Research On Partially Bioabsorbable Interbody Fusion Cage For Lumbar Fusion Of Rhesus Monkey Models

Interbody fusion cage which can significantly improve the effect ofspinal fusion has been applied to clinical use. The long-term clinicalfollow-up studies found that cage is still defective in materials andbiomechanics. Our research group proposes a new design conception that isproviding compressive stress stimulation, promoting interbody fusion atearly stage, to prevent the collapse of the interbody at lately stage based onmaterials science and biomechanics. According to the Rhesus monkeylumbar physiological and anatomical structure, partially absorbableinterbody fusion cage (PBIFC) with the combination of absorbable lid andnon-absorbable base was designed and made out. Rhesus monkey waschose as experimental animals for the similarity in biomechanical of spinewith human. Fusion surgeries were operated on Rhesus monkey lumbar,PBIFC and non-absorbable cage were implanted in the L3/L4and L4/5respectively and randomly, at post-operation instancy,4weeks,12weeksand24weeks, the results of the fusions were recorded based on generalobservations, radiology and histology, to provide reliable evidences forfurther clinical experiments and applications. Objectives:1. To design the partially bioabsorbable interbody fusion cage (PBIFC)applied on Rhesus monkey, and make the sample of PBIFC.2. Lumbar fusion experiments in the Rhesus monkeys, to assess fusioneffect of PBIFC.Methods:1. Authorized by the ethics committee of the second affiliated hospitalof Chongqing Medical University, by the X-ray and CT scan examination oflumbar, vertebral anatomical structures of rhesus monkey were expounded,combined with PBIFC design conception. PBIFC samples were producedand sterilized.2. Rhesus monkeys (4male and4female) were selected for the lumbarinterbody fusion experiment. The PBIFC and n-HA/PA66cage fully loadediliac implant in L3-L5vertebral space random respectively with each rhesusmonkey. Experimental animals with successful surgery in the immediate andpostoperative4,12and24weeks take the X-ray examination, measurementand calculation the disc space height (DSH) changes. after24weeks theanimals were sacrificed and underwent CT and Micro CT scans to observethe interbody fusion; subsequently taken the fusion segments specimens tocut into hard slice, toluidine blue and VG staining were observed toabsorbable material’s degradation in vivo, as well as bone graft andintervertebral fusion and new bone formation. Results:1. The shape of PBIFC is similar to renal, the absorbable poly-aminoacid/calcium sulfate lid is light yellow, the non-absorbable n-HA/PA66recess is beige; the upper and lower surfaces polished to increase thecoefficient of friction, smooth rounded corner, The total length:12mm, thetotal width:8.5mm, the total height:5mm, wall thickness:3mm.Absorbable lid length:12mm, width:8.5mm, high:2mm, wall thickness:1mm, thickness:1mm.2. In lumbar fusion in Rhesus monkey models, preoperative,postoperative instancy and4weeks, DSHs have no significant difference inall groups; significant differences appeared in DSH at postoperative12weeks and24weeks, the DSH of PBIFC group reduced more comparedwith n-HA/PA66group, in PBIFC group, a small amount of new bone canbe seen in the interbody space, the visible part of the flux lines, but not yetintegrated at postoperative4weeks; the apparent new bone has grown intoPBIFC at postoperative12weeks;7interbody fusions and1partialinterbody fusion were seen in PBIFC group versus4interbody fusions and4partial interbody fusions were found in n-HA/PA66group atpostoperative24weeks. All cages did not suffer the displacement anddamaged. Postoperative24weeks, CT scans showed that the bonyconnection between bone graft and vertebral can be seen in most specimensof PBIFC group; but partial bony connection of bone graft and vertebral only can be found in n-HA/PA66group’s, the degrees of fusion based onCT scans: compared with full mark of30, PBIFC group with26.25±2.49is significant higher than n-HA/PA66group with19.16±3.06(P<0.05).Postoperative24weeks, the Micro CT scan results showed that thetrabecular bone of internal cage of PBIFC group integrated with the upperand lower vertebral trabecular bone fully, trabecular bone morphology isnormal as vertebral, without any morphological differences. In n-HA/PA66group, some trabecular bone fracture, smaller and thinner trabecular bonecompared with vertebral can be seen. Through the parametric analysis,PBIFC group shared the better quality in BMD, BVF, BS/BV, BV/TV, Tb.N, Tb. Th and Tb. Sp than n-HA/PA66group. Postoperative24weeks,mineralized bone tissue slices toluidine blue and VG staining showed thatthe poly-amino acid/sulfuric acid calcium lid has been completely degradedand absorbed, non-absorbable recess was firmly fitted with bone tissue; amount of new bone in cage formatted at the boundary between vertebraland PBIFC, the fusion of bone graft was complete at interface in PBIFC,newborn trabecular owned good shape, accordance with the vertebraltrabecular morphology, the upper and lower ends of the trabecular is fullyconnected in PBIFC group; in n-HA/PA66cage, the trabecular morphologyis smaller and shorter than PBIFC group, and the upper and lower ends oftrabecular was not completely connected, cartilage exist in cage central. Conclusions:1. PBIFC satisfies the Rhesus monkey lumbar physiological andanatomical structure, combines with absorbable poly-amino acid/calciumsulfate lid and non-absorbable n-HA/PA66recess. Strength of PBIFC issuitable, lid and recess combine closely, and the elastic modulus ofmaterials is similar with vertebral.2. Lumbar fusion experiments with PBIFC on Rhesus monkey,confirmed that the PBIFC group was better than the same-shapednon-absorbable cage in fusion rate, fusion quality and intervertebral heightmaintain by radiological and histological evaluation within the observationtime.