| BackgroundSpinal fusion (spondylodesis) is a common surgical procedure used to treat instabilityor spondylolisthesis caused by degenerative changes or trauma, by fusing two adjacentvertebrae together to restore mechanical stability. In the United States, there are over200,000patients underwent such surgical procedure. Autologous iliac bone graft wereinitially used to achieve spinal fusion, since such graft are prone to fractures, donor sitecomplications and limited availability, medical practitioners gradually began to exploreother artificial interbody fusion cages.The invention of Bagby and Kuslich fusion cage (BAK cage) brought spine fusion toa new era, from autogenous bone graft to cage. People had been searching for better cagedesigns and materials eversince. And cylinderical mesh, spiral and box designs came intobeing. Based upon materials, fusion cages came to two categories: Metallic fusion cages,as made of titanium and its alloys, and non-metallic fusion cages, which contain twosubcategories as biodegradable cages and non-biodegradable cages. The main purpose ofusing fusion cage was to open and hold the height of the inter-vertebra space, restorephysiological curvature of the spine, and regain its stability, until bony fusion wereachieved. Early practice of fusion cages indeed yielded good clinical results with fewer complications. As practice became widespread, there were cage related complicationsworthy of notice. Some of these complications were due to defects in cage designs,especially on spiral cages. Others were caused by material itself, which could be seen inboth metallic cages and non-metallic cages. Metallic cages tended to have a high elasticmodulus, as stainless steel had an elastic modulus of220GPa, and titanium alloy of110GPa, both were much higher than that of the human vertebrae, which was only3.8-11GPa (10-57fold). Cages of high modulus could shield stress and prevent theosteoblasts inside the cage from getting enough stress stimuli, lack of which could slowbone formation and even lead to osteolysis.High modulus cages may cause osteoporosis of the adjacent vertebra, subsidence, andloss of height of inter-vertebra space. What’s more, metallic fusion cages are notradiolucent, which interferes fusion evaluation under X ray. It can also cast artifactsduring CT scans, which makes it difficult to evaluate bone ingrowth.Compared with metallic cages, carbon fiber cage shields much less stress. But thebits broke off the carbon fiber cage often induce aseptic inflammation. PEEK cage mayhave an elastic modulus of3.57GPa, which was a close match to cortical bone, yet it isbio-inert and non-biodegradable. This makes it hard to form intimate contact with bonetissue. And further observations are required to determine whether there were any longterm complications. Unlike PEEK material, the mechanical properties of the PLLA cageare less sufficient. PLLA often degrades before bony fusion occurs. Thus cage collapse,disc height loss and spine instability are often observed. And degradation products likelactic acid, once concentration gets too high, could induce aseptic inflammation andosteoporosis.In conclusion, as the clinical application of current cages suffering from all kinds ofdefects, it’s imperative to search for new cage material. Recently, novel PCL-TCPmaterial has showed excellent biocompatibility both in vivo and in vitro. It has acompressive modulus6.38MPa, close to cancelous bone. The scaffolds are manufacturedusing fused deposition modeling technique (FDM). It has a hive like layout of0/60/120°,and a porosity of75%, which facilitates bone ingrowth. Therefore, the novel PCL-TCP material has the potential to compete with other existing cage materials. CylindericalPCL-TCP fusion cage were made with a radius of5mm，and height of5mm. And itsfusion efficacy was evaluated compared with the Ti6Al4V fusion cage.ObjectiveDesign and preparation of absorbable cylinderical PCL-TCP cervical fusion cage insheep. To evaluate fusion performance of PCL-TCP cage without bone graft in sheepcervical fusion model, and to compare with commonly used Ti6Al4V cage withautologous bone, as to bone ingrowth, osteointegration capabilities, and their efficacy inrebuilding local mechanical stability.MethodsThe shape of the fusion cage was designed upon analysis of the current availablecages. Levels in sheep vertebra, sharing similarities with human cervical vertebra, werechosen for discectomy.18adult female Small Tailed Han Sheep (Age:16-24months;Weight:36-48kg) were used for this study. Group1: PCL-TCP cages without bone graft;Group2: Ti6Al4V cages with autologous iliac bone graft. Each sheep underwent anteriorcervical discectomy and fusion with a PCL-TCP cage or Ti cage at C2/C3or C3/C4levelrandomly. At6,9,12month post-operation, experimental animals were sacrificed toobtain specimens, which were subsequently subjected to X-ray, Micro-CT and histologyanalysis. Mechanical evaluations were performed on6and12month groups. Fig.1：Experiment design18sheep were implanted with PCL-TCP cage alone orTi6Al4V cage with autograft at C2/C3or C3/C4level randomly. X-ray, Micro-CT andhistology analysis were performed at6,9,12month post-op. Mechanical evaluation wereperformed at6and12month post-op.ResultsThe radiological and histological analyses showed no evident cage migration,fracture or infection during the study period. The biodegradable PCL-TCP cage was aseffective as the Ti6Al4V cage combined with autogenous bone graft in performinganterior cervical fusion in sheep model. With the help of autologous bone graft, bonybridge occurred earlier in Ti6Al4V group than PCL-TCP group (6month vs9month).Although Ti6Al4V group showed better preliminary mechanical improvements thanPCL-TCP group at6month, they all achieved bony fusion at12month, and there were nosignificant difference on the mechanical properties between the two groups (ROM andstiffness tests, p>0.05). At12month post-operation, the PCL-TCP group showed a higherB/I (bone/interspace ratio) of88.10±3.63%, which is2.6fold of the Ti6Al4V group(33.74±2.78%)(p<0.05). The lower B/I ratio in the Ti6Al4V group implied the stressshielding effects inside the metallic cages, and hence the less bone volume in the pores.The PCL-TCP group also exhibited superior osteointegration capability with a higher boneand cage contact surface/cage perimeter ratio (CS/PC) of79.31±3.15%, which is1.35foldof the Ti6Al4V group (58.44±2.43%)(p<0.05).Conclusions1、The biodegradable PCL-TCP cage alone had the same efficacy as the Ti6Al4V cageaided with autograft in performing anterior cervical spine fusion in sheep cervical model.Once fused, there were no significant differences on mechanical properties between thetwo groups;2、Compared with the Ti6Al4V cage, the biodegradable PCL-TCP cage had a lower elasticmodulus， hence less stress shielding effect and higher bone volume fraction andbone/interspace ratio (B/I); 3、Due to excellent biocompatibility, the biodegradable PCL-TCP cage was albe to formintimate contact with bone. Compared with the Ti6Al4V cage, the biodegradablePCL-TCP cage had superior contact surface/cage perimeter ratio (CS/PC)(p<0.05), whichmeans superior osteointegration and less cage migration and subsidence;4、 The biodegradable PCL-TCP cage was capable of keeping the height of theinter-vertebra space until bony fusion achieved. So, with excellent biocompatibility and notoxicity，the PCL-TCP showed great potential to be an ideal cage material.5、Autologous bone graft was effective in promoting early bony fusion in the implantationarea, but it had no evident effects on promoting early bone ingrowth in the pores ofTi6Al4V cages. |
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