| Objective: To investigate the biomechanical changes and segmental stability, formation of the bone, fusion and degradation of the PDLLA (poly-dl-lactic acid, PDLLA) lumbar interbody fusion cage in the process of degradation in vivo. Methods: (1) For this study 48 healthy adult domestic pigs were randomly divided into experimental group(n=24) , underwent L4-5 discectomy and fusion with PDLLA cages were packed with autologous fragmented cancellous bone harvested locally, control group(n=24) the same amount of autologous bone. The specimens removed when pigs had been killed at 1 month, 3, 6, 9, 12and18 months after surgery. (2) The specimens were then biomechanical tested with flexion and extension,left and right axial rotations, left and right lateral bending in spine tester, to observe the stability of the intergration segment. Biomechanical testing of the motion segment L4-5 was performed under flexion, extension, lateral bending and axial rotation. The range of motion were determined. (3) Using X-ray, computed tomographic scans and other imaging equipments to observe the fusion of the segment surgery of the 48 surgical specimens. The fusion was assessed according to Baramki's criteria, with the cut-off point of degree II or over. (4) The samples was obviously to know the fusion of bone graft and degradation of materials with optical microscopy and electronic microscope at different phases. Results: (1) The non-destructive biomechanical testing has shown: the most stable in the right axial rotation and the most unstable in the extension, extension at the 3-12 months the stability of the experimental group was worse than the control group(P <0.05), the experimental group was stable than the control group at 18 on the left flexor and right axial rotation (P <0.05). The measurements at 6 months and 1 month in the same state (other than the flexion, left lateral bending in the experimental group and the extension in the control group) compared to a clear statistical difference (P <0.05).(2) The results of X-ray examination showed that no signs of integration at 1 month after the operation. At 3 months, there was on signs of bony integration in the experimental group. Two cases can be seen limited fusion in the control group. At 6 months, the osteopontin of calcify was formed in 1 case in the experimental group. 1 case incomplete fused and three cases fused can be seen in the control group. At 9 months, there was a little of Trabecular bone connectivity in 1case and most fused in 3 cases in the experimental group. 4 cases of the control group were completely fused. At 12 months, the fusion was good in the both groups, which was close to the normal bone tissue. At 18 months, there was a solid fusion which is basically same as the bone tissue around in both groups. Computed tomographic scans showed no signs of fusion in the two groups, at 1 month afer the surgery. At 3 months, there were more trabecular in the surgery region in the experimental group, but no integration. In the control group, there was 1 case began to fusion, the others had more trabecular bone. At 6 months, the surgical site can be seen "void", "island"-like structure in the experimental group, of which 1 case showed incomplete fusion. In the control group, the calcification of bone bridge (about 60%-78%) was formed, and 2 cases had been fused. At 9 months, in the experimental group, about 70%-90% of the material of the bone graft and the parts of the degradation were filled with the bridge formed by the calcified bone, expressed fusion. Four cases expressed fusion and about 75%-90% of bone bridge and calcification formation in the control group. At 12 months, the two groups had been fused, both about 85%-95% of bone bridge and calcification formation. At 18 months, two groups of the surgical site and surrounding tissue were similar, and about 90%-100% of bone bridge and calcification formation. The process of bony fusion was completely finished. (3) The histological observation showed that: there was no response inflammatory in the surgical site and in surgical time periods. At 1 month, the degradation of Materials was not obvious, but the outline of fusion cage was existed. The material degradation accelerated after 3 months, and reached the peak at 6 months, and the material degradation to the large fragments. At 9 months the materials degradation to sand-like, and mostly was absorpted. The fusion in the experimental group was worse than that of the control group 9 months ago. After that, the speed of fusion speed-up in the experimental group. At 12 months, no signs of defferences can been found in both groups. Ultimately, both groups reached a good fusion of the bone. 12 months later, the material was basically completed. (4) At 1 month some minor stratification in the surface of the cage, the irregular arrangement pore-like holes were visible fissures after amplified. At 3 months, the speed of the materials degradation accelerate, the cage figure seriously damaged. The hole-like fissures was increase and deepen compared with at 1 month. The surface was uneven,there was amplified grid, fracture decomposition and micro-spherical particles after enlarge, and the degradation accelerate from the inside to the out. At 6 months, the cage degradation for the fragments, showing honeycomb. Part of the region degradation for in powder. At 9 months, the material degradation for sand-like, the chips existence in the fibers and bone tissue. At 12 months, electron microscope can be seen a small amount of tiny particles existence in fibers and fresh bone tissue, At the same time, the vast majority of material was absorbed. At 18 months, The materials have been complete degradation and absorption by the body. Conclusion: (1) The biodegradable absorbable lumbar interbody fusion cage was the most stable in right axial rotation, then the stability was weak in extension. The mechanical properties of the bioabsorbable lumbar interbody fusion cage was not bad than autograts in extension. (2) The operation collapsed intervertebral segment wasn't observed through the X-ray examination and CT scan in the experimental stage. There was not a premature collapse of fusion before 6 months ago, and the fusion rate of the experimental group slower than that of the control group. The rate of the fusion speeded up after 6 months. Lastly, at 18 months the fusion of the two groups are similar. But there was no obvious differences at different time periods, and PDLLA cage autogenous bone can be used as a substitute. (3) The PDLLA material has good biocompatibility, and is able to maintain the mechanical stability before the fusion, even if the initial material have the impact on the fusion, but in the end the material was completely degraded and was replaced by new bone tissue, so as to bony integration was achieved. (4) The PDLLA cage beginning gradually degrade Since implanted into the body. At 1 month, the surface of the cage in touch with the organizations had degraded change, but not obviously. At 3-6 months the degradation of material was the fastest with the implanted time increased. At 9-18 months a slower rate of material degradation, access to speed up period of the absorption. At 12 months, that the material has basic fully degradation and absorption. At 18 months the material had completely absorbed.
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