| Spinal disease is very common. Cervical and lumbar diseases have the highest incidence, which account for60%-80%of morbidity rate in adults. For patients with spinal disease in which non-surgical therapies are ineffective, surgery is often needed. Intervertebral fusion, as a therapy for spinal disease and injury, has been used for a few decades; it is a milestone in the history of spinal surgery. The autologous iliac bone graft implant as the gold standard has high healing rate, though the complications are the major problem which cannot be neglected. It is reported that the incidence of complications associated with autologous iliac bone graft implant is25.3%. Moreover, autologous iliac bone graft is likely to subside due to insufficient strength, resulting in the loss of intervertebral height. The capacity of vertebral column and pressure-reducing effect will be directly affected. Bagby was the first to implant BAK cage system into human body in1988, which successfully achieved the desired purposes of maintaining intervertebral space height, increasing the stability of lumbar interbody fusion surgery and reducing the pressure on a nerve root. However, intervertebral cage still has some defects with respect to material and design, and therefore requires further modification.Depending on the used material, intervertebral cage is divided into non-absorbable cage and absorbable cage. Metal cage is the first applied non-absorbable lumbar cage. It is mainly made of stainless steel or titanium alloy, which has high stiffness and poor histocompatibility. Metal cage was then phased out as it affected post-operative MRI scan. Carbon fiber cage, PEEK cage and nHA/PA66cage are also made of non-absorbable materials, having modulus of elasticity similar to that of normal adult skeleton. But as the bone graft is degraded and absorbed, the fusion cage is not being accordingly absorbedand degraded. As a result, the compressive stress of interface between the vertebra and the bone graft will decrease. The stress shielding effect thus produced will affect the fusion speed and effect.Extensive studies have demonstrated that appropriate compressive stress will stimulate fracture recovery and accelerate bone graft fusion. Allograft bone is absorbable but it has restricted application due to its limited source, subsidence risk and disease dissemination in later stage. Intervertebral fusion cage made of bioabsorbable materials is being intensively studied both in China and internationally. Polylactic acid (PLA) is one of the most important candidate materials. Absorbable cage made of PLA overcomes the defect of stress shielding effect caused by the degradation of bone graft. Polylactic acid cage is not free from the following defects:â‘ Poor mechanical strength;â‘¡Rapid degradation of polylactic acid in human bodies, which often results in mechanical strength insufficient for the load on thoracolumbar spine before fusion finish. PLA cage tends to subside under the axial load of spinal column, and such phenomena as loss of intervertebral height and spinal instability are possible occurrences;â‘¢Poor histocompatibility. The accumulation of lactic acid may also lead to sterile inflammation and osteolysis. For all these reasons, polylactic acid fusion cage has found not extensive application.In view of the disadvantages of both absorbable and non-absorbable cages, we propose a new design in which the compressive stress is high at early stage in order to stimulate the fusion of bone graft, and cage subsidence is prevented at later stage by combining absorbable and non-absorbable materials. The new-type case combines n-HA/PA66with calcium sulfate/multi-(amino acid) copolymer composite to achieve these goals.Objective:To develop partially bioabsorbable interbody fusion cage (PBIFC), whose biomechanical performance was assessed by finite element analysis and in vitro mechanical test. The fusion effect of PBIFC was also assessed by lumbar interbody fusion experiment in large animals, in order to investigate the feasibility of such PBIFC to be applied as lumbar interbody fusion cage and to provide experimental basis for clinical application.Method:1. The new PBIFC was designed based on nHA/PA66cage which has been clinically applied for many years, and was jointly manufactured with a company.2. The lumbar spine of a29-year old male volunteer was scanned at Radiology Department at the Second Affiliated Hospital of Chongqing Medical University. The scanned images were inputted to the computer via the workstation. The finite element model of the third and fourth lumbar vertebrae was constructed and confirmed. Using this model, the implantation of partially absorbable cage or non-absorbable cage via the anterior approach was simulated; the models immediately and4weeks after lumbar intervertebral fusion surgery were respectively constructed, including the models immediately after the implantation of partially absorbable cage and n-HA/PA66cage and the models4weeks after the implantation of partially absorbable cage and n-HA/PA66cage. ANSYS12.10was used for finite element analysis. Axial load of400N and torque of lONm were applied respectively on L3, in order to simulate the movements of anterior flexion, posterior extension, rotation and lateral flexion. The stress on the intervertebral fusion cage and bone graft in different movements and the stress distribution on L4end plate were obtained.3. Separate spinal segments were made from lumbar vertebrae of fresh calf. Part of the fibrous ring and nuclecus pulposus were resected by anterior approach. Then the new PBIFCs were respectively implanted (in combination or separately). The implants were divided into different groups depending on the experiment purposes. The mechanical performance of combination-type cage under compressive load was first tested, followed by pull-out test on combination-type cage with or without bone graft. Finally, the stability of combination-type cage was assessed by testing the movement angles under vertically compressive load and torsion respectively.4. Goats were selected for in vivo lumbar intervertebral fusion experiment.16female goats were randomly divided into three groups:Group A: partially absorbable cage filled with auto logo us bone graft; Group B: n-HA/PA66cage filled with autologous bone graft; and autologous bone graft group, in which autologous cortical iliac bone graft was implanted. Either of the two types of intervertebral fusion cages and one autologous iliac bone graft was implanted in intervertebral spaces at L2-L5in each animal. X-ray examination was performed on the lumbar spine of goats in lateral position immediately after lumbar fusion surgery and every month. The fusion of bone graft as well as the changes in intervertebral space height before and after the surgery was observed. All the goats were sacrificed24weeks after the surgery for CT scan. Lumbar interbody fusion was observed and scored.Result:1. Partially absorbable cage was successfully designed and manufactured.2. Three-dimensional finite element model of L3/L4was constructed. Immediately after surgery, the stress on the bone graft in partially absorbable cage was higher than that in n-HA/PA66cage; the stress on the end plate in partially absorbable cage was lower than that in n-HA/PA66cage. The stress profiles of the end plate in the two models were not significantly different. The stress difference between the two models increased four weeks after surgery, compared with that immediately after the surgery. The stress profile of end plate in partially absorbable cage was greater than that in n-HA/PA66cage.3. The in vitro mechanical performance test of partially absorbable cage showed that it had similar torsion resistance, pull-out resistance and compression resistance as n-HA/PA66cage; the difference was not significant (P>0.05).4. In goat lumbar intervertebral fusion experiment, DSH before and immediately after surgery was not significantly different among the three groups; from4weeks to24weeks after surgery, DSH in nHA/PA66cage group was significantly different from that in partially absorbable fusion cage group and iliac bone graft group (P<0.05). DSH in partially absorbable fusion cage group and in iliac bone graft group was significantly decreased. From4weeks to16weeks after surgery, DSH was not significantly different from partially absorbable fusion cage group and iliac bone graft group. But the difference between the two groups was significant at the20st week to the24th week after surgery. DSH in iliac bone graft group decreased even more significantly. The lumbar intervertebral fusion with partially absorbable cage was not significantly different from that using iliac bone graft, as indicated by X-ray examination. However, nHA/PA66had the highest degree of lumbar intervertebral fusion (P<0.05).24weeks after the surgery, the lumbar intervertebral fusion at CT layers was scored in the three groups. The score of iliac bone graft group was higher than that of partially absorbable fusion cage group, but the difference was not significant (P>0.05); the score of PBIFC group was higher than that of nHA/PA66cage group, and the difference was significant (P<0.05).Conclusion:1. PBIFC designed and manufactured in this study has the appearance, internal structure and surface processing which match well with lumbar intervertebral structure. The strength and modulus of elasticity of PBIFC correspond to those of human skeleton.2. The compressive stress on the bone graft in PBIFC group can maintain at a steady level and that on the end plate is even smaller.3. PBIFC has sufficient compression resistance, pull-out resistance and torsion resistance.4. When applied in goat lumbar interbody fusion, PBIFC is able to maintain intervertebral space height. Lumbar interbody fusion was observed within the specified period, and the effect of lumbar interbody fusion was better than that of non-absorbable fusion cage with the same external structure.
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