A Novel Biodegradable Hybrid Fixation System Incorporating Mg Screw Coated with Degradation-Resistant Polymer Film for Fracture Fixation

摘要

Bone fracture is common and fracture healing especially osteoporotic fracture healing is a major challenge issue in orthopaedic clinics. The incidence of osteoporotic fractures was reported to be over 30% in people aged 50 years or above. Fixation of osteoporotic fracture is demanding as the current metallic implants developed for fracture fixation are made of permanent metals, such as stainless steel or titanium (Ti) that are too rigid, which induces stress shielding effects and even accelerates bone loss of the fractured bone after fixation. In addition, the healing process of the osteoporotic fracture is also impaired because of the decrease in number and function of bone stem cells from both bone marrow and periosteum.;Biodegradable magnesium (Mg) as a potential biomaterial for orthopaedic application has attracted great attention in recent years. Biodegradability, similar mechanical properties and osteopromotive effect make Mg as the appropriate candidates for orthopaedic implants. However, Mg alone cannot provide sufficient mechanical support over the period of time required for fracture fixation and healing due to its rapid degradation in the early stage. Therefore, a thin layer of polymer coating film was prepared on the surface of Mg implant to control the degradation rate and it was combined with the common titanium (Ti) implants together to develop a hybrid fixation system. In this innovative design, Ti plate and screw(s) were able to provide enough mechanical strength and Mg screw could provide initial stability at the fracture line and its degradation could promote the fracture healing. Furthermore, the coating film can protect the Mg screw(s) from direct metal-to-metal contact with Ti-plate, therefore prevent the electrochemical corrosion of Mg screw head and also reduce its degradation.;This thesis investigated the effect of Ti-Mg hybrid fixation system for osteoporotic long bone fixation and its enhancement for fracture repair. There were both in vitro and in vivo studies involved in this thesis. In the in vitro experiments, Mg pins were prepared with a series of polymer coating films with various preparation conditions to investigate the corrosion resistance enhancement by the coating films. Two different batches of immersion tests were performed to prove the enhancement of corrosion resistance of polymer-coated Mg pins and obtained the best coating films which showed the slowest corrosion rate. Particularly, Mg pins coated with biocompatible polymer, poly-lactic acid (PLA) using developed wet-casting method showed excellent protection effect and these coating films were prepared on the surface of pure Mg screws for in vivo experiments.;In the in vivo experiments, eighty-four female New Zealand White Rabbits were used as the sizable experimental animals. An innovative "Z" shape osteotomy was conducted at the right tibia shaft of the rabbit, which was more closely to the clinical indications. A specifically designed dynamic compression plate (DCP) made of titanium and common Ti screws were used to fix the fractured bones. Coated Mg screw was put in the middle part of the plate where just through the fracture site (Mg group). The control group (Ti group) used fixation method with Ti screws only to fix the fractured bones. Radiography analysis, four-point bending mechanical test, histological and histomorphometric analysis were performed to evaluate the structural changes of new bone formation and fracture healing quality in tibiae.;The radiographic images showed that there was very significant difference (50%) in generated callus tissue areas between the Mg group and Ti group (n=6, p<0.001). Mechanical test results indicated that the Ti-Mg hybrid fixation system showed comparable ultimate strength and failure energy compared to Ti fixation system (n=8). Histomorphometric analysis revealed that the bone formation rate in Mg group was however significantly higher than that of Ti group. Hematoxylin & eosin (H & E) staining showed more callus formation in Mg group compared to Ti group while toluidine blue staining results showed significantly higher level of endochondral ossification in Mg group compared to Ti group. The immunohistochemistry staining results further indicated that the osteoporotic fracture healing process in Mg group was accelerated compared to that of Ti group.