Biomechanical evaluation of diagonal fixation in pedicle screw instrumentation

Clinical role of crosslink augmentation during short-segment posterior lumbar interbody fusion with total facetectomy: A propensity score-matched analysis

We investigated the clinical role of crosslink augmentation during PLIF with total facetectomy. We retrospectively reviewed 376 patients who underwent one- or two-segment PLIF with unilateral or bilateral total facetectomy between January 2017 and March 2022. The patients were categorized into two groups based on whether a crosslink was instrumented or not. Radiological outcome measurements included fusion status and implant failure rates on 12-month postoperative computed tomography. Patient-reported outcome measures included the Numerical Rating Scale for lower back pain, Oswestry Disability Index, and Short Form-12 Physical Component Summary scores, which were assessed preoperatively and at 12 months postoperatively. Surgery-related complications included surgical site infection, symptomatic implant failure, and epidural hematoma. Propensity score matching was performed to compare both groups after adjusting for confounding factors, including baseline and surgical procedural characteristics. A total of 200 cases were included in the propensity score calculation, and one-to-one matching was performed, resulting in 56 pairs with and without a crosslink. The fusion status (88.7 % vs. 85.5 %), screw loosening (14.3 % vs. 14.3 %), cage subsidence (17.9 % vs. 16.1 %), and cage dislodgement (5.4 % vs. 0 %) showed no significant difference between those with and without a crosslink, respectively. No significant differences were observed in the patient-reported outcomes or surgery-related complication rates between the groups. Crosslink augmentation during PLIF with total facetectomy is not recommended because it does not provide any radiological or clinical benefit and is associated with avoidable expenses.

Cervical Lift-up Basket Laminoplasty after Resection of Spinal Intramedullary Tumors: A Finite Element Analysis and Clinical Image Evaluation

Although reconstructive laminoplasty is commonly performed after resection of spinal intramedullary tumors of the cervical spine, its biomechanical rigidity of laminoplasty framework remains unclear. The objective of this study was to examine the structural reliability of our unique method of cervical lift-up basket laminoplasty by using computed tomography (CT)-based finite element analysis (FEA) and clinical radiological evaluation. A finite element model of cervical laminoplasty was created based on CT images using FEA software. Cervical lift-up basket laminoplasty (Basket) was compared with the standard style of open-door basket laminoplasty (Open-door). Clinical subjects for radiological evaluation comprised 33 patients who underwent cervical lift-up basket laminoplasty after resection of spinal intramedullary tumors. An FEA-equivalent stress histogram showed that stress was moderately dispersed around the basket. Virtual displacement of the spinous process of the Basket model was equivalent to that of the Open-door model in any direction of posterior-to-anterior, right-to-left, or top-to-bottom force. In the clinical analysis, radiological data with a minimum postoperative period of 6 months were obtained in a total of 28 out of 33 patients. No patients underwent revision surgery because of implant-related complications. No significant differences in C2-C7 angle or cervical tilt angle were observed between pre- and postoperatively. The structural rigidity of cervical lift-up basket laminoplasty was equivalent to the open-door style on the FEA. Clinical radiological evaluation suggested that there were no serious adverse events associated with cervical laminoplasty, although the longer postoperative follow-up is mandatory.

Biomechanical Evaluation of the Cross-link Usage and Position in the Single and Multiple Segment Posterior Lumbar Interbody Fusion

OBJECTIVE

Previous studies have neither explored the usage of cross-links nor investigated the optimal position of the cross-links in posterior lumbar interbody fusion (PLIF). This study evaluates biomechanical properties of cross-links in terms of different fixation segments and optimal position in single- and multi-segment posterior lumbar interbody fusion.

METHODS

Two finite element (FE) models of instrumented lumbosacral spine with single-(L4/5) and multi-segment (L3-S1) PLIF surgery were simulated. On the basis of the two models, the benefits of the usage of cross-links were assessed and compared with the status of no application of cross-links. Moreover, the effects of position of cross-links on multi-segment PLIF surgery were studied in Upper, Middle, and Lower positions.

RESULTS

No significant difference was found in the range of motion (ROM), intersegmental rotational angle (IRA) of adjacent segments, and intradiscal pressure (IDP) regardless of the usage of cross-links in the single-segment PLIF surgery, while the cross-link increased the maximum von Mises stress in the fixation (MSF) under the axial rotation (53.65 MPa vs 41.42 MPa). In the multi-segment PLIF surgery, the usage of cross-links showed anti-rotational advantages indicated by ROM (Without Cross-link 2.35^o , Upper, 2.24^o ; Middle, 2.26^o ; Lower, 2.30^o ) and IRA (Without Cross-link 1.19^o , Upper, 1.08^o ; Middle, 1.09^o ; Lower, 1.13^o ). The greatest values of MSF were found in without cross-link case under the flexion, lateral bending, and axial rotation (37.48, 62.61, and 86.73 MPa). The application of cross-links at the Middle and Lower positions had lower values of MSF (48.79 and 69.62 MPa) under the lateral bending and axial rotation, respectively.

CONCLUSION

The application of cross-links was not beneficial for the single-segment PLIF, while it was found highly advantageous for the multi-segment PLIF. Moreover, the usage of cross-links at the Middle or Lower positions resulted in a better biomechanical stability.

Posterior spinal instrumentation and decompression with or without cross-link?

Background

Posterior lumbar instrumentation requires sufficient primary stiffness to ensure bony fusion and to avoid pseudarthrosis, screw loosening, or implant failure. To enhance primary construct stiffness, transverse cross-link (CL) connectors attached to the vertical rods can be used. Their effect on the stability of a spinal instrumentation with simultaneous decompression is yet not clear. This study aimed to evaluate the impact of CL augmentation on single-level lumbar instrumentation stiffness after gradual decompression procedures.

Methods

Seventeen vertebral segments (6 L1/2, 6 L3/4, 5 L5/S1) of 12 fresh-frozen human cadavers were instrumented with a transpedicular screw–rod construct following the traditional pedicle screw trajectory. Range of motion (ROM) of the segments was sequentially recorded before and after four procedures: (A) instrumented before decompression, (B) instrumented after unilateral laminotomy, (C) instrumented after midline bilateral laminotomy, and (D) instrumented after unilateral facetectomy (with transforaminal lumbar interbody fusion [TLIF]). Each test was performed with and without CL augmentation. The motion between the cranial and caudal vertebrae was evaluated in all six major loading directions: flexion/extension (FE), lateral bending (LB), lateral shear (LS), anterior shear (AS), axial rotation (AR), and axial compression/distraction (AC).

Results

ROM was significantly reduced with CL augmentation in AR by Δ0.03–0.18° (7–12%) with a significantly higher ROM reduction after more extensive decompression. Furthermore, slight reductions in FE and LB were observed; these reached statistical significance for FE after facetectomy and TLIF insertion only (Δ0.15; 3%). The instrumentation levels did not reveal any subgroup differences.

Conclusion

CL augmentation reduces AR-ROM by 7–12% in single-level instrumentation of the lumbar spine, with the effect increasing along with the extensiveness of the decompression technique. In light of the discrete absolute changes, CL augmentation may be warranted for highly unstable vertebral segments rather than for standard single-level posterior spinal fusion and decompression.

Cross-links in posterior pedicle screw-rod instrumentation of the spine: a systematic review on mechanical, biomechanical, numerical and clinical studies

Purpose

Dorsal screw-rod instrumentations are used for a variety of spinal disorders. Cross-links (CL) can be added to such constructs, however, no clear recommendations exist. This study aims to provide an overview of the available evidence on the effectiveness of CL, potentially allowing to formulate recommendations on their use.

Methods

A systematic literature review was performed on PubMed and 37 original articles were included and grouped into mechanical, biomechanical, finite element and clinical studies. The change in range of motion (ROM) was analyzed in mechanical and biomechanical studies, ROM, stiffness and stress distribution were evaluated in finite element studies and clinical outcome parameters were analyzed in clinical studies.

Results

A relative consistent reduction in ROM in axial rotation with CL-augmentation was reported, while minor and less consistent effects were observed in flexion–extension and lateral bending. The use of CLs was clinical beneficial in C1/2 fusion, while the limited clinical studies on other anatomic regions show no significant benefit for CL-augmentation.

Conclusion

While CL provides some additional axial rotation stability in most situations, lateral bending and flexion–extension are less affected. Based on clinical data, CL-augmentation can only be recommended for C1/2 instrumentations, while for other cases, further clinical studies are needed to allow for evidence-based recommendations.

Biomechanical effects of posterior pedicle fixation techniques on the adjacent segment for the treatment of thoracolumbar burst fractures: a biomechanical analysis

Posterior pedicle fixation technique is a common method for treating thoracolumbar burst fractures, but the effect of different fixation techniques on the postoperative spinal mechanical properties has not been clearly defined, especially on adjacent segments. A finite element model of T10-L2 with moderate T12 vertebra burst fracture was constructed to investigate biomechanical behavior of three posterior pedicle screw fixation techniques. Compared with traditional short-segment 4 pedicle screw fixation (TS-4) and intermediate long-segment 6 pedicle screw fixation (IL-6), mono-segment 4 pedicle screw fixation (MS-4) provides a safer surgical selection to prevent the secondary degeneration of adjacent segments in the long-term.

Do the Position and Orientation of the Crosslink Influence the Stiffness of Spinal Instrumentation?

STUDY DESIGN

Biomechanical study of double-level pedicle screw constructs with or without crosslinks (CL) in an unstable model.

OBJECTIVES

The purpose of this study is to investigate the optimal position and orientation of the CL.

SUMMARY OF BACKGROUND DATA

Several reports have described biomechanical research on such CL, but no definite consensus has been reached regarding the effects. Very few studies have examined the position and orientation of the CL. The question of where and how the CL should be clinically set remains unanswered.

METHODS

Ten cadaveric lumbar spines (L3-L5) of boars were used and 7 models were prepared by the sequential damage and spinal instrumentation of each specimen. Bending stiffness was measured in flexion, extension, lateral bending, and axial rotation for each model using 6-axis material tester under torque of 0 to ±3 N m. Results for each configuration were compared using analysis of variance and the Turkey-Kramer test.

RESULTS

In flexion, extension, and lateral bending, 7 models showed similar stiffness with no significant differences. In axial rotation, stiffness increased significantly (P<0.05) in the cephalic, central, caudal, and oblique CL models in comparison with that of the no CL model, and stiffness of the horizontal 2 CL and oblique 2 CL models was significantly higher than that of cephalic, central, caudal, and oblique CL models (P<0.05). However, no significant differences in stiffness were seen between cephalic, central, and caudal CL models, between the central and oblique CL models, or between the horizontal and oblique 2 CL models.

CONCLUSIONS

Concomitant use of CLs significantly increased axial rotational stiffness, even though stiffness in flexion, extension, and lateral bending was not increased. In addition, stiffness in axial rotation significantly improved with the use of 2 CLs instead of a single CL, and stiffness was unchanged by position and orientation of CL.

Biomechanical evaluation of the fixation strength of lumbar pedicle screws using cortical bone trajectory: a finite element study

OBJECT

Cortical bone trajectory (CBT) maximizes thread contact with the cortical bone surface and provides increased fixation strength. Even though the superior stability of axial screw fixation has been demonstrated, little is known about the biomechanical stiffness against multidirectional loading or its characteristics within a unit construct. The purpose of the present study was to quantitatively evaluate the anchorage performance of CBT by the finite element (FE) method.

METHODS

Thirty FE models of L-4 vertebrae from human spines (mean age [± SD] 60.9 ± 18.7 years, 14 men and 16 women) were computationally created and pedicle screws were placed using the traditional trajectory (TT) and CBT. The TT screw was 6.5 mm in diameter and 40 mm in length, and the CBT screw was 5.5 mm in diameter and 35 mm in length. To make a valid comparison, the same shape of screw was inserted into the same pedicle in each subject. First, the fixation strength of a single pedicle screw was compared by axial pullout and multidirectional loading tests. Next, vertebral fixation strength within a construct was examined by simulating the motions of flexion, extension, lateral bending, and axial rotation.

RESULTS

CBT demonstrated a 26.4% greater mean pullout strength (POS; p = 0.003) than TT, and also showed a mean 27.8% stronger stiffness (p < 0.05) during cephalocaudal loading and 140.2% stronger stiffness (p < 0.001) during mediolateral loading. The CBT construct had superior resistance to flexion and extension loading and inferior resistance to lateral bending and axial rotation. The vertebral fixation strength of the construct was significantly correlated with bone mineral density of the femoral neck and the POS of a single screw.

CONCLUSIONS

CBT demonstrated superior fixation strength for each individual screw and sufficient stiffness in flexion and extension within a construct. The TT construct was superior to the CBT construct during lateral bending and axial rotation.

Biomechanical stability of transverse connectors in the setting of a thoracic pedicle subtraction osteotomy

BACKGROUND CONTEXT

Transverse connectors (TCs) are often used to improve the rigidity of posterior spinal instrumentation as previous investigations have suggested that TCs enhance torsional rigidity in long-segment thoracic constructs. Posterior osteotomies, such as pedicle subtraction osteotomy (PSO), are used in severe thoracic deformities and provide a significant amount of correction; as a consequence, however, PSOs also induce three-column spinal instability. In theory, augmentation of longitudinal constructs with TC after a thoracic PSO may provide additional rigidity, but the concept has not been previously evaluated.

PURPOSE

To evaluate the biomechanical contribution of TC to the rigidity of a long-segment pedicle screw-rod construct after a thoracic PSO.

STUDY DESIGN

An in vitro fresh-frozen human cadaveric biomechanical analysis.

METHODS

Seven human cadaveric thoracic spines were prepared and instrumented from T4-T10 with bilateral pedicle screws/rods and a PSO was performed at T7. Intact range of motion (ROM) testing was performed with nondestructive loading and analyzed by loading modality (axial rotation [AR], flexion/extension [FE], and lateral bending [LB]). Range of motion analysis was performed in the unaugmented construct, the construct augmented with one TC, and the construct augmented with two TCs.

RESULTS

After PSO and an unaugmented longitudinal pedicle screw-rod construct, T4-T10 (overall construct) and T6-T8 (PSO site) ROMs were significantly reduced in all planes of motion compared with intact condition (AR: 11.8° vs. 31.7°; FE: 2.4° vs. 12.3°; 3.4° vs. 17.9°, respectively, p<.05). Augmentation of longitudinal construct with either one or two TCs did not significantly increase construct rigidity in FE or LB compared with the unaugmented construct (p>.05). In contrast, during AR, global ROM was significantly reduced by 43% and 48% at T6-T8 (1.7° and 1.2° vs. 2.38°, respectively) after addition of one and two TCs (p<.05), respectively. One TC did not significantly reduce torsional ROM from the intact state.

CONCLUSIONS

Two TCs significantly improved torsional rigidity of the entire construct and at the PSO site, with no differences in rigidity for FE and LB or with the addition of only one TC. In the setting of a PSO and long-segment pedicle screw-rod construct, augmentation with at least two TCs should be considered to improve torsional rigidity.

Cross-Links Do Not Improve Clinical or Radiographic Outcomes of Posterior Spinal Fusion With Pedicle Screws in Adolescent Idiopathic Scoliosis: A Multicenter Cohort Study

STUDY DESIGN

Retrospective, comparative analysis.

OBJECTIVES

Comparative analysis was performed to determine the differences, if any, between adolescent idiopathic scoliosis (AIS) patients who underwent posterior spinal fusion (PSF) with and without cross-links.

SUMMARY OF BACKGROUND DATA

Cross-links are frequently used during PSF for AIS. It is unclear whether they provide any advantage for patients with all-pedicle screw constructs.

METHODS

A prospectively collected multicenter database of patients with AIS undergoing spinal fusion was retrospectively queried. Study inclusion criteria were primary PSF with all-pedicle screw fixation (greater than 90% fixation points) and minimum 2 years' follow-up. Collected data included demographics, radiographic measures, complications, Scoliosis Research Society-22r and Spinal Appearance Questionnaire (SAQ) scores.

RESULTS

A total of 500 patients were included (377 cross-link and 123 non-cross-link). Age, body mass index, gender, and preoperative major Cobb angle were not different between groups. Except for a slightly decreased lumbar Cobb angle (2.7°) in the cross-link group, no other radiographic measures were different at follow-up. Complications were not significantly different between groups: 21 of 377 (6%) crosslink and 9 of 123 (7%) non-cross-link. Infection occurred in 1 patient in the cross-link group and none in the non-cross link group. Reoperation occurred in 4 patients, all with cross-links (3 for implant removal and 1 for distal adding-on). Scoliosis Research Society-22r scores, total and individual domains, improved by a similar amount in both groups. At follow-up, parent and patient SAQ appearance scores were not significantly different. The SAQ expectations domain scores were similar for all visits and improved for both patients and parents.

CONCLUSIONS

There do not appear to be significant clinical or radiographic outcome differences in patients with AIS undergoing PSF based on the use of cross-links at 2-year follow-up. Surgeons should consider eliminating cross-links in patients with AIS who have PSF with all-pedicle screw constructs. This may have substantial cost savings without affecting patient outcome.

Spine fusion cross-link causing delayed dural erosion and CSF leak: case report

The past 2 decades have seen a considerable increase in the number of lumbar spinal fusion surgeries. To enhance spinal stabilization and fusion, make the construct resistant to or stiffer for axial stress loading, lateral bending, and torsional stresses, cross-links and connectors were designed and included in a rod-screw construct. The authors present the case of a 49-year-old woman who presented 11 years after undergoing an L4-5 decompression and fusion in which a pedicle screw-rod construct with an integrated cross-link was designed to attach onto the pedicle screws. The patient's response at the time to the initial surgery was excellent; however, at the time of presentation 11 years later, she had significant postural headaches, severe neurogenic claudication, and radiculopathy. Imaging revealed canal compression across the instrumented levels and a possible thickened adherent filum terminale. Reexploration of the level revealed a large erosive dural defect with a CSF leak, spinal canal compression, and a thickened filum at the level of the cross-link. To the author's knowledge, such complications have not been reported in literature. The authors discuss this rare complication of spinal fusion and the need to avoid dural compression when cross-links are used.

Effectiveness of cross-linking posterior segmental instrumentation in adolescent idiopathic scoliosis: a 2-year follow-up comparative study

BACKGROUND CONTEXT

Surgeons continue to debate the need for a cross-link (CL) in posterior spinal instrumentation constructs with segmental pedicle screws in adolescent idiopathic scoliosis (AIS). Advantage of CLs is increased stiffness of the construct, and disadvantages include added expense and risk of late operative-site pain and pseudarthrosis.

PURPOSE

To compare the effectiveness of using CLs versus using no cross-links (NCLs) in posterior segmental instrumentation in AIS.

STUDY DESIGN

Retrospective comparative study, level of evidence 3.

PATIENT SAMPLE

Seventy-five AIS patients less than 21 years of age, who underwent posterior spinal instrumentation with segmental pedicle screws (25 with CLs and 50 with NCLs) at a single institution with 2-year follow-up, are described.

OUTCOME MEASURES

Physiologic measures include imaging: thoracic and lumbar Cobb angles, correction rate, apical vertebral translation (AVT), and apical vertebral rotation (AVR); self-report measures include Scoliosis Research Society (SRS) domain outcome scores.

METHODS

Preoperative (pre-op) and postoperative first erect, 1-year, and 2-year follow-up radiographs were measured. Instrumentation-related complications and normalized SRS scores were recorded. Independent sample t test, χ(2) test, and repeated-measures analysis of variance were used for analyses.

RESULTS

The average age at surgery was 14 years, the mean pre-op Cobb angle was 57°, and the mean number of levels fused was 10.9. The groups were similar preoperatively with respect to age, sex, Lenke curve, Cobb angle, AVT, and Risser grade and were similar intraoperatively for levels fused and anchor density. There was no difference in AVR, Cobb angle, correction rate, or AVT between the groups (p>.05). Complications included one wound infection in the CL group and one painful scar in the NCL group. There were no differences in SRS domain scores.

CONCLUSION

We observed no differences in maintenance of correction, SRS scores, and complications with or without cross-linking posterior segmental instrumentation in AIS patients over 2-year follow-up. Further follow-up is necessary.

Influence of an auxiliary facet system on intervertebral discs and adjacent facet joints

BACKGROUND CONTEXT

Facet supplementation stabilizes after facetectomy and undercutting laminectomy. It is indicated in degenerative spondylolisthesis with moderate disc degeneration and dynamic stenosis.

PURPOSE

To determine the influence of an auxiliary facet system (AFS) on the instrumented disc, adjacent levels' discs, and facet joints and to compare it with fusion.

STUDY DESIGN

Finite element study.

METHODS

L3-L4, L4-L5, and L5-S1 were studied using a validated finite element model with prescribed displacements for an intact spine, lesion by facetectomy and undercutting laminectomy, AFS, and fusion at L4-L5. The distribution of segmental range of motion (ROM) and applied moments, von Mises stress at the annulus, and facet joint contact forces were calculated with rotations in all planes. Institutional support for implant evaluation and modeling was received by Clariance.

RESULTS

In flexion-extension and lateral bending, fusion decreased L4-L5 ROM and increased adjacent levels' ROM. Range of motion was similarly distributed with intact lesion and AFS. In axial rotation, L4-L5 ROM represented 33% with intact, 55% after lesion, 25% with AFS, and 21% with fusion. Fusion increased annulus stress at adjacent levels in flexion-extension and lateral bending, but decreased stress at L4-L5 compared with AFS. In axial rotation, von Mises stress was similar with fusion and AFS. Facet loading increased in extension and lateral bending with fusion. It was comparable for fusion and AFS in axial rotation.

CONCLUSIONS

This study suggests that the AFS stabilizes L4-L5 in axial rotation after facetectomy and undercutting laminectomy as fusion does. This is because of the cross-link that generates an increased annulus stress in axial rotation at adjacent levels. With imposed displacements, without in vivo compensation of the hips, the solicitation at adjacent levels' discs and facet joints is higher with fusion compared with AFS. Fusion decreases intradiscal stress at the instrumented level.

The use of X-shaped cross-link in posterior spinal constructs improves stability in thoracolumbar burst fracture: a finite element analysis

Posterior instrumentation is a common fixation method used to treat thoracolumbar burst fractures. However, the role of different cross-link configurations in improving fixation stability in these fractures has not been established. A 3D finite element model of T11-L3 was used to investigate the biomechanical behavior of short (2 level) and long (4 level) segmental spine pedicle screw fixation with various cross-links to treat a hypothetical L1 vertebra burst fracture. Three types of cross-link configurations with an applied moment of 7.5 Nm and 200 N axial force were evaluated. The long construct was stiffer than the short construct irrespective of whether the cross-links were used (p < 0.05). The short constructs showed no significant differences between the cross-link configurations. The XL cross-link provided the highest stiffness and was 14.9% stiffer than the one without a cross-link. The long construct resulted in reduced stress to the adjacent vertebral bodies and screw necks, with 66.7% reduction in bending stress on L2 when the XL cross-link was used. Thus, the stability for L1 burst fracture fixation was best achieved by using long segmental posterior instrumentation constructs and an XL cross-link configuration. Cross-links did not improved stability when a short structure was used.

Should we cross the cross-links?

STUDY DESIGN

Retrospective study.

OBJECTIVE

To assess critically if cross-links are necessary adjuvants in posterior spinal constructs.

SUMMARY OF BACKGROUND DATA

Although numerous biomechanical studies are available in the literature, there has been no clinical study that has evaluated the need for cross-links in clinical situations.

METHODS

The spinal constructs of patients of varied etiology who underwent surgery between July 2007 and July 2011 without the usage of cross-links were evaluated. The immediate postoperative erect radiographs were compared with the erect radiographs at the last follow-up by 2 independent observers (spine fellows not involved in the management of the patients) critically for any rotational instability using the Nash-Moe technique of assessment of vertebral rotation as well as for any "parallelogram effect." The intraobserver and interobserver reliability was analyzed.

RESULTS

There were 208 cases included in the study during the study period that satisfied the criteria. The total number of motion segments fused was 707 ranging from 1 to 15 involving various etiologies. The average follow-up was 15 months (12-36 mo). Barring one patient with a thoracolumbar fracture with rotational instability (AO [Arbeitsgemeinschaft für Osteosynthesefragen] type C) who had undergone a short-segment fixation, none of the cases demonstrated any rotational instability in the follow-up radiographs. Interestingly, the rotational instability (parallelogram effect) in that patient got corrected spontaneously once anterior reconstruction was performed. The intraobserver reliability was 100% and the interobserver reliability was 92.83%. This variability was in assessing the grade of vertebral rotation only; none of the levels had a change in rotation irrespective of variation in grade assessment in the final postoperative radiograph.

CONCLUSION

This study concludes that use of cross-links in clinical practice may be avoidable. The derivations from biomechanical studies do not translate into clinical advantages. Eliminating the usage of cross-links reduces the operative time as well as the overall total hospital costs (a single cross-link may cost anywhere between $1500 and $2000 and surgeons tend to use single or multiple cross-links). Additionally, prominence of implants, corrosion, infection, implant failure, and pseudarthrosis are the other complications attributed to cross-links in the literature that can be eliminated by preventing their incorporation in spinal constructs.

LEVEL OF EVIDENCE

N/A.

Effect of pilot hole on biomechanical and in vivo pedicle screw-bone interface

PURPOSE

To experimentally study the influence of pilot hole diameter (smaller than or equal to the internal (core) diameter of the screw) on biomechanical (insertion torque and pullout strength) and histomorphometric parameters of screw-bone interface in the acute phase and 8 weeks after pedicle screw insertion.

METHODS

Fifteen sheep were operated upon and pedicle screws inserted in the L1-L3 pedicles bilaterally. The pilot hole was smaller (2.0 mm) than the internal diameter (core) of the screw on the left side pedicle and equal (2.8 mm) to the internal diameter (core) of the screw on the right side pedicle. Ten animals were sacrificed immediately (five animals were assigned to pullout strength tests and five animals were used for histomorphometric bone-screw interface evaluation). Five animals were sacrificed 8 weeks after pedicle screw insertion for histomorphometric bone-screw interface evaluation.

RESULTS

The insertion torque and pullout strength were significantly greater in pedicle screws inserted into pilot holes smaller than internal (core) diameter of the screw. Histomorphometric evaluation of bone-screw interface showed that the percentage of bone-implant contact, the area of bone inside the screw thread and the area of bone outside the screw thread were significantly higher for pilot holes smaller than the internal (core) diameter of the screw immediately after insertion and after 8 weeks.

CONCLUSION

A pilot diameter smaller than the internal (core) diameter of the screw improved the insertion torque and pullout strength immediately after screw insertion as well the pedicle screw-bone interface contact immediately and 8 weeks after screw placement in sheep with good bone mineral density.

Development and kinematic verification of a finite element model for the lumbar spine: application to disc degeneration

The knowledge of the lumbar spine biomechanics is essential for clinical applications. Due to the difficulties to experiment on living people and the irregular results published, simulation based on finite elements (FE) has been developed, making it possible to adequately reproduce the biomechanics of the lumbar spine. A 3D FE model of the complete lumbar spine (vertebrae, discs, and ligaments) has been developed. To verify the model, radiological images (X-rays) were taken over a group of 25 healthy, male individuals with average age of 27.4 and average weight of 78.6 kg with the corresponding informed consent. A maximum angle of 34.40° is achieved in flexion and of 35.58° in extension with a flexion-extension angle of 69.98°. The radiological measurements were 33.94 ± 4.91°, 38.73 ± 4.29°, and 72.67°, respectively. In lateral bending, the maximum angles were 19.33° and 23.40 ± 2.39, respectively. In rotation a maximum angle of 9.96° was obtained. The model incorporates a precise geometrical characterization of several elements (vertebrae, discs, and ligaments), respecting anatomical features and being capable of reproducing a wide range of physiological movements. Application to disc degeneration (L5-S1) allows predicting the affection in the mobility of the different lumbar segments, by means of parametric studies for different ranges of degeneration.

Transverse connectors providing increased stability to the cervical spine rod-screw construct: an in vitro human cadaveric study

Object

The object of this study was to determine if the addition of transverse connectors (TCs) to a rod-screw construct leads to increased stabilization of the cervical spine.

Methods

Eleven human cadaveric cervical spines (C2–T1) were used to examine the effect of adding connectors to a C3–7 rod-screw construct in 3 models of instability: 1) C3–6 wide laminectomy, 2) wide laminectomy and 50% foraminotomy at C4–5 and C5–6, and 3) wide laminectomy with full medial to lateral foraminotomy. Following each destabilization procedure, specimens were tested with no TC, 1 TC between the C-5 screws, and 2 TCs between the C-4 and C-6 screws. Testing of the connectors was conducted in random order. Specimens were subjected to ± 2 Nm of torque in flexion and extension, lateral bending, and axial rotation. Range of motion was determined for each experimental condition. Statistical comparisons were made between the destabilized and intact conditions, and between the addition of TCs and the absence of TCs.

Results

The progressive destabilization procedures significantly increased motion. The addition of TCs did not significantly change motion in flexion and extension. Lateral bending was significantly decreased with 2 connectors, but not with 1 connector. The greatest effect was on axial rotation. In general, 2 TCs were more restrictive than 1 TC, and decreased motion 10% more than fixation alone.

Conclusions

Regardless of the degree of cervical destabilization, 1 or 2 TCs decreased motion compared with rods and screws alone. Axial rotation was most affected. Transverse connectors effectively increase the rigidity of rod-screw constructs in the cervical spine. Severe cervical instability can be overcome with the use of 2 TCs, but in cases in which 2 cannot be used, 1 should be adequate and superior to none.

The biomechanics of pedicle screw-based instrumentation

There are three basic concepts that are important to the biomechanics of pedicle screw-based instrumentation. First, the outer diameter of the screw determines pullout strength, while the inner diameter determines fatigue strength. Secondly, when inserting a pedicle screw, the dorsal cortex of the spine should not be violated and the screws on each side should converge and be of good length. Thirdly, fixation can be augmented in cases of severe osteoporosis or revision. A trajectory parallel or caudal to the superior endplate can minimise breakage of the screw from repeated axial loading. Straight insertion of the pedicle screw in the mid-sagittal plane provides the strongest stability. Rotational stability can be improved by adding transverse connectors. The indications for their use include anterior column instability, and the correction of rotational deformity.

Biomechanical evaluation of short-segment posterior instrumentation with and without crosslinks in a human cadaveric unstable thoracolumbar burst fracture model

STUDY DESIGN

This study evaluates the biomechanical characteristics of spinal instrumentation constructs in a human unstable thoracolumbar burst fracture model simulated by corpectomy.

OBJECTIVE

To compare the biomechanical characteristics of short-segment posterior instrumentation, with and without crosslinks, in a human unstable burst fracture model simulated by corpectomy.

SUMMARY OF BACKGROUND DATA

Unstable thoracolumbar burst fractures are serious injuries, and their management remains controversial. Some authors advocate the use of short-segment posterior instrumentation for certain burst fractures. Whether crosslinks contribute additional stability has not been determined.

METHODS

Six fresh frozen human spines (T10-L2) were potted to isolate the T11-L1 segments, and biomechanically tested in axial rotation, lateral bending, flexion, and extension. A custom spine testing system was used that allows motion with 6 degrees of freedom. After testing was completed on intact specimens, a corpectomy was performed at T12 to simulate an unstable burst fracture with loss of anterior and middle column support. Short-segment transpedicular instrumentation was then performed from T11 to L1. Each specimen was retested with 1, 2, or no crosslinks. Construct stiffness and motion data were analyzed with each intact specimen serving as its own internal control.

RESULTS

Torsional stiffness in axial rotation was significantly increased (P < 0.05) in short-segment fixation constructs with 1 and 2 crosslinks, but none was restored to the preinjury baseline level. Significant reductions in standardized motion were also achieved with 1 and 2 crosslinks compared to no crosslinks (P < 0.05), but they remained greater than baseline. Crosslinks significantly increased stiffness and decreased motion in lateral bending, beyond the baseline level (P < 0.05). In flexion, all constructs had significantly decreased stiffness and increased motion compared to the intact specimen (P < 0.05), with crosslinks providing no additional benefit. Conversely, none of the constructs demonstrated a significant change in extension compared to baseline (P > 0.05). When attempting to load the constructs to failure, screw pullout was seen in all specimens.

CONCLUSION

Crosslinks, when added to short-segment posterior fixation, improve stiffness and decrease motion in axial rotation, but do not restore baseline stability in this corpectomy model. Short-segment posterior fixation is also inadequate in restoring stability in flexion with injuries of this severity. Short-segment posterior instrumentation alone can achieve baseline stability in lateral bending, and crosslinks provide even greater stiffness.

Biomechanical contribution of transverse connectors to segmental stability following long segment instrumentation with thoracic pedicle screws

STUDY DESIGN

An in vitro biomechanical cadaver study of long segment thoracic pedicle screw constructs with transverse connectors (TC).

OBJECTIVE

To determine the resultant degree of motion of the instrumented thoracic spine after segmental pedicle screw instrumentation with and without TC. SUMMARY OF BACKGROUND DATA.: TC are generally not thought to be necessary with thoracic pedicle screw constructs, yet to date no study has reported the effect of TCs after all pedicle screw long thoracic fusions.

METHODS

Eight human cadaveric spines were potted and then instrumented from T4-T10 with bilateral 5.5 mm multiaxial titanium (Ti) pedicle screws and 5.5 mm contoured Ti rods. Specimens were tested with a six-degree-of-freedom spine stimulator in the intact condition, after instrumentation, after placement of 1 TC (3 different locations) and after placement of both TCs. Data were analyzed by loading modality (axial rotation, flexion-extension, and lateral bending) using one-way analysis of variance with an alpha of 0.05. Paired t tests were used for post hoc analysis with correction for multiple comparisons.

RESULTS

There was no difference with the addition of 1 or 2 TCs in terms of flexion-extension or lateral bending when compared to the instrumented condition (P > 0.05). Biomechanical testing of the long-segment thoracic constructs in axial rotation (torsion) loading modes generated the most significant findings of this study. After instrumentation with thoracic pedicle screws, T4-T10 full ROM was significantly reduced from the intact condition (P < 0.05). On average, TPS alone resulted in a 65% decrease in ROM. However, the addition of a transverse connector at 1 of the 3 positions tested yielded another 20% improvement in axial segmental stability as represented by further ROM reduction. These differences were significant from the TPS only group (no TCs), regardless of the TC position (P < 0.05). Furthermore, 2 TCs placed at the proximal and distal ends of the construct provided the greatest biomechanical axial stability to the instrumented specimens (P < 0.05). This was highlighted by an average of 35% ROM reduction from the stability level achieved with the TPS only constructs (P < 0.05), or an additional 15% improvement in axial stability over a single TC.

CONCLUSION

For long thoracic pedicle screw constructs, the addition of 1 or 2 TCs significantly decreases construct axial rotation, which is the primary plane of motion for the thoracic spinal region. A single TC contributed to a significant reduction of T4-T10 ROM (an additional 20%) relative to TPS fixation alone (P < 0.05), while the location of the TC within the construct was irrelevant. A second TC had an additive effect (an additional 15% reduction) on axial stability. (P < 0.05) Flexion-extension and lateral bending are not affected. Single TC significantly improves axial rotation stability in long thoracic pedicle screw constructs. Two crosslinks, however, are better than one.

Biomechanical testing of a novel four-rod technique for lumbo-pelvic reconstruction

STUDY DESIGN

A biomechanical testing protocol was used to study different lumbo-pelvic fixation techniques in a human cadaveric lumbar spine model.

OBJECTIVE

To compare the in vitro biomechanics of a novel four-rod lumbo-pelvic reconstruction technique with and with out cross-links, to that of a conventional cross-linked two-rod technique.

SUMMARY OF BACKGROUND DATA

Numerous lumbo-pelvic reconstruction methods based on the Galveston two-rod technique have been proposed for cases involving total sacrectomy. Recently a technique that proposes novel use of 4 supporting longitudinal rods across the lumbo-pelvic junction has been reported. No comparative in vitro biomechanical testing has been previously done to evaluate these different reconstruction methods.

METHODS

Five spines were evaluated in flexion, extension, left-right lateral bending and left-right axial rotation in a human total sacrectomy model. The model was comprised of cadaveric lumbar spines (L1-L5) with custom fabricated polyethylene blocks used to simulate pelvic fixation. Three conditions were evaluated: Linked Four-Rod, Linked Two-Rod, and Four-Rod (no cross-links). Flexibility and motion data were compared using a one-way repeated measures analysis of variance and SNK tests.

RESULTS

The Linked Four-Rod and Four-Rod conditions significantly decreased flexibility and reduced L5-Pelvic motion over the Linked Two-Rod construct in flexion and extension. The Linked Four-Rod condition significantly decreased flexibility in left-right axial rotation compared with the Four-Rod and Linked Two-Rod conditions. No significant differences occurred in relative lateral movement between left and right pelvic polyethylene blocks.

CONCLUSION

The four-rod technique improved fixation stability over the conventional linked two-rod technique in flexion and extension, and when cross-linked, in left-right axial rotation. The four-rod technique also significantly reduced L5-Pelvic junction movement in flexionand extension, which may have implications for bony fusion. The use of cross-links is recommended.

[Biomechanical study of a ventral stand-alone cage for the lumbar spine with and without additional posterior fixation]

AIM

Biomechanical study to examine the stability of a stand-alone cage (SAC; Topaz, Ulrich Medizintechnik, Ulm) incorporating integrated anterior fixation with and without additional posterior fixation.

METHOD

Six human spinal specimens (L4/5) were loaded in a spine tester in the three main motion planes in the following states: (a) intact, (b) defect (nucleotomy), (c) SAC, (d) SAC+internal fixator (IF), and (e) SAC+translaminar facet screws (TFS). Facet joint translation (FJT) and range of motion (ROM) were measured and used to evaluate the stability of the tested states.

RESULTS

The SAC stabilized the segment in comparison to the intact (a) and defect-containing (b) segments. The most rigid fixation was found for SAC+IF compared with the other states (a, b, c, e). The ROM and FJT of the SAC with the additional IF (d) showed a significant higher stability in all three motion planes. All differences in ROM and FJT between the tested states were statistically significant (p<0.05) except for the FJT SAC and SAC+TFS.

CONCLUSION

The ventral Topaz SAC is a stable implant for the lumbar spine. Additional dorsal stabilization is an option to increase the stability.

Are the spines of calf, pig and sheep suitable models for pre-clinical implant tests?

Pre-clinical in vitro tests are needed to evaluate the biomechanical performance of new spinal implants. For such experiments large animal models are frequently used. Whether these models allow any conclusions concerning the implant's performance in humans is difficult to answer. The aim of the present study was to investigate whether calf, pig or sheep spine specimens may be used to replace human specimens in in vitro flexibility and cyclic loading tests with two different implant types. First, a dynamic and a rigid fixator were tested using six human, six calf, six pig and six sheep thoracolumbar spine specimens. Standard flexibility tests were carried out in a spine tester in flexion/extension, lateral bending and axial rotation in the intact state, after nucleotomy and after implantation. Then, the Coflex interspinous implant was tested for flexibility and intradiscal pressure using another six human and six calf lumbar spine segments. Loading was carried out as described above in the intact condition, after creation of a defect and after implantation. The fixators were most easily implantable into the calf. Qualitatively, they had similar effects on ROM in all species, however, the degree of stability achieved differed. Especially in axial rotation, the ROM of sheep, pig and calf was partially less than half the human ROM. Similarly, implantation of the Coflex interspinous implant caused the ROM to either increase in both species or to decrease in both of them, however, quantitatively, differences were observed. This was also the case for the intradiscal pressure. In conclusion, animal species, especially the calf, may be used to get a first idea of how a new pedicle screw system or an interspinous implant behaves in in vitro flexibility tests. However, the effects on ROM and intradiscal pressure have to be expected to differ in magnitude between animal and human. Therefore, the last step in pre-clinical implant testing should always be an experiment with human specimens.

Mechanical stiffness of segmental versus nonsegmental pedicle screw constructs: the effect of cross-links

STUDY DESIGN

A biomechanical study in porcine spines of the construct stiffness effects of segmental pedicle screws. Stiffness effects of supplementation of nonsegmental screw constructs with cross-links was also evaluated.

OBJECTIVE

To assess the biomechanical differences between constructs using segmental versus nonsegmental pedicle screw-based instrumentation as well as the effect of cross-links.

SUMMARY OF BACKGROUND DATA

An in vitro biomechanical comparison of segmental versus nonsegmental pedicle screw constructs with and without cross-links using porcine lumbar vertebrae was performed. Mechanical trade-offs of reducing the number of pedicle screws in a given construct and substituting a cross-link for a pair of screws are not well understood.

METHODS

Three, 4, and 5-vertebral segments from 18 porcine spines were instrumented with segmental and nonsegmental pedicle screw constructs, and with nonsegmental screws augmented with cross-links. Unconstrained biomechanical testing in flexion, extension, and axial rotation with 6 degree-of-freedom motion tracking was performed. Statistical comparisons of stiffness data were conducted using 2-tailed paired t tests.

RESULTS

There was a statistically significant increase in stiffness between models with segmental pedicle screws compared to nonsegmental pedicle screws in 6 of the 9 mechanical tests. The remaining 3 tests approached but did not reach statistical significance (P = 0.087, 0.062, and 0.078). When cross-links were added to the nonsegmental models, differences in stiffness compared to segmental pedicle screws were largely eliminated, decreasing well below statistical significance in 8 of 9 tests. The highest difference in nonsegmental models with cross-links and segmental pedicle screw models was observed for the 5-vertebrae fusion models, for which axial rotation testing maintained statistically significant differences (P = 0.006), and flexion testing approached significance (P = 0.062).

CONCLUSIONS

Segmental pedicle screw constructs increased mechanical stiffness compared to nonsegmental constructs in our fusion models. Placement of a single cross-link with nonsegmental screws eliminated statistical differences for 3 and 4-vertebral level constructs, and may be a satisfactory alternative in this clinical setting. Caution in applying these results inlonger constructs is recommended, given persistent increased stiffness found for the segmental 5-vertebral level models.

Effects of charité artificial disc on the implanted and adjacent spinal segments mechanics using a hybrid testing protocol

STUDY DESIGN

Finite element model of L3-S1 segment and confirmatory cadaveric testing were used to investigate the biomechanical effects of a mobile core type artificial disc (Charité artificial disc; DePuy Spine, Raynham, MA) on the lumbar spine.

OBJECTIVE

To determine the effects of the Charité artificial disc across the implanted and adjacent segments.

SUMMARY OF BACKGROUND DATA

Biomechanical studies of artificial discs that quantify parameters, like the load sharing and stresses, are sparse in the literature, especially for mobile-type core artificial disc designs. In addition, there is no standard protocol for studying the adjacent segmental effects of such implants.

METHODS

Human osteo-ligamentous spines (L1-S1) were tested before and after L5-S1 Charité artificial disc placement. The data were used to validate further an intact 3-dimensional (3-D) nonlinear L3-S1 finite element model. The model was subjected to 400-N axial compression and 10.6 Nm of flexion/extension pure moments (load control) or pure moments that produced the overall rotation of the L3-S1 Charité model equal to the intact case (hybrid approach). Resultant motion, load, and stress parameters were analyzed at the experimental and adjacent levels.

RESULTS

Finite element model validation was achieved only with the load-controlled experiments. The hybrid approach, believed to be more clinically relevant, revealed that Charité artificial disc leads to motion increases in flexion (19%) and extension (44%) at the L5-S1 level. At the instrumented level, the decrease in the facet loads was less than at the adjacent levels; the corresponding decrease being 26% at L3-L4, 25% at L4-L5, and 13.4% at L5-S1 when compared to the intact. Intradiscal pressure changes in the L4-L5 and L3-L4 segments were minimal. Shear stresses at the Charité artificial disc-L5 endplate interface were higher than those at S1 interface. However, in the load control mode, the increase in facet loads in extension was approximately 14%, as compared to the intact case.

CONCLUSIONS

The hybrid testing protocol is advocated because it better reproduces clinical observations in terms of motion following surgery, using pure moments. Using this approach, we found that the Charité artificial disc placement slightly increases motion at the implanted level, with a resultant increase in facet loading when compared to the adjacent segments, while the motions and loads decrease at the adjacent levels. However, in the load control mode that we believe is not that clinically relevant, there was a large increase in motion and a corresponding increase in facet loads, as compared to the intact.

Thoracic transfacet pedicle screw fixation: a new instrumentation technique

Object.Pedicle screw instrumentation of the thoracic spine remains technically challenging. Transverse process and costotransverse screw fixation techniques have been described as alternatives to pedicle screw fixation (PSF). In this study, the authors introduce thoracic transfacet PSF and compare its experimental biomechanical results with those of standard PSF in short-term cyclic loading in cadaveric thoracic specimens.

Methods.Specimens were tested intact for six cycles at compressive loads of 250 N offset by 1 cm along appropriate axes to induce flexion, extension, and left and right lateral bending. The specimens were then fixed with either a pedicle screw/rod construct or transfacet pedicle screws and retested in the same fashion. After this sequence, specimens were loaded until failure in flexion mode at a rate of 5 mm/minute was observed.

Both fixation constructs provided significantly greater stiffnesses than that demonstrated when the specimen was intact (p < 0.05, two-way analysis of variance). Additionally, the two constructs were statistically equivalent in terms of stiffness and load-to-failure values (p < 0.05, two-tailed nonpaired t-test). The only difference observed was that the low midthoracic region (T7–9) was biomechanically weaker than the upper midthoracic and lower thoracic areas in flexion after the destabilization and instrumentation-augmented stabilization procedures.

Conclusions.In selected thoracic surgical procedures, transfacet PSF may, after analysis of long-term biomechanical data, potentially become a reasonable alternative to conventional PSF.

A Biomechanical Study of Posterior Spinal Instrumentation Using Pedicle Screws With and Without Cross-Links

OBJECTIVE

It is not clear from the studies reported in the literature whether the addition of cross-links to bilateral pedicle screws increases the stiffness of the construct significantly to fix multilevel vertebral bodies. The current study addresses the question of how cross-links change the rotational stiffness of constructs with 6, 12, and 20 pedicle screws.

METHODS

Seven thoracolumbar 10-level spine segments (T4-L1), from individuals of average age 72 years (range 46-83 years), were instrumented with pedicle screws (6, 12, and 20) and tested in torsion with and without cross-links. As the cadaver specimens had signs of osteopenia, constructs consisting of 10 wood blocks 35 x 35 x 78 mm (weight x height x length) were also tested following an identical protocol as in the cadavers to prove the concept.

RESULTS

For both wood blocks and cadavers, the rotational stiffness increased linearly with number of screws with and without cross-links. Two cross-links increased rotational stiffness significantly for both 12 and 20 screws (nonsignificant for 6 screws) for wood block constructs. However, the rotational stiffness values of the cadaver constructs with or without cross-links failed to show any significant differences.

CONCLUSIONS

This work has raised the need for developing a suitable animal model for the study of multilevel vertebral bodies to simulate the fixation of scoliosis corrections.

Torsional stability of cross-link configurations: a biomechanical analysis

BACKGROUND CONTEXT

Cross-link systems have been used to augment segmental spinal instrumentation since the earliest introduction of these fixation systems. Although transverse cross-links have little impact on sagittal motion of spinal constructs, cross-linkage does affect torsional rigidity. Despite the wide variety of cross-link designs, almost all have been configured as transverse devices. The relative mechanical benefit of different cross-link configurations is not known.

PURPOSE

The purpose of this study was to compare the torsional stability of three different cross-link configurations.

STUDY DESIGN

Biomechanical analysis of segmental instrumentation constructs using porcine spines.

METHODS

Thoracic porcine spines (T4 to T10) were instrumented with 6.5-mm conical pedicle screws and 7.0-mm connecting rods from T5 to T9. Terminal vertebrae were embedded in polymethylmethacrylate (PMMA) after a T7 corpectomy. Four cross-link configurations were tested in a randomized manner: Un-cross-linked Control (CONT); Transverse Rod-Rod (RR); Transverse Screw-Screw (SS); and Diagonal Screw-Screw (DX) Cross-links. The specimens were rotated to 3 Nm at a rate of 0.2 degrees/s and cycled six times with data acquisition over the final two cycles. Stiffness, rotation, and energy data were normalized to each control. A Newman-Keuls repeated measures analysis of variance was used to infer significant differences.

RESULTS

Diagonal cross-link configurations provided the most rigid construct. Transverse cross-links did not significantly change torsional behavior compared with the unlinked control. Rotation and energy expended were not significantly greater torsional stiffness compared with other constructs tested (p<.01).

CONCLUSIONS

The diagonal cross-link configuration provided increased torsional stiffness as compared with unlinked or transverse configurations. This observation should be considered in future cross-link designs.

In vitro biomechanical comparison of pedicle screws, sublaminar hooks, and sublaminar cables

OBJECT

Three types of posterior thoracolumbar implants are in use today: pedicle screws, sublaminar titaniumcables, and sublaminar hooks. The authors conducted a biomechanical comparison of these three implants in human cadaveric spines.

METHODS

Spine specimens (T5-12) were harvested, radiographically assessed for fractures or metastases, and their bone mineral density (BMD) was measured. Individual vertebrae were disarticulated and fitted with either pedicle screws, sublaminar cables, or bilateral claw hooks. The longitudinal component of each construct consisted of bilateral 10-cm rods connected with two cross-connectors. The vertebral body was embedded in cement, and the rods were affixed to a ball-and-socket apparatus for the application of a distraction force. The authors analyzed 1) 20 vertebrae implanted with screws; 2) 20 with hooks, and 3) 20 with cables. The maximum pullout (MPO) forces prior to failure (mean +/- standard deviation) for the screw, hook, and cable implants were 972 +/- 330, 802 +/- 356, and 654 +/- 248 N, respectively (p = 0.0375). Cables allowed significantly greater displacement (6.80 +/- 3.95 mm) prior to reaching the MPO force than hooks (3.73 +/- 1.42 mm) and screws (4.42 +/- 2.15 mm [p = 0.0108]). Eleven screw-implanted vertebrae failed because of screw pullout. All hook-and-cable-implanted vertebrae failed because of pedicle, middle column, or laminar fracture.

CONCLUSIONS

These findings suggest that screws possess the greatest pullout strength of the three fixation systems. Sublaminar cables are the least rigid of the three. When screw failure occurred, the mechanism was generally screw back-out, without vertebral fractures.