investigation into the biomechanical properties of a dynamized anterolateral compression implant that allows controlled subsidence.

OBJECTIVES

To determine the extent to which both modes of the anterolateral compression implant (controlled collapsing and rigid) are able to reestablish the stability of the lumbar spine after L4 corpectomy.

SUMMARY OF BACKGROUND DATA

Over time, anterior and posterior spinal implants have been associated with progressive angulation, and occasionally implant failure and breakage. To circumvent this occurrence and provide better graft loading, dynamized or collapsing devices for clinical use have been developed.

METHODS

Eight fresh calf spines (L1-L6) were placed in a biomechanical testing frame. Pure moments of 6 Nm were loaded onto the intact spine in six directions: flexion, extension, right and left lateral bending, and right and left axial rotation. A total L4 corpectomy then was performed, and the defect grafted with a wooden dowel. Loading was repeated after the specimens were stabilized using the two modes of the anterolateral compression implant in succession.

RESULTS

The results showed that both modes of the implant (the rigid mode in particular) restore the stiffness of the unstable spine to normal levels of flexion, extension, and right and left lateral bending, even to levels exceeding normal. These devices, however, fall short of achieving normal stability in right and left axial rotation.

CONCLUSION

In the cadaveric calf spine after L4 corpectomy, restoration of stability with a dynamized anterior spinal implant is possible in flexion, extension, and right and left lateral bending, but not in axial rotation.