Design And Experimental Research Of New Type Of Versatile Assemble Interbody Fusion Cage

Background and ObjectiveInterbody cages have been certified to restore disc height and to increase stability of the spinal segment, and thereby enhance fusion in the surgical treatment of low back pain,spondylolisthesis and degenerative lumber disease.Concomitantly, the use of these cage devices for lumbar interbody fusion has rapidly gained popularity, cage design and choice of cage material also play a crucial role in long-term results. Techniques of interbody fusion include approaching from anterior, posterior, lateral, posterolateral and transformainal.Traditional open surgery has been evolving toward more minimally invasive techniques in efforts to reduce approach-related morbidity, improve cosmesis, and speed recovery and return to normal activity. Approaches to lumbar discectomy and interbody fusion have been greatly influenced by recent advances in minimally invasive spine surgery, and most techniques have been completed through the use of tubular exposures and endoscopic visualization. It is necessary that new types of cage devices be designed for suitable tubular exposures and endoscopic visualization. As we know, general cages have more large volume comparison with various systerms of narrow tubes and endoscope, sometimes working coaxially through tubular portals limits anatomical access and instrument manipulation, and reliance on two-dimensional images for visualization can be challenging. These limitations may result in compromising objectives for the minimally invasive benefits. A novel, assemble cage , which has relative little volume when on condition of separative ,may be easily access to the interbody via a tubular and endoscopic route; meanwhile it can be used in open surgery for interbody fusion depending on practical requirement when combinational state.Differences in cage design/materical and cage/surgical technique may also significantly affect the biomechanics behavior, fusion effect and even stability of the fused spine segment. The purpose of this study was to design and manufacture a type of ALIFC,and verificate its biomechanics behavior, stability of calf lumbar segment in vitro, fusion effect in goat models and contribution to recovery stability of lumbar isthmic spondylolysis. So experiment study consisted of five parts. First, new type of assemble cage was designed and manufactured for double purposes based on the requirement of MILIF.Secondly, to certificate ALIFC biomechanics behavior by mean of created two FEMs, FEMs were conducted in all directions including axial compression, flexion, extension, lateral bending, and rotation. The evaluation results will focus on endplate stress distribution, peak stress of von Mises, stress of cage.Thirdly, to evaluation its biomechanical behavior through calf specimens study in vitro. Calf lumbar specimens were randomly divided and tested for range of motion (ROM) under the condition of flexion/extension,bending laterally and axial rotation(left/right),axial compressing load and pull-out strength. Forthly, to evaluation cage fusion effect in establishing goat models of lumber interbody fusion. ALIFC were implanted into intervertable space in healthy adult goat moedls,these goat were nured,observed and evaluated for fusion effect through X-ray observation in postoperational time until finish of experiment.Fifthly,to evaluate ALIFC function in treating lumbar instability of calf specimen in vitro, instabile lumbar specimen in calf with single isthmus cleavage were made and put them into biomechanicis test after implanted three types of cages with pedicle screw fixation.Methods①To design the new type of versatile assemble interbody fusion cage and it was manufactured by professional medical instrument company.②3D finite element models were developed, reproducing the human L3-L4 spinal unit in intact condition and after implantation of two different cage models. The instrumented models reproduced the post-operative conditions resulting after implant of the different cages including assemble or separate ALIFC. Simulations were run imposing various loading conditions including axial compression, flexion, extension, lateral bending, and rotation under a constant compressive preload. The evaluation results derived from FEMs data will focus on endplate stress distribution, peak stress of von Mises, stress of cage . Stress distributions on the bony surface were evaluated and discussed.③To evaluation ALIFC biomechanical behavior in vitro by mean of FSU of calf ,twenty-four calf lumbar specimens were randomly divided into 6 groups with 4 specimens in each group including control group,destabilized group,autogenous graft group,traditional PEEK cage group,ALIFCage-1 group,ALIFCage-2 group.All groups were tested for range of motion (ROM) under the condition of flexion,extension,bending laterally and axial rotation(left/right),axial compressing load and pull-out strength using a spinal three dimension analysis system.The strenghth, stiffness of of ALIFC, the different mechanical behaviour influence on FSU motion of calf lumbar when its situation of separate or combined were biomechanically tested and compared to each other by mean of tested data in all direction.④The ALIFC was implanted into intervertebral disc of goat models to find out its influence on spine bone fusion at different phases by X-ray compared with normal interbody fusion cage. Sixteen healthy adult goat were used in this experiment, these ALIFC were implanted into intervertable space,then these goat were nured,observed and evaluated for fusion effect in postoperational time until finish of experiment. During the animal application, fusion rate of ALIFC and fusion process with time change were observated by X-ray exposure and compared with normal bone graft group .⑤To evaluate ALIFC function in treating lumbar instability of calf specimen in vitro, instabile lumbar specimen with single isthmus cleavage were made and put them into biomechanicis test after implanted two types of cages or simultaneously with pedicle screw fixation. The procedure is as follows : A normal FSU→B FSU of single isthmus cleavage→C FSU of single isthmus cleavage with ALIFC implantation (in separate state)→D FSU of single isthmus cleavage with ALIFC implantation (in assemble state)→E FSU of single isthmus cleavage with left pedicle screw fixation with ALIFC implantation (in separate state)→F FSU of single isthmus cleavage with left pedicle screw fixation with ALIFC implantation (in assemble state)→G FSU of single isthmus cleavage with bilateralis pedicle screw fixation with ALIFC implantation (in separate state).Corresponding changes of left/right axial torsion moment and angular displacement,lumbar intervertebral segmental stiffness under various conditions were measured by dynamic multidimensional biomechanical fatigue testing machine and homogeneity of variance test setting the level of statistical significance to 0.05.Result:①Based on the requirement of MILIF,new type of assemble cage was designed and manufactured for double purposes of either MILIF or open interbody fusion surgery. The ALIFC was rectangle shape and made of pure PEEK material according to the practical need of MILIF.It is convenient that new cage be easily fabricated in preoperative preparation and instrument manipulation of inserting fusion segment space through tubular and endoscopic system in operation period.②The FEM analyses approximated the loading situation existing in the initial period after implantation the cage. Following cage insertion, high strains and stresses were concentrated in the contact areas between the cage and endplate. Contact stresses around the implants intend to be concentrated around the periphery of the device. After implantation of ALIFC, The stiffness of new cage in assemble condition was similar to the traditional cage on biomechanical data in FEM. The stresses were symmetrical distribution in lateral areas of the endplate when a separate cage was used in the place of a combination cage, but comparison with the separate cage, the stresses in the endplate decreased significantly at center places where the cage wasnot contacting the endplate .The stress of cage was high in rotation moment, the maximum stress was found in assemble model. The largest difference of stress was found in rotation. In extension, difference of stress was least because almost no stress of cage was measured in both models , and most stress was distributed at the posterior part of the cage. A comparison of peak von Mises stresses on the endplates for a 500-N compression and a 15N·m torsional moment in various loading conditions for spines in the new cage. The stresses were symmetrically distributed in lateral areas of the endplate when a separate cage was used in place of a assemble cage. A maximum stress of 43×107 MPa was seen on the endplate in bending with the separate model, as compared with 25×107 MPa in assemble model .③The maximal compressive load and pull-out force of the ALIFC was 9038N and 726N respective and was in coincident with the maximal demand of safety in human spine. ROM:Statistically significant difference was observed between ALIFCage-1 group and intact group,destabilized group,AG group,under the condition of flexion/extension,bending laterally,but no statistically significant difference compared with T-PEEKCage group and ALIFCage-2 group;Under the condition of axial rotation there was no significant difference between ALIFCage-1 group and intact group,AG group but statistically significant difference was existed compared with T-PEEKCage group,ALIFCage-2 group.Axial compressing load and pull-out strength:The axial compressing load and maximum pull out strength of ALIFCage-1 group were increased and decread compared with T-PEEKCage group, ALIFCage-2 group respectively, which was no statistically significant difference.④During the animal application, all goat models were living and checked by X-ray observation. In fusion test study, ALIFC has high fusion rate alike autogenous bone graft , it can anastate and retain the lumbar physiological antecurvatur, the middle and posterior of the intervetebral disc height, supply enough intension and time for the whole progress of bone graft fusion, prevent the subsidence and migration of the cage.⑤The resection of posterior zygapophysial joint lead to a mechanical instabilityof axial compression, flexion, extension, left/right bending, left/right axial torsion. zygapophysial joint had an important sense on torsional stability,especially on the opposite side.Using ALIFC to treat lumbar instability of calf specimen of single isthmus cleavage had good simulation outcome, it can get all directions stabilization combind with pedicle screw.ConclusionALIFC have enough biomechanical stress and strength on assemble or separate condition ,it can provide enough primary stability for lumbar intervertebral fusion and new cage is a suitable device for load-bearing implants. This type of ALIFC has perspective use to human in the future.