Development of Polymers for Minimally Invasive Orthopedic Implants

An overview of the history of orthopedic implants is presented along with a review of polymers used in orthopedics. Minimally invasive implants are introduced and discussed. Design requirements for minimally invasive orthopedic implants are discussed along with design classifications for minimally invasive implants. Ultrahigh Molecular Weight Polyethylene (UHMWPE) was introduced and the use of the material as a bearing surface is described. A compression molding process is developed and modifications to the molds and mold surfaces are also discussed. An acrylic polymer cement system was conceived and developed for use in a novel minimally invasive implant. The cement serves as a main structural component for a new type of femoral fracture repair device that is installed through a small (~2.5 cm) incision. The physical, chemical, and mechanical characteristics of the polymer are determined and compared to currently available cement. Improvements to the acrylic polymer system were pursued. Acrylic fibers, developed for the hip fracture device, were incorporated into the acrylic polymer. Incorporation of fibers up to 15% was attempted. 5% fibers by weight increased fatigue life of the acrylic polymer system approximately 8 times over the non-fiber reinforced material. Blending of the fibers into the powder became increasingly more difficult with higher levels of fibers. 15% fibers were difficult to blend uniformly, with large variations in properties. Nanometer sized barium sulfate was added to the blends replacing conventional sized barium sulfate. Increases in fatigue life were observed. Light initiated curing of acrylic cement was investigated. The object was to initiate curing of the acrylic system over a distance of several inches. Camphorquinone was used as the initiator. Mechanical properties of the cured samples were determined.