Internal forces and moments in transpedicular spine instrumentation. The effect of pedicle screw angle and transfixation--the 4R-4bar linkage concept

Pelvic Fixation Construct Trends in Spinal Deformity Surgery

Purpose

Although many techniques exist, spinopelvic fixation continues to present challenges in the management of adult spinal deformity. Shear forces, complex anatomy, and bone quality are common reasons why spine surgeons continue to explore options for fixation.

Methods

A retrospective chart reviewed of patients receiving pelvic fixation for adult spinal deformity over a 12-year period was conducted. Patients were divided into 3 cohorts based on date of surgery: (1) 2010 to 2013, (2) 2014 to 2017, and (3) 2018 to 2021. Pelvic fixation constructs in the study included traditional iliac screws, stacked S2-alar-iliac (S2AI screws), and triangular titanium implants.

Results

Of the 494 patients with multiple implant constructs who met the inclusion criteria for this study, patients undergoing pelvic fixation surgery who received at least 2 implants increased by approximately 5% every 4 years (90.2%, 94.6%, 99.1% respectively). Over the 12-year span, the implementation of the S2AI screw grew 120%.

Conclusion

At our institution, there is a trend toward using multiple bilateral implant constructs for pelvic fixation, with nearly a tenfold percentage increase between the most recent cohorts. These include iliac screws with S2AI screws, multiple stacked S2AI screws, and S2AI screws used in conjunction with triangular titanium implants in hopes to decrease implant failure.

Systematic Review of Traumatic Thoracic Spondyloptosis and Presentation of a Novel Approach for Management With Quad Rod Construct

BACKGROUND

Traumatic thoracic spondyloptosis (TTS) is a rare but devastating spinal injury often secondary to high-impact trauma. TTS is typically managed with surgical fusion and stabilization.

OBJECTIVE

To evaluate current surgical management of TTS while presenting a novel surgical technique for reduction and fusion.

METHODS

We performed a systematic review of surgical management of TTS using Preferred Reporting Items for Systematic Reviews and Meta-Analyses of Individual Participant Data guidelines with 2 independent reviewers. We identified patient demographics, level of spondyloptosis, American Spinal Injury Association grade, level of spinal fusion, surgical approach, type of construct used, and reduction of fracture.

RESULTS

Seventeen articles with 37 patients with TTS managed surgically were identified. The male:female ratio was 31:6. The average age was 33 years (±15). Motorized injury including motor vehicle accident, road traffic accident, and motor vehicle collision (16 patients, 43%) and fall including fall from height, stairs, train, or standing (16 patients, 43%) were equivalent. The middle (15 patients 40%) and lower (18 patients, 49%) thoracic regions were similar for the level of spondyloptosis. Thirty-four patients (92%) were American Spinal Injury Association A. Thirty-six patients (97.3%) underwent posterior only surgery and 1 (2.7%) underwent a combined posterior-anterior approach. There were 29 (78%) dual rod constructs and 8 (22%) dual rod with connectors or crosslinks. Complete reduction was obtained in 24 (65%) patients, incomplete in 11 (30%), and 2 (5%) patients were not reported. Two of our patients underwent novel quad rod reconstruction with complete reduction.

CONCLUSION

Surgical management of TTS is typically posterior only with complete fracture reduction. We have presented a novel quad rod approach for reduction of TTS.

Biomechanical Effects of a Cross Connector in Sacral Fractures - A Finite Element Analysis

Background: Spinopelvic fractures and approaches of operative stabilization have been a source of controversial discussion. Biomechanical data support the benefit of a spinopelvic stabilization and minimally invasive procedures help to reduce the dissatisfying complication rate. The role of a cross connector within spinopelvic devices remains inconclusive. We aimed to analyze the effect of a cross connector in a finite element model (FE model). Study Design: A FE model of the L1-L5 spine segment with pelvis and a spinopelvic stabilization was reconstructed from patient-specific CT images. The biomechanical relevance of a cross connector in a Denis zone I (AO: 61-B2) sacrum fracture was assessed in the FE model by applying bending and twisting forces with and without a cross connector. Biomechanical outcomes from the numerical model were investigated also considering uncertainties in material properties and levels of osseointegration. Results: The designed FE model showed comparable values in range-of-motion (ROM) and stresses with reference to the literature. The superiority of the spinopelvic stabilization (L5/Os ilium) ± cross connector compared to a non-operative procedure was confirmed in all analyzed loading conditions by reduced ROM and principal stresses in the disk L5/S1, vertebral body L5 and the fracture area. By considering the combination of all loading cases, the presence of a cross connector reduced the maximum stresses in the fracture area of around 10%. This difference has been statistically validated (p < 0.0001). Conclusion: The implementation of a spinopelvic stabilization (L5/Os ilium) in sacrum fractures sustained the fracture and led to enhanced biomechanical properties compared to a non-reductive procedure. While the additional cross connector did not alter the resulting ROM in L4/L5 or L5/sacrum, the reduction of the maximum stresses in the fracture area was significant.

The Effect of Transpedicular Screw Design on Its Performance in Vertebral Bone Under Tensile Loads: A Parametric Study

STUDY DESIGN

A biomechanical study using bovine thoracolumbar spines.

OBJECTIVE

To study investigated whether thread design parameters aimed at altering the state of stress at the screw-bone interface increase the screw's holding power.

SUMMARY AND BACKGROUND DATA

Internal spinal fixators utilizing transpedicular screw fixation are used to achieve early stabilization of the injured spine in a range of clinical conditions. Despite advances in the design of internal spinal fixation systems, implant loosening, and catastrophic failures at the screw-bone interface remains a serious complication in adult spine surgery. Although the performance of the screws in the vertebral bone critically depends on ability of screw thread design to provide and maintain adequate bone purchase, the effect of individual thread design parameters on screw performance and the failure process of the screw-bone interface, remains unclear.

METHODS

On the basis of the AO Schanz thread, this parametric study used 96 lumbar bovine vertebrae instrumented with 19 screw designs to investigate the effects of pitch, ratio of major to minor diameter, screw insertion depth, and major diameter, on screw performance under pure tensile loading. The effect of vertebral morphometry on screw performance and the extent of damage within the failed screw-bone interface were evaluated.

RESULTS

The increase in screw insertion depth, screw pitch, and the ratio of major to minor diameter, significantly affected screw performance under tensile loads. Complex interactions existed between the major diameter and each of the design variables. Vertebral morphometry had little effect on screw performance while the damage within the failed bone-screw interface confined to the immediate region of the screw threads.

CONCLUSIONS

Design variables, able to reduce shear stresses or modify the complex stress profile at the bone-screw interface, are more effective in preventing early failure of the interface.

Pedicle screw-rod fixation: a feasible treatment for dogs with severe degenerative lumbosacral stenosis

Background

Degenerative lumbosacral stenosis is a common problem in large breed dogs. For severe degenerative lumbosacral stenosis, conservative treatment is often not effective and surgical intervention remains as the last treatment option. The objective of this retrospective study was to assess the middle to long term outcome of treatment of severe degenerative lumbosacral stenosis with pedicle screw-rod fixation with or without evidence of radiological discospondylitis.

Results

Twelve client-owned dogs with severe degenerative lumbosacral stenosis underwent pedicle screw-rod fixation of the lumbosacral junction. During long term follow-up, dogs were monitored by clinical evaluation, diagnostic imaging, force plate analysis, and by using questionnaires to owners.

Clinical evaluation, force plate data, and responses to questionnaires completed by the owners showed resolution (n = 8) or improvement (n = 4) of clinical signs after pedicle screw-rod fixation in 12 dogs. There were no implant failures, however, no interbody vertebral bone fusion of the lumbosacral junction was observed in the follow-up period. Four dogs developed mild recurrent low back pain that could easily be controlled by pain medication and an altered exercise regime.

Conclusions

Pedicle screw-rod fixation offers a surgical treatment option for large breed dogs with severe degenerative lumbosacral stenosis with or without evidence of radiological discospondylitis in which no other treatment is available. Pedicle screw-rod fixation alone does not result in interbody vertebral bone fusion between L7 and S1.

ASTM F1717 standard for the preclinical evaluation of posterior spinal fixators: Can we improve it?

Preclinical evaluation of spinal implants is a necessary step to ensure their reliability and safety before implantation. The American Society for Testing and Materials reapproved F1717 standard for the assessment of mechanical properties of posterior spinal fixators, which simulates a vertebrectomy model and recommends mimicking vertebral bodies using polyethylene blocks. This set-up should represent the clinical use, but available data in the literature are few. Anatomical parameters depending on the spinal level were compared to published data or measurements on biplanar stereoradiography on 13 patients. Other mechanical variables, describing implant design were considered, and all parameters were investigated using a numerical parametric finite element model. Stress values were calculated by considering either the combination of the average values for each parameter or their worst-case combination depending on the spinal level. The standard set-up represents quite well the anatomy of an instrumented average thoracolumbar segment. The stress on the pedicular screw is significantly influenced by the lever arm of the applied load, the unsupported screw length, the position of the centre of rotation of the functional spine unit and the pedicular inclination with respect to the sagittal plane. The worst-case combination of parameters demonstrates that devices implanted below T5 could potentially undergo higher stresses than those described in the standard suggestions (maximum increase of 22.2% at L1). We propose to revise F1717 in order to describe the anatomical worst case condition we found at L1 level: this will guarantee higher safety of the implant for a wider population of patients.

Finite Element Analysis and Biomechanical Comparison of Short Posterior Spinal Instrumentation with Divergent Bridge Construct versus Parallel Tension Band Construct for Thoracolumbar Spine Fractures

The ideal treatment for unstable thoracolumbar fractures remains controversial with posterior reduction and stabilization, anterior reduction and stabilization, combined posterior and anterior reduction and stabilization, and even nonoperative management advocated. Short segment posterior osteosynthesis of these fractures has less comorbidities compared with the other operative approaches but settles into kyphosis over time. Biomechanical comparison of the divergent bridge construct versus the parallel tension band construct was performed for anteriorly destabilized T11-L1 spine segments using three different models: (1) finite element analysis (FEA), (2) a synthetic model, and (3) a human cadaveric model. Outcomes measured were construct stiffness and ultimate failure load. Our objective was to determine if the divergent pedicle screw bridge construct would provide more resistance to kyphotic deforming forces. All three modalities showed greater stiffness with the divergent bridge construct. The FEA calculated a stiffness of 21.6 N/m for the tension band construct versus 34.1 N/m for the divergent bridge construct. The synthetic model resulted in a mean stiffness of 17.3 N/m for parallel tension band versus 20.6 N/m for the divergent bridge (p = 0.03), whereas the cadaveric model had an average stiffness of 15.2 N/m in the parallel tension band compared with 18.4 N/m for the divergent bridge (p = 0.02). Ultimate failure load with the cadaveric model was found to be 622 N for the divergent bridge construct versus 419 N (p = 0.15) for the parallel tension band construct. This study confirms our clinical experience that the short posterior divergent bridge construct provides greater stiffness for the management of unstable thoracolumbar fractures.

Validation of an Improved Method to Calculate the Orientation and Magnitude of Pedicle Screw Bending Moments

Previous methods of pedicle screw strain measurement have utilized complex, time consuming methods of strain gauge application, experience high failure rates, do not effectively measure resultant bending moments, and cannot predict moment orientation. The purpose of this biomechanical study was to validate an improved method of quantifying pedicle screw bending moment orientation and magnitude. Pedicle screws were instrumented to measure biplanar screw bending moments by positioning four strain gauges on flat, machined surfaces below the screw head. Screws were calibrated to measure bending moments by hanging certified weights a known distance from the strain gauges. Loads were applied in 30 deg increments at 12 different angles while recording data from two independent strain channels. The data were then analyzed to calculate the predicted orientation and magnitude of the resultant bending moment. Finally, flexibility tests were performed on a cadaveric motion segment implanted with the instrumented screws to demonstrate the implementation of this technique. The difference between the applied and calculated orientation of the bending moments averaged (±standard error of the mean (SEM)) 0.3 ± 0.1 deg across the four screws for all rotations and loading conditions. The calculated resultant bending moments deviated from the actual magnitudes by an average of 0.00 ± 0.00 Nm for all loading conditions. During cadaveric testing, the bending moment orientations were medial/lateral in flexion–extension, variable in lateral bending, and diagonal in axial torsion. The technique developed in this study provides an accurate method of calculating the orientation and magnitude of screw bending moments and can be utilized with any pedicle screw fixation system.

Computed tomography-guided navigation of thoracic pedicle screws for adolescent idiopathic scoliosis results in more accurate placement and less screw removal

STUDY DESIGN

Retrospective study of computed tomography-guided navigation (CTGN) of thoracic pedicle screw placement in patients with adolescent idiopathic scoliosis (AIS).

OBJECTIVE

To compare the accuracy and safety of thoracic pedicle screw placement and frequency of intraoperative removal using CTGN versus conventional freehand technique in AIS.

SUMMARY OF BACKGROUND DATA

Even in experienced hands, more than 10% of the thoracic pedicle screws are misplaced. CTGN may improve accuracy and safety, but there is little published data on its efficacy.

METHODS

We reviewed intraoperative computed tomographic images in a consecutive series of AIS cases undergoing posterior fusion during a 1-year period. Three types of screws were identified: an optimal screw--the central axis is in the plane and axis of the pedicle with the tip completely within the vertebral body; an acceptable screw--the majority of its shank is outside the central axis of the pedicle, but not potentially unsafe; and a potentially unsafe screw--(1) the central axis of the screw traversed the canal, (2) left anterior/lateral vertebral body perforation, risking the aorta, or (3) any screw repositioned or removed after the postimplant computed tomography.

RESULTS

In 42 patients, 485 screws were evaluable with a visible pedicle and screw (300 navigated and 185 non-navigated). Screws were classified as follows: optimal screws, 74% CTGN versus 42% non-navigated; acceptable screws, 23% CTGN versus 49% non-navigated; and potentially unsafe, 3% CTGN versus 9% non-navigated (P < 0.001). A potentially unsafe screw was 3.8 times less likely to be inserted with navigation (P = 0.003). The odds of a significant medial breach were 7.6 times higher without navigation (P < 0.001). A screw was 8.3 times more likely to be removed intraoperatively in the non-navigated cohort (P = 0.003).

CONCLUSION

CTGN resulted in more optimally placed thoracic pedicle screws, fewer potentially unsafe screws, and fewer screw removals.

Free-hand placement of high thoracic pedicle screws with the aid of fluoroscopy: evaluation of positioning by CT scans in a four-year consecutive series

OBJECTIVE: To evaluate the feasibility, safety and accuracy of pedicle screw placement in the upper thoracic spine using the free-hand technique with the aid of fluoroscopy; to analyze the methods used to verify correct screw positioning intra and postoperatively. METHOD: All patients with instability of the cervicothoracic or upper thoracic spine and at least one screw placed in the segment T1-T6 as part of a posterior construct entered the study. Only C-arm intraoperative fluoroscopy was used to guide screw placement. RESULTS: We obtained excellent positioning in 98.07% of the screws. CT scans precisely demonstrated pedicle wall and anterolateral body violations. There was no hardware failure, no neurological or vascular injury and no loss of alignment during the follow-up period. CONCLUSION: Pedicle screws can be safely placed in the upper thoracic spine when strict technical principles are followed. Only a CT scan can precisely demonstrate vertebral body and medial pedicle cortical violations.

Long-term results of pediculo-body fixation and posterolateral fusion for lumbar spondylolisthesis

Grob et al. (Eur Spine J 5:281-285, 1996) illustrated a new fixation technique in inveterate cases of grade 2-3 spondylolisthesis (degenerative or spondylolytic): a fusion without reduction of the spondylolisthesis. Fixation of the segment was achieved by two cancellous bone screws inserted bilaterally through the pedicles of the lower vertebra into the body of the upper slipped vertebra. Since 1998 we have been using this technique according to the authors' indications: symptomatic spondylolisthesis with at least 25% anterior slippage and advanced disc degeneration. Afterwards this technique was used also in spondylolisthesis with low reduction of the disc height and slippage less than 25%. In every case we performed postero-lateral fusion and fixation with two AO 6.5 Ø thread 16 mm cancellous screws. From 1998 to 2002 we performed 62 fusions for spondylolisthesis with this technique: 28 males (45.16%) and 34 females (54.84%), mean age 45 years (14-72 years). The slipped vertebra was L5 in 57 cases (92%), L4 in 2 cases (3.2%), L3 in 1 case (1.6%), combined L4 and L5 in 2 cases (3.2%). In all cases there was an ontogenetic spondylolisthesis with lysis. Lumbar pain was present in 22 patients and lumbar-radicular pain was present in 40 patients. The mean preoperative VAS was 6.2 (range 5-8) for lumbar pain, and 5.5 (range 4-7) for leg pain. The fusion area was L5-S1 in 53 cases (85.5%), L3-L4 in 1 case (1.6%), L4-S1 in 8 cases (12.9%). A decompression of the spinal canal by laminectomy was performed in 33 procedures (53%). When possible a bone graft was done from the removed neural arc, and from the posterior iliac crest in the other cases. The mean blood loss was about 254 ml (100-1,000). The mean operative time was 75 min (range 60-90). The results obtained by computerized analysis at follow-up at least 5 years after surgery showed a significant improvement in preoperative symptoms. The patients were asymptomatic in 52 cases (83.9%); strained-back pain was present in 8 cases (12.9%), and there was persistent lumbar-radicular pain in 2 cases (3.2%). The mean ODI score was 2.6%, the mean VAS back pain was 1.3, the mean VAS leg pain 0.7. Some complications were observed: a nerve root compression by a screw invasion of intervertebral foramen, resolved by screw removal; an iliac artery compression by a lateral exit screw from pediculo, resolved by screw removal; a deep iliac vein phlebitis with thrombosis caused by external compression due to a wrong intraoperative position, treated by medicine. Two cases of synthesis mobilization and two cases of broken screws was detected. No cases of pseudoarthrosis and immediate or late superficial or deep infection were observed. The analysis of the long-term results of the spondylolisthesis surgical treatment with direct pediculo-body screw fixation and postero-lateral fusion gave a very satisfactory response. The technique is reliable in allowing an optimal primary stability, creating the best biomechanical conditions to obtain a solid fusion.

Significantly improved outcomes with a less invasive posterior lumbar interbody fusion incorporating total facetectomy

STUDY DESIGN

Original study.

OBJECTIVE

Prospective comparison of clinical outcomes after a standard posterior lumbar interbody fusion (ST-PLIF) and after a limited exposure PLIF incorporating total facetectomy (LI-PLIF).

SUMMARY OF BACKGROUND DATA

Most groups have reported significantly improved clinical outcomes after ST-PLIF. To our knowledge, however, a comparison of outcomes between ST-PLIF and the LI-PLIF that we herein describe has not been reported before.

METHODS

Patients were included who had suffered chronic low back pain for a minimum of 2 years that was unresponsive to conservative treatment. N = 114 consecutive patients underwent ST-PLIF, whereas n = 209 underwent LI-PLIF. All patients underwent pre- and postoperative evaluations for Oswestry Disability Index (ODI), short-form 36 (SF-36), and visual analogue scores (VAS). The minimum follow-up for either group was 2 years.

RESULTS

There was a significant improvement in the ODI (22.5 +/- 1.0, P < 0.001), VAS for back pain (3.8 +/- 0.1, P = 0.003), VAS for leg pain (4.0 +/- 0.2, P = 0.002), and SF-36 for bodily pain (14.7 +/- 0.9, P = 0.012) after ST-PLIF. However, there was a significantly greater improvement in all scores after LI-PLIF: ODI (28.8 +/- 1.4 vs. 22.5 +/- 1.0, P < 0.001), VAS for back pain (5.4 +/- 0.2 vs. 3.8 +/- 0.1, P = 0.001), VAS for leg pain (5.1 +/- 0.2 vs. 4.0 +/- 0.2, P < 0.001), and SF-36 for bodily pain (18.5 +/- 0.8 vs. 14.7 +/- 0.9, P = 0.003). There was a significantly shorter duration of hospital stay after LI-PLIF (2.24 +/- 0.057 days) than after ST-PLIF (4.04 +/- 0.13 days) (P = 0.005). Operative complications occurred in 19.3% of ST-PLIF and in 6.7% of LI-PLIF.

CONCLUSION

Clinical outcomes were significantly improved after both ST-PLIF and LI-PLIF. However, outcomes were significantly better after LI-PLIF than after ST-PLIF. Significantly shortened hospital stay with LI-PLIF probably reflected the "less invasive" technique per se. Significantly better clinical outcomes with fewer complications after LI-PLIF, however, potentially reflected maneuvers singular to LI-PLIF: (1) preservation of posterior elements, (2) avoidance of far lateral dissection over the transverse processes, (3) bilateral total facetectomy, (4) fewer neurologic complications, and (5) avoidance of iliac crest autograft. LI-PLIF is therefore recommended over ST-PLIF.

Use of instrumented pedicle screws to evaluate load sharing in posterior dynamic stabilization systems

BACKGROUND CONTEXT

Dynamic stabilization is an alternative to fusion intended to eliminate or at least minimize the potential for adjacent level degeneration. Different design approaches are used in pedicle screw-based systems that should have very different effects on the loading of the posterior column and intervertebral disc. If the implant system distributes these loads more evenly, loads in the pedicle screws will be reduced, and screw loosening will be prevented.

PURPOSE

The purpose of this study was to determine how two different design approaches to dynamic stabilization systems, Dynesys System and the Total Posterior Spine (TOPS) System, affect the load carried by the pedicle screws.

STUDY DESIGN/SETTING

A controlled laboratory study in which the magnitude of the moments on pedicle screws during flexion-extension and lateral bending were measured after implantation of two posterior dynamic stabilization devices into cadaveric spines.

METHODS

Five lumbar spines were tested in flexion-extension and lateral bending. Specimens were tested sequentially: first intact, then with the Dynesys system implanted, and finally with the TOPS system implanted. Range of motion (ROM) for each construct was measured with a 210N and 630N compressive load. The pedicle screws were instrumented with strain gages, which were calibrated so that the moments on the screws could be determined from the strain measurements.

RESULTS

Compared with intact values, ROM decreased in flexion-extension and lateral bending when the Dynesys System was implanted. With implantation of the TOPS System, ROM returned to values that were not significantly different from the intact values. The moments in the screws with the Dynesys System were significantly higher than with the TOPS System with increases of as much as 56% in flexion-extension and 86% in lateral bending.

CONCLUSIONS

The design of the posterior stabilization device influences the amount of load seen by the pedicle screws and therefore the load sharing between spinal implant and bone.

ROD CANTILEVER TECHNIQUES

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

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.

Limitations of the cervical porcine spine in evaluating spinal implants in comparison with human cervical spinal segments: a biomechanical in vitro comparison of porcine and human cervical spine specimens with different instrumentation techniques

STUDY DESIGN

Porcine and human cervical spine specimens were in vitro biomechanically compared with different instrumentation techniques.

OBJECTIVES

To evaluate whether subaxial porcine cervical spines are a valid model for implant testing in a single level corpectomy.

SUMMARY OF BACKGROUND DATA

Biomechanical in vitro tests are widely used for implant tests, mainly with human spine specimens. The availability of human cadavers is limited and the properties of the specimen regarding age, bone mineral density, and grade of degenerative changes is inhomogeneous.

METHODS

Six porcine and six human cervical specimens were loaded nondestructively with pure moments: 1) in an intact state; 2) after a corpectomy of C5 and substitution by a cage with integrated force sensor; 3) after additional instrumentation with a posterior screw and rod system with: a) lateral mass and b) pedicle screws; 4) after instrumentation with an anterior plate; and 5) with a circumferential instrumentation. The unconstrained motion and the axial loads occurring in the corpectomy gap were measured, as well as the bone mineral density of the specimen before testing.

RESULTS

The range of motion in the intact state, as well as for the different instrumentations, was comparable for flexion-extension. In lateral bending and axial rotation, marked differences in the intact state as well as for pedicle screw instrumentations occurred.

CONCLUSIONS

The subaxial porcine cervical spine is a potential model in flexion-extension because of its biomechanical similarity. For lateral bending and axial rotation, the marked differences severly restrict the comparability.

A biomechanical analysis of an instrumented spinal fixator under torsional loads

The effect of design features of an internal spinal fixator on the loading of its individual components is paramount to the understanding of the interaction between the fixator and the instrumented spine. Using a corpectomy injury model, a strain gauge instrumented spinal fixation device was employed to investigate the role of clamp tightening torque and the inclusion of transverse bars on the distribution of bending and torsional moments acting on the fixator under torsional loading. The increase in clamp torque from 5 to 10 Nm caused a marked decrease (40%) in torsional moments acting on the vertical rods, an increase of 24% in torsional moments acting on the screws and an increase of 44% in bending moments acting on the rods along the sagittal plane of the fixator. The inclusion of transverse elements significantly increased (132%) the bending moment acting on the rods and decreased (92%) the bending moments acting on the screws along the sagittal plane. The change in both design parameters significantly reduced the response hysteresis and decreased the asymmetry of loading. A theoretical model, developed to elucidate the load path mechanisms underlying this response, successfully predicted the external response of the fixator. This model suggested both design parameters would affect the internal force and moment distribution across the fixator and the relative role of each load response mechanism in effecting this response. The changes in load patterns across the fixator will influence both its ability to augment the process of spinal fusion and the long-term performance of its components.

Prospective evaluation of thoracic pedicle screw placement using fluoroscopic imaging

BACKGROUND

In this prospective 18-month study, 29 patients underwent posterior thoracic instrumentation with placement of 209 transpedicular screws guided by intraoperative fluoroscopic imaging and anatomic landmarks. We assessed the safety, accuracy, complications, and early stability of this technique.

METHODS

Pedicle and pedicle-rib units were measured, and screw cortical penetrations were graded on anatomy and depth of penetration. All 29 patients underwent preoperative computed tomographic (CT) imaging, and 28 underwent postoperative CT imaging (199/209 screws).

RESULTS

From T2 to T12, screw diameters were >or=5 mm with mean medial screw angulation measuring 20-25 degree. Of the 209 screws placed from T1 to T12, 111 had diameters greater than or equal to the pedicle width. From T3 to T9, the mean diameter of the pedicle screws exceeded the mean pedicle width. Lateral pedicle wall penetration occurred significantly more often than superior, inferior, and medial pedicle wall penetrations and anterolateral vertebral body penetration. Five of six high-risk screw penetrations occurred in one patient when intraoperative technique was compromised. We observed no new postoperative neurologic deficits, visceral injuries, or pedicle screw instrumentation failures. The three high-risk anterolateral vertebral body penetrations at T1 and T2 were associated with a significantly decreased mean screw transverse angle; the three high-risk medial pedicle wall penetrations occurring from T3 to T9 were associated with a significantly increased mean screw transverse angle. Among all 26 patients available at postoperative follow-up (mean 11.9 months), the mean loss of kyphosis correction was 2.0 degree.

CONCLUSIONS

Guided by intraoperative fluoroscopic imaging and anatomic landmarks, thoracic pedicle screws can be placed safely. Early clinical follow-up reveals excellent results with minimal loss of kyphosis correction.

Posterior thoracic extrapedicular fixation: a biomechanical study

STUDY DESIGN

In vitro biomechanical testing of thoracic spine specimens using a standardized three-dimensional spine flexibility protocol.

OBJECTIVES

To compare the mechanical stability of the intrapedicular and extrapedicular technique for pedicle screw placement. The hypothesis was that extrapedicular screw placement provides an equally rigid construct.

SUMMARY OF BACKGROUND DATA

Pedicle screws provide rigid fixation of instabilities in the lumbar and lumbosacral spine. Anatomic considerations and the potential risk of neurologic complications are the main reasons to hesitate using pedicle screws in the thoracic spine. Extrapedicular fixation would allow safer insertion due to an increased distance to the spinal canal.

METHODS

Twelve human cadaveric thoracic spines (six intra-, six extrapedicular) were instrumented with the USS system, using computed tomography-based computer navigation to ensure accurate placement. The specimens were tested in flexion-extension, torsion, and lateral bending. The ROM was measured using an optoelectronic system, and the two methods were compared before and after implantation of the USS construct and before and after fatigue testing of the construct.

RESULTS

The ROM of the instrumented spine was reduced to less than 50% that of its original ROM. There were no statistically significant differences in the ROM reduction between the intra- and the extrapedicular technique. Cyclic fatiguing of the construct did not significantly increase the ROM.

CONCLUSIONS

The extrapedicular technique provides a construct for stabilization of the thoracic spine that is as rigid as the conventional intrapedicular technique, but has the advantage of a safer surgical screw insertion.

Surgical techniques for total sacrectomy and spinopelvic reconstruction

The surgical management of sacral tumors requires partial or total sacrectomy and spinopelvic reconstruction. These lesions present a great surgical challenge, because most spine surgeons are unfamiliar with the techniques required for these procedures. The authors describe a step-by-step operative technique and provide several illustrations. Total sacrectomy is performed by sequential anterior and posterior approaches that involve a rectus abdominis pullthrough pedicle flap reconstruction. The anterior procedure is an intraperitoneal approach used to expose the anterior aspect of the tumor, to ligate the main tumor vessels, and to conduct an anterior partial sacrectomy. After this, the rectus abdominis myocutaneous flap, based on the inferior epigastric vessel, is prepared, and a posterior sacrectomy is performed, dividing all sacral nerve roots in the thecal sac. After complete en bloc extirpation of the sacrum with tumor, spinopelvic reconstruction and closure with a myocutaneous flap are performed. Spinopelvic reconstruction is undertaken using a modified Galveston technique or double iliac screw fixation combined with posterior lumbar segmental fixation. These provide a long lever arm within the ilium to counteract the forces exerted by the lumbar spine. Understanding the nature of the disease as well as the biomechanics of the lumbosacral pelvic area and spinopelvic fixation will help surgeons select the appropriate treatment for sacral tumors.

Mechanical performance of the new posterior spinal implant: effect of materials, connecting plate, and pedicle screw design

STUDY DESIGN

A newly designed spinal implant was tested to evaluate multicycle stiffness and fatigue resistance.

OBJECTIVES

To investigate the effect of different materials, connecting plate, and pedicle screw design on the mechanical performance of the spinal implant.

SUMMARY OF THE BACKGROUND DATA

The addition of cross-linkages did not significantly increase implant compression/flexion stiffness, but accelerated fatigue failure at the rod junctions. Both Ti-6Al-4V spinal implants and the 316L stainless-steel counterparts have been used extensively for clinical cases; however, design factors establishing the proposed superiority of the Ti-6Al-4V implant for fatigue resistance have not, as yet, been extensively studied.

METHODS

Twenty implants with connecting plates (two materials by two screw designs by five implants) and five implants without connecting plates were assembled to UHMWPE blocks and cyclically loaded from 60 N to 600 N at a frequency of 5 Hz.

RESULTS

Failure sites for the tested prototypes were at the cephalic screw hubs or rod-plate junctions. All Ti-6Al-4V implants demonstrated reduced stiffness compared to the structurally identical 316L analogs. The use of connecting plates raised the stiffness of the 316L prototypes without cross-links. However, elimination of the connecting plate avoided stress concentration at the rod/plate junctions and increased fatigue life. The Ti-6Al-4V new system with the minimal notch effect at the screw hubs achieved greater fatigue resistance than its 316L counterpart. By contrast, enlargement of the inner-hub diameter resulted in greater gains for fatigue resistance than for stiffness, especially for Ti-6Al-4V variants.

CONCLUSIONS

Although Ti-6Al-4V was superior to 316L for endurance-limit properties, structural design of the Ti-6Al-4V implant dramatically affects fatigue resistance. This may explain the differences between existing studies and the current report, comparing fatigue life for implants made from these two materials. Our results reveal that Ti-6Al-4V must be carefully treated because of sensitivity to notch, with special consideration given to screw-hub design.

Spinal instability and deformity due to neoplastic conditions

In addition to tumor resection, a major goal of spine surgery involving tumors is the preservation or achievement of spinal stability. The criteria defining stability, originally developed for use in trauma, are not directly applicable in the setting of neoplasia. The authors discuss the most common patterns of tumor-related instability and deformity at all levels of the spinal column and review the surgical options for treatment.

Effectiveness of transfixation and length of instrumentation on titanium and stainless steel transpedicular spine implants

This study compares the effectiveness of transfixation on the stiffness of two pedicle screw-rod constructs of different manufacture, implant design, and alloy, applied in one-and two-level instability. Four screws composed of either stainless steel or Titanium were assembled in pairs to two polymethylmethacrylate blocks to resemble one-and two-level corpectomy models and the construct underwent nondestructive torsional, extension, and flexion loading. In every loading test, each construct was tested using stainless steel or titanium rods of 4.9-mm diameter in two different lengths (short, 10 cm; long, 15 cm), not augmented or augmented with different transfixation devices or a pair of devices. The authors compared the stiffness of stainless steel and titanium constructs without cross-link with the stiffness of that reinforced with single or double Texas Scottish Rite Hospital (TSRH) cross-link, closed new-type cross-link (closed NTC), or open new-type cross-link (open NTC). The results showed that augmentation or no augmentation of short rods conferred significantly more stiffness than that of long rods of the same material in all three loading modes. The closed NTC provided the greatest increase of torsional, extension, and flexion stiffness, and single TSRH provided the least amount of stiffness. Torsional stiffness of short stainless steel rods augmented or not augmented was significantly greater than that of their titanium counterparts. Torsional stiffness of long titanium rods was always greater than that of their stainless steel counterparts. Extension stiffness of short nonaugmented titanium rods was superior to that of long titanium rods, whereas extension stiffness of nonaugmented short and long stainless steel rods was similar. Nonaugmented short titanium rods showed greater flexion stiffness than that of long titanium rods. Long stainless steel rods displayed significantly greater flexion stiffness than did their titanium counterparts. This nondestructive study showed that cross-links increase the torsional stiffness significantly but less so the flexion and extension stiffness of both titanium and stainless steel posterior transpedicular constructs. This increase was proportional to the cross-sectional diameter of the cross-link. Titanium constructs showed more torsional stiffness when used in two-level instability and steel showed more torsional stiffness in one-level instability, particularly when they are reinforced. Stainless steel constructs showed greater flexion stiffness when they were used in two-level and titanium showed greater flexion stiffness in one-level instability, particularly when they were reinforced with stiff cross-links. The effect of transfixation on extension forces was obvious when thick cross-links were used.

The use of pedicle-screw internal fixation for the operative treatment of spinal disorders

Pedicle screws have dramatically improved the outcomes of spinal reconstruction requiring spinal fusion. Short-segment surgical treatments based on the use of pedicle screws for the treatment of neoplastic, developmental, congenital, traumatic, and degenerative conditions have been proved to be practical, safe, and effective. The Funnel Technique provides a straightforward, direct, and inexpensive way to very safely apply pedicle screws in the cervical, thoracic, or lumbar spine. Carefully applied pedicle-screw fixation does not produce severe or frequent complications. Pedicle-screw fixation can be effectively and safely used wherever a vertebral pedicle can accommodate a pedicle screw--that is, in the cervical, thoracic, or lumbar spine. Training in pedicle-screw application should be standard in orthopaedic training programs since pedicle-screw fixation represents the so-called gold standard of spinal internal fixation.

Torsional rigidity of scoliosis constructs

STUDY DESIGN

A biomechanical study of the rigidity of various scoliosis constructs instrumented with and without caudal pedicle screw anchors and with none, one, or two cross-link devices.

OBJECTIVES

To determine whether the increased torsional rigidity provided by distal pedicle screw fixation might make cross-linking unnecessary.

SUMMARY OF BACKGROUND DATA

Pedicle screws and cross-linking devices have been shown to increase the structural rigidity of spinal constructs. Their relative contributions to scoliosis construct rigidity has not been determined.

METHODS

"Short" (T2-T11) and "long" (T2-L3) scoliosis constructs were mounted on an industrially fabricated spine model and tested in a hydraulic testing machine. Four different short and four different long constructs were tested: hooks only, hooks with concave side thoracic sublaminar wires, hooks with distal pedicle screw anchors, and hooks, distal pedicle screw anchors, and concave thoracic sublaminar wires. There were four iterations for each construct tested: no cross-links, one superior cross-link at T4-T5, one inferior cross-link at T9-T10, and two cross-links. Torsional rigidity was tested by applying a rotational torque at T2. Vertebral body motion was recorded with a three-dimensional video analysis system.

RESULTS

Constructs with distal pedicle screws were statistically more rigid in torsion than those with hooks as distal anchors. The additional torsional rigidity from one or more cross-links was negligible compared with that provided by pedicle screws.

CONCLUSIONS

With pedicle screws as distal anchors in scoliosis constructs, cross-linking with one or two devices adds very little additional rotational stiffness and may be unnecessary in many cases.

Lumbar interbody fusion using the Brantigan I/F cage for posterior lumbar interbody fusion and the variable pedicle screw placement system: two-year results from a Food and Drug Administration investigational device exemption clinical trial

STUDY DESIGN

A carbon fiber-reinforced polymer cage implant filled with autologous bone was designed to separate the mechanical and biologic functions of posterior lumbar interbody fusion.

OBJECTIVES

To test the safety and efficacy of the carbon cage with pedicle screw fixation in a 2-year prospective study performed at six centers under a protocol approved by the Food and Drug Administration, and to present the data supporting the Food and Drug Administration approved indications.

SUMMARY OF BACKGROUND DATA

The success of posterior lumbar interbody fusion has been limited by mechanical and biologic deficiencies of the donor bone. Some failures of pedicle screw fixation may be attributable to the absence of adequate load sharing through the anterior column. Combining an interbody fusion device with pedicle screw fixation may address some limitations of posterior lumbar interbody fusion or pedicle screw fixation in cases that are more complex mechanically.

METHODS

This clinical study of posterior lumbar interbody fusion with pedicle screw fixation involved a prospective group of 221 patients.

RESULTS

Fusion success was achieved in 176 (98.9%) of 178 patients. In the management of degenerative disc disease in patients with prior failed discectomy surgery, clinical success was achieved in 79 (86%) of 92 patients, and radiographic bony arthrodesis in 91 (100%) of 91 patients. Disc space height, averaging 7.9 mm before surgery, was increased to 12.3 mm at surgery and maintained at 11.7 mm at 2 years. Fusion success was notdiminished over multiple fusion levels. These results were significantly better than those reported in prior literature. Although significant surgical complications occurred, those attributable to the implant devices occurred less frequently and generally were minor.

CONCLUSIONS

The Brantigan I/F Cage for posterior lumbar interbody fusion and the Variable Screw Placement System are safe and effective for the management of degenerative disc disease.

Successful short-segment instrumentation and fusion for thoracolumbar spine fractures: a consecutive 41/2-year series

STUDY DESIGN

A retrospective review of all the surgically managed spinal fractures at the University of Missouri Medical Center during the 41/2-year period from January 1989 to July 1993 was performed. Of the 51 surgically managed patients, 46 were instrumented by short-segment technique (attachment of one level above the fracture to one level below the fracture). The other 5 patients in this consecutive series had multiple trauma. These patients were included in the review because this was a consecutive series. However, they were grouped separately because they were instrumented by long-segment technique because of their multiple organ system injuries.

OBJECTIVES

The choice of the anterior or posterior approach for short-segment instrumentation was based on the Load-Sharing Classification published in a 1994 issue of Spine. The purpose of this review was to demonstrate that grading comminution by use of the Load-Sharing Classification for approach selection and the choice of patients with isolated fractures who are cooperative with spinal bracing for 4 months provide the keys to successful short-segment treatment of isolated spinal fractures.

SUMMARY OF BACKGROUND DATA

The current literature implies that the use of pedicle screws for short-segment instrumentation of spinal fracture is dangerous and inappropriate because of the high screw fracture rate.

METHODS

Charts, operative notes, preoperative and postoperative radiographs, computed tomography scans, and follow-up records of all patients were reviewed carefully from the time of surgery until final follow-up assessment. The Load-Sharing Classification had been used prospectively for all patients before their surgery to determine the approach for short-segment instrumentation. Denis' Pain Scale and Work Scales were obtained during follow-up evaluation for all patients.

RESULTS

All patients were observed over 40 months except for 1 patient who died of unrelated causes after 35 months. The mean follow-up period was 66 months (51/2 years). No patient was lost to follow-up evaluation. Prospective application of the Load-Sharing Classification to the patients' injury and restriction of the short-segment approach to cooperative patients with isolated spinal fractures (excluding multisystem trauma patients) allowed 45 of 46 patients instrumented by the short-segment technique to proceed to successful healing in virtual anatomic alignment.

CONCLUSIONS

The Load-Sharing Classification is a straightforward way to describe the amount of bony comminution in a spinal fracture. When applied to patients with isolated spine fractures who are cooperative with 3 to 4 months of spinal bracing, it can help the surgeon select short-segment pedicle-screw-based fixation using the posterior approach for less comminuted injuries and the anterior approach for those more comminuted. The choice of which fracture-dislocations should be strut grafted anteriorly and which need only posterior short-segment pedicle-screw-based instrumentation also can be made using the Load-Sharing Classification.

Factor analysis of the effectiveness of transfixation and rod characteristics on the TSRH screw-rod instrumentation

The effects of Texas Scottish Rite Hospital (TSRH) hardware parameters (rod length and diameter and cross-link) and their interaction on the stiffness of the TSRH pedicle screw-rod construct were evaluated. Four TSRH screws were assembled in pairs to two polymethyl-methacrylate blocks to resemble a one-level or more corpectomy model and the construct underwent nondestructive torsional, extension, and flexion loading. In every loading test, each construct was tested using TSRH rods of different lengths (10, 15, and 20 cm) and diameters (4.9 and 6.5 mm) and different cross-links (TSRH and two new types made for this experiment). We compared the stiffness of the construct without cross-linking with that with single or double TSRH cross-linking, or either the closed new-type cross-link (closed NTC) or the open new-type cross-link (open NTC) using factor analysis. There was no axial slipping of one rod versus the other up to a force of 100 kg. The stiffness of the construct in all three loading modes increased as the rod length decreased, the rod diameter increased, and the construct was augmented with a cross-link. The closed NTC provided the greatest stiffness and the single TSRH provided the least stiffness. Unaugmented 10-cm-long rods showed two or three times more torsional stiffness than did that of the longer unaugmented rods independent of rod diameter. In addition, the closed NTC offered the maximal increase in flexion stiffness of the construct with thick rods and 10-, 15-, and 20-cm-long rods at a maximum of 40%, 27%, and 30%, respectively. This rigid closed NTC increased the extension stiffness of the same construct with 10- and 15-cm-long rods at 40% and 6%, respectively, whereas it had no influence on the extension stiffness of 20-cm-long rods.

Load-sharing characteristics of stabilized lumbar spine segments

STUDY DESIGN

Load sharing in stabilized spinal segments was evaluated using sequential injury and stabilization with a posterior instrumentation system under an in vitro flexibility protocol.

OBJECTIVE

To analyze the partitioning of applied loads between anatomic and implanted structures of lumbar functional spinal units stabilized with a posterior instrumentation system. To identify surgical indications for which the risk of fixator breakage in vivo is high.

SUMMARY OF BACKGROUND DATA

Relatively few groups have experimentally measured the in vitro and in vivo forces and/or moments supported by posterior instrumentation systems, and no analysis, of the load sharing in these systems has been performed. This information will provide novel insight into implant fatigue life, and the degree to which the spinal anatomy is shielded from the applied load and will allow the verification of mathematical models for new injury scenarios.

METHODS

Specimen kinematics were determined using an optoelectronic tracking system. Intradiscal pressure and the forces and moments supported by the implants were measured using, respectively, a needle-mounted pressure sensor and strain gauges mounted on the spinal implants.

RESULTS

A large majority of the applied moments were supported by an equal and opposite force pair between the intervertebral disc and fixator rods in flexion and extension and an equal and opposite force pair between the left and right fixator rods in lateral bending. Torsional moments were shared approximately equally between the posterior elements, intervertebral disc, an equal and opposite shear force pair in the transverse plane between the right and left fixators and internal fixator moments.

CONCLUSIONS

When posterior instrumentation devices are used to stabilize severe anterior column injuries, they are at risk of fracture secondary to reversed bending moments.

Spinal-pelvic fixation in patients with lumbosacral neoplasms

OBJECT

Primary and metastatic neoplasms of the lumbosacral junction frequently pose a complex problem for the surgical management and stabilization of the spine because of the anatomical and biomechanical factors of this transition zone between spine and pelvis. The authors have used a modification of the Galveston technique, originally described by Allen and Ferguson in the treatment of scoliosis, to achieve rigid spinal-pelvic fixation in patients with lumbosacral neoplasms. The authors retrospectively reviewed their experience, with particular attention to method, pain relief, and neurological status.

METHODS

From July 1994 through December 1998, 13 patients at the authors' institution have required spinal-pelvic fixation secondary to instability caused by primary (eight cases) or metastatic (five cases) neoplasms. Previous treatment included spinal surgery in 10 (77%), radiation therapy in seven (54%), and/or chemotherapy in six (46%). Following tumor resection, fixation was achieved by intraoperative placement of contoured titanium rods bilaterally into the ilium. These rods were attached to the lumbar spine with pedicle screws and subsequently crosslinked. Arthrodesis was performed. In the follow-up period of 3 to 50 months (average 20 months), nine (69%) of 13 patients were still alive. There were no cases of surgery-related death. Seven weeks postoperatively instrumentation failure occurred in one patient and was corrected by performing double L-rod spinal-pelvic fixation. Two patients experienced neurological dysfunction (ankle weakness and neurogenic bladder) that was thought to be related to tumor resection rather than the fixation procedure. Neurological status improved in four patients and remained unchanged in seven patients. Ambulatory status improved in 62% (eight patients), remained unchanged in 23% (three patients), and worsened in 15% (two patients). Spinal pain, as measured by a visual analog pain scale and determined by medication consumption was significantly reduced in 85% (11 cases).

CONCLUSIONS

In selected patients with primary or metastatic lumbosacral tumors, resection followed by modified Galveston L-rod spinal-pelvic fixation is an effective means of achieving stabilization that can provide significant pain relief and preserve ambulatory capacity.

The effects of design and configuration on the biomechanical response of an internal spinal fixator

This study examines the biomechanical performance of an internal spinal fixator and the effects of specific design features under a range of loading modes. The commercial device was mounted on plastic vertebrae in a corpectomy injury model and attached by a series of experimental jigs to an appropriate material testing machine and tested under axial compression, torsion and flexion and extension moments. Results from the torsional tests indicated that increasing the clamp tightening torque from 5 to 15 N m significantly increased the rigidity of the fixation system. The inclusion of the transverse elements resulted in a significant increase in the torsional stiffness, with the increase largely overriding the effect of clamp tightening torque. By contrast, under compressive and both flexion and extension loads, neither of the design features of the fixator had a marked effect on the overall measured stiffness of the system. However, under extension loads, there were specific interactions between the two design parameters. The present study clearly indicates the need for the optimization of the design of the clamps and for alternative configurations of the transverse elements to enhance their performance under sagittal loads.

Mechanical stability of thoracolumbar pedicle screw fixation. The effect of crosslinks

STUDY DESIGN

Pedicle screw fixation for unstable thoracolumbar spine injuries is relatively new. The effect of one or two crosslinks on rotational and lateral bending stiffness was studied.

OBJECTIVE

To determine the rotational and bending stiffness values of thoracolumbar fractures fixed by the AO's internal fixation system with zero, one, or two crosslinks.

METHODS

Eight embalmed thoracolumbar spine segments. (T12-L2) were instrumented at T12 and L2 with a pedicle screw-rod system. Rotational stiffness was determined for 10 cycles to 2.5 degrees, 3.5 degrees, and 5 degrees of rotation, with and without one or two crosslinks, and lateral bending stiffness for 10 cycles to 0.25, 0.40, and 0.50 inch. The results showed a clear trend toward increased stiffness with crosslinks.

RESULTS

The stiffness values of the two-crosslink construct at 2.5 degrees and 3.5 degrees of rotation were significantly higher than those of the zero-crosslink system. Also, the bending stiffness of the two-crosslink construct was significantly higher than that of no-crosslink system at all of the displacements.

CONCLUSIONS

Rotational stiffness values of the two-crosslink construct were significantly higher than those of the zero-crosslink system, at 2.5 degrees and 3.5 degrees of rotation. Lateral bending stiffness of the two-crosslink system was higher than that of the zero-crosslink system at all levels of displacement.

Mechanical evaluation of cross-link designs in rigid pedicle screw systems

STUDY DESIGN

This study was designed to evaluate the biomechanical performance of 5 different cross-link brands to determine which design characteristics are biomechanically desirable.

METHODS

The Cotrel-Dubousset, Isola, Puno Winter Byrd, Rogozinski, and Texas Scottish Rite Hospital systems were assembled to vertebral models according to the manufacturer's specifications. Three constructs were tested for each brand of instrumentation: without cross-links, with one cross-link, and with two cross-links. Four modes of loading: axial, torsional, flexion-extension, and lateral-flexion were used. Load-displacement curves were plotted. The stiffness was calculated from the slope of these curves.

OBJECTIVES

Five different rigid pedicle screw systems were tested to determine: 1) what are the characteristics of cross-link design that are most effective in limiting torsional motion; 2) whether two cross-links are more effective than one; and 3) whether cross-linkage increases the construct stiffness in lateral bending.

SUMMARY OF BACKGROUND DATA

Cross-linkage has been shown to increase the torsional stiffness of rod and screw constructs. Increased construct stiffness has been correlated with higher fusion rates.

RESULTS

Increases in axial, flexion-extension, or lateral-flexion stiffness, with the addition of one or two cross-links, were not statistically significant. In torsional loading, increases in stiffness within brands were statistically significant in every case. The average increase was 44% with one added cross-link and 26% with two. The magnitude of the increase in torsional stiffness was compared with the cross-sectional area of the respective cross-link. Greater stiffness correlated with larger cross-sectional area (r = 0.81 for one cross-link, and r = 0.60 for two).

CONCLUSION

The use of cross-linkage in spinal fusion increases torsional stiffness in pedicle screw and hook constructs. This study 1) confirmed the effectiveness of cross-linkage in limiting torsional motion and showed the superiority of two cross-links to one cross-link in limiting torsional motion, 2) showed that increase of torsional stiffness of a cross-linked construct is proportional to the cross-sectional area of the cross-link, and 3) demonstrated that cross-links do not increases stiffness in the lateral flexion mode.

Biomechanics of transfixation in pedicle screw instrumentation

STUDY DESIGN

The biomechanical role of transfixation in pedicle screw instrumentation was investigated using flexibility tests and finite element analyses.

OBJECTIVE

To assess the stabilizing effect of use and position of transfixators.

SUMMARY OF BACKGROUND DATA

Transfixation is common in pedicle screw instrumentation, however, its biomechanical role and optimal position are not completely understood.

METHODS

Specimens underwent nondestructive flexibility tests using a three-dimensional motion analysis system. Tests compared the intact spine with instrumentation with and without transfixators Rotational angles of the superior vertebra, resulting from the maximum moment of 6.4 Nm, were compared. Three-dimensional finite element models investigated transfixator position. Rotations of the superior vertebra were compared for cases with and without transfixators to determine the position providing the greatest stability.

RESULTS

Biomechanical test showed that only axial rotational stability significantly improved with transfixators compared with instrumentation alone. Pimte element models predicted improvement in lateral bending and axial rotation with transfixators compared with the case with no transfixator. With one transfixator, the greatest improvement in axial rotation stability occurred with the transfixator at the proximal 1/4 position of the rods. When two transfixators were used, the optimal locations were with one transfixator in the middle and the second at the proximal 1/8 position.

CONCLUSIONS

Transfixators improved the stabilizing effects of pedicle screw instrumentation. The greatest axial rotation stability was obtained with two transfixators; one in the middle and the other at the proximal 1/8 position of the longitudinal rods.

A device for the measurement of pedicle screw moments by means of internal strain gauges

Pedicle screws are commonly used in spinal reconstruction, and failure of pedicle screws due to bending is a significant clinical problem. To measure the moments typically placed on pedicle screws in situ we instrumented 7 mm Cotrel-Dubousset (CD) pedicle screws with internally mounted strain gauges. The screws were designed to measure flexion-extension moments at a single cross-section as dictated by strain gauge placement. It is possible to measure moments of up to 12 Nm at any location along the length of the screw by constructing transducers with varying strain gauge placements. These transducers are capable of measuring moments at points located within the vertebra including the pedicle, which is where failure usually occurs clinically. Transducer output was both linear and reproducible. These transducers are being used to investigate the load transfer characteristics between the pedicle screw and the vertebra. This technique could be applied to investigations of load sharing in reconstruction plates, lag-screws, and cross-locked intramedullary nails.

The role of pediculolaminar fixation in compromised pedicle bone

STUDY DESIGN

This in vitro study analyzed the effects of a supralaminar hook on pedicle screw fixation in compromised pedicle bone.

OBJECTIVES

To determine the ability of pediculolaminar fixation to restore pedicle screw pull-out strength after stripping of senile pedicle bone.

SUMMARY OF BACKGROUND DATA

Despite improvements in pedicle screw design, the bone-screw interface remains the "weakest link" in pedicle screw fixation. This interface is especially vulnerable in osteoporotic bone previously instrumented pedicles, and at the ends of long instrumentation constructs.

METHODS

Side-to-side testing between a pedicle screw and a pedicle screw supplemented with a supralaminar hook (pediculolaminar fixation) was performed in human cadaveric lumbar vertebrae. Comparisons were made for intact and compromised pedicle bone.

RESULTS

Pediculolaminar fixation restored 89% of intact pedicle screw pull-out strength whereas the pedicle screw alone restored only 19% of intact pull-out strength. The role of pediculolaminar fixation was greatest in weaker bone. Significant differences were noted in energy to failure and post-failure energy. In intact bone, the pediculolaminar construct did not increase pull-out strength or energy to failure, although it did have a greater post-failure energy.

CONCLUSIONS

Pediculolaminar fixation can augment pedicle screw fixation in pedicle bone compromised by previous stripping or significant osteoporosis or both.