Biomechanical comparisons of spinal fracture models and the stabilizing effects of posterior instrumentations

Biomechanical Effects of Lateral Inclination C1 and C2 Pedicle Screws on Atlantoaxial Fixation

Objective

To assess the biomechanical effect of lateral inclination C₁ and C₂ pedicle screws on the atlantoaxial fixation through vitro human cadaveric study.

Methods

From January 2016 to December 2017, fresh‐frozen cadaveric cervical spines with intact ligaments from eight donated cadavers at an average age of 71.5 ± 10.6 years, comprising of six males and two females, were collected. There were no fracture and congenital malformation in all specimens according to the imaging examination. The range of motion (ROM) of the specimens were tested in their intact condition and destabilized condition. Next, the specimens were randomly divided into two groups to ensure no differences in sex and age: Group 1 was medial inclination C₁ pedicle screw and C₂ pedicle screws (C₁MPS‐C₂PS) and Group 2 was lateral inclination C₁ pedicle screw and C₂ pedicle screws (C₁LPS‐C₂PS). The ROM of the fixation scenarios were recorded. Thereafter, all the specimens with fixation constructs were tested for 1,000 cycles of axial rotation and tensile loading to failure was carried out collinearly to the longitudinal axis of all the screws, the data were documented as screw pullout strength (SPS) in newtons. All the recorded data subjected to quantitative analysis.

Results

The ROM of specimens was increased significantly in destabilized condition and significantly reduced in fixation condition compared with intact condition. In C₁LPS‐C₂PS groups, the C₁‐C₂ cervical segment showed 3.96° ±1.21° and 3.75° ± 1.33° in flexion and extension direction, 2.85° ± 0.91° and 2.96° ± 0.71° in right and left lateral bending, 2.20° ± 0.43° and 2.15° ± 0.40° in right and left axial rotation. In C₁MPS‐C₂PS groups, it showed 4.24° ±1.31° and 3.98° ± 1.21° in flexion and extension direction, 2.76° ± 1.10° and 3.23° ± 0.62° in right and left lateral bending, 2.20° ± 0.46° and 2.21° ± 0.42° in right and left axial rotation. There was no statistically significant difference on ROM and screw pullout strengths (764.29 ± 129.00 N vs 714.55 ± 164.63 N) between the two groups. However, there was one specimen in the C₁MPS‐C₂PS group showing rupture the inferior wall of the left screw trajectory owing to the relatively thin posterior arch of the atlas, the screw pullout strength was significantly reduced (left pullout strength value: 430.5 N, right pullout strength value: 748.4 N). Therefore, in the case of the thin posterior arch of the atlas, the C₁LPS‐C₂PS group had strong long‐term biomechanics.

Conclusion

The lateral inclination C₁ pedicle screw can achieve the same biomechanical strength as the traditional atlas pedicle screw. However, for the case where the posterior arch of the atlas is relatively thin, a lateral inclination C₁ pedicle screw is more suitable.

Computed Tomography-Based Feasibility Study of C1 Posterior Arch Crisscrossing Screw Fixation

Study Design

Retrospective radiographic analysis.

Purpose

Posterior fixation of C1 using screws is the most popular technique among the various methods for C1 stabilization, but it places the surrounding neurovascular structures at risk. Approximately 20% of the population has an anomalous groove for the vertebral artery; therefore, salvage methods are necessary. Therefore, we analyzed the feasibility of a newer C1 posterior arch crisscrossing screw fixation technique and studied its feasibility in the Indian population on the basis of the anatomy of the C1 posterior arch.

Overview of Literature

Multiple techniques have been described for C1–C2 fixation, such as wiring techniques, interlaminar clamps, transarticular screws, screw-plate/screw-rod system fixation, and hook-screw system fixation techniques, to provide rigid C1–C2 stability. However, although C1 fixation has evolved with time, it is not complication-free.

Methods

A 100 computed tomography (CT) scans of cervical spines with 1 mm slice thickness in the axial and sagittal sections obtained were randomly selected for the evaluation. Atlantoaxial anomalies due to trauma, deformities, infections, and tumors were excluded. All the images were measured for height of the posterior tubercle, width of the posterior arch, and length of the screw, and the screw projection angle was calculated. Demographic data were collected for all the subjects.

Results

Out of the 88 CT scans analyzed, the mean height of the posterior tubercle was 7.4 mm, wherein 84.09% exceeded 7 mm, and the width of the posterior tubercle was 5.4 mm, wherein 88.6% (n=78) had posterior arch width >3.5 mm. A total of 13.6% (n=12) vertebrae were not suitable for screw placement, whereas 75% (n=66) vertebrae could accommodate 3.5×15 mm or longer screws. The screw projection angles ranged from 11.2° to 35° on the right and from 15.6° to 38.2° on the left.

Conclusions

C1 posterior arch screw fixation is a feasible and safe method because it poses little risk of injury to the surrounding neurovascular structures.

Pedicle screws with a thin hydroxyapatite coating for improving fixation at the bone-implant interface in the osteoporotic spine: experimental study in a porcine model

OBJECTIVE

Instrumentation failure caused by the loosening of pedicle screws (PSs) in patients with osteoporosis is a serious problem after spinal surgery. The addition of a thin hydroxyapatite (HA) surface coating applied by using a sputtering process was reported recently to be a promising method for providing bone conduction around an implant without a significant risk of coating-layer breakage. In this study, the authors evaluated the biomechanical and histological features of the bone-implant interface (BII) of PSs with a thin HA coating in an in vivo porcine osteoporotic spine model.

METHODS

Three types of PSs (untreated/standard [STPS], sandblasted [BLPS], and HA-coated [HAPS] PSs) were implanted into the thoracic and lumbar spine (T9–L6) of 8 mature Clawn miniature pigs (6 ovariectomized [osteoporosis group] and 2 sham-operated [control group] pigs). The spines were harvested from the osteoporosis group at 0, 2, 4, 8, 12, or 24 weeks after PS placement and from the control group at 0 or 24 weeks. Their bone mineral density (BMD) was measured by peripheral quantitative CT. Histological evaluation of the BIIs was conducted by performing bone volume/tissue volume and bone surface/implant surface measurements. The strength of the BII was evaluated with extraction torque testing.

RESULTS

The BMD decreased significantly in the osteoporosis group (p < 0.01). HAPSs exhibited the greatest mean extraction peak torque at 8 weeks, and HAPSs and BLPSs exhibited significantly greater mean torque than the STPSs at 12 weeks (p < 0.05). The bone surface/implant surface ratio was significantly higher for HAPSs than for STPSs after 2 weeks (p < 0.05), and bonding between bone and the implant surface was maintained until 24 weeks with no detachment of the coating layer. In contrast, the bone volume/tissue volume ratio was significantly higher for HAPSs than for BLPSs or STPSs only at 4 weeks.

CONCLUSIONS

Using PSs with a thin HA coating applied using a sputtering process strengthens bonding at the BII, which might improve early implant fixation after spinal surgery for osteoporosis. However, the absence of increased bone mass around the screw remains a concern; prescribing osteoporosis treatment to improve bone quality might be necessary to prevent fractures around the screws.

Pseudarthrosis in adult and pediatric spinal deformity surgery: a systematic review of the literature and meta-analysis of incidence, characteristics, and risk factors

We conducted a systematic review with meta-analysis and qualitative synthesis. This study aims to characterize pseudarthrosis after long-segment fusion in spinal deformity by identifying incidence rates by etiology, risk factors for its development, and common features. Pseudarthrosis can be a painful and debilitating complication of spinal fusion that may require reoperation. It is poorly characterized in the setting of spinal deformity. The MEDLINE, EMBASE, and Cochrane databases were searched for clinical research including spinal deformity patients treated with long-segment fusions reporting pseudarthrosis as a complication. Meta-analysis was performed on etiologic subsets of the studies to calculate incidence rates for pseudarthrosis. Qualitative synthesis was performed to identify characteristics of and risk factors for pseudarthrosis. The review found 162 articles reporting outcomes for 16,938 patients which met inclusion criteria. In general, the included studies were of medium to low quality according to recommended reporting standards and study design. Meta-analysis calculated an incidence of 1.4% (95% CI 0.9-1.8%) for pseudarthrosis in adolescent idiopathic scoliosis, 2.2% (95% CI 1.3-3.2%) in neuromuscular scoliosis, and 6.3% (95% CI 4.3-8.2%) in adult spinal deformity. Risk factors for pseudarthrosis include age over 55, construct length greater than 12 segments, smoking, thoracolumbar kyphosis greater than 20°, and fusion to the sacrum. Choice of graft material, pre-operative coronal alignment, post-operative analgesics, and sex have no significant impact on fusion rates. Older patients with greater deformity requiring more extensive instrumentation are at higher risk for pseudarthrosis. Overall incidence of pseudarthrosis requiring reoperation is low in adult populations and very low in adolescent populations.

An in Vitro Biomechanical Model of Differing Pedicle Screw Configurations for Long Construct Segmental Thoracic Fixation

BACKGROUND

The optimum pattern of pedicle screw (PS) fixation during long-segment thoracic fixation has not been determined.

OBJECTIVE

To evaluate rod stress and construct stability with minimal, alternating, skipped, and bilateral PS constructs in the iatrogenically destabilized thoracic spine.

METHODS

Eight cadaveric thoracic specimens (T3-T12) were initially tested intact to ±5 Nm using a custom 6 degree-of-freedom spine testing apparatus in flexion-extension (FE), lateral bending (LB), and axial rotation. Specimens were instrumented with T4-T10 bilateral PS, with Ponte osteotomies to introduce instability. Rods were bent to fit the PS and then spines were tested with the minimal, alternating, skipped, and bilateral fixation patterns. Range of motion (ROM) was calculated from T4-T10 and segmentally. In addition, strain gauges fixed to the spinal rods measured rod stress under FE and LB. Results were compared using ANOVA and post hoc Holm Sidak tests.

RESULTS

All fixation patterns provided significant reductions in ROM with respect to the intact spine. In all motion planes, minimal provided the least amount of rigidity, while bilateral provide the greatest; however, no statistically significant differences were detected in FE. In LB and axial rotation, skipped, alternating, and bilateral were all significantly more rigid than minimal (P < .01). Rod strains were greatest under LB and correlated with overall construct ROM, where bilateral had significantly lower strain than the other patterns (P < .05).

CONCLUSION

All constructs effectively decreased thoracic ROM. There was significant improvement in stabilization and decreased rod stress when more fixation points beyond the minimal construct were included.

Biomechanical study of novel unilateral C1 posterior arch screws and C2 laminar screws combined with an ipsilateral crossed C1-C2 pedicle screw-rod fixation for atlantoaxial instability

INTRODUCTION

Current surgical methods to treat atlantoaxial instability pose potential risks to the surrounding blood vessels and nerves of operative approach. Therefore, more secure and highly effective methods are expected. This study sought to assess the biomechanical efficacy of a novel unilateral double screw-rod fixation system by comparing with traditional and emerging fixation methods in cadaveric models.

MATERIALS AND METHODS

Ligamentous cervical spines (C0-C7) from ten fresh cadaveric specimens were used to complete range of motion (ROM) test in their intact condition (control group), destabilization, and stabilization after different fixations, including unilateral C1-C2 pedicle screws (PS) with a screw-rod system (Group A), bilateral C1-C2 PS with screw-rod systems (Group B), unilateral C1 posterior arch screws (PAS) and C2 laminar screws (LS) combined with an ipsilateral paralleled C1-C2 PS-rod (Group C), and unilateral C1 PAS and C2 LS combined with an ipsilateral crossed C1-C2 PS-rod (Group D). After that, pullout strength test was performed between PS and PAS using ten isolated atlas vertebras.

RESULTS

All fixation groups reduced flexibility in all directions compared with both control group and destabilization group. Furthermore, comparisons among different fixation groups showed that bilateral C1-C2 PS-rod (Group B), unilateral C1 PAS + C2 LS combined with an ipsilateral paralleled C1-C2 PS-rod (Group C) and unilateral C1 PAS + C2 LS combined with an ipsilateral crossed C1-C2 PS-rod (Group D) could provide a better stability, respectively, in all directions than unilateral C1-C2 PS-rod (Group A). However, no statistical significance was observed among Groups B, C, and D. Data from pullout strength test showed that both C1 PS (585 ± 53 N) and PAS (463 ± 49 N) could provide high fixed strength, although PS was better (P = 0.009).

CONCLUSION

The surgical technique of unilateral C1 PAS + C2 LS combined with a ipsilateral crossed C1-C2 PS-rod fixation could provide a better stability than the traditional unilateral PS-rod fixation and a same stability as bilateral PS-rod fixation, but with less risk of neurovascular injury. Therefore, this new technique may provide novel insight for an alternative of atlantoaxial instability treatment.

Unilateral C-1 posterior arch screws and C-2 laminar screws combined with a 1-side C1–2 pedicle screw system as salvage fixation for atlantoaxial instability

OBJECT

Atlantoaxial instability often requires surgery, and the current methods for fixation pose some risk to vascular and neurological tissues. Thus, new effective and safer methods are needed for salvage operations. This study sought to assess unilateral C-1 posterior arch screws (PASs) and C-2 laminar screws (LSs) combined with 1-side C1–2 pedicle screws (PSs) for posterior C1–2 fixation using biomechanical testing with bilateral C1–2 PSs in a cadaveric model.

METHODS

Six fresh ligamentous human cervical spines were evaluated for their biomechanics. The cadaveric specimens were tested in their intact condition, stabilization after injury, and after injury at 1.5 Nm of pure moment in 6 directions. The 3 groups tested were bilateral C1–2 PSs (Group A); left side C1–2 PSs with an ipsilateral C-1 PAS + C-2 laminar screw (Group B); and left side C1–2 PSs with a contralateral C-1 PAS + C-2 LS (Group C). During the testing, angular motion was measured using a motion capture platform. Data were recorded, and statistical analyses were performed.

RESULTS

Biomechanical testing showed that there was no significant difference among the stabilities of these fixation systems in flexion-extension and rotation control. In left lateral bending, the bilateral C1–2 PS group decreased flexibility by 71.9% compared with the intact condition, the unilateral C1–2 PS and ipsilateral PAS+LS group decreased flexibility by 77.6%, and the unilateral C1–2 PS and contralateral PAS+LS group by 70.0%. Each method significantly decreased C1–2 movements in right lateral bending compared with the intact condition, and the bilateral C1–2 PS system was more stable than the C1–2 PS and contralateral PAS+LS system (p = 0.036).

CONCLUSIONS

A unilateral C-1 PAS + C-2 LS combined with 1-side C-1 PSs provided the same acute stability as the PS, and no statistically significant difference in acute stability was found between the 2 screw techniques. These methods may constitute an alternative method for posterior atlantoaxial fixation.

Visualizing the stress distribution within vascular tissues using intravascular ultrasound elastography: a preliminary investigation

A methodology for computing the stress distribution of vascular tissue using finite element-based, intravascular ultrasound (IVUS) reconstruction elastography is described. This information could help cardiologists detect life-threatening atherosclerotic plaques and predict their propensity to rupture. The calculation of vessel stresses requires the measurement of strain from the ultrasound images, a calibrating pressure measurement and additional model assumptions. In this work, we conducted simulation studies to investigate the effect of varying the model assumptions, specifically Poisson's ratio and the outer boundary conditions, on the resulting stress fields. In both simulation and phantom studies, we created vessel geometries with two fibrous cap thicknesses to determine if we could detect a difference in peak stress (spatially) between the two. The results revealed that (i) Poisson's ratios had negligible impact on the accuracy of stress elastograms, (ii) the outer boundary condition assumption had the greatest effect on the resulting modulus and stress distributions and (iii) in simulation and in phantom experiments, our stress imaging technique was able to detect an increased peak stress for the vessel geometry with the smaller cap thickness. This work is a first step toward understanding and creating a robust stress measurement technique for evaluating atherosclerotic plaques using IVUS elastography.

C1 posterior arch crossing screw fixation for atlantoaxial joint instability

STUDY DESIGN

Anatomical measurements and in vitro biomechanical testing were performed to evaluate a new method for posterior C1 fixation.

OBJECTIVE

This study sought to assess C1 posterior arch crossing screw fixation for posterior C1-C2 fixation, using anatomical measurements and biomechanical testing with traditional C1 pedicle screws (PS) in a cadaveric model.

SUMMARY OF BACKGROUND DATA

Atlantoaxial instability often requires surgery, and the current methods for atlas fixation incur some risk to the vascular and neurological tissues. Thus, new, effective, and safe methods are needed for salvage operations.

METHODS

Morphometric analysis of the C1 posterior arch was performed using 3-dimensional computed tomography. Six fresh ligamentous human cervical spines (C0-C4) were evaluated for their biomechanics. The specimens were tested in their intact condition and after stabilization (C1-C2 PS, C1 posterior arch screws [PAS] with C2 PS) and injury due to 1.5 N·m of pure moment in flexion, extension, lateral bending, and axial rotation. During testing, 3-dimensional angular motion was measured with a motion capture platform (Vicon Nexus). Data for all scenarios were recorded, and statistical analysis was performed.

RESULTS

Anatomical assessment indicated that 91.51% of C1 posterior tubercles exceeded 7 mm in thickness, 93.40% had a width of the posterior arch of greater than 3.5 mm, and 65.57% had a unilateral screw length of greater than 15 mm, indicating that the posterior arch fixation could be achieved by two 3.5 × 15-mm screws placed in a crossed manner. Twenty-two cases (11%) were not suitable for crossing screw placement because the posterior arch was flat and the entry point was present on the same side. Biomechanical testing showed that the PS and PAS rod-screw systems significantly reduced flexibility in flexion, extension, and rotation compared with the intact position. For lateral bending, there was a trend for the C1 PS and PAS systems toward decreased flexibility in comparison with the intact position. At the same time, C1 PAS decreased C1-C2 movement by 33.0% in left bending (P = 0.171) and 24.4% in right bending (P = 0.095); however, no significant difference was observed for left bending with C1 PAS compared with C1 PS, and the C1 PS and PAS systems significantly reduced the flexibility more than destabilization.

CONCLUSION

Crossing screw fixation of the C1 posterior arch is straightforward and imposes little risk of injury to the neural and vascular structures as long as the implants remain intraosseous. According to the results of our anatomical and biomechanical study, C1 posterior arch crossing screw fixation may constitute an alternative method for posterior atlantoaxial fixation.

LEVEL OF EVIDENCE

3.

Intraoperative 3-dimensional imaging (O-arm) for assessment of pedicle screw position: Does it prevent unacceptable screw placement?

Background

Pedicle screws are biomechanically superior over other spinal fixation devices. When improperly positioned, they lose this advantage and put adjacent structures at risk. Accurate placement is therefore critical. Postoperative computed tomography (CT) scans are the imaging gold standard and have shown malposition rates ranging from 2% to 41%. The O-arm (Medtronic Navigation, Louisville, Colorado) is an intraoperative CT scanner that may allow intervention for malpositioned screws while patients are still in the operating room. However, this has not yet been shown in clinical studies. The primary objective of this study was to assess the usefulness of the O-arm for evaluating pedicle screw position by answering the following question: What is the rate of intraoperative pedicle screw revision brought about by O-arm imaging information? A secondary question was also addressed: What is the rate of unacceptable thoracic and lumbar pedicle screw placement as assessed by intraoperative O-arm imaging?

Methods

This is a case series of consecutive patients who have undergone spine surgery for which an intraoperative 3-dimensional (3D) CT scan was used to assess pedicle screw position. The study comprised 602 pedicle screws (235 thoracic and 367 lumbar/sacral) placed in 76 patients, and intraoperative 3D (O-arm) imaging was obtained to assess screw position. Action taken at the time of surgery based on imaging information was noted. An independent review of all scans was also conducted, and all screws were graded as either optimal (no breach), acceptable (breach ≤2 mm), or unacceptable (breach >2 mm). The rate of pedicle screw revision, as detected by intraoperative 3D CT scan, was determined.

Results

On the basis of 3D imaging information, 17 of 602 screws (2.8%) in 14 of 76 cases (18.4%) were revised at the time of surgery. On independent review of multiplanar images, 11 screws (1.8%) were found to be unacceptable, 32 (5.3%) were acceptable, and 559 (92.9%) were optimal. All unacceptable screws were revised to an optimal or acceptable position, and an additional 6 acceptable screws were revised to an optimal position. Thus, by the end of the cases, none of the 602 pedicle screws in the 76 surgical procedures was in an unacceptable position.

Conclusion

The new-generation intraoperative 3D imaging system (O-arm) is a useful tool that allows more accurate assessment of pedicle screw position than plain radiographs or fluoroscopy alone. It prompted intraoperative repositioning of 2.8% of pedicle screws in our series. Most importantly, it allowed identification and revision of all unacceptably placed pedicle screws without the need for reoperation.

Pediatric pedicle screws: comparative effectiveness and safety: a systematic literature review from the Scoliosis Research Society and the Pediatric Orthopaedic Society of North America task force

BACKGROUND

Pedicle screws are widely used in spinal surgery. There is extensive published literature concerning the use of pedicle screw instrumentation for spinal surgery in adults. Now there is a trend to use pedicle screws in pediatric patients, including the very young. A systematic review of the current English-language literature on the use of pedicle screw instrumentation in the pediatric age group was performed to specifically determine (1) the pedicle screw placement accuracy in patients with spine deformity and (2) the effect size of all-pedicle screw constructs compared with other methods of spinal instrumentation in terms of the percentage of scoliosis correction.

METHODS

English-language studies of pedicle screw use in pediatric patients (defined as those younger than eighteen years of age) were included. Descriptive statistics synthesized the accuracy of pedicle screw placement. Accuracy rates were compared between pediatric and adult patients. The effect of pedicle screw instrumentation on scoliosis correction was calculated with use of Cobb angle measurements.

RESULTS

On the basis of the literature search, 1181 articles were screened, 320 abstracts were examined, and ninety full-text articles representing 5761 patients were reviewed in detail. Seventeen studies met the inclusion criteria for the analysis of pedicle screw placement accuracy. A total of 13,536 pedicle screws were placed in 1353 pediatric patients. The overall placement accuracy rate in pediatric patients was 94.9%, which was higher than the rate of 91.5% reported for adults. The weighted, geometric, and 5% trimmed mean accuracy rates of pedicle screw placement were 91.9%, 88.5%, and 89.1%, respectively (standard deviation = 10%; interquartile range = 10%). Sixteen comparative studies met the inclusion criteria for the analysis of the effect of pedicle screw instrumentation on scoliosis correction. Pedicle screw constructs had a significantly larger percentage of Cobb angle correction compared with hooks (Cohen's d = 1.14) and hybrid constructs (Cohen's d = 0.49).

CONCLUSIONS

The accuracy of pedicle screw placement in the pediatric spine exceeds the accuracy rate reported in adults. Pedicle screw instrumentation constructs are significantly more effective for scoliosis correction, as determined on the basis of Cobb angle measurements, than are hook constructs and hybrid constructs.

Montagens curtas com instrumentação do nível fracturado e moldagem in situ no tratamento das fracturas tipo "burst" da coluna toracolombar

OBJECTIVO: Avaliação dos resultados clínicos e radiográficos do tratamento cirúrgico de fracturas tipo burst da coluna toracolombar por fixação posterior curta, com instrumentação do nível fracturado e moldagem in situ. MÉTODOS: Entre Novembro de 2007 e Janeiro de 2009, foram seleccionados pacientes que sofreram fractura da coluna toracolombar tipo burst a um nível, com presença de instabilidade neurológica ou mecânica, submetidos a fixação posterior curta com instrumentação do nível fracturado e moldagem in situ. Foi realizada avaliação radiográfica e clínica no pré-operatório, no pós-operatório imediato e, pelo menos, um ano após a cirurgia. RESULTADOS: Foram incluídos 12 pacientes com uma média de idades de 39,1 anos. O seguimento médio foi de 14,5 meses. Um paciente abandonou a consulta aos dois meses, e dois não compareceram à consulta de avaliação clínica. Dois pacientes apresentavam défices neurológicos à entrada (Frankel B). Obtivemos uma melhoria da angulação vertebral de 14,2º, da deformidade cifótica de 11,2º e 27,2% de recuperação da altura vertebral anterior. Ao tempo de seguimento final, verificaram-se perdas de 2,7º, 3,8º e 6,1%, respectivamente. Registou-se um Oswestry Disability Index (ODI) médio de 6,2 e uma Visual Analogic Scale (VAS) de 1,6. Os dois pacientes com lesões neurológicas melhoraram para um nível D de Frankel. Não se observou qualquer caso de desmontagem ou falência do material. CONCLUSÕES: A instrumentação do nível da fractura aumenta a rigidez da montagem, protegendo a vértebra fracturada de cargas anteriores, garantindo um ponto de fixação adicional que permite uma melhor correcção por moldagem in situ. Os resultados obtidos, radiográficos e clínicos, são bons e mantêm-se ao longo do tempo.

Biomechanical evaluation of an atlantoaxial lateral mass fusion cage with C1-C2 pedicle fixation

STUDY DESIGN

A biomechanical testing protocol was used to evaluate atlantoaxial fixation techniques in a human cadaveric model.

OBJECTIVE

To compare in vitro biomechanics of atlantoaxial lateral mass fusion cage combined with C1-C2 pedicle screw technique with those of C1-C2 pedicle screw technique alone and C1-C2 transarticular screws combined with Gallie wires.

SUMMARY OF BACKGROUND DATA

An atlantoaxial lateral mass fusion cage was designed, knowing that the cage, when rigidly combined with C1-C2 pedicle screws, could offer other fusion spots for atlantoaxial stabilization in cases when the posterior arch of the atlas is absent or removed for decompression and a Gallie fixation is impossible. No comparative in vitro biomechanical test has been conducted previously to evaluate the feasibility of this method.

METHODS

Anatomic measurements of the atlantoaxial lateral masses were taken using computed tomography in normal human subjects. Six fresh-frozen human cadaveric cervical spines (C0-C4) were used in the biomechanical study. Specimens were tested in their intact condition, after destabilization via transverse-alar-apical ligament disruption, and after implantation of 3 fixation constructs: (1) transarticular screws combined with Gallie wires, (2) C1-C2 pedicle screws, and (3) atlantoaxial lateral mass fusion cage combined with C1-C2 pedicle screws. Pure moment loading up to 1.5 Nm in flexion/extension, right-left lateral bending, and right-left axial rotation was applied to the occiput, and relative intervertebral rotations were determined using stereophotogrammetry. Range of motion for the intact, destabilized, and 3 fixation scenarios were determined.

RESULTS

The anatomic data indicated that feasible cage design were in 3 sizes: 11/8, 12/9, and 13/10 mm for length/width, and 3.5, 4, and 4.5 mm for height. The biomechanical data indicated that transverse-alar-apical ligament disruption significantly increased C1-C2 motion for all directions. All the 3 fixation techniques significantly reduced motion compared with the intact and destabilized cases. There were no statistically significant differences among the 3 fixation techniques.

CONCLUSION

The biomechanical study indicated that, contrary to expectation, addition of a cage did not increase the stability compared with C1-C2 pedicle screw alone. However, the C1 + C2 + Cage technique may be a viable alternative for atlantoaxial stabilization when the posterior arch of the atlas is absent or removed for decompression and a Gallie fixation is impossible.

Translaminar facet joint screws to enhance segmental fusion of the lumbar spine

From 1984 to 1988, 70 consecutive lumbar and lumbosacral spine fusions enhanced with translaminar facet joint screws were performed for segmental degenerative disease. Twenty patients had partial decompression of central stenosis, 15 concomitant discectomy, and 19 lateral nerve root decompression. The mean time to fusion was 4.5 months (range 2-7 months). At followup (average 45 months; range 24-74 months) 98.5% were judged to have solid fusion. Satisfactory results were observed in 84% of cases, 91% without previous surgery and 75% after previous discectomy. Supplementation of posterolateral fusion by translaminar facet screws significantly improved time to fusion, fusion rate, and clinical outcomes with no significant increased complications.

Functional outcome of computer-assisted spinal pedicle screw placement: a systematic review and meta-analysis of 23 studies including 5,992 pedicle screws

A number of studies have shown increased accuracy of pedicle screw placement in spine with the help of computer-assisted navigation. The literature is lacking in regard to functional benefit derived from this technique. The aim of this systematic review was to look at the functional outcomes following computer-assisted pedicle screw placement in spine. A 'Dialog Datastar' search was used using optimized search strategy covering the period from 1950 to July 2009; 23 papers were finally included which met our inclusion criteria. We report on a total of 1,288 patients with 5,992 pedicle screws. The comparison of neurological complications in two groups demonstrated an odds ratio of 0.25 (95% CI 0.06, 1.14) in favour of using navigation for pedicle screw insertion (p = 0.07). Comparative trials demonstrated a significant advantage in terms of accuracy of navigation over conventional pedicle screw insertion with a relative risk of 1.12 (95% CI 1.09, 1.15) (p < 0.00001). Navigation does not show statistically significant benefit in reducing neurological complications and there was insufficient data in the literature to infer a conclusion in terms of fusion rate, pain relief and health outcome scores.

An analysis of decision making and treatment in thoracolumbar metastases

STUDY DESIGN

Systematic review of literature.

OBJECTIVE

To determine whether surgical approach and technique to be used in thoracolumbar metastases is influenced by anatomic region of the spine.

SUMMARY OF BACKGROUND DATA

There is a significant debate about the best surgical approach for the treatment of thoracolumbar metastasis.

METHODS

Two research questions below were determined through a consensus among a panel of spine experts. A systematic review of literature was conducted using Pubmed. The search terms included "spin*" and "metasta*." This was to include the terms, "metastatic," "metastasis," "metastases," "spinal," and "spine." 1. In the T2 to T5 region, what is the impact of different surgical approaches (anterior, posterior, combined anteroposterior [AP]) on local recurrence, adverse events, pain alleviation, and neurologic recovery? 2. Within the thoracolumbar spine (T6-T10; T11-L2; and L3-L5) what is the impact of different surgical approaches on outcomes as per question one. The results of the systematic review were discussed with spine oncology experts through a modified Delphi technique to arrive at treatment recommendations.

RESULTS

From the search terms used 5176 abstracts were found. Based on the review of these abstracts, 161 were deemed acceptable. These abstracts were reviewed according to an inclusion and exclusion criteria, leaving 60 articles. These 60 articles were reviewed in detail leaving 32 articles for inclusion. There was no level I study. There was 1 level II study, 5 level III studies, and 26 level IV studies. Most of these studies selected their approach by tumor topography. The quality of evidence was very low.

CONCLUSION

There is very low quality evidence to support the superiority of one approach over another. There is a strong recommendation for posterior or posterior-lateral approach from T2 through T5. For the T6-L5 regions of the spine we recommend either anterior, posterior, or combined anterior and posterior surgery depending on the clinical presentation, surgeon and patient preference.

Comparison of surgical outcomes in thoracolumbar fractures operated with posterior constructs having varying fixation length with selective anterior fusion

PURPOSE

Surgical treatment in the case of thoracolumbar burst fractures is very controversial. Posterior instrumentation is most frequently used, however, but the number of levels to be instrumented still remains a matter of debate.

MATERIALS AND METHODS

A total of 94 patients who had a single burst fracture between T11 and L2 were selected and were managed using posterior instrumentation with anterior fusion when necessary. They were divided into three groups as follows; Group I (n = 28) included patients who were operated by intermediate segment fixation, Group II (n = 32) included patients operated by long segment fixation, and Group III (n = 34) included those operated by intermediate segment fixation with a pair of additional screws in the fractured vertebra. The mean follow-up period was twenty one months. The outcomes were analyzed in terms of kyphosis angle (KA), regional kyphosis angle (RA), sagittal index (SI), anterior height compression rate, Frankel classification, and Oswestry Disability Index questionnaire.

RESULTS

In Groups II and III, the correction values of KA, RA, and SI were much better than in Group I. At the final follow up, the correction values of KA (6.3 and 12.1, respectively) and SI (6.2 and 12.0, respectively) were in Groups II and III found to be better in the latter.

CONCLUSION

The intermediate segment fixation with an additional pair of screws at the fracture level vertebra gives results that are comparable or even better than long segment fixation and gives an advantage of preserving an extra mobile segment.

Pedicle screw surface coatings improve fixation in nonfusion spinal constructs

STUDY DESIGN

Biomechanical and histologic analysis.

OBJECTIVE

To compare the strength of the bone-screw interface of standard uncoated pedicle screws with screws treated with hydroxyapatite (HA), titanium plasma spray (TPS), and a composite HA-TPS coating.

SUMMARY OF BACKGROUND DATA

Transpedicular screw fixation has become the gold standard in the treatment of various thoracolumbar spinal conditions. Pedicle screw loosening, however, has been reported, especially in mechanically demanding constructs or in vertebrae with low bone mineral density.

METHODS

Six mature porcine were instrumented with 4 types of titanium monoaxial pedicle screws (uncoated, HA-only coated, TPS-only coated, and HA-TPS composite coated) in a systematically varied, single-blinded fashion. After a 3-month survival period, the spines were harvested en-bloc and "time zero" control screws were instrumented in adjacent vertebrae. Screw placement and bone mineral density were evaluated with a postharvest computed tomography, and the strength of the tissue-implant interface was evaluated with a torsional screw extraction analysis (60 screws) and a nondecalcified histologic analysis (16 screws).

RESULTS

At 3 months postoperative, peak torque increased for all 3 types of coated screws (increased fixation) and decreased significantly for the uncoated screws (P < 0.001). Although 3-month peak torque was not statistically different between the 3 screw coatings, 4 of 10 TPS-only coated screws had a peak torque that was nearly 0 (<0.1 N m) versus only 1 of 10 HA-only screws and 0 of 10 HA-TPS composite screws. Histologic analysis confirmed the biomechanical findings with improved osseointegration in the HA-only and HA-TPS composite screws.

CONCLUSION

Pedicle screw coatings that promote mechanical interlocking, TPS, or direct osteoblast bonding(HA) increased screw fixation in this nonfusion model. More non-HA coated screws, however, were thought to be "loose" with a nearly zero peak extraction torque and fibrous encapsulation. Increased osseointegration with HA may result in a decreased incidence of screw loosening and improved outcomes of transpedicular spinal instrumentation in nonfusion procedures.

Transpedicular plate fixator as effective system of spine stabilisation: biomechanical characteristics

INTRODUCTION

Zespol fixator, which was created in Poland by Ramatowski and Granowski, has an angular stable connection of screws and plate. These properties of this plate fixator, that is effective and not an expensive system of osteosynthesis of shaft of long bone widely used in Poland, impelled us to adapt it as a transpedicular plate fixator of spine.

AIM

The aim of our in vitro study was to measure loads acting on spine stabilized by transpedicular plate fixator and to determine if its stability is comparable with uninjured spine. We also hypothesized that the spine stability with examined fixator had similar properties as spine fixators constructed with screws and rods.

MATERIALS AND METHODS

We tested its biomechanical properties and compared it with a CD device by using specimens of four human spines. Each spine with damage induced in laboratory conditions was stabilised by one of those stabilisers in one (L4-L5) or two (Th12-L2) motion segments and subsequently were subject to load. The spines without and with one of transpedicular stabilization were subject to an unsymmetrical shift of +3/-4 mm for extension-compression and symmetrical shift for bending, in the frontal plane (+0.14/-0.14 rad) and the sagittal plane (+0.11/-0.11 rad), respectively.

RESULTS

Loads during extension-compression and bending in the sagittal plane were similar to the uninjured spine for short stabilization by using both stabilizers and amounted to 92.3 and 98.26%, respectively, of the load range sums of healthy spines. For long stabilization these loads amounted to 93.2 and 84.4%, respectively. Only following short and long stabilization for both devices in case of bending in the frontal plane the increase in loads up to 144.2 and 163.3% of the range sums of uninjured spines was achieved.

CONCLUSION

It corroborates the fact that the application of the modified Zespol device for spine stabilisation provides the possibility of restoring its load transfer capacity similar to that in the healthy spine and comparable with the CD device.

Two column lesions in the thoracolumbar junction: anterior, posterior or combined approach? A comparative biomechanical in vitro investigation

There are various surgical techniques for the treatment of spinal fractures in the thoracolumbar region. Several implants have been developed for anterior or posterior instrumentation. Optimal treatment of unstable thoracolumbar osseous and ligamentous injuries remains controversial. To compare the stabilizing effects of an antero-lateral, thoracoscopically implantable plate system (macsTL, Aesculap, Germany) with the stability provided by a fixateur interne (SOCON, Aesculap, Germany), this in vitro investigation examined six human bisegmental (T12-L2) spinal units. Specimens were tested intact, and with simulation of osseous lesions in the anterior and ligamentous lesions in the posterior column (combined A/B-fracture). While loaded in the main anatomical planes such as flexion/extension, left and right lateral bending and left and right axial rotation with a bending moment of 7.5 Nm in a special testing jigs, motion analysis was performed. Quantitative interpretation of the stabilizing effect was achieved using a contactless three-dimensional motion analysis system. Each specimen was tested in four different scenarios: the first step measured movements of intact spinal segments. For the second step, specimens underwent simulation of combined A/B-fracture provided with bisegmental (T12/L2) antero-lateral fixation and bone strut graft from the iliac crest. For the third step, segments were additionally stabilized by the fixateur interne. The last measurement (fourth step) was performed after removing the anterior instrumentation. Range of motion (ROM) values were compared and statistically evaluated. Compared to the intact specimens the anterior instrumentation of the combined lesion, simulated A/B-fracture, leads to a stabilizing effect in flexion/extension and lateral bending. In contrast to these findings the torsional instability increased for the upper segment and bisegmentally. A maximum rigidity, beyond intact values, was registered for each anatomical plane with the combined instrumentation: antero-lateral and fixateur interne. After removing the anterior screw plate system maximum movements, in all segments for flexion/extension and lateral bending, bisegmentally and for the upper segment in axial rotation, were less than ROM values measured with the anterior system only. With respect to these findings a combined ventro-dorsal stabilization procedure should be considered for ligamentous disruptions of the posterior column in combination with A-fractures in the thoracolumbar junction.

Pedicle screw fixation of the thoracic spine: an in vitro biomechanical study on different configurations

STUDY DESIGN

An in vitro biomechanical study of different pedicle screw configuration usage on the thoracic spine using a cadaveric model.

OBJECTIVES

To investigate the degree of motion afforded different pedicle screw configurations in the thoracic spine using a cadaveric model with 2 different degrees of intrinsic stability.

SUMMARY OF BACKGROUND DATA

Recently, thoracic pedicle screws have become an alternative to hook and wire fixation, and have gained popularity. Clinically, pedicle screw use has ranged from application to every segment, to skipping every other level. There exists no clear consensus as to which strategy is most appropriate.

METHODS

The load-displacement behavior of 6 different constructs was determined on 8 fresh frozen cadaver spine specimens (T4-T12). Each construct was evaluated on 2 destabilization models, including minimum destabilization (bilateral facetectomy) and maximum destabilization (facetectomy and annulotomy). Pure moments were applied, and the resultant range of motion for each scenario was determined.

RESULTS

Facetectomy did not significantly destabilize the thoracic spine. Annulotomy and facetectomy created gross instability that rendered testing of this destabilization model impossible. All constructs significantly reduced the range of motion compared to intact or facetectomized specimens (P < or = 0.001). When different constructs were compared to each other, a pattern of continuously increasing stability emerged, with the "maximum" construct being the most stable and "minimum" configuration being the least, with varying degrees of statistical significance.

CONCLUSIONS

Our results suggest that the most important factor for the acute postoperative stability of spinal fixation is the degree of preoperative or iatrogenic destabilization. The minimum amount of pedicle screws provides adequate stability when there is minimal destabilization of the spine. On the other hand, when anterior column release has been performed or instability exists before surgery, segmental pedicle screw fixation may be necessary to achieve adequate stability.

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.

Is a single anterolateral screw-plate fixation sufficient for the treatment of spinal fractures in the thoracolumbar junction? A biomechanical in vitro investigation

Controversy exists about the indications, advantages and disadvantages of various surgical techniques used for anterior interbody fusion of spinal fractures in the thoracolumbar junction. The purpose of this study was to evaluate the stabilizing effect of an anterolateral and thoracoscopically implantable screw-plate system. Six human bisegmental spinal units (T12-L2) were used for the biomechanical in vitro testing procedure. Each specimen was tested in three different scenarios: (1) intact spinal segments vs (2) monosegmental (T12/L1) anterolateral fixation (macsTL, Aesculap, Germany) with an interbody bone strut graft from the iliac crest after both partial corpectomy (L1) and discectomy (T12/L1) vs (3) bisegmental anterolateral instrumentation after extended partial corpectomy (L1), and bisegmental discectomy (T12/L1 and L1/L2). Specimens were loaded with an alternating, nondestructive maximum bending moment of +/-7.5 Nm in six directions: flexion/extension, right and left lateral bending, and right and left axial rotation. Motion analysis was performed by a contact-less three-dimensional optical measuring system. Segmental stiffness of the three different scenarios was evaluated by the relative alteration of the intervertebral angles in the three main anatomical planes. With each stabilization technique, the specimens were more rigid, compared with the intact spine, for flexion/extension (sagittal plane) as well as in left and right lateral bending (frontal plane). In these planes the bisegmental instrumentation compared to the monosegmental case had an even larger stiffening effect on the specimens. In contrast to these findings, axial rotation showed a modest increase of motion after bisegmental instrumentation. To conclude, the immobilization of monosegmental fractures in the thoracolumbar junction can be secured by means of bone grafting and the implant used in this study for all three anatomical planes. After bisegmental anterolateral stabilization a sufficient reduction of the movements was registered for flexion/extension and lateral bending. However, the observed slight increase of the range of motion in the transversal plane may lead to loosening of the implant before union. Therefore, the use of an additional dorsal fixation device should be considered.

Biomechanical testing of posterior atlantoaxial fixation techniques

STUDY DESIGN

An in vitro biomechanical study of C1-C2 posterior fusion techniques was conducted using a cadaveric model.

OBJECTIVE

To investigate and compare the acute stability afforded by a novel rod-based construct that uses direct polyaxial screw fixation to C1 and C2 with contemporary transarticular screw and wire techniques.

SUMMARY AND BACKGROUND DATA

Acute stability of the atlantoaxial complex is required to achieve bony consolidation. Various forms of posterior wiring were the first standardized procedures advocated to achieve C1-C2 fixation, but because of insufficient construct stability, these techniques have been coupled with transarticular screw fixation. Significant technical difficulties, however, including the possibility of neurovascular compromise during implantation are associated with transarticular screw placement. A novel technique that uses direct polyaxial screw fixation to C1 and C2 and bilateral longitudinal rods was developed recently. However, there are no published reports detailing the biomechanical characteristics of this new construct.

METHODS

In this study, 10 fresh-frozen human cadaveric cervical spines with occiput (C0-C4) were used. Osteoligamentous specimens were tested in their intact condition after destabilization via odontoidectomy, and after two different Gallie wiring techniques. Each specimen was assigned to one of the two screw fixation groups. Five specimens were implanted with the polyaxial screw-rod construct and tested. The remaining five specimens were tested after application of bilateral C1-C2 transarticular screws with Gallie wiring (Magerl-Gallie technique). Pure-moment loading, up to 1.5 Nm in flexion and extension, right and left lateral bending and right and left axial rotation, was applied to the occiput, and relative intervertebral rotations were determined using stereophotogrammetry (motion analysis system). Range of motion data for all fixation scenarios were normalized to the destabilized case, and statistical analysis was performed using one-way analysis of variance with Fisher's least significant difference PLSD post hoc test for multiple comparisons.

RESULTS

The data indicate that destabilization via odontoidectomy significantly increased C1-C2 motion. Both screw techniques significantly decreased motion, as compared with both Gallie wiring methods in lateral bending and axial rotation (P < 0.02 for all) and tended toward reduced motion in flexion-extension. There was no statistically significant difference between the two screw techniques.

CONCLUSIONS

The results clearly indicate the screw-rod system's equivalence in reducing relative atlantoaxial motion in a severely destabilized upper cervical spine, as compared with the transarticular screw-wiring construct. These findings mirror the previously reported clinical results attained using this new screw-rod construct. Thus, the decision to use either screw construct should be based on safety considerations rather than acute stability.

Biomechanical evaluation of a bipedicular spinal fixation system: a comparative stiffness test

STUDY DESIGN

This biomechanical study using cadaver thoracic spines evaluated the initial stiffness of two different fixation constructs using a new spinal implant: the bipedicular spinal fixation device (BSF).

OBJECTIVE

To compare the biomechanical stiffness of a new construct using BSF with a regular construct using pedicular and laminar hooks.

SUMMARY OF BACKGROUND DATA

Disadvantages of thoracic posterior implants and developments in in situ rod contouring led to the creation of a new implant for spine deformity surgery that would provide immediate stiffness to preserve spine correction, allow efficient postoperative rehabilitation, and obtain a good fusion rate.

METHODS

Two age-paired groups of six human thoracic spines each (T3-T12) were compared: a regular group whose construct was in accordance with the Cotrel-Dubousset technique and the BSF group. In both groups, the spines were tested intact and then after injury. An injury was induced by transections of interspinous and anterior longitudinal ligaments and anterior discectomies. A three-dimensional ultrasonic measurement device, the Zebris 3D Motion Analyzer, was used to record the motion of the T6 relative to the T8 vertebra under loads, and to determine the ranges of motion (ROMs) between intact spines and the spine construct.

RESULTS

In flexion-extension, the regular construct showed a significantly greater mean of relative ROMs than the BSF construct for principal rotation (88% and 69% respectively, P = 0.015). However, no significant differences were demonstrated in any of the other motions.

CONCLUSION

The BSF construct showed stiffness similar to that of the regular construct, encouraging clinical investigation.

Effects of bone cement volume and distribution on vertebral stiffness after vertebroplasty

STUDY DESIGN

The biomechanical behavior of a single lumbar vertebral body after various surgical treatments with acrylic vertebroplasty was parametrically studied using finite-element analysis.

OBJECTIVES

To provide a theoretical framework for understanding and optimizing the biomechanics of vertebroplasty. Specifically, to investigate the effects of volume and distribution of bone cement on stiffness recovery of the vertebral body.

SUMMARY OF BACKGROUND DATA

Vertebroplasty is a treatment that stabilizes a fractured vertebra by addition of bone cement. However, there is currently no information available on the optimal volume and distribution of the filler material in terms of stiffness recovery of the damaged vertebral body.

METHODS

An experimentally calibrated, anatomically accurate finite-element model of an elderly L1 vertebral body was developed. Damage was simulated in each element based on empirical measurements in response to a uniform compressive load. After virtual vertebroplasty (bone cement filling range of 1-7 cm3) on the damaged model, the resulting compressive stiffness of the vertebral body was computed for various spatial distributions of the filling material and different loading conditions.

RESULTS

Vertebral stiffness recovery after vertebroplasty was strongly influenced by the volume fraction of the implanted cement. Only a small amount of bone cement (14% fill or 3.5 cm3) was necessary to restore stiffness of the damaged vertebral body to the predamaged value. Use of a 30% fill increased stiffness by more than 50% compared with the predamaged value. Whereas the unipedicular distributions exhibited a comparative stiffness to the bipedicular or posterolateral cases, it showed a medial-lateral bending motion ("toggle") toward the untreated side when a uniform compressive pressure load was applied.

CONCLUSION

Only a small amount of bone cement ( approximately 15% volume fraction) is needed to restore stiffness to predamage levels, and greater filling can result in substantial increase in stiffness well beyond the intact level. Such overfilling also renders the system more sensitive to the placement of the cement because asymmetric distributions with large fills can promote single-sided load transfer and thus toggle. These results suggest that large fill volumes may not be the most biomechanically optimal configuration, and an improvement might be achieved by use of lower cement volume with symmetric placement.

Stereotactic navigation for placement of pedicle screws in the thoracic spine

OBJECTIVE

Pedicle screw fixation in the lumbar spine has become the standard of care for various causes of spinal instability. However, because of the smaller size and more complex morphology of the thoracic pedicle, screw placement in the thoracic spine can be extremely challenging. In several published series, cortical violations have been reported in up to 50% of screws placed with standard fluoroscopic techniques. The goal of this study is to evaluate the accuracy of thoracic pedicle screw placement by use of image-guided techniques.

METHODS

During the past 4 years, 266 image-guided thoracic pedicle screws were placed in 65 patients at the University of Michigan Medical Center. Postoperative thin-cut computed tomographic scans were obtained in 52 of these patients who were available to enroll in the study. An impartial neuroradiologist evaluated 224 screws by use of a standardized grading scheme. All levels of the thoracic spine were included in the study.

RESULTS

Chart review revealed no incidence of neurological, cardiovascular, or pulmonary injury. Of the 224 screws reviewed, there were 19 cortical violations (8.5%). Eleven (4.9%) were Grade II (< or =2 mm), and eight (3.6%) were Grade III (>2 mm) violations. Only five screws (2.2%), however, were thought to exhibit unintentional, structurally significant violations. Statistical analysis revealed a significantly higher rate of cortical perforation in the midthoracic spine (T4-T8, 16.7%; T1-T4, 8.8%; and T9-T12, 5.6%).

CONCLUSION

The low rate of cortical perforations (8.5%) and structurally significant violations (2.2%) in this retrospective series compares favorably with previously published results that used anatomic landmarks and intraoperative fluoroscopy. This study provides further evidence that stereotactic placement of pedicle screws can be performed safely and effectively at all levels of the thoracic spine.

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.

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

STUDY DESIGN

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

OBJECTIVES

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

BACKGROUND DATA

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Biomechanical testing sequelae relevant to spinal fusion and instrumentation

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

Changes in cadaveric cancellous vertebral bone strength in relation to time. A biomechanical investigation

STUDY DESIGN

A biomechanical study was conducted during a 3.5-day period to test for changes occurring in pullout strengths of cancellous screws inserted into human cadaveric vertebral bodies.

OBJECTIVES

To quantify, within the testing time of 3.5 days, the possible changes to the mechanical properties of cadaveric vertebral bodies, resulting from structural degradation caused by postmortem, time-dependent, autolytic processes during mechanical testing of implant-bone biomechanics.

SUMMARY OF BACKGROUND DATA

Biomechanical testing of whole spinal implants and analysis of the screw-bone interface of spinal implants is an area of clinical interest that frequently requires the use of cadaveric spine specimens. Changes in vertebral bone properties during the testing period may invalidate experimental results, but no data are available on degradation of bone during the testing period.

METHODS

Anterior oblique cancellous screws were inserted into human vertebral bodies from which the ventral cortex had been removed. The pullout strength was measured at 0, 24, 60, and 84 hours after insertion. The tests were performed on 48 human vertebral bodies, which were stored by freezing to -23 C, thawed for testing, and kept at room temperature during the testing time for as long as 84 hours.

RESULTS

The axial pullout strength showed no statistically significant change during 84 hours (P = 0.15). There were no significant differences attributable to vertebral level from T4 to L4, probably because the ventral cortices had been removed (P = 0.7).

CONCLUSIONS

During 3.5 days, there were no changes in pullout strength of vertebral cancellous bone. In biomechanical studies during a maximum period of 3 days with a small number of cadaveric spines (e.g., four spine specimen) the time-dependent changes in pullout strength play a less significant role than do the interspine differences. Interspine differences should be regarded as an important factor to be considered in the design of biomechanical tests.

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

STUDY DESIGN

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

OBJECTIVES

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

SUMMARY OF BACKGROUND DATA

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Transpedicle instrumentation in the degenerative spine

Controversy continues regarding the use of pedicle instrumentation in fusion surgery on the degenerative lumbar spine. Recently, however, published reports are increasingly demonstrative of the positive effect of pedicle fixation on fusion and outcome success in the management of these disorders. Recent developments in surgical technique and implant design have improved operative risk and implant related complications. However, patient selection and demonstration of the underlying pathophysiology of degenerative lumbar motion segment pain remain 2 of the main challenges to clinical practice. The importance of well designed clinical studies are of paramount consequence in the solution of the existing controversies surrounding surgical treatment of the degenerative lumbar spine. Equally significant is the need to establish general and accepted measures of outcome to assess and compare treatment options. Combining study design and accepted outcome assessment will permit the resolution of the remaining controversies. The future of pedicle instrumentation of the degenerative lumbar spine is of necessity tied to the resolution of these remaining points of controversy-only thereafter can the current rate of advances continue.

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

STUDY DESIGN

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

OBJECTIVES

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

BACKGROUND DATA

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Outcome analysis for adults with spondylolisthesis treated with posterolateral fusion and transpedicular screw fixation

The outcomes of 52 adult patients with symptomatic low-grade spondylolisthesis treated with autologous posterolateral arthrodesis and pedicle screw fixation were retrospectively reviewed. Although a 90% rate of successful fusion was obtained using this technique, only 60% of patients were considered to have good outcomes. Treatment failures consisted mostly of back pain and were not predicted by preoperative symptoms. Compensation claims and smoking had very significant adverse impacts on both employment and pain results despite high fusion rates, particularly in patients under the age of 55 years. Overall, patients who required more than one operation demonstrated poor outcomes compared to those who only needed one. However, patients with at least two prior operations or preoperative pseudoarthrosis fared particularly poorly, whereas those who had undergone only one prior surgery and had no attendant compensation issue reported good results. A trend toward poor outcome was observed in patients with postlaminectomy spondylolisthesis, versus those with isthmic or degenerative etiologies. Gender did not exert an impact on outcome. The authors conclude that autologous posterolateral arthrodesis combined with pedicle screw fixation resulted in a high fusion rate, and contributed to successful outcomes in the treatment of certain subgroups of adults with spondylolisthesis. In the absence of other risk factors, patients may obtain significant benefit from surgery despite older age and a single failed operation. Careful patient selection appears critical in predicting the maximum benefit from this technique.

Variables affecting pedicle screw plate fixation of an unstable L3-L4 defect

Fresh frozen human cadaveric spinal specimens (T8-S1) were subjected to pure flexion extension bending moment and pure axial torque loadings while intervertebral rotations were recorded at the L3-L4, L2-L3, and Ll-L2 discs. A standardized unstable defect was created at the L3-L4 disc, and loading tests were repeated after application of bilateral Steffee plates in 2 configurations: a short plate with 2 pedicle screws (spanning the defect) and a longer plate with 3 pedicle screws (spanning the defect and 1 disc above). Each plating configuration was tested in the unlocked state (nuts compressing the plate down onto the spine) and locked state (nuts above and below the plate tightened against each other to clamp the plate to the screws). Locking the plates to the screws had no effect on any intervertebral rotation at any disc level. Use of a longer plate that also spanned the disc above the defect offered no advantage in controlling flexion extension rotations at the defect site. However, mean torsional rotation at the defect site with the 3-screw plate was approximately 50% of the mean for a 2-screw plate. Extension and torsional rotations at the L2-L3 disc (1 level above the defect site) were unaffected by application of a 2-screw plate; flexion rotation at this level increased slightly after plating. All motions at the L2-L3 disc were reduced (as would be expected) when the 3-screw plate spanned this uninjured disc. Plating the defect had no effect on disc rotations at the L1-L2 disc (2 levels above the fracture site).

Comparison of residual stability in thoracolumbar spine fractures using neutral zone measurements

Because treatment algorithms for spinal injuries depend largely on the clinical assessment of stability after injury, this study both quantified and compared the mechanical stability after three different patterns of injury in the thoracolumbar spine. We created compression fractures, burst fractures, and flexion-distraction injuries in 26 thoracolumbar specimens from human cadavers in order to compare residual stability as a function of type of injury. Spinal stability was evaluated using measurements of the boundaries of the neutral zone, which provide a measure of spinal laxity in various directions of motion. An increase after injury was indicative of greater spinal laxity and hence reduced residual stability. Geometric characteristics (or parameters) of the neutral zone boundaries were used for statistical comparison between the types of injury. Of the three groups, burst fractures retained the least residual stability and compression fractures, the greatest. The angular ranges of motion in the neutral zone for burst fractures demonstrated increases (compared with average values for intact specimens) of 154% in flexion-extension, 134% in lateral bending, and 108% in torsion after injury. The results for flexion-distraction injuries were similar to those for burst fractures in flexion-extension (126%) and torsion (62%); however, more residual stability was retained in lateral bending than was seen for burst fractures (48%). Compression fractures retained the most residual stability, with increases in motion of 40% in flexion-extension, 56% in lateral bending, and 3% in torsion. These findings may be useful in determining the necessity for surgical stabilization of the spine and selection of the appropriate system of fixation.

Early surgery for thoracolumbar spinal cord injury: initial experience from a developing spinal cord injury centre in India

The spinal cord injury centre of Nizam's Institute of Medical Sciences, Andhra Pradesh, India has been functioning now for 8 months and offers its services to the population of 80 million in the state. To date, 92 patients with a spinal cord injury have been treated; 51 had a thoracolumbar spinal injury. This report presents the results of the management of these 51 patients. Preoperatively both CT and MRI were performed and the radiological findings were correlated with outcome. Twenty five had a thoracic and 26 a lumber location. Twenty nine patients underwent surgical treatment (15 thoracic and 14 lumbar) and the others were treated conservatively (10 thoracic and 12 lumbar). All these operations were carried out within 2 weeks following trauma, and methylprednisolone therapy was instituted in those who reached the hospital early. Contraindications for surgery included a delay in admission of more than 3 weeks following trauma, a focus of sepsis, bedsores, a generalised bone disorder such as osteopenia, and medical illnesses. Transpedicular screw-plate fixation was performed in 27 patients, and two patients underwent decompressive laminectomy and interlaminar bone and wire fixation. Delayed spinal decompression was offered to one patient to relieve radiculopathy. Fracture-dislocation spinal injury and those with transection of the spinal cord had the worst outcome, whilst patients with a wedge compression fracture and cord oedema fared better. Operated cases had a shorter hospital stay, and complications of immobilisation were limited. Positive psychological influence of mobilisation and early acclimatisation to the altered style of living with their disability were the most significant outcomes following surgery.

Distraction and compression loads enhance spine torsional stiffness

We have assessed the degree to which compression and distraction forces applied to the spine of the magnitude achievable through instrumentation systems increase torsional stiffness, thereby possibly enhancing the environment for arthrodesis. A functional relationship between spine torsional stiffness and axial compression and distraction forces is described. To establish this relationship whole human thoracolumbar spines as well as individual motion segments were tested in torsion under a variety of axial loading conditions consistent with forces applied clinically. These studies indicate that applying axial loads to the whole thoracolumbar spine through the action of compression or distraction increases the stiffness of the spine in torsion. Compression and distraction forces increased the torsional stiffness of thoracic segments, but only compression forces were found to significantly increase the stiffness of lumbar segments. The soft tissue structures of both the anterior and the posterior columns have the ability to increase torsional stiffness with axial loading.

Transpedicular screw-rod fixation of the lumbar spine: operative technique and outcome in 104 cases

A total of 104 patients underwent transpedicular spinal instrumentation using the Cotrel-Dubousset (71 cases) or the Texas Scottish Rite Hospital (33) screw-rod system. Surgery was performed for lumbar vertebral column instability secondary to fractures (28 cases), spondylolisthesis (29), tumors (four), vertebral osteomyelitis (two), or postoperative causes (41). Pseudoarthrodesis due to failure of a prior fusion was present in 37 cases. The 55 men and 49 women (mean age 47 years, range 18 to 87 years) all presented with severe back pain. Signs or symptoms of neural compression were noted in 96 patients. Surgery consisted of neural decompression, internal fixation, and autogenous iliac bone grafting. Spondylolistheses were fused in situ, without reduction; otherwise, major spinal deformities were corrected. A total of 516 pedicle screws were placed. The mean extent of fusion was 2.7 motion segments (range one to six motion segments). A 96% fusion rate was obtained with a mean follow-up period of 20 months. There were no operative deaths. Major complications included one spinal epidural hematoma, three isolated nerve root deficits (two transient, one permanent), and three wound infections (two deep, one superficial). Instrument failure eventually developed in 18 patients; nine were asymptomatic with a solid fusion and did not require further treatment and the other nine were symptomatic or had a pseudoarthrosis and required operative revision. Pedicle screw-rod fixation offers biomechanical advantages compared to other forms of internal fixation for the lumbar spine. It enables short-segment fixation with preservation of lumbar lordosis and adjacent normal motion segments. This technique provides a highly successful method to obtain arthrodesis, even with prior pseudoarthrosis.

Three-dimensional geometrical and mechanical modelling of the lumbar spine

The main objective of this study is to design a three-dimensional geometrical and mechanical finite element model of the lumbar spine. The model's geometry is constructed using six parameters per vertebra. These parameters are digitized from two X-rays (anterio-posterior and lateral), thus yielding an individualized model which can be arrived at from the radiographs of a tested specimen. This procedure makes the model validation easier, as geometry is generally a factor of dispersion in experimental results. The geometrical reconstruction, in the form of a finite elements mesh, was effected for the whole lumbar spine. The global coherence of the model was verified.