Biomechanics of transfixation in pedicle screw instrumentation

Biomechanical Study of Horizontal Screw-screw Crosslink in C1-2 Pedicle Screw-rod Fixation

STUDY DESIGN

This is a biomechanical study in vitro.

OBJECTIVE

To investigate the biomechanical differences between horizontal rod-rod crosslink (hR-R CL) and the horizontal screw-screw crosslink (hS-S CL) implementation in C1-2 pedicle screw-rod (C1-2 PSR) fixation.

SUMMARY OF BACKGROUND

To improve internal fixation stability, transverse connector (TC) is used in C1-2 PSR to increase torsional stiffness. The connection mode of horizontal connection includes hR-R CL and hS-S CL. hS-S CL adopted in C1-2 PSR was rarely reported and its biomechanics are still unclear.

MATERIALS AND METHODS

Six fresh cadaveric cervical spine specimens were each tested as an Intact model, then modified and tested as an Instability model (unstable odontoid fractures), and then as 3 internal fixation models, including C1-2 PSR, C1-2 pedicle screw-rod+horizontal rod-rod crosslink (C1-2 PSR+ hR-R CL), C1-2 pedicle screw-rod+horizontal screw-screw crosslink (C1-2 PSR+ hS-S CL). The ROM of the C1-2 segments was measured by applying 1.5 nm load in 6 loading conditions, including flexion-extension (FE), left and right lateral bending (LB), and left and right axial rotation (AR).

RESULTS

The C1-2 PSR+hR-R CL and C1-2 PSR+hS-S CL models, respectively, showed 60% and 75% lower ROM than the C1-2 PSR model in LB and AR conditions ( P <0.05). ROM was comparable between the C1-2 PSR+hR-R CL and the C1-2 PSR+hS-S CL models in all loading conditions ( P >0.05).

CONCLUSION

Both types of crosslinks showed superior C1-2 stability under LB and AR conditions than PSR without crosslinks. The C1-2 segment stability was comparable between the 2 types of crosslinks themselves.

Mechanical Analysis of Posterior Pedicle Screw System Placement and Internal Fixation in the Treatment of Lumbar Fractures

Objective

Image segmentation technology is applied to separate a single vertebra from the three-dimensional model of the spine, so as to separate a single vertebra image with smaller error, higher degree of automation, and better results. The objectives are to study the biomechanical characteristics of posterior short-segment pedicle screw fixation by three-dimensional finite element method, analyze the mechanical characteristics of posterior pedicle screw rod fixation system under different factors, and demonstrate the feasibility of its application in the treatment of lumbar fracture.

Methods

The authors searched the database for articles about the treatment of lumbar spine fracture, screw rod internal fixation system, and its mechanical parameters. The threshold segmentation method based on region segmentation method was used to segment the image, and the three-dimensional finite element model was used to analyze the biomechanical characteristics of different posterior internal fixation for lumbar spine fracture.

Results

The posterior pedicle internal fixation system for the treatment of multilevel spinal fractures is a mature surgical technique and has fewer postoperative complications. Transpedicle fixation is effective and reliable. It can effectively restore the coronal and sagittal curvature of the vertebral body and restore the stability of the spine better. But the choice of internal fixation method should be individualized based on fracture type, identification of critical and secondary injury sites, and stability assessment. Only after mastering the biomechanical characteristics of the posterior screw rod system for the treatment of lumbar fracture, selecting the appropriate method, and fixing the appropriate movement unit can the best fixation be achieved.

Conclusion

Threshold method is the most direct and simple image segmentation method. The core technology of thresholding is the selection of threshold, which will affect the final segmentation effect. The most common segmentation method is to calculate the segmentation threshold by histogram. The threshold method has less computation and good segmentation effect for the image with large contrast between background and target. Posterior pedicle screw rod system internal fixation has the advantages of less trauma, good reduction, reliable fixation, and less complications. The design, placement angle and depth of various internal fixation systems, and the number of fixed segments all show different mechanical characteristics. As long as we master the above characteristics, choose the appropriate method and fix the appropriate motor unit, and we can get the best fixation; it can be used as an effective treatment for lumbar fracture.

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.

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

Purpose

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

Methods

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

Results

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

Conclusion

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

The Role of Transverse Connectors in C1-C2 fixation for Atlantoaxial Instability: Is It Necessary? A Biomechanical Study

OBJECTIVE

To investigate the biomechanical effect of C1 lateral mass-C2 pedicle screw-rod (C1LM-C2PS) fixation with and without transverse connectors (TC) in an atlantoaxial instability (AAI) model.

METHODS

Ten freshly frozen cadaveric specimens were tested using an industrial robot under the following conditions: intact model, AAI model, C1-C2 model, C1-C2 with one TC model, and C1-C2 with two TCs model. Three types of motion, flexion-extension (FE), lateral bending (LB), and axial rotation (AR), were applied (1.5 Nm) to the specimens. The range of motion (ROM) and neutral zone (NZ) between C1 and C2 in all directions were measured.

RESULTS

Compared with those of the intact and AAI models, the C1-C2 ROM and NZ of all instrumented groups were decreased significantly in each direction of loading motion (P < 0.05). The mean FE ROM in the no TC, 1 TC, and 2 TC groups was 2.12° ± 0.41°, 2.29° ± 0.42°, and 2.04° ± 0.69°, respectively (P = 0.840, 0.981, 0.628, respectively); the mean LB ROM in the 3 intervention groups was 1.26° ± 0.67°, 1.02° ± 0.51° and 1.03° ± 0.57°, respectively (P = 0.489, 0.501, 1.000, respectively). During AR, the ROM and NZ of the no TC group (3.19° ± 0.89° and 1.51° ± 0.42°) were significantly reduced by more than 60% compared with those in the 1 (0.98° ± 0.28° and 0.40° ± 0.11°) and 2 TC groups (1.17° ± 1.69° and 0.42° ± 0.61°) (P < 0.001). Two TCs were equivalent for all loading motions to 1 TC (P > 0.05).

CONCLUSIONS

Adding TCs to C1LM-C2PS can effectively decrease the axial rotation ROM and enhance the stability of C1-C2 segment. Therefore, it is necessary to use TC-strengthened C1 lateral mass -C2 pedicle screw-rod fixation in patients with instability of C1-C2.

The Influence of Cross-Links on Long-Segment Instrumentation Following Spinal Osteotomy: A Finite Element Analysis

OBJECTIVE

To develop finite element models of spine following osteotomy and evaluate the effect of number and location of cross-links (CLs) on long-segment instrumentation.

METHODS

A finite element model of instrumented spine following osteotomy was created from computed tomography images of a postoperative male patient with thoracolumbar kyphotic deformity. Five fixation models were established to simulate different number and location of CLs. Four loading conditions (flexion, extension, lateral bending, and axial rotation) were applied on the models. Range of motion (ROM), maximum value and distribution of stress on implants, and stress on vertebrae were compared between models.

RESULTS

With increased number of CLs, average ROM of instrumented segments was reduced by 2.37%, 1.89%, and 2.49% in flexion, extension, and lateral bending. ROM was reduced by 21.98% in loading axial rotation condition. With increased number of CLs, ROM tended to be limited. Peak stresses were located on rods during axial rotation, on proximal pedicle screws during flexion, and on the osteotomy site during extension and lateral bending. CLs had an effect of dispersing stress concentration.

CONCLUSIONS

The application of CLs enhanced the rigidity of the construct. With increased number of CLs, ROM of the construct was decreased, especially in axial rotation. CLs can also disperse the stress concentration. After comparing various CL configurations in different motion conditions, we believe that the optimal method is to place 2 CLs at the osteotomy site and the proximal segment.

Biomechanical Stability of a Cross-Rod Connection with a Pedicle Screw System

Background

Surgery with pedicle screw instrumentation does not provide sufficient torsional stability. This leads to pseudoarthrosis, loosening of the pedicle screws, and, ultimately, implant failure.

Material/Methods

Functional spinal units from 18 deer were evaluated using a 6-axis material testing machine. As specimen models, we prepared an intact model, a damaged model, a cross-rod model, and a cross-link model. We measured the range of motion (ROM) during bending and rotation tests.

Results

The range of motions of cross-rod model were almost equal to those of cross-link model during the bending test. In the rotation test, the average ranges of motion of the intact, cross-rod, and cross-link models were 2.9°, 3.1°, and 3.9° during right rotation and 2.9°, 3.1°, and 4.1° during left rotation, respectively. The range of motions of the cross-rod model were significantly smaller than those of the cross-link model during the rotation test. The range of motions of the intact model were significantly smaller than those of the cross-link model during the rotation test, but there were no statistically significant differences between the range of motions of intact model and cross-rod model during the rotation test.

Conclusions

The stability of spinal fixation such as cross-rod model is equal to the fixation using the pedicle screw system during bending tests and equal to that of the intact spine during rotation tests.

[Common pedicle screw placement under direct vision combined with dome shaped decompression via small incision for double segment thoracolumbar fracture with nerve injury]

Objective

To determine the feasibility, safety, and efficacy of common pedicle screw placement under direct vision combined with dome shaped decompression via small incision for double segment thoracolumbar fracture with nerve injury.

Methods

A retrospective analysis was performed on the clinical data of 32 patients with double segment thoracolumbar fracture with nerve injury undergoing common pedicle screw placement under direct vision combined with dome shaped decompression via small incision between November 2011 and November 2015 (combined surgery group), and another 32 patients undergoing traditional open pedicle screw fixation surgery (traditional surgery group). There was no significant difference in gender, age, cause of injury, time of injury-to-surgery, injury segments and Frankel classification of neurological function between two groups ( P>0.05). The length of soft tissue dissection, the operative time, the blood loss during surgery, the postoperative drainage, the visual analogue scale (VAS) of incision after surgery, and recovery of neurological function after surgery were evaluated.

Results

All cases were followed up 9 to 12 months (mean, 10.5 months) in combined surgery group, and 8 to 12 months (mean, 9.8 months) in traditional surgery group. The length of soft tissue dissection, the operative time, the blood loss during surgery, the postoperative drainage, and the postoperative VAS score in the combined surgery group were significantly better than those in the traditional surgery group ( P<0.05). Dural rupture during surgery and pedicle screw pulling-out at 6 months after surgery occurred in 2 cases and 1 case of the combined surgery group; dural rupture during surgery occurred in 1 case of the traditional surgery group. The X-ray films showed good decompression, and fracture healing; A certain degree of neurological function recovery was achieved in two groups.

Conclusion

Common pedicle screw placement under direct vision combined with dome shaped decompression via small incision can significantly reduce iatrogenic trauma and provide good nerve decompression. Therefore, it is a safe, effective, and minimally invasive treatment method for double segment thoracolumbar fracture with neurological injury.

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

STUDY DESIGN

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

OBJECTIVES

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

SUMMARY OF BACKGROUND DATA

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Statistical Interspace Models (SIMs): Application to Robust 3D Spine Segmentation

Statistical shape models (SSM) are used to introduce shape priors in the segmentation of medical images. However, such models require large training datasets in the case of multi-object structures, since it is required to obtain not only the individual shape variations but also the relative position and orientation among objects. A solution to overcome this limitation is to model each individual shape independently. However, this approach does not take into account the relative position, orientations and shapes among the parts of an articulated object, which may result in unrealistic geometries, such as with object overlaps. In this article, we propose a new Statistical Model, the Statistical Interspace Model (SIM), which provides information about the interaction of all the individual structures by modeling the interspace between them. The SIM is described using relative position vectors between pair of points that belong to different objects that are facing each other. These vectors are divided into their magnitude and direction, each of these groups modeled as independent manifolds. The SIM was included in a segmentation framework that contains an SSM per individual object. This framework was tested using three distinct types of datasets of CT images of the spine. Results show that the SIM completely eliminated the inter-process overlap while improving the segmentation accuracy.

Posterior C1-C2 screw and rod instrument for reduction and fixation of basilar invagination with atlantoaxial dislocation

PURPOSE

To report the surgical technique and preliminary clinical results for the treatment of basilar invagination (BI) with atlantoaxial dislocation (AAD) by posterior C1-C2 pedicle screw and rod instrument.

METHODS

Between July 2012 and August 2013, 33 patients who had BI with AAD underwent surgery at our institution. Pre and postoperative three-dimensional computed tomographic (CT) scans were performed to assess the degree of dislocation. Magnetic resonance (MR) imaging was used to evaluate the compression of the medulla oblongata. For all patients, reduction of the AAD was conducted by two steps: fastening nuts and rods was performed to achieve the horizontal reduction. Distraction between C1 and C2 screws was performed to obtain the vertical reduction.

RESULTS

No neurovascular injury occurred during surgery. Follow-up ranged from 6 to 15 months (mean 10.38 months) in 32 patients. Post-operative three-dimensional CT showed that complete horizontal reduction was obtained in 30/33 (90.9%), and complete vertical reduction was obtained in 31/33 (93.9%). The repeated three-dimensional CT and MR image demonstrated that bony fusion and the decompression of the medulla oblongata were obtained in all patients. Clinical symptoms improved significantly 3 months after surgery.

CONCLUSIONS

This C1-C2 pedicle screw and rod instrument is a promising technique for the treatment of BI with AAD.

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

BACKGROUND CONTEXT

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

PURPOSE

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

STUDY DESIGN

Retrospective comparative study, level of evidence 3.

PATIENT SAMPLE

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

OUTCOME MEASURES

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

METHODS

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

RESULTS

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

CONCLUSION

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

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

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

Should we cross the cross-links?

STUDY DESIGN

Retrospective study.

OBJECTIVE

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

SUMMARY OF BACKGROUND DATA

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

METHODS

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

RESULTS

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

CONCLUSION

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

LEVEL OF EVIDENCE

N/A.

Bone properties affect loosening of half-pin external fixators at the pin-bone interface

INTRODUCTION

Local bone yielding at the pin-bone interface of external fixation half-pins has been known to initiate fixator loosening. Deterioration of bone properties due to ageing and disease can lead to an increase in the risk of pin loosening. This study determines the extent, locations and mechanics of bone yielding for unilateral external fixation systems at the tibial midshaft with changes in age-related bone structure and properties. The study also evaluates the effect of the number of pins used in the fixation system and use of titanium pins (in place of steel) on bone yielding.

METHODS

We employ nonlinear finite element (FE) simulations. Strain-based plasticity is used to simulate bone yielding within FE analyses. Our analyses also incorporate contact behaviour at pin-bone interfaces, orthotropic elasticity and periosteal-endosteal variation of bone properties.

RESULTS

The results show that peri-implant yielded bone volume increases by three times from young to old-aged cases. The use of three, rather than two half-pins (on either side of the fracture), reduces the volume of yielded bone by 80% in all age groups. The use of titanium half-pins resulted in approximately 60-65% greater volumes of yielded bone.

CONCLUSIONS

We successfully simulate half-pin loosening at the bone-implant interface which has been found to occur clinically. Yielding across the full cortical thickness may explain the poor performance of these devices for old-aged cases. The models are able to identify patients particularly at risk of half-pin loosening, who may benefit from alternative fixator configurations or techniques such as those using pre-tensioned fine wires.

The position of the aorta changes with altered body position in single right thoracic adolescent idiopathic scoliosis: a magnetic resonance imaging study

STUDY DESIGN

A prospective clinical magnetic resonance imaging study.

OBJECTIVE

To explore the differences in the position of the aorta relative to the spine in patients with single right thoracic adolescent idiopathic scoliosis (RT-AIS) in 2 different body positions (supine and prone).

SUMMARY OF BACKGROUND DATA

Pedicle screws are used widely in scoliosis surgery. With an increase in the incidence of vascular complications that result from misplaced pedicle screws, studies regarding the spatial relationship of the aorta and the vertebral body have also gradually increased and show that the aorta is positioned more posteriorly in patients with RT-AIS than in normal subjects. In these imaging studies, the patients received computed tomographic or magnetic resonance (MR) scans in the supine position. Recent studies of subjects without a spinal deformity found that the aorta moves from a posterolateral to an anteromedial position when the subject changes from a supine position to a prone position. However, no studies investigated aorta shifting with changing body position in patients with AIS.

METHODS

Twenty-six patients with single RT-AIS were recruited into this study. Each patient received an axial MR scan from T5 through L3 in both the supine and prone positions. In the Cartesian coordinate system, the left pedicle-aorta (LtP-Ao) angle, LtP-Ao distance, and vertebral rotation angle were measured from T5 through L3 in the axial plane MR images. We also simulated misplacement of the pedicle screw with commonly used length and 20° direction error, and the potential risk of aorta impingement was defined as the virtual pedicle screw crossing the aorta. The paired sample t test was used to compare these parameters between the 2 body positions.

RESULTS

The mean LtP-Ao angle and mean LtP-Ao distance differed between the body positions at each level. At the T5-T10 levels, the patients in the prone position exhibited significantly smaller LtP-Ao angles (26.2° vs. 38.8°; P, 0.01) and distances (27.0 vs. 30.7 mm; P, 0.01) than those in the supine position. The vertebral rotation angle was larger in the prone position than in the supine position at periapical levels, although this difference did not reach statistical significance (P . 0.05). The percentage of potential risk of aorta impingement was significantly higher in the prone position than in the supine position at the T5-T10 levels (19.7% vs. 6.6%, respectively; P, 0.05). CONCLUSION.: The aorta shifts more anteromedially and more closely to the spine at the T5-T10 levels when patients with RT-AIS change from the supine to the prone position. Thus, in the prone position, the aorta is potentially at a higher risk for injury from anterior and lateral cortex penetration by the left pedicle screws. The spinal surgeon should be aware of these altered conditions to avoid injury to the aorta during pedicle screw insertion in patients with RT-AIS who are in the prone position.

Biomechanical analysis of the C2 intralaminar fixation technique using a cross-link and offset connector for an unstable atlantoaxial joint

BACKGROUND CONTEXT

C2 intralaminar screws offer the advantage of avoiding the vertebral artery; however, biomechanical studies have demonstrated inferiority of C2 intralaminar screw fixation compared with C2 intrapedicular fixation in the presence of an odontoid fracture. Addition of a transverse cross-link may improve stability afforded by the lamina screws but will require the use of offset connectors to complete the construct.

PURPOSE

The aims of this project were to evaluate whether transverse cross-links can add adequate stability to atlantoaxial constructs using C1 lateral mass and C2 intralaminar screw fixation. The secondary objective was to determine the biomechanical contribution of the C2 offset connectors.

STUDY DESIGN

In vitro human cadaveric biomechanical study.

METHODS

Ten cadaveric specimens were obtained and instrumented with C1 lateral mass, C2 pedicle, and C2 intralaminar screws. After intact spine testing, each C1-C2 construct was nondestructively evaluated under axial rotation (AR), flexion extension (FE), and lateral bending (LB). Intralaminar fixation was tested with and without offset connectors, which allowed for cross-link addition to the construct. After normal state evaluation, the odontoid was resected and analyses were repeated.

RESULTS

Postreconstruction range of motion in AR, FE, and LB showed no significant differences between the four fixation constructs in the stable specimens. Transpedicular fixation at C2 proved superior to intralaminar techniques without a cross-link in AR and LB after destabilization with an odontoidectomy. The addition of a cross-link to the intralaminar construct improved segmental AR and LB stability to the level afforded by the transpedicular fixation. Offset connectors appeared to marginally weaken the intralaminar fixation, but the findings were not significant.

CONCLUSIONS

Coupled with an offset connector and a cross-link, C2 intralaminar screws offer similar segmental stability to intrapedicular fixation in the presence of an unstable dens fracture. Lateral offset connectors at C2 do not significantly compromise stability of C1 lateral mass-C2 intralaminar fixation.

The biomechanical effect of transverse connectors use in a pre- and postlaminectomy model of the posterior cervical spine: an in vitro cadaveric study

STUDY DESIGN

An in vitro biomechanical study investigating the effect of transverse connectors on posterior cervical stabilization system in a laminectomy model.

OBJECTIVE

To evaluate the optimal design, number, and location of the transverse connectors in stabilizing long segment posterior instrumentation in the cervical spine.

SUMMARY OF BACKGROUND DATA

In the cervical spine, lateral mass screw (LMS) fixation is used for providing stability after decompression. Transverse connectors have been used to augment segmental posterior instrumentation. However, in the cervical region the optimal design, number, and the location of transverse connectors is not known.

METHODS

Seven fresh human cervicothoracic cadaveric spines (C2-T1) were tested by applying ±1.5 Nm moments in flexion (F), extension (E), lateral bending (LB), and axial rotation (AR). After testing the intact condition, LMS/rods were placed and then were tested with two different transverse connectors (top-loading connector [TL] and the head-to-head [HH] connector) in multiple levels, pre- and postlaminectomy (PL).

RESULTS

LMS significantly reduced segmental motion by 77.2% in F, 75.6% in E, 86.6% in LB, and 86.1% in AR prelaminectomy and by 75.4% in F, 76% in E, 80.6% in LB, and 76.4% in AR postlaminectomy compared to intact (P < 0.05). Only in AR, PL constructs with HH connectors at C3 & C7, TL connectors at C4-C5 & C5-C6, and at C3-C4 & C6-C7 significantly reduced the range of motion by 12.9%, 11.9%, and 11.9%, respectively, compared to PL LMS (P < 0.05). No statistical significance was observed between TL connector and HH connector in all loading directions.

CONCLUSION

The biomechanical advantage of transverse connectors is significant in AR, when using two connectors at the proximal and distal ends, compared to one connector. In a clinical setting, this data may guide surgeons on transverse connector configurations to consider during posterior cervical instrumentation.

Effects of nonlinearity in the materials used for the semi-rigid pedicle screw systems on biomechanical behaviors of the lumbar spine after surgery

Recently, various types of semi-rigid pedicle screw fixation systems have been developed for the surgical treatment of the lumbar spine. They were introduced to address the adverse issues commonly found in traditional rigid spinal fusion--abnormally large motion at the adjacent level and subsequent degeneration. The semi-rigid system uses more compliant materials (nitinol or polymers) and/or changes in rod design (coiled or twisted rods) as compared to the conventional rigid straight rods made of Ti alloys (E = 114 GPa, υ = 0.32). However, biomechanical studies on the semi-rigid pedicle screw systems were usually limited to linear modeling of the implant and anatomic elements, which may not be capable of reflecting realistic post-operative motions of the spine. In this study, we evaluated the effects of nonlinearity in materials used for semi-rigid pedicle screw fixation systems to evaluate the changes in biomechanical behaviors using finite element analysis. Changes in range of motion (ROM) and center of rotation (COR) were assessed at the operated and adjacent levels. Actual load-displacement results of the semi-rigid rod from mechanical test were carried out to reflect the nonlinearity of the implant. In addition, nonlinear material properties of various spinal ligaments studies were used for the finite element modeling. The post-operative models were constructed by modifying the previously validated intact model of the L1-S1 spine. Eight different post-operative models were made to address the effects of nonlinearity-with a traditional stiffness modulus rod (with linear ligaments, case 1; with nonlinear ligaments, case 5), with a rigid rod (with linear ligaments, case 2; with nonlinear ligaments, case 6), with a soft rod (with linear ligaments, case 3; with nonlinear ligaments, case 7), and with a nonlinear rod (with linear ligaments, case 4; with nonlinear ligaments, case 8). To simulate the load on the lumbar spine in a neutral posture, follower load (400 N) was applied and then the hybrid loading condition was applied to measure the ROM and COR in the sagittal plane. The more the nonlinearity was included in the model the closer the motion behavior of the device was to that of the intact spine. Furthermore, our results showed that the nonlinearity of the semi-rigid rod was a more sensitive factor than the nonlinearity of the spinal ligaments on biomechanical behavior of the lumbar spine after surgery. Therefore, for better understanding of the surgical effectiveness of the spinal device, more realistic material properties such as nonlinearity of the device and anatomic elements should be considered. In particular, the nonlinear properties of the semi-rigid rod were considered more than the nonlinearity of spinal ligaments.

Two-level noncontiguous versus three-level anterior cervical discectomy and fusion: a biomechanical comparison

STUDY DESIGN

Biomechanical study.

OBJECTIVE

To determine biomechanical forces exerted on intermediate and adjacent segments after two- or three-level fusion for treatment of noncontiguous levels.

SUMMARY OF BACKGROUND DATA

Increased motion adjacent to fused spinal segments is postulated to be a driving force in adjacent segment degeneration. Occasionally, a patient requires treatment of noncontiguous levels on either side of a normal level. The biomechanical forces exerted on the intermediate and adjacent levels are unknown.

METHODS

Seven intact human cadaveric cervical spines (C3-T1) were mounted in a custom seven-axis spine simulator equipped with a follower load apparatus and OptoTRAK three-dimensional tracking system. Each intact specimen underwent five cycles each of flexion/extension, lateral bending, and axial rotation under a ± 1.5 Nm moment and a 100-Nm axial follower load. Applied torque and motion data in each axis of motion and level were recorded. Testing was repeated under the same parameters after C4-C5 and C6-C7 diskectomies were performed and fused with rigid cervical plates and interbody spacers and again after a three-level fusion from C4 to C7.

RESULTS

Range of motion was modestly increased (35%) in the intermediate and adjacent levels in the skip fusion construct. A significant or nearly significant difference was reached in seven of nine moments. With the three-level fusion construct, motion at the infra- and supra-adjacent levels was significantly or nearly significantly increased in all applied moments over the intact and the two-level noncontiguous construct. The magnitude of this change was substantial (72%).

CONCLUSION

Infra- and supra-adjacent levels experienced a marked increase in strain in all moments with a three-level fusion, whereas the intermediate, supra-, and infra-adjacent segments of a two-level fusion experienced modest strain moments relative to intact. It would be appropriate to consider noncontiguous fusions instead of a three-level fusion when confronted with nonadjacent disease.

The biomechanics of pedicle screw-based instrumentation

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

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

STUDY DESIGN

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

OBJECTIVE

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

SUMMARY OF BACKGROUND DATA

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

METHODS

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

RESULTS

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

CONCLUSION

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

The cervical end of an occipitocervical fusion: a biomechanical evaluation of 3 constructs. Laboratory investigation

OBJECT

Stabilization with rigid screw/rod fixation is the treatment of choice for craniocervical disorders requiring operative stabilization. The authors compare the relative immediate stiffness for occipital plate fixation in concordance with transarticular screw fixation (TASF), C-1 lateral mass and C-2 pars screw (C1L-C2P), and C-1 lateral mass and C-2 laminar screw (C1L-C2L) constructs, with and without a cross-link.

METHODS

Ten intact human cadaveric spines (Oc-C4) were prepared and mounted in a 7-axis spine simulator. Each specimen was precycled and then tested in the intact state for flexion/extension, lateral bending, and axial rotation. Motion was tracked using the OptoTRAK 3D tracking system. The specimens were then destabilized and instrumented with an occipital plate and TASF. The spine was tested with and without the addition of a cross-link. The C1L-C2P and C1L-C2L constructs were similarly tested.

RESULTS

All constructs demonstrated a significant increase in stiffness after instrumentation. The C1L-C2P construct was equivalent to the TASF in all moments. The C1L-C2L was significantly weaker than the C1L-C2P construct in all moments and significantly weaker than the TASF in lateral bending. The addition of a cross-link made no difference in the stiffness of any construct.

CONCLUSIONS

All constructs provide significant immediate stability in the destabilized occipitocervical junction. Although the C1L-C2P construct performed best overall, the TASF was similar, and either one can be recommended. Decreased stiffness of the C1L-C2L construct might affect the success of clinical fusion. This construct should be reserved for cases in which anatomy precludes the use of the other two.

Effects of facetectomy and crosslink augmentation on motion segment flexibility in posterior lumbar interbody fusion

STUDY DESIGN

Biomechanical assessment using calf lumbar motion segments.

OBJECTIVE

To determine whether facetectomy affects the primary stability of posterior lumbar interbody fusion.

SUMMARY OF BACKGROUND DATA

To improve visualization and access to the disc space, the facet joints frequently are removed. Previous biomechanical studies have indicated a fundamental role for the facet joints in maintaining spinal segment stability.

METHODS

Single motion segments from calf lumbar spines were tested for pure-moment flexibility in flexion-extension (FE), lateral bending (LB), and axial rotation (AR). After testing intact, an interbody cage and pedicle screw system were implanted. Next, a bilateral facetectomy was performed, and finally a crosslink was added. Flexibility testing was repeated at each stage of implantation. Data are reported for range of motion (ROM), neutral zone (NZ), and a new compliance parameter (COM), based on the slopes of the moment-angle curve in the neutral and elastic regions.

RESULTS

With posterior lumbar interbody fusion implantation, ROM in FE was reduced 82% +/- 4% (mean +/- standard deviation) and NZ 78% +/- 7% over intact (P < 0.015: Wilcoxon). Reduction in LB was slightly more, whereas reduction in AR was considerably less and did not achieve statistical significance for NZ. After facetectomy, ROM in FE increased 0.3 degrees (P < 0.05), on average, and NZ did not change. In LB neither changed significantly. In AR, ROM increased 0.6 degrees (P < 0.05), and NZ increased 0.2 degrees (P < 0.05). The addition of a crosslink changed ROM and NZ less than 0.1 degrees in FE and LB, whereas in AR it restored half of the stability lost due to facetectomy in ROM (P < 0.05), and had a similar trendwise effect on NZ. The new compliance measure, COM, was found to agree with the direction of change in ROM more consistently than did NZ.

CONCLUSION

Facetectomy causes a nominal increase in ROM and NZ in FE and LB, which are not affected by the addition of a crosslink. Although the effect of facetectomy is greater in AR-and crosslink has a measurable restoring effect-all differences are within a few tenths of a degree under this loading paradigm. Thus, the clinical utility of adding a crosslink may not be justified based on these small biomechanical changes. COM can serve as a complement to ROM and NZ, or even as a surrogate when its 2 components are reported together, as it shows strong agreement with ROM, effectively distinguishes between lax and elastic region behaviors, and provides a measure of flexibility independent of the load range.

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

STUDY DESIGN

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

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

STUDY DESIGN

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

OBJECTIVE

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

SUMMARY OF BACKGROUND DATA

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

METHODS

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

RESULTS

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

CONCLUSION

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

Biomechanics of unilateral compared with bilateral lumbar pedicle screw fixation for stabilization of unilateral vertebral disease

Object

An in vitro flexibility experiment was performed to compare the biomechanical stability of asymmetrical lumbar pedicle screw fixation (longer hardware attached ipsilaterally to a 1-sided lesion), short and long fixation, and fixation with and without interconnection to the involved vertebra.

Methods

Seven human cadaveric specimens (T12–S1) were studied intact; after simulated unilateral lesions were created at L2–3 and L3-4, the segments were stabilized by 1) L2–4 unilateral fixation (L-3 excluded), 2) L2–4 bilateral fixation (L-3 included contralaterally), 3) L2–5 unilateral fixation (L-3 excluded), 4) L2–5 fixation ipsilateral (L-3 excluded) and L2–4 fixation contralateral (L-3 included), 5) L2–5 bilateral fixation (L-3 included contralaterally), and 6) L2–5 bilateral fixation (L-3 excluded). The testing order varied among specimens. Angular range of motion (ROM) and lax zone were recorded optically while loading to 6.0 Nm was created with nonconstraining pure moments.

Results

Unilateral short fixation provided significantly worse stabilization than any other construct tested in all loading modes (p < 0.05, repeated-measures analysis of variance). There was a mean 56% reduction in ROM across the lesion after adding 1 additional level rostrally and caudally. Asymmetrical long/short stabilization provided similar stability to symmetrical long stabilization. Minimal additional stability was gained by including L-3 in the long bilateral fixation construct.

Conclusions

Unilateral fixation is inadequate for stabilizing a 2-level unilateral lesion. Bilateral fixation, whether symmetrical or asymmetrical, provides good stabilization for this injury. It is not important for stability to include the level of the lesion within the long construct contralaterally.

[Biomechanical evaluation of posterior instrumentation for lumbar burst fracture: comparison of two internal devices]

PURPOSE OF THE STUDY

Burst fractures generally occur due to trauma to the thoracolumbar spine. Surgery is indicated for unstable fractures. Posterior instrumentation with pedicular screws is generally proposed. In certain circumstances, hooks may be preferred due to excessive risk of insertion of the pedicular screw. The purpose of this study was to compare two posterior instrumentations, one using pedicular screws on either side of the fracture each protected by hoods and a second composed of the same pedicular screws inserted under the fracture hooks above.

MATERIAL AND METHODS

Twelve spinal specimens from human cadavers composed of segments T10 to L2 were used. Range of flexion, extension, lateral inclination, and rotation were noted on T10 up to application of 7 Nm. Spinal segments were tested first intact, then in four configurations: 1) instrumented without lesion, 2) lesion simulating burst fracture of L1 without section of the interspinous ligament, 3) and with section of the interspinous ligament, and 4) with L1 corporectomy. Finally a test to rupture was performed by applying a flexion moment up to fracture.

RESULTS

Mean flexion-extension of the instrumented spine was limited compared with the intact spine for both instrumentation configurations and irrespective of the lesion. The same behavior was observed for lateral inclination with less pronounced motion with the first instrumentation. For rotation, the range of motion increased clearly with the second instrumentation and this with the first lesion while with the first instrumentation, rotation amplitude remained below that of the intact spine. There was however an increase in the vertical displacement during flexion-extension for both instrumentations. For the rupture test, the mean flexion moment at rupture was 14.4 Nm (10.6-22 Nm) with no difference between the two instrumentations.

DISCUSSION

This mode simulating burst fractures of the spine appears to be reproducible and more realistic than corporectomy. Attention should be taken concerning the limits of this type of study since fractures can occur for forces as small as 10.6 Nm. Thus we observed that pedicle screw configurations and also fractures produced mean ranges of motion greater than intact segments irrespective of the type of lesion simulated. However, the net increase in motion was observed during rotation movements when hooks were used, even when they were placed only below the fracture. Putting pressure on the hooks does not prevent them from slipping along the lamina. But neither of these two configurations controls the fracture gap. A vertebral reinforcement might be necessary.

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

STUDY DESIGN

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

OBJECTIVE

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

SUMMARY OF BACKGROUND DATA

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Less invasive posterior fixation method following transforaminal lumbar interbody fusion: a biomechanical analysis

BACKGROUND CONTEXT

Current surgical trends increasingly emphasize the minimization of surgical exposure and tissue morbidity. Previous research questioned the ability of unilateral pedicle screw instrumentation to adequately stabilize posterior fusion constructs. No study to date has addressed the effects of reduced posterior instrumentation mass on interbody construct techniques. Unilateral surgical exposure for transforaminal lumbar interbody fusion (TLIF) allows ipsilateral pedicle screw placement. Theoretically, percutanous contralateral facet screw placement could provide supplemental construct support without additional surgical exposure.

PURPOSE

Identify the biomechanical effects of reduced spinal fusion instrumentation mass on interbody construct stability.

STUDY DESIGN

An in vitro biomechanical study using human lumbar spines comparing stability of TLIF constructs augmented by: (1) bilateral pedicle screw fixation, (2) unilateral pedicle screw fixation, or (3) a novel unilateral pedicle screw fixation supplemented with contralateral facet screw construct.

METHODS

Seven fresh frozen human cadaveric specimens were tested in random construct order in flexion/extension, lateral bending, and axial rotation using +/-5.0 Nm torques and 50 N axial compressive loads. Analysis of torque rotation curves determined construct stability. Using paired statistical methods, comparison of construct stiffness and total range of motion within each specimen were performed using the Wilcoxon signed ranks test with a Holm-Sidák multiple comparison procedure (alpha=0.05).

RESULTS

In flexion/extension, lateral bending, and axial rotation, there were no measurable differences in either stiffness or range of motion between the standard bilateral pedicle screw and the novel construct after TLIF. After TLIF, the unilateral pedicle screw construct provided only half of the improvement in stiffness compared with bilateral or novel constructs and allows for significant off-axis rotational motions, which could be detrimental to stability and the promotion for fusion.

CONCLUSIONS

All tested TLIF constructs with posterior instrumentation decreased segmental range of motion and increased segmental stiffness. While placing unilateral posterior instrumentation decreases overall implant bulk and dissection, it allows for significantly increased segmental range of motion, less stiffness, and produces off-axis movement. The technique of contralateral facet screw placement provides the surgical advantages of unilateral pedicle screw placement with stability comparable to TLIF with bilateral pedicle screws.

Multiaxial pedicle screw designs: static and dynamic mechanical testing

STUDY DESIGN

Randomized investigation of multiaxial pedicle screw mechanical properties.

OBJECTIVES

Measure static yield and ultimate strengths, yield stiffness, and fatigue resistance according to an established model. Compare these measured properties with expected loads in vivo.

SUMMARY OF BACKGROUND DATA

Multiaxial pedicle screws provide surgical versatility, but the complexity of their design may reduce their strength and fatigue resistance. There is no published data on the mechanical properties of such screws.

MATERIALS AND METHOD

Screws were assembled according to a vertebrectomy model for destructive mechanical testing. Groups of five assemblies were tested in static tension and compression and subject to three cyclical loads. Modes of failure, yield, and ultimate strength, yield stiffness, and cycles to failure were determined for six designs of screw.

RESULTS

Static compression yield loads ranged from 217.1 to 388.0 N and yield stiffness from 23.7 to 38.0 N/mm. Cycles to failure ranged from 42 x 10(3) to 4,719 x 10(3) at 75% of static ultimate load. There were significant differences between designs in all modes of testing. Failure occurred at the multiaxial link in static and cyclical compression.

CONCLUSIONS

Bending yield strengths just exceeded loads expected in vivo. Multiaxial designs had lower static bending yield strength than fixed screw designs. Five out of six multiaxial screw designs achieved one million cycles at 200 N in compression bending. "Ball-in-cup" multiaxial locking mechanisms were vulnerable to fatigue failure. Smooth surfaces and thicker material appeared to be protective against fatigue failure.

Biomechanical investigation of pedicle screw-vertebrae complex: a finite element approach using bonded and contact interface conditions

This study used finite element simulation to investigate the load transfer mechanisms within the screw/vertebra complex under different interface conditions, and under varying screw lengths. Both bonded and contact conditions were employed to demonstrate the interface between the screw and vertebra. Loadings were applied at the superior surface of the vertebra and screw unthreaded end, respectively, to represent two modes of flexion loads. The results indicated that the screw within the vertebra underwent a series of discontinuities of loading, identified by the localized high contact pressures, thus creating localized bending moments. The peak stress of screw was located at the junction of the screw's hub and thread, which is consistent with the location of screw failure observed in a clinical setting and the values of peak stress in the screw were proportional to the amount of moments generated by the two loading modes. The interface condition plays an important role in transferring the force within the screw/vertebra complex. A contact interface condition induces significantly higher stress in the screw than the bonded condition. Therefore providing a binding surface (with HA, or porosity coating on the screw surface) between the screw and the vertebra might be the most effective way to prevent screw failure. The influences of screw length on the peak stress in the screw become negligible when the screw is of sufficient length to extend fully into the vertebral body.

Physical characteristics of polyaxial-headed pedicle screws and biomechanical comparison of load with their failure

STUDY DESIGN

Pedicle screw strength or load to failure was biomechanically evaluated, and the geometric characteristics of pedicle screw instrumentation systems were compared.

OBJECTIVES

To compare the features of pedicle screw systems, and to demonstrate the failure point of the polyaxial pedicle screw head.

SUMMARY OF BACKGROUND DATA

Many pedicle screw instrumentation systems are currently available to the spine surgeon. Each system has its unique characteristics. It is important for the surgeon to understand the differences in these pedicle screw systems. Pedicle screw load to failure has not been subjected to a comparison study.

METHODS

The physical characteristics of each pedicle screw instrumentation system were determined. Features of rods, instruments, and pedicle screws were cataloged. Biomechanical testing of the pedicle screw construct was performed to determine the site and force of the load to failure. Nine pedicle screw systems were evaluated. Testing was performed with a pneumatic testing system under load control. Three polyaxial screws were used for each test at a load rate of 100 N/second. The load failure value was the force at which the pedicle screw or polyaxial head-screw interface initially deflected.

RESULTS

Biomechanical testing demonstrated in all instances that the polyaxial head coupling to the screw was the first failure point. Although there have been subtle design differences in the instruments over time, the features of the pedicle screw instrument sets have become remarkably similar.

CONCLUSIONS

Biomechanical pedicle screw load-to-failure data demonstrated that the polyaxial head coupling to the screw is the first to fail and may be a protective feature of the pedicle screw, preventing pedicle screw breakage. Knowing the physical characteristics of the available pedicle screw instrumentation systems may allow the choice of pedicle screw best suited for a given clinical situation.

Finite element analysis in spine research

Finite element analysis is a widely accepted tool used in many industries and research activities. It allows new designs to be thoroughly 'tested' before a prototype is even manufactured, components and systems which cannot readily be experimented upon to be examined, and 'diagnostic' investigations to be undertaken. Finite element models are already making an important contribution to our understanding of the spine and its components. Models are being used to reveal the biomechanical function of the spine and its behaviour when healthy, diseased or damaged. They are also providing support in the design and application of spinal instrumentation. The spine is a very complex structure, and many of the models are simplified and idealized because of the complexity and uncertainty in the geometry, material properties and boundary conditions of these problems. This type of modelling simplification is not peculiar to spinal modelling problems. Indeed, the idealization is often a strength when there is such uncertainty and variation between one individual and another, allowing cause-effect relationships to be isolated and fully explored, and the inherent variability of experimental tests to be eliminated. This paper reviews the development of finite element analysis in spinal modelling. It shows how modelling provides a wealth of information on our physiological performance, reduces our dependence on animal and cadaveric experiments and is an invaluable complement to clinical studies. It also leads to the conclusion that, as computing power and software capabilities increase, it is quite conceivable that in the future it will be possible to generate patient-specific models that could be used for patient assessment and even pre- and inter-operative planning.

Biomechanical evaluation of diagonal fixation in pedicle screw instrumentation

STUDY DESIGN

Flexibility tests and finite element analyses were performed for the biomechanical evaluation of diagonal transfixation in pedicle screw instrumentation.

OBJECTIVE

To assess the biomechanical advantages of diagonal transfixation compared with conventional horizontal transfixation.

SUMMARY AND BACKGROUND DATA

A few pedicle screw instrumentation systems allow the use of cross-links in the diagonal direction. Such a diagonal transfixation is anticipated to improve the surgical construct stability, but its biomechanical qualities have not been completely evaluated.

METHODS

Flexibility tests were performed on 10 calf lumbar spines (L2-L5). Specimens were subjected to pure moments up to 8.2 Nm in flexion, extension, lateral bending, and extension while the resulting movements of L3 and L4 were measured by a three-dimensional motion analysis system. The tested cases included (1) intact, (2) pedicle screw fixation without transfixation after total removal of the L3-L4 disc, (3) pedicle screw fixation with diagonal transfixation, and (4) pedicle screw fixation with horizontal transfixation. Three-dimensional finite element models of the tested surgical constructs were also developed by use of three-dimensional beam elements to investigate the effect of diagonal transfixation and horizontal transfixation on the construct stability and the corresponding stress changes in the screws.

RESULTS

When compared with no transfixation, horizontal transfixation significantly improved the lateral bending and axial rotation stability by 15.7% and 13.9%, respectively, but there was no improvement of stability in flexion and extension. By contrast, diagonal transfixation significantly improved the flexion and extension stability by 12% and 10.7%, respectively, but not the lateral bending and axial rotation stability in comparison with no transfixation. Comparison between horizontal transfixation and diagonal transfixation showed that the stabilizing effect of diagonal transfixation was greater in flexion and extension (13% and 11%, P < 0.01) than that of horizontal transfixation but smaller in lateral bending (11%, P < 0.05) and axial rotation (6.6%, P > 0.1). Finite element model predictions of the motion changes were similar to the changes observed in flexibility tests. In horizontal transfixation, the load changes, compared with no transfixion, were a 0.02% increase in flexion-extension, a 27.5% increase in lateral bending, and a 58% decrease in axial rotation, and the magnitudes of the moments applied on both the right and left pedicle screws were identical. However, when diagonal transfixation was achieved by connecting the left superior screw and the right inferior screw, the loads in the left screw were increased by 11.5% in flexion-extension, 43.6% in lateral bending, and 7.9% in axial rotation, whereas the loads in the right screw were decreased by 10.9% in flexion-extension, increased by 0.06% in lateral bending, and decreased by 18.1% in axial rotation.

CONCLUSIONS

The results of this study showed that diagonal transfixation provides more rigid fixation in flexion and extension but less in lateral bending and axial rotation in comparison with horizontal transfixation. Furthermore, greater stresses in the pedicle screws were predicted in the diagonal transfixation model. These limitations of diagonal transfixation should be considered carefully for clinical application.

Morphologic changes in the lumbar intervertebral foramen due to flexion-extension, lateral bending, and axial rotation: an in vitro anatomic and biomechanical study

STUDY DESIGN

A biomechanical and anatomic study with human cadaveric lumbar spine.

OBJECTIVES

The purpose of this study is to examine the morphologic changes in the intervertebral foramen during flexion, extension, lateral bending, and axial rotation of the lumbar spine and to correlate these changes with the flexibility of the spinal motion segments.

SUMMARY OF BACKGROUND DATA

Previous studies showed morphologic changes in the intervertebral foramen during flexion and extension; however, those changes during lateral bending and axial rotation were not well known.

METHODS

There were 81 motion segments obtained from 39 human cadaveric lumbar spines (mean age 69 years). The motion segments were imaged with CT scanner with 1-mm thick consecutive sections. For biomechanical testing each motion segment was applied with incremental pure moments of flexion, extension, lateral bending, and axial rotation. Rotational movements of the motion segment were measured using VICON cameras. After application of the last load, the specimens were frozen under load, and then CT was performed with the same technique described above. Six parameters of the intervertebral foramen were measured, including foraminal width (maximum and minimum), foraminal height, disc bulging, thickness of ligamentum flavum, and cross-sectional area of the foramen.

RESULTS

Flexion increased the foraminal width (maximum and minimum), height, and area significantly while significantly decreasing the disc bulging and thickness of ligamentum flavum (P < 0.05). However, extension decreased the foraminal width (maximum and minimum), height, and area significantly. Lateral bending significantly decreased the foraminal width (maximum and minimum), height, and area at the bending side, whereas lateral bending significantly increased the foraminal width (minimum), height, and area at the opposite side of bending. Likewise, axial rotation decreased the foraminal width (minimum) and area at the rotation side significantly while significantly increasing the foraminal height and foraminal area at the opposite side. The percent change in the foraminal area was found significantly correlated with the amount of segmental spinal motion except for the extension motion.

CONCLUSIONS

This study showed that the intervertebral foramen of the lumbar spine changed significantly not only on flexion-extension but also on lateral bending and axial rotation. The percent change in cross-sectional foraminal area was correlated with the amount of segmental motion except for extension motions. Further studies are needed to assess the morphologic changes in the intervertebral foramen in vivo and to correlate clinically.

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 effect of disc degeneration and facet joint osteoarthritis on the segmental flexibility of the lumbar spine

STUDY DESIGN

A biomechanical and imaging study of human cadaveric spinal motion segments.

OBJECTIVE

To investigate the effect of both disc degeneration and facet joint osteoarthritis on lumbar segmental motion.

SUMMARY OF BACKGROUND DATA

Spinal degeneration includes the osteoarthritic changes of the facet joint as well as disc degeneration. Disc degeneration has been reported to be associated with spinal motion. The association of facet joint osteoarthritis with lumbar segmental motion characteristics and the combined influence of disc degeneration and facet osteoarthritis has not yet been investigated.

METHODS

A total of 110 lumbar motion segments (52 female, 58 male) from 44 human lumbar spines were studied (mean age = 69 years). Magnetic resonance images were used to assess the disc degeneration from Grade I (normal) to Grade V (advanced) and the osteoarthritic changes in the facet joints in terms of cartilage degeneration, subchondral sclerosis, and osteophytes. Disc height, endplate size, and facet joint orientation and width also were measured from the computed tomographic images. Rotational movements of the motion segment in response to the flexion, extension, lateral bending, and axial rotational moments were measured using a three-dimensional motion analysis system.

RESULTS

Female motion segments showed significantly greater motion (lateral bending: P < 0. 001, flexion: P < 0.01, extension: P < 0.05) and smaller endplate size (P < 0.001) than male ones. The segmental motion increased with increasing severity of disc degeneration up to Grade IV, but decreased in both genders when the disc degeneration advanced to Grade V. In male segments, the disc degeneration-related motion changes were significant in axial rotation (P < 0.001), lateral bending (P < 0.05), and flexion (P < 0.05), whereas female segments showed significant changes only in axial rotation (P < 0.001). With cartilage degeneration of the facet joints, the axial rotational motion increased, whereas the lateral bending and flexion motion decreased in female segments. In male segments, however, motion in all directions increased with Grade 3 cartilage degeneration and decreased with Grade 4 cartilage degeneration. Subchondral sclerosis significantly decreased the motion (female: axial rotation, P < 0. 05; extension, P < 0.05 vs.- male:flexion,P < 0.05). Severity of osteophytes had no significant association with the segmental motion.

CONCLUSION

Axial rotational motion was most affected by disc degeneration, and the effects of disc degeneration on the motion were similar between genders. Facet joint osteoarthritis also affected segmental motion, and the influence differed for male and female spines. Further studies are needed to clarify whether the degenerative process of facet joint osteoarthritis differs between genders and how facet joint osteoarthritis affects the stability of the spinal motion segment.

Primary lumbosacral stability after open posterior and endoscopic anterior fusion with interbody implants: a roentgen stereophotogrammetric analysis

STUDY DESIGN

After posterior stabilization of the spondylolytic lumbosacral level, mobility of the fused vertebrae could be studied before and after an additional anterior endoscopic interbody fusion using roentgen stereophotogrammetric analysis.

OBJECTIVE

To determine the in vivo primary lumbosacral stability of additional anterior interbody fusion after transpedicular screw fixation.

SUMMARY OF BACKGROUND DATA

In vitro studies indicate a significant decrease in segmental motion after pedicle screw fixation and additional anterior fusion. Roentgen stereophotogrammetric studies demonstrate the adequacy of transpedicular lumbar instrumentation in posterolateral fusions. There are no studies examining the effect of additional anterior interbody fusion after posterior instrumentation in vivo.

METHODS

In this study, 15 patients with low-grade spondylolisthesis at L5-S1 underwent a two-stage open posterior and endoscopic anterior lumbar fusion using carbon fiber (Brantigan I/F) cages. At surgery, tantalum markers were implanted into the fifth lumbar (L5) and the first sacral (S1) vertebra. All the patients were examined by roentgen stereophotogrammetric analysis after the first and second surgical procedures.

RESULTS

After implantation of the posterior pedicle system only, the mean intervertebral mobility determined by roentgen stereophotogrammetric analysis was 0.23 mm in the transverse (x), 0.54 mm in the vertical (y), and 1.2 mm in the sagittal (z) axes. After additional anterior endoscopic fusion with carbon cages, the remaining translation between the fused segment L5/S1 decreased to 0.17 mm in the x, 0.16 mm in the y, and 0.44 mm in the z axes.

CONCLUSION

Anterior endoscopic lumbosacral fusion significantly increases the primary stability of the posterior fusion with a pedicle system in two axes of motion.

Biomechanical effect of anterior grafting devices on the rotational stability of spinal constructs

In the thoracolumbar spine, frequently strut grafting is used to restore the anterior and middle column defects. Biomechanical stability of the surgical construct may be altered significantly depending on the type of anterior grafting devices. In this study, a biomechanical flexibility test was conducted to compare the stabilizing role of various types of anterior grafting devices, such as a polymethylmethacrylate block, tricortical iliac crest bone graft, one large Harms cage, and two small Harms cages using a calf lumbar corpectomy model. The Harms cage, especially one large cage, improved the axial rotational stability significantly in both anterior and posterior fixation groups as compared with the iliac bone or polymethylmethacrylate. No significant difference in the stabilizing role was found among different grafting devices in lateral bending, flexion, and extension. These results suggest that a more rigid spinal construct can be obtained by using a metal cage with improved friction at the cage-bone interface.

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.

Efficacy of pedicle screw fixation in the treatment of spinal instability and failed back surgery: a 5-year review

OBJECT

The goal of this study was to review retrospectively the outcome of 95 patients with various disorders leading to instability of the thoracolumbar and lumbar spine who were treated consecutively via a posterior surgical approach with pedicle screw fixation in which the Texas Scottish Rite Hospital system was used.

METHODS

All cases were managed according to the same protocol. Follow-up review averaged 29.6 months. Radiographic evidence of osseous union and the patient's current status were analyzed. Four screws were malpositioned, and there were two dural lacerations of a nerve root and one pedicle fracture. Deep wound infections developed in five patients (5.2%), and three patients had postoperative radicular pain. In one case, the rods disengaged from the screws; in four cases, hardware was removed but there were no broken screws. Neurological deficits improved in 85% of the surviving patients, and no patient was worse neurologically after surgery. The rate of osseous union was 96.8%. Three patients developed pseudarthrosis, one of whom was asymptomatic. Back pain improved in 80 patients. A solid bone fusion, however, was not necessarily associated with decreased back pain.

CONCLUSIONS

These results support the use of pedicle screw fixation as an effective and safe procedure for fusion of the thoracolumbar and lumbar spine and support the finding that complications can be minimal when a meticulous surgical technique is used. The proper selection of patients for surgery is probably the most important factor associated with good outcomes.