Biomechanical evaluation of Caspar cervical screws: comparative stability under cyclical loading

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.

Use of an integrated anterior cervical plate and cage device (PCB) in cervical anterior fusion

The aim of this study is to evaluate an integrated cage and plate device (the plate cage Benezech, PCB) filled with autogenous bone in anterior cervical discectomy and fusion. The fused segment height, lordosis, and fusion were assessed by postoperative radiographic examination at different intervals. Patients were evaluated using Odom's criteria and the Short Form (SF)-36 Health Survey questionnaire. The mean follow-up duration was 4.1 years. Fusion was achieved in 90.0%, 96.0% and 100% of patients at 3 months, 6 months and at final visit, respectively. The fused segment height and lordosis were restored and maintained. Cage subsidence (3mm) occurred at one level and settling was observed at three levels. An excellent-to-good result was achieved in 81.8% of patients. The data from the SF-36 questionnaire revealed significant postoperative improvement (p<0.01) except for social function and mental health. This study suggests that patients instrumented with PCB can obtain good radiographic and clinical results and that PCB is a safe and effective device in cervical anterior fusion.

Anterior cervical discectomy and fusion with a locked plate and wedged graft effectively stabilizes flexion-distraction stage-3 injury in the lower cervical spine: a biomechanical study

STUDY DESIGN

An in vitro three-dimensional (3D) flexibility test of human C3-C7 cervical spine specimens.

OBJECTIVE

To test the hypothesis that anterior cervical fusion with a wedged graft and a locked plate can effectively stabilize the cervical spine after complete anterior and posterior segmental ligamentous release.

SUMMARY OF BACKGROUND DATA

Distraction-flexion Stage 3 injuries of the lower cervical spine (bilateral facet dislocations) are usually reduced under awake cranial traction. When the magnetic resonance imaging reveals a traumatic disc prolapse, anterior cervical discectomy and fusion (ACDF) is usually recommended. Most authors advise combining ACDF with posterior instrumentation to address the insufficiency of the posterior elements. However, there is clinical evidence that ACDF with a locked plate alone suffices for the treatment of these injuries, especially in young patients. Still, there are no biomechanical studies on the effect of a locked plate on the complete anterior and posterior ligamentous-deficient young cervical spine under physiologic preload.

METHODS

Eight fresh frozen human lower cervical spines (C3-C7) from young donors (age, 44.5 years; range, 21-63 years) were used. A 3D flexibility test was conducted using a moment of 0.8 Nm without preload. Flexion-extension was additionally tested using a moment of 1.5 Nm under 0 and 150 N follower preload. Spines were tested first intact, then after complete C5-C6 discectomy with posterior longitudinal ligament resection and ACDF with a wedged bone graft and a rigid locked plate, and finally after complete release of the supraspinous, interspinous, and intertransverse ligaments; the facet capsules; and ligamentum flavum. RESULTS.: When tested under 0.8 Nm moment without preload, complete posterior and anterior ligamentous release did not significantly increase the ROM of the ACDF construct in flexion-extension (P > 0.025), lateral bending (P > 0.025), and axial rotation (P > 0.025). When tested under 1.5 Nm moment with or without a compressive preload, the complete posterior and anterior ligamentous release did not significantly affect the ROM of the ACDF construct (P > 0.01). The application of preload significantly reduced the motion at the C5-C6 ACDF construct with ligamentous disruption in comparison with the motion in the absence of a preload (P < 0.01).

CONCLUSION

Anterior cervical fusion with a wedged graft and a rigid constrained (locked) plate can effectively stabilize the nonosteoporotic cervical spine after complete posterior element injury when excessive ROM is prevented (for example, by the use of postoperative external immobilization). Even when the construct is subjected to higher moments, adequate stability can be achieved when physiologic preload is present. Osteoporosis and lack of sufficient preload due to poor neuromuscular control may affect long-term screw stability, and additional external immobilization may be needed until fusion matures.

4- and 5-level anterior fusions of the cervical spine: review of literature and clinical results

In the future, there will be an increased number of cervical revision surgeries, including 4- and more-levels. But, there is a paucity of literature concerning the geometrical and clinical outcome in these challenging reconstructions. To contribute to current knowledge, we want to share our experience with 4- and 5-level anterior cervical fusions in 26 cases in sight of a critical review of literature. At index procedure, almost 50% of our patients had previous cervical surgeries performed. Besides failed prior surgeries, indications included degenerative multilevel instability and spondylotic myelopathy with cervical kyphosis. An average of 4.1 levels was instrumented and fused using constrained (26.9%) and non-constrained (73.1%) screw-plate systems. At all, four patients had 3-level corpectomies, and three had additional posterior stabilization and fusion. Mean age of patients at index procedure was 54 years with a mean follow-up intervall of 30.9 months. Preoperative lordosis C2-7 was 6.5 degrees in average, which measured a mean of 15.6 degrees at last follow-up. Postoperative lordosis at fusion block was 14.4 degrees in average, and 13.6 degrees at last follow-up. In 34.6% of patients some kind of postoperative change in construct geometry was observed, but without any catastrophic construct failure. There were two delayed unions, but finally union rate was 100% without any need for the Halo device. Eleven patients (42.3%) showed an excellent outcome, twelve good (46.2%), one fair (3.8%), and two poor (7.7%). The study demonstrated that anterior-only instrumentations following segmental decompressions or use of the hybrid technique with discontinuous corpectomies can avoid the need for posterior supplemental surgery in 4- and 5-level surgeries. However, also the review of literature shows that decreased construct rigidity following more than 2-level corpectomies can demand 360 degrees instrumentation and fusion. Concerning construct rigidity and radiolographic course, constrained plates did better than non-constrained ones. The discussion of our results are accompanied by a detailed review of literature, shedding light on the biomechanical challenges in multilevel cervical procedures and suggests conclusions.

Bicortical screw fixation of distal fibula fractures with a lateral plate: an anatomic and biomechanical study of a new technique

One of the potential drawbacks of lateral plating of distal fibula fractures is less than satisfactory fixation of unicortical screws commonly placed in the distal fragment to avoid implant penetration of the ankle joint. This study examines the anatomy of the distal fibula, proposes new techniques for bicortical screw fixation and radiographic evaluation of screw placement, and compares pullout strength of unicortical versus bicortical screws in this area. Sixteen pairs of human cadaver feet were used in this study. It was found that a large percentage of the surface area of the distal fibula is nonarticular and that the distal fibula could be divided into 3 zones with distinct anatomic features. Zone I is defined as the distal most 1.5 cm of the fibula, zone II is the next 1 cm of fibula proximal to zone I, and zone III is defined as the fibula above the ankle joint, starting at just over 2.5 cm proximal to the tip of the fibula. We determined a safe corridor for bicortical screw placement by means of a lateral plate in each zone. An improved radiographic view is described for confirmation of extraarticular screw placement. Screw pullout testing was performed on 8 pairs of fresh-frozen human cadaver fibulas. In both zone I and zone II, the bicortical screw fixation was significantly stronger than the unicortical screw fixation. In zone I, the average pullout strength for the bicortical screw fixation was 2.3 times higher than the unicortical screw fixation. In zone II, the average pullout strength for the bicortical screw fixation was 3.3 times higher than the unicortical screw fixation. This study shows that not only is bicortical screw placement in the distal fibula technically feasible, but it is also biomechanically stronger than unicortical placement in this area.

Trabecular bone density of male human cervical and lumbar vertebrae

The objective of this study was to determine the bone mineral density (BMD) of cervical vertebrae and correlate with the lumbar spine. Fifty-seven young adult healthy male volunteers, ranging from 18 to 41 years of age, underwent quantitative computed tomography (QCT) scanning of C2-T1 and L2-L4 vertebrae. To account for correlations, repeated measures techniques were used to compare data as a function of spinal level and region. Linear regression methods were used (+/-95% CI) to compare data as a function of spinal level and region. The mean age and body height were 25.0 +/- 5.8 years and 181.0 +/- 7.6 cm. BMD decreased from the rostral to caudal direction along the spinal column. Grouped data indicated that the neck is the densest followed by the first thoracic vertebra and low back with mean BMD of 256.0 +/- 48.1, 194.3 +/- 44.2, and 172.2 +/- 28.4 mg/cm(3), respectively; differences were statistically significant. While BMD did not vary significantly between the three lumbar bodies, neck vertebrae demonstrated significant trends. The matrix of correlation coefficients between BMD and spinal level indicated that the relationship is strong in the lumbar (r = 0.92-0.96) and cervical (r = 0.73-0.92) spines. Data from the present study show that the trabecular bony architecture of the neck is significantly different from the low back. These quantitative BMD data from a controlled young adult healthy human male volunteer population may be valuable in establishing normative data specifically for the neck. From a trabecular bone density perspective, these results indicate that lumbar vertebrae cannot act as the best surrogates for neck vertebrae. Significant variations in densities among neck vertebrae, unlike the low back counterpart, may underscore the need to treat these bones as different structures.

Biomechanical comparison of anterior cervical spine locked and unlocked plate-fixation systems

Three different anterior plate-fixation systems are available for the stabilisation of the cervical spine: (1) the cervical spine locking plate (CSLP), (2) dynamic plates allowing vertical migration of the fixation screws, and (3) various types of plates that are secured with either monocortical or bicortical unlocked screws. Unicortical screw purchase does not involve the risk of posterior cortex penetration and possible injuries to the spinal cord. The development of locking plates with unicortical screw-fixation and intrinsic stability of the screw-plate interface, via an angle-stabilised connection, was an attempt to increase the stability of unicortical screw-fixation systems. The aim of the study was to compare the biomechanical properties of a non-locking, anterior-plate system with 4.5 mm screw fixation and a locking anterior-plate system, in a single destabilised cervical spine-motion segment. Using fresh cadaveric cervical spine specimen C3-C7, multidirectional flexibility was measured at the C4-C5 level in an unconstrained test system, before and after destabilisation and fixation with an anterior plate with either locked or unlocked screw purchase. Direct comparison of the fixed cervical spine segments with unlocked and locked anterior-plate fixation did not demonstrate significant differences. This in vitro study documented that neither locked nor unlocked anterior-plate fixation can increase stability in all modes of testing. H-plate spondylodesis with unlocked screws seems to provide sufficient mechanical integrity in most cases of monosegmental lesions.

Biomechanical analysis of anterior cervical spine plate fixation systems with unicortical and bicortical screw purchase

Anterior plate fixation with unicortical screw purchase does not involve the risk of posterior cortex penetration and possible injuries of the spinal cord. However, there are very few biomechanical data about the immediate stability of non-locking plate fixation with unicortical or bicortical screw placement. The aim of the present study was to evaluate the immediate biomechanical properties in terms of flexibility of a non-locking anterior plate system with 4.5-mm screw fixation and unicortical or bicortical screw purchase applied to a single destabilized cervical spine motion segment. Using fresh cadaveric cervical spine specimens C3-C7, multidirectional flexibility was measured at the level C4-C5 before and after destabilization and fixation with an anterior plate with either unicortical or bicortical screw purchase. The results showed that fixed cervical spine segments with anterior plate and bicortical screw purchase were more rigid than intact specimens in all modes of testing. The difference was statistically significant for flexion and extension ( P<0.001). Plate fixation with unicortical screw purchase had statistically significant decreased ranges of motion compared to the intact specimen only in extension. Neither unicortical nor bicortical screw purchase decreased the range of motion significantly in axial rotation compared to the intact specimens. This in vitro study documented that neither unicortical nor bicortical screw purchase with non-locking plate fixation can increase stability in all modes of testing, in axial rotation in particular. Direct comparison between the group with uni- and that with bicortical screw fixation did not reveal significant differences, and therefore no advantage was shown for either type of screw fixation. Therefore, we demonstrated that both uni- and bicortical screw purchase with non-locking plate fixation can decrease immediate flexibility of the tested motion segment, with better results for bicortical purchase. No significant differences were found comparing the two groups of screw fixation. These data suggest that unicortical screw fixation can be used for anterior plate fixation with a comparable immediate stability to bicortical screw fixation.

The cortical shell architecture of human cervical vertebral bodies

STUDY DESIGN

An anatomic study of cervical vertebral bodies.

OBJECTIVES

To provide quantitative information on the cortical shell architecture of the middle and lower cervical vertebral bodies.

SUMMARY OF BACKGROUND DATA

Some external dimensions have been measured, but little quantitative data exists for the cortical shell architecture of the vertebral bodies of the cervical spine.

METHODS

Twenty-one human cervical vertebral bodies (C3-C7) were sectioned along parasagittal planes into five 1.7-mm thin slices for each vertebra. Radiographs of each slice were digitized, and external and internal dimensions were measured. Averages and standard deviations were computed. Single factor analysis of variance was used to determine significant (P < 0.05) differences between the vertebral levels.

RESULTS

The superior endplate was thickest in the posterior region (range 0.74-0.89 mm) and thinnest in the anterior region (range 0.44-0.56 mm). The inferior endplate was thickest in the anterior region (range 0.61-0.81 mm) and thinnest in the posterior region (range 0.49-0.62 mm). In the central region, the superior endplate (range 0.42-0.58 mm) was thinner than the inferior endplate (range 0.53-0.64 mm). Variation with vertebral level was dependent on the dimension studied.

CONCLUSIONS

Comprehensive quantitative anatomic data of the middle and lower cervical vertebral bodies have been obtained. This may be useful in improving the understanding of the three-column and other vertebral-fracture theories, the fidelity of the finite element models of cervical spine, and the designs of surgical instrumentation.

A new anterior cervical instrumentation system combining an intradiscal cage with an integrated plate: an early technical report

STUDY DESIGN

To provide a technical report on a new device. Early outcome assessment of patients undergoing anterior cervical decompression and stabilization with the described technique.

OBJECTIVES

To detail the operative and technical aspects of the new anterior cervical instrumentation and to identify its suitability for an extensive clinical study.

SUMMARY OF BACKGROUND DATA

The controversy over the need for fusion in the surgery of cervical disc disease is in part fueled by the absence of an ideal technique. However, the design of the integrated anterior cervical plate and cage device (PCB) appears to provide immediate stability and to restore disc height and cervical lordosis in addition to reducing graft recipient and donor site-related complications.

METHODS

After a standard anterior cervical discectomy and preparation of the disc space, the correct size of the cage with integrated plate is inserted. Self-tapping screws are inserted without image intensifier. Cancellous bone chips are used. The technique was used in 29 patients that were followed prospectively from 5 to 24 months (median 8 months, interquartile range 6-15 months).

RESULTS

The technique was easy to learn and implement. The authors' experience has been free of intraoperative complications. After surgery no screw backout or device failure was identified. Twenty-seven patients improved clinically. Donor site morbidity was trivial. No collars were applied after surgery.

CONCLUSIONS

The design of the system appears to prevent bone-graft recipient site and donor site complications, provides immediate biomechanical stability, prevents screw backout or breakage, and restores posterior interbody height and lordosis. The device is of sufficient promise to warrant further close evaluation focusing on long-term outcome. The new radiolucent version has potential advantages for the assessment of fusion.

Stabilizing potential of anterior cervical plates in multilevel corpectomies

STUDY DESIGN

An in vitro investigation of three-dimensional kinematics of cervical spine models of one- and three-level corpectomy with anterior plate fixation.

OBJECTIVES

To evaluate the capability of an anterior plate to stabilize the reconstructed cervical spine under simulated physiologic motions, and to study the effects of fatigue loading.

SUMMARY OF BACKGROUND DATA

Clinical studies have found high failure rates of multilevel anterior cervical plate fusions, indicating suboptimal stabilization. However, no biomechanical studies have been done to investigate the stabilizing capabilities of long-plate instrumentations in corpectomy models.

METHODS

Seven fresh human cadaveric cervical spine specimens (C2-T1) were used. Flexibility tests consisted of flexion, extension, and bilateral torsion, and lateral bending, each with a pure moment of 0.25, 0.5, 0.75, and 1.0 Nm. Stabilizing potential indices [(MotionIntact-MotionInstrumented)/MotionIntact] for ranges of motion and neutral zones obtained from the flexibility tests, were measured when the specimen was intact and after one-level (C5) and three-level (C4, C5, and C6) corpectomies and anterior plate stabilizations). The stabilizing potential indices were re-measured after a 1000-cycle fatigue loading (1 Nm flexion and extension moments at C5 vertebra at 0.14 Hz).

RESULTS

The differences in stabilizing potential indices of range of motion and neutral zone between one-level and three-level plates were not significant before fatigue. However, after fatigue, the stabilizing potential indices significantly decreased (P < 0.05) for the three-level model, but not for the one-level plate model.

CONCLUSIONS

The capability of an anterior cervical plate to stabilize the spine after three-level corpectomy was significantly reduced with fatigue loading.

The effect of locking fixation screws on the stability of anterior cervical plating

STUDY DESIGN

Current anterior cervical plate systems were tested with locked and unlocked fixation screws and with unicortical and bicortical fixation screws to determine fixation rigidity and pull-off strengths.

OBJECTIVES

To evaluate the effects of screw-plate locking and screw length on fixation strength and stability of anterior cervical plates.

SUMMARY OF BACKGROUND DATA

New plate systems provide for rigid locking of the screw-plate interface, theoretically increasing construct rigidity, allowing unicortical fixation, and preventing screw back-out. There are few data on the effects of locking screws on the stability of anterior cervical plating.

METHODS

Eighty fresh lamb vertebrae (C3-T1) were used. Test systems included: Cervical Spine Locking Plate (CSLP; Synthes, Paoli, PA, Orion plate (Sofamor-Danek, Memphis, TN), and Acroplate (AcroMed, Cleveland, OH). The CSLP and Orion plates were tested with fixation screws, locked and unlocked, and the AcroMed plate with unicortical and bicortical screw purchase. Biomechanical testing of the screw-plate constructs was performed to determine the initial bone-plate rigidity and pull-off strength. A 2.5-Nm cyclic bending moment was then applied to additional constructs for 10(5) cycles, and these constructs retested.

RESULTS

Locked CSLP and Orion constructs were more rigid than all unlocked unicortical systems initially and after cyclic loading (P < 0.05). After cycling, the rigidity of all unlocked unicortical constructs decreased significantly (P < 0.05). There was no significant difference in pull-off strengths between the CSLP, the Orion, and the unicortical AcroMed plate. However, all had significantly less pull-off strength than the AcroMed plate with bicortical screws. A negative correlation was observed between initial pull-off strength and sagittal vertebral body diameter.

CONCLUSIONS

Locking screws significantly increased the rigidity of the tested screw-plate systems initially and after cyclic loading. Because pull-off strength was affected by the vertebral body diameter, use of longer unicortical screws may be clinically beneficial in the patient with larger cervical vertebrae.

Biomechanical evaluation of a newly developed monocortical expansion screw for use in anterior internal fixation of the cervical spine. In vitro comparison with two established internal fixation systems

STUDY DESIGN

The primary biomechanical stability of anterior internal fixation of the cervical spine obtained with a new monocortical expansion screw in vitro was evaluated.

OBJECTIVES

To determine whether the anterior internal fixation of the spine obtained with the new monocortical expansion screw provides biomechanical stability comparable with that obtained with bicortical fixation.

SUMMARY OF BACKGROUND DATA

The anterior plate instrumentation used with bicortical screw fixation in the cervical spine provides a primary stability superior to that associated with monocortical screw fixation. However, bicortical screws have the potential to perforate the posterior cortex. Therefore, monocortical instrumentation systems were developed, but without the biomechanical stability associated with bicortical systems. A new expansion screw for monocortical fixation was developed to improve biomechanical stability of monocortical systems.

METHODS

Three different internal fixation systems were compared in this study: 1) H-plate with AO 3.5-mm bicortical screws, 2) cervical spine locking plate with monocortical screws, and 3) H-plate with the new monocortical expansion screws. Eight fresh human cadaver spine segments from C4 to C7 were tested in flexion-extension, axial rotation, and lateral bending using pure moments of +/- 2.5 Nm without axial preload. Five conditions were investigated consecutively: 1) intact spine; 2) uninstrumented spine with the segment C5-C6 destabilized; 3-5) instrumentation of the segment C5-C6 with the three implants mentioned above after removal of the disc and insertion of an interbody spacer.

RESULTS

Between bicortical and monocortical expansion screw H-plate fixation, no significant differences were observed in all load cases concerning range of motion and neutral zone. The neutral zone and range of motion were significantly larger for the cervical spine locking plate than for bicortical and monocortical expansion screw fixation in all load cases, except neutral zone for axial rotation versus bicortical screw fixation. The instrumented cases only had a significantly lower range of motion and neutral zone than the intact cases in extension-flexion, whereas for lateral bending and axial rotation no significant differences could be observed. Because the experimental design precluded any cyclic testing, the data represent only the primary stability of the implants.

CONCLUSIONS

In anterior instrumentation of the cervical spine using a H-plate, the new monocortical expansion screw provides the same biomechanical stability as the bicortical 3.5-mm AO screw and a significantly better biomechanical stability than the cervical spine locking plate. Therefore, the expansion screw may be an alternative to the bicortical fixation and does not involve the risk of penetration of the posterior vertebral body cortex.

Comparative pull-out strength of tapped and untapped pilot holes for bicortical anterior cervical screws

STUDY DESIGN

This biomechanical study analyzed the axial pull-out strength of tapped versus untapped pilot holes for bicortical screws in the anterior cervical spine.

OBJECTIVE

To determine which pilot hole preparation method was mechanically better.

SUMMARY OF BACKGROUND DATA

Tapping pilot holes in the lumbar spine was previously shown significantly to reduce pull-out strength of pedicle screws. No study was found investigating the effect of tapping on pilot holes for anterior cervical bicortical screws.

METHODS

Twenty-five unembalmed human cadaveric cervical vertebrae (C3-C7) were tested. Two identical pilot holes were drilled into each vertebra: one pilot hole was tapped, and the control pilot hole was not tapped. A fully threaded cortical bone screw was inserted into each pilot hole. Screw pull-out strength was determined using a servocontrolled hydraulic materials testing system and an axial load cell. Force-deformation and failure curves were obtained.

RESULTS

There were no statistically significant differences between the axial pull-out strength of tapped and untapped pilot holes at any vertebral level. Mean force to-failure was 386 +/- 42 N in the untapped pilot holes and 397 +/- 48 N in the tapped pilot holes.

CONCLUSIONS

Tapping a pilot hole for bicortical screws of the anterior cervical spine neither weakens nor strengthens the axial pull-out strength of fully threaded cortical bone screws. Tapping may be unnecessary; however, it may be desirable in patients with dense bone to cut the thread profile into the bone or if the screws have dull tips and threads.

Justification for decreasing the number of views taken in radiologic follow-up of Casper plate cervical spine hardware

RATIONALE AND OBJECTIVES

We tested the null hypothesis that there would be no difference between the diagnostic yield of the lateral radiograph alone and the yield of the lateral and anteroposterior views combined when assessing patients after placement of an anterior cervical appliance.

METHODS

We evaluated 630 radiographic examinations obtained from 117 consecutive patients who had anterior cervical plating. For each examination, we looked at the lateral radiograph first and then at the anteroposterior radiography to determine whether there would be any additional information on the anteroposterior film.

RESULTS

Of 501 examinations in which the lateral and anteroposterior views were available, we found that the anteroposterior view added information in 18. In eight of the 18, the added information consisted only of clinically insignificant tilting of the plate. Significant findings were seen in 209 examinations in the lateral view and in 219 examinations with the lateral and anteroposterior views combined. This was not significantly different.

CONCLUSION

The anteroposterior view rarely adds significant information. It probably should be obtained early postoperatively to check for tilting of the plate but after that only for specific indications. This will save money and reduce the patient's exposure to radiation.

Radiographic and clinical follow-up review of Caspar plates in 49 patients

Although they are excellent clinical tools, Caspar anterior cervical plates have not been studied closely with regard to their mechanisms of failure. As more extensive operations are contemplated on older, sicker patients, it is imperative to know when a plating system might be prone to failure and what the mechanism of that failure might be. Therefore, the authors reviewed 49 patients undergoing Caspar plate placement in whom sufficient radiographs were obtained to determine if the fate of the hardware was related to the patient's age, type of operation, and the length of construct. Eleven of 49 patients suffered hardware failure, defined as any amount of screw backout or breakage, plate pullout, or pseudarthrosis. Four patients underwent hardware removal; one underwent posterior fusion for pseudarthrosis. Only two required treatment in a halo brace. There was an eventual fusion rate of 100%, including one fibrous union, and one of the patients who underwent repeat surgery was lost to follow-up review. No graft extrusions or new neurological deficits were incurred as a result of hardware failure. Plate length predicted plate failure in a statistically significant manner. Increasing age and reoperation correlated with plate failure but were not statistically significant in this small number of patients. Telescoping of the bone graft and vertebral bodies, with concomitant migration of the plate and slippage of the screws, was common. However, telescoping was more profound in the group in which the plates failed. The authors conclude that Caspar plate failures are more likely to occur in the elderly and in patients who need longer constructs. Bone fusion can be expected even when the hardware loosens.

Biomechanical evaluation of Caspar and Cervical Spine Locking Plate systems in a cadaveric model

There exist two markedly different instrumentation systems for the anterior cervical spine: the Cervical Spine Locking Plate (CSLP) system, which uses unicortical screws with a locking hub mechanism for attachment, and the Caspar Trapezial Plate System, which is secured with unlocked bicortical screws. The biomechanical stability of these two systems was evaluated in a cadaveric model of complete C5-6 instability. The immediate stability was determined in six loading modalities: flexion, extension, right and left lateral bending, and right and left axial rotation. Biomechanical stability was reassessed following fatigue with 5000 cycles of flexion-extension, and finally, the spines were loaded in flexion until the instrumentation failed. The Caspar system stabilized significantly in flexion before (p < 0.05) but not after fatigue, and it stabilized significantly in extension before (p < 0.01) and after fatigue (p < 0.01). The CSLP system stabilized significantly in flexion before (p < 0.01) but not after fatigue, and it did not stabilize in extension before or after fatigue. The moment needed to produce failure in flexion did not differ substantially between the two plating systems. The discrepancy in the biomechanical stability of these two systems may be due to differences in bone screw fixation.

Performance of ventral spondylodesis screws in cervical vertebrae of varying bone mineral density

STUDY DESIGN

Biomechanical parameters affecting the strength of screw-to-bone fixations of ventral cervical spondylodeses were determined.

OBJECTIVES

The rate of implant loosening with ventral cervical spondylodeses is high. Types of failure and how they can be avoided are presented.

SUMMARY OF BACKGROUND DATA

The number of sound studies on corresponding thoraco-lumbar spinal implants is large. However, no comparative study has been published thus far covering the strength of screw-to-bone fixation in the cervical region.

METHODS

Human cervical vertebrae were obtained at autopsy. Their bone mineral density related to calcium-hydroxyapatite was determined by single energy quantitative computed tomography scan. Standard cancellous screws with a diameter of 3.5 mm and 4.5 mm "rescue" screws were screwed down to failure into 43 single vertebral bodies. The applied torque T and the resultant axial force Fax at the "washer's" position were measured simultaneously by a specially designed electronic testing machine.

RESULTS

A strong correlation among Fax, T, and bone mineral density was found (0.4636 < r < 0.7545). Application of standard screws reveals that Fax and T linearly respond to the effective thread length, whether the posterior vertebral cortex is engaged or not (paired t test: P < 0.05, n = 38). When "rescue" screws are used and the posterior vertebral cortex is included, a significantly higher torque T must be applied to achieve the same revolution. Surprisingly, the resulting force Fax hardly alters. If under similar circumstances the posterior vertebral cortex is not included in the construct, Fax is significantly lower (paired t test: P < 0.05, n = 32). A stable fixation of 3.5 mm screws cannot be achieved if bone mineral density remains below 150 mg/ml.

CONCLUSIONS

Thus, determining bone mineral density before surgery by quantitative computed tomography is recommended to ensure a proper selection of screw type and thereby increase the success of surgical fixation.

Biomechanical analysis of bone mineral density, insertion technique, screw torque, and holding strength of anterior cervical plate screws

✓ The bone mineral density (BMD) of 99 cadaveric cervical vertebral bodies (C3–7) was determined using dual x-ray absorptiometry. The vertebral bodies were randomly assigned to receive either a unicortical (51 bodies) or bicortical (48 bodies) Caspar cervical plating screw. The initial insertion torque was measured using a digital electronic torque wrench, and the force required to withdraw the screw from the vertebral body was determined. The mean BMD for the total group of 99 was 0.787 ± 0.154 g/cm2, the mean insertion torque was 0.367 ± 0.243 newton-meters, and the mean pullout force was 210.4 ± 158.1 newtons. A significant correlation was noted between BMD and torque (p < 0.0001, r = 0.42), BMD and pullout force (p < 0.0001, r = 0.54), and torque and pullout force (p < 0.0001, r = 0.88). Although the BMD of the unicortical and bicortical groups was equivalent (p = 0.92), the insertion torque and pullout force differed significantly (p = 0.02 and p = 0.008, respectively) for the unicortical and bicortical groups. A holding index for each screw and insertion technique was defined as the product of the BMD and insertion torque. The calculated holding index and resultant pullout force were significantly correlated for both techniques of screw insertion (r = 0.92), and a significant difference in holding index was observed with unicortical versus bicortical screw placement (p = 0.04). The determination of BMD and measurement of insertion torque to create a unique holding index provides an assessment of bone—screw interaction and holding strength of the screw, both of which impact on the resultant stability of cervical instrumentation. As the number of cervical plating systems increases, the determination of a holding index for various screws and insertion techniques may assist in the comparison of cervical instrumentation.