Effects of in Vivo Bone Fracture Fixation on the physico-mechanical Properties of poly-5D/95L-lactide Bone Plates

An in vivo degradation study of poly-5D/95L-lactide (PLA95) bone plates was performed to determine the interaction between the fixation plate and the bone fracture. The PLA95 bone plates and screws were prepared by injection molding. Male adult New Zealand rabbits were used to establish a fracture gap vertical to the mid-portion of the right mandible. A mandibular fracture/repair model was designed to mimic the exact clinical fracture conditions using a combination of a micro-saw and an ultra-thin osteotome procedure. Resorbable PLA95 plates and screws were implanted and animals were sacrificed at 0, 1, 4, 8, 12, 16, 26, and 39 weeks. The physicochemical changes of the PLA95 fixation devices, including 3-point bending strength, molecular weight, and thermal analysis, were investigated after each in vivo aging period. Observations of the implantation time-dependent histology of the tissues around the interface of the screw insertion and fracture gap revealed healing progress under bone plate fixation. The molecular weight of the PLA95 plate and screw monotonically decreased with implantation time. On the other hand, the bending strength of the PLA95 plates first increased until week 8, after which it dramatically decreased with time. The results of the thermal analysis together with the peak separation analysis showed multiple endothermic peaks. The load of the fractured bone being transferred to the PLA95 bone plate might have played an important role in enhancing the bending strength of the fixation plate. Under a load-bearing situation, the PLA95 bone plate crystal underwent transformation either from the β-form to the α-form or from a slightly imperfect crystal to a better one, during in vivo degradation.