The design and analysis of a laminated partially degradable composite bone plate for fracture fixation.

Abstract

During the early stages of fracture healing, rigid internal fixation maintains alignment and promotes primary osseous union. Unfortunately, as healing progresses rigid fixation from bone plating can cause bone in the region of the plate to undergo stress protection atrophy. This can result in significant loss of bone mass and osteoporosis. Refracture of the bone upon device removal is a widely reported complication. In an effort to minimize or eliminate stress protection atrophy, we have designed a partially absorbable, fiber-reinforced bone plate. Ideally, such a plate gradually loses rigidity as the fracture heals, increasingly transferring stress to the bone. Stress protection may be avoided and removal of the device after healing may be unnecessary. Composite theory was used to determine an optimum fiber layup for a composite bone plate. Composite analysis suggested the mechanical superiority of a 0 degree/ +/ -45 degree laminae layup. Given this laminated design, a thermoplastic absorbable polymer (polylactic acid polymer) was reinforced with high-modulus carbon fiber to produce a semiabsorbable composite. Implant evaluation included optimizing fabrication techniques, thorough mechanical device testing, and implantation on canine femurs to determine biocompatibility and efficacy. The composite design proved to have superior static and fatigue properties to laminated or random fiber designs used previously. Two techniques for hole fabrication were tested. The production of screw holes during the molding process rather than machining postmolding, improved the mechanical integrity of the finished plate. Although the 0 degree/ +/- 45 degree carbon/polylactic acid composite possessed superior mechanical properties, it was unsuccessful in the in vivo environment. Water absorption and subsequent delamination made the plate flexible. Hypertrophic nonunions developed. Further development to prevent water intrusion and premature loss of mechanical properties is necessary.