Biomechanical Evaluation On Primary Stability Of Wallis Interspinous Dynamic Stabilization Combined With Modified Facet Bone Graft

BackgroundDegenerative disc disease is one of the most common spinal disorders. Epidemiology showed that over 80 percent of population suffered from low back pain (LBP) in lifetime. Interspinous instability could occur due to degenerative disc disease, and finally lead to low back pain.Lumbar rigid internal fixation combined with fusion is an effective treatment for patients with symptomatic degenerative disc disease. Long-term follow up results showed that the fusion rate of lumbar fusion could reach to 90 percent, while the clinical satisfactory rate was only 30 percent. Widely literature showed that rigid internal fixation could increase the fusion rate, rather than increase the clinical satisfactory rate apparently. The reason was that rigid internal fixation increased the stress of adjacent segment. The abnormal stress focused on the adjacent disc and articular process, and resulted in abnormal range of motion and corresponding pathological changes, and finally led to secondary stenosis, spondylolisthesis,and instability. Some of the patients'low back pain recurred due to the above secondary diseases and subjected to re-operation.Ghiselli studied 215 patients with lumbar fusion and found that the re-operation rate of 5 years and 10 years postoperatively was 16.5 and 36.1 percent respectively. At the same time, he initiated two possible solutions:they are preserving the motion function and dynamic stabilization combined with fusions. The former was accepted and used widely in clinical until now. While the dynamic fixation systems include the follow 4 types:1 interspinous process distraction system; 2 interspinous process compression system; 3 ligament system with pedicle screw fixation; 4 semi-fixation with pedicle screw. Wallis system is one of the interspinous process distraction systems.The interspinous process distraction systems is a non-fusion dynamic fixation. The principle was to distract the spinous process of the segmental lesions, maintain the intervertebral height, absorbe the oscillation, load the burden of vertebrae and facet joints, keep the motion segments in a slight buckled state, distract the crooked ligamentum flavum to release the narrowness of the nerve root canal and the pressure of the spinal canal, and finally release the low back pain. In summary, the Wallis system not only prevented degeneration and instability caused by over motion, but also prevented the impact of complete fusion on adjacent segments.Senegas carried out a prospective, non-random, controlled trial, the clinical evaluation included nervous system examination, VAS score, ODI score. He found that Wallis system was superior than control group when referred to the above parameters. He concluded that Wallis system could effectively release the residual low back pain. Folman investigated 37 patients (23 males,14 females) implanted with Wallis systems, and the results showed that the ODI score dropped from 43 to 17 after implantation, the VAS of low back pain score dropped from 6.6 to 1.4 after implantation, while the VAS of low extremity dropped from 8.2 to 1.5. We could conclude from these results that the application of Wallis system achieved a satisfactory effectiveness. Many long-term follow up from multiple-center studies indicated that the Wallis system could effectively prevent the lumbar degeneration, and release the low back pain caused by adjacent segmental degeneration.Nevertheless, it is still controversial that whether Wallis system could reduce the recurrence of degenerative disc disease. Some scholars argued that some complications may caused by Wallis system, such as implants loosening, displacement, fracture of facet joints, and recurrence of disc herniation. Jacques carried out a long-term survival analysis about postoperative Wallis implantation. In this study,142 patients with Wallis implantation was followed up during 1987-1994. He found that 30 patients of 142 (21.1%) were subject to re-operation. Among them, 26 patients'Wallis systems were removed.Faced with such a high rate of re-operation, we tried to investigate the dynamic stabilization combined with fusion technique, and we have applied for the 2009 Scientific Research Project of Shenzhen:the clinical application of Wallis fixation system combined with facet joints fusions in the treatment of degenerative disc disease (NO.200902057), modified bone graft fusion of facet joints was taken. Some other representative dynamic fixation techniques combined with fusion were Twin flex dynamic fixation, AO/Dynamic Compression Plate system, MOSS fixation. However, there are problems unsolved about the feasibility and clinical effectiveness of this novel technique, the fusion rate of facet joints under dynamic fixation and the postoperative primary stability. Biomechanical evaluation should be carried out to confirm the theoretical basis of this technique. There are no relative studies reported.Based on the above mentioned arguments, we carried out biomechanical evaluation on primary stability of Wallis interspinous dynamic stabilization combined with interspinous fusions. Our study was designed to provide experimental data for clinical application.Objective1,To explore the feasibility of modified bone graft fusion of bilateral facet joints on lumbar specimens.2,To measure the three-dimension lumbar active degree after single segment Wallis interspinous dynamic stabilization combined with bilateral interspinous fusions postoperatively.3,to provide biomechanical data for the clinical application of this new technique and lead a guidance for whether adorning orthosis or not.Methods9 segments (3**L1/L2,3*L3/L4,3*L5/L6) of lumbar specimens were obtained from three homologous newborn calves (2-3 months young,450-500N weight, Shenzhen bright farm). We remove the muscle around the vertebrae, and preserve the ligaments and facet joints. Before formal experiment, pre-experiment of lumbar facet joints bone graft fusion and Wallis implantation were taken on one calf specimen. Auto agglutinative polymethacrylate was used to pouring on the vertebrae and adjacent discs. Among these 9 lumbars, two broken specimens were excluded, and the residual 7 specimens were subject to complete tests. Three-dimensional motion tester (MTS 858Mini Bionix) was used to measure the stability of six different motion directions:anterior flexion/posterior extension, left/right lateral bending and left/right rotation. The loading moment was 8Nm. Unloading was repeated three times to collected the figure to calculate the variance degree of the center area, and then load the moment again, calculate the ROM in the biggest loading burden. The motion direction was taken by random order to eliminate the system error caused by loading ways. To insure the proper negative loading rate, there were10 minutes interval between two different loading ways. To reduce the impact of spine viscoelasticity, we collected the data in 30s after every formal loading. Figures were collected by 3D digital system, and then input into the computer. We used DICM to analyze the figure data, calculate the shift quantities of the inter-vertebrae. The measure accuracy was 0.1°. Complete ROM (range of motion) and Nz (Neutral zone) were calculated. Specimens were divided into three groups:normal group (non-operation), Wallis group (segmental specimen instrumented with Wallis), Wallis-fusion group (segmental specimen instrumented with Wallis system combined with bilateral interspinous fusions). We compared the variation of lumbar active degree among these three groups under different states.Modified bone graft fusion of bilateral facet joints were taken after exposure of capsule of facet joints. Longitudinal incision of 1cm was taken according to the projection line of facet joint. Articular cartilage of facet joints was removed to expose the bone area and form implant bed of 1cm* 5mm* 2-3 mm. cancellous bone was filled. Finally, capsule of facet joints was sutured.All statistical analyses were conducted using SPSS software package (version13.0; SPSS Inc). Data were shown as means±standard deviation (SD). Statistical analysis of ROM among different states was performed by analysis of variance with LSD test. Green house-Geisser calibration was used when the comparison did not satisfy the condition of sphericity test. Difference was considered to be statistically significant at P<0.05.Results1. Modified bone graft fusion of bilateral facet joints is a simplified operation. It could obtain good exposure, removal of articular cartilage, adequate quantity of bone graft and complete repair of facet joint capsule.2. In flexion and extension state, the ROM of Wallis-fusion group was significantly decreased when compared to the normal group (p<0.05), while there was no significant difference of ROM between Wallis-fusion group and Wallis group (p>0.05); the Nz of Wallis-fusion group was increased when compared to the Wallis group (p<0.05), while there was no significant difference of Nz between Wallis-fusion group and normal group (p>0.05).3. In lateral bending and rotation state, there were no significant differences of ROM and Nz among the three groups (p>0.05). Both of the ROM and Nz of Wallis group were significantly reduced when compared to normal group in flexion and extension state (p<0.05). While in the lateral bending and rotation state, there were no significantly differences of ROM and Nz between Wallis group and normal group (p>0.05).Conclusion1. Wallis interspinous dynamic stabilization combined with modified bone graft fusion of bilateral facet joints is a simplified operation. It could obtain good exposure, removal of articular cartilage, adequate quantity of bone graft and complete repair of facet joint capsule.2. Wallis interspinous dynamic stabilization combined with bilateral interspinous fusions could increase the postoperative primary stability in flexion and extension state. Though the stability under lateral bending and rotation was as normal, the stability under flexion and extension state was less than single Wallis fixation. It suggested that orthosis should be adorned postoperatively.3. Wallis implantation could increase the stability under flexion and extension state; while it did not impact the stability under lateral bending and rotation.