Preliminary Study on Composite Hip Prostheses Made by Resin Transfer Molding

Abstract

Metal hip implants used in arthroplastic surgery have an elastic modulus which is an order of magnitude higher than the modulus of the surrounding cortical bone. Therefore the metal implant assumes most of the applied loads resulting in resorption of the unloaded bone material by the human body which can cause the impant to loosen. In addition, patients may experience allergic reaction due to the release of metal ions or particles caused by friction, wear or enzymatic effects. Composite materials offer potential benefits such as tailorable mechanical properties, enhanced damage tolerance and fatigue life and improved biocompatibility. Based on structural analysis and mold filling simulation, prototypes of a hip prosthesis were manufactured using the resin transfer molding process. The preform of the prototype consists of braided high strength carbon fiber socks which are wrapped around a balsa wood insert. Two fiber architectures with different fiber angles (20° and 15° relative to the stem axis) were used. The maximum average fiber volume fraction obtained was about 38%. The mechanical performance of the composite hip prostheses was evaluated by static, fatigue and impact testing. Static testing verified that composite prostheses with the 20° braid carbon fiber preform exhibited an ultimate load of 7.5 kN, which is ten times the body weight of a 75 kg person. Fatigue testing showed that two million load cycles can be performed with a maximum load of 5 kN for the implants with the 20° reinforcement and 5.5 kN with 15° preform. The use of this type of prostheses in press fit applications was verified by conducting impact tests, simulating the hammer blows executed during the surgery. The study lays out the scientific base for a new manufacturing technique for composite hip prostheses using the resin transfer molding process and shows that prostheses with similar stiffness to the surrounding bone can be manufactured. All steps of the development cycle, including design, structural analyis, mold filling simulation, manufacturing and evaluation of performance, were performed on a basic level. The mechanical performance, especially fatigue and impact resistance were found to be excellent. The design of the implant and the mold need further revision in order to enhance mechanical performance, to ease manufacturing and to improve quality.