A Sufficient Compression Fixation By Pedicel Screw System For Promoting Posterior Lumbar Interbody Fusion In Treatment Lumbar Dislocations, Spondylisthesis And Spondylolysis--The Biomechanics Study And Clinical Research

IntroductionLumbar spondylolisthesis is not an infrequent finding and has been reported in approximately 6% of the adult population. Spondylolisthesis causes segmental instability with symptomatic nerve root compression that is unresponsive to conservative treatment, and it may necessitate decompression with stabilization. The surgical management of spondylolisthesis is a challenge because of the difficulties in achieving a reliable arthrodesis in the face of high mechanical forces in lumban spine. The lumbar spine can provide a hostile environment for successful bone fusion as it bears substantial axial, torsional, and translational loads. Translational loads are particularly relevant at the lumbosacral junction where the local anatomy acts to convert axial loads into translational and angular resultant force vectors. Furthermore, it is difficult to resect a disc completely or to perform a complete corpectomy. On one hand, aggressive endplate debridement may result in the loss of integrity of this vital component of the axial load - bearing complex. On the other hand, incomplete debridement may result in pseudar-throsis due to inadequate preparation of the graft bed.Several surgical techniques have been conducted using various internal fixation and fusion devices. Posterior lumbar interbody fusion (PLIF) is one of the established methods of treatment for spondylolisthesis. The advantages of PLIF over other types of lumbar fusion are that it dynamically decompresses the neural structures by holding the vertebral bodies apart, fusing them into a single motionsegment, and it may have a higher rate of success in single - level fusion. Pedicle screw fixation has been used to increase the fusion rate, to correct deformities, and to provide early stabilization. However, construct failure often occurs following PLIF because of the large lumbosacral loads and cantilever pullout forces at this region.A new method of lumbosacral PLIF that has been used successfully in spon-dylolisthesis at our institution involves the posterior decompression, interverte-bral foraminal enlargement, reduction and posterior interbody compression fusion achieved through compression on the segmental pedicle screw system to make the posterior walls of adjacent vertebra contact. As lumbosacral fixation techniques and theories continue to develop, further understanding in terms of their comparative biomechanical properties is necessary. In the present study, we try to provide some insight into the biomechanical, theoretical, as well as practical findings about interbody compression fusion for the treatment of spondylolisthesis. The purpose of the current study was to evaluate and analyze the biomechanical features of interbody compression fusion using a calf spine model of simulated spondylolisthesis and a three - dimensional nonlinear finite element method respectively.MethodsBiomechanical evaluation in a Calf Spine ModelSpecimen PreparationA total of 48 fresh calf lumbar spines (Lj.j motion segments) were used in this investigation. All specimens were examined by radiograph to exclude occult fracture or abnormality. In preparation for biomechanical testing, the calf spinal specimens were thawed at room temperature and stripped of all residual musculature, with care taken to preserve all ligamentous attachments and maintain segmental integrity. The proximal and distal ends of the specimen were fixed to the materials testing machine ( BIONDC 858, Material Test System, Eden, Prairie, MN, USA) using the self-made clamp and bone cement (PMMA).Biomechanical testingNondestructive tests were performed under axial compression ( 500 N) , flexion ( 10.0 Nm) , extension ( 10. ONm) , right lateral bending (10. 0 Nm) , and left lateral bending (10. ONm). Load displacement curves were recorded. Each test was performed over 3 full cycles and data from the third cycle were obtained for further analysis. A cyclic compression force was applied as conditioning (500 10 N at 1 Hz for 100 cycles) to remove excess fluid from the disc and return the disc to its predeath height.Each intact lumbar spine was tested first (intact). Consequently, an L4 laminectomy, a bilateral L4 ₅ facetectomy, and a radical discectomy ( with resection of both anterior and posterior longitudinal ligaments) were performed according to Panjabi method. The spondylolisthesis were stabilized by the conventional PLJF and interbody compression fusion (ICF) achieved through compression on the segmental pedicle screw system to make the posterior walls of adjacent vertebra contact respectively. All through the destabilizing and restabilizing procedures, great care was taken to avoid damaging the bilateral facet joints, and the capsules overlying the facet were left intact. The specimens were kept moist with saline spraying throughout the biomechanical testing to avoid the detrimental effects of ligament desiccation. These spondylolisthesis specimens were retested (PLIFand ICF).AnalysisStiffness values were calculated from the load displacement curves. A specific value (ratio of PLIF or ICF to intact) was calculated to evaluate the stiffness. Independent - samples T test analysis was performed to determine whether significant differences existed in stiffness and displacement between different surgical interventions. All significance comparisons were at P <0. 05. Statistical analyses were performed with statistical product and service solutions (SPSS) software.Finite element AnalysisNormal ModelFor the preparation of three - dimensional finite element model, L4 -L5 motion segment data were obtained from computed tomography (CT, Marconi MX8000, Philips) (at 2 mm wide increments) of the lumbar spine of a 19 -year - old man who has no abnormal findings on roentgenography. In the global coordinate system, the cross - sectional plane on CT was considered to be the XY plane, and the Z axis was set vertical to the cross - sectional plane of the center of the intervertebral disc. To simplify preparation of the models, the differences in shape of the L4 L5 vertebral bodies were ignored. L4 L5 motion segments comprising a total of 146411 elements and 227847 nodes were developed.A three - dimensional isotropic solid element was used for modeling the cortical bone, cancellous bone, endplate, annulus fibers and nucleus pulposus. The material properties of each element were determined from the literature. Ligaments were modeled using the cable element, and each element was arranged in the anatomic direction. The cross - sectional area of each ligament and its nonlinear stress - strain behavior were obtained from the literature. The facet joint was treated as a nonlinear frictionless three - dimensional contact problem.Spondylolisthesis and fusion ModelsSpondylolisthesis model was prepared by resecting supraspinous, interspi-nous, posterior longitudinal flavum ligament and bilateral facet joints of L4 - 5 , as well as posterior half nucleus pulposus and vertebral arc on the normal model described above.The interbody fusion model was prepared by replacing the material properties of the outermost layer of the annulus of the standard model with those of cortical bone and by replacing the material properties inside the annulus and nucleus with those of cancellous bone.The conventional PLIF model (PLIF model) was prepared by adding pedicle screw system to the interbody fusion model described above, which consist of a total of 435968 elements and 84017 nodes. The interbody compression fusion model (ICF model) was prepared by making the posterior walls of adjacent vertebra contact and adding pedicle screw system to the interbody fusion model, the ICF model comprised a total of 633059 elements and 116946 nodes.Loading ConditionsWith the PLIF model and ICF model, the stress of 1200 N was applied to the screws. The inferior surface of the L5 vertebral body and its inferior facetwere fixed in all directions. 5 loads were used in this study. In compression loading, an axial compressive force of up to 500N was loaded on the center of the surface of the L4 vertebral body. In flexion - extension, lateral bending and torsion loading, the moment was applied on the surface of the L4 vertebral body up to 10 Nm. Each model was loaded while the response to compressive load, the response to flexion - extension, lateral bending and torsion loads, and the distribution of vonMises stress were analyzed and compared.Clinical evaluation17 patients, who suffered from isthmic and degenerative spondylolisthesis with more than 2mm displacement on the plain radiograph, were treated consecutively with posterior decompression, intervertebral foraminal enlargement, reduction and posterior interbody compression fusion achieved through compression on the segmental pedicle screw system to make the posterior walls of adjacent vertebra contact. The mean age at the time of surgery was 63.6 years ( range, 33 -76 years). Patients were followed up and evaluated for complications, neurological function and fusions.ResultsBiomechanical evaluation in a Calf Spine ModelThe interbody compression fusion showed higher rigidity than conventional PLIF. The rates of increase in stiffness in the compression, flexion and extension test were about 43% , 48% and 51% respectively (p < 0.05). The specific value of stiffness for ICF was 1.4 — 1.5 times greater than that of conventional PLIF.Finite element AnalysisResponse to loadsAt axial compression force, the ICF model showed the highest rigidity, followed by the PLIF and normal model. The rate of increase of the stiffness in the ICF model was about 31.6% , compared with the PLIF model. In all flexion -extension, lateral bending and torsion loads, the ICF model showed the highest rigidity. The rotation angles of flexion and extension decreased 37.6% and 39.9%. Above findings were similar to those in calf spine specimens.VonMises Stress DistributionRegarding the VonMises stress distributions under compression loading, in the ICF models, the stress in the vertebral body and in interbody was up to 81. 1MPa and 46. 332 MPa respectively, while in the PLIF models, stress was 5. 609 MPa and 9.049 MPa.In both internal fixation models with flexion - extension, lateral bending and torsion loading, stress concentration was located the junction of screws and cortical bone, and the stress in response to torsion was the largest, up to 112. 752 MPa.Clinical evaluationAll patients underwent successful follow - up for an average of 24. 5 months, ranging from 12 to 32 months. No intraoperative and postoperative complications were observed. The neurological function of the patients was restored partially or complete. No patient had neurologic deterioration after surgery. The lumbar spine was stable during physical examination and all patients were satisfied with the surgery. Radiographs showed implants to be satisfactorily positioned in all cases. There were no cases of hardware failure, loss of reduction, or painful hardware requiring removal. No significant movement was detected on the dynamic films and all tomograms confirmed the presence of a bridging fusion mass. Radiographic evaluation showed close conjunction of adjacent posterior edges of vertebral body in 12 of 17 cases, and the distance between the posterior edge less than 3mm in the other 5 patients.ConclusionsThis biomechanical study showed that the new surgical procedure, interbody compression fusion by pedicle screw system, for spondylolisthesis was a stable construct compared with conventional PLIF techniques.It showed that the finite element technique can be used to predict clinically relevant biomechanical parameters in the lumbar spine. Finite element analysis confirmed the observation that interbody compression fusion was more rigid than...