Bio-Integration and Bone Fixation Performance of Continuous Mineral Fiber-Reinforced Implants

CATEGORY: Basic Sciences/Biologics; Other INTRODUCTION/PURPOSE: The integration of an implant into surrounding tissue as remodeling occurs is a characteristic associated with bone grafts or bone fillers, some of which are osteoconductive or osteostimulative. Many bioabsorbable polymer implants lack quiescent degradation and are associated with adverse inflammation. Recently, new bio-integrative bone fixation implants comprised of continuous mineral fibers and polymer were introduced. The implant’s high mineral content is intended to encourage an increased bio-integrative response, while the continuous fiber structure provides mechanical bone fixation strength. The study objectives were to evaluate the implant’s long-term bio-integration and ability to maintain fixation in a load bearing in- vivo model. METHODS: Twenty-four rabbits were studied over 104-week period to evaluate the bio-integration of fiber-reinforced bone fixation pins. The continuous reinforcing mineral fibers made up approximately 50% of the implant, comprised of elements found in native bone, including calcium, silica, and magnesium. The other 50% was comprised of poly (L-lactide-co-D, L lactide) (PLDLA) at a 70:30, L:DL ratio. Pins were implanted bilaterally, with three fiber-reinforced pins (test) implanted into the mid-shaft of one femur and three PLDLA polymer pins (control) into the mid-shaft of the other femur. Implantation sites were scored histologically at multiple timepoints to assess bio-integration by means of implant degradation profile, surrounding bone quality and tissue ingrowth. A separate group of twelve rabbits was studied clinically, radiographically and histologically over 12 weeks to evaluate the fiber-reinforced implant performance, compared to a stainless-steel implanted group, in a lateral femur condyle osteotomy model under full load bearing conditions. RESULTS: At 104 weeks, implant material fully eliminated in 11 out of 12 fiber-reinforced implants and in 6 out of 12 PLDLA implants. The fiber-reinforced group showed increased propensity for bio-integration throughout the course of the study, demonstrating a gradual degradation profile and much higher score of tissue ingrowth. Amount of polymer decreased from a score of 4.0 at 4 weeks to score 1.7 at week 26, score 1.0 at week 78 and 0.1 at 104 weeks. The polymer control underwent abrupt late stage degradation, with amount of polymer dropping from a score of 4.0 to 0.7 from 78 to 104 weeks. In the load bearing osteotomy model, the fiber-reinforced implants performed comparable to stainless-steel, demonstrating tighter bone-to- implant interface with no intervening fibrotic tissue. CONCLUSION: This study represents the first long term in-vivo evaluation of mineral fiber-reinforced implants demonstrating both bio-integration and orthopedic fixation. Quiescent bio-integration is a significant challenge for degradable orthopedic fixation implants. The implants must be mechanically strong for stable fixation while able to gradually integrate with surrounding bone without adverse effects. Continuous fiber reinforced implants proved the unique potential to meet this challenge with a fiber structure that provides fixation strength and is comprised entirely of minerals found in native bone. An increased level of mesenchymal tissue ingrowth, combined with the absence of local or systemic adverse response, demonstrates excellent bio- integration.