Preliminary study on composite hip prostheses made by resin transfer molding

Metal hip implants used in arthroplastic surgery have an elastic modulus wh ich is an order of magnitude higher than the modulus of the surrounding cor tical bone. Therefore the metal implant assumes most of the applied loads r esulting in resorption of the unloaded bone material by the human body whic h can cause the impant to loosen. In addition, patients may experience alle rgic reaction due to the release of metal ions or particles caused by frict ion, wear or enzymatic effects. Composite materials offer potential benefit s such as tailorable mechanical properties, enhanced damage tolerance and f atigue life and improved biocompatibility. Based on structural analysis and mold filling simulation, prototypes of a h ip prosthesis were manufactured using the resin transfer molding process. T he 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 degrees and 15 degrees relative to the ste m axis) were used. The maximum average fiber volume fraction obtained was a bout 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 degrees braid carbon fiber preform exhibited an ult imate 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 degrees reinforcement a nd 5.5 kN with 15 degrees preform. The use of this type of prostheses in pr ess fit applications was verified by conducting impact tests, simulating th e hammer blows executed during the surgery. The study lays out the scientific base for a new manufacturing technique fo r composite hip prostheses using the resin transfer molding process and sho ws that prostheses with similar stiffness to the surrounding bone can be ma nufactured. All steps of the development cycle, including design, structura l analysis, mold filling simulation, manufacturing and evaluation of perfor mance, were performed on a basic level. The mechanical performance, especia lly fatigue and impact resistance were found to be excellent. The design of the implant and the mold need further revision in order to enhance mechani cal performance, to ease manufacturing and to improve quality.