A. Reinhardt
S.G. Advani, M.H. Santare, F. Miller
Department of Mechanical Engineering
University of Delaware
Newark, Delaware 19176
INTRODUCTION: Metal hip prostheses have a modulus which is an order of magnitude higher than the surrounding cortical bone. The prosthesis therefore assumes the major part of the load, resulting in resorption of the unloaded bone material by the human body. Further, metal implants can cause allergic reactions due to the release of metal particles or ions caused by friction or enzymatic effects. The use of composite materials will allow the designer to tailor the mechanical properties to match the bone properties while maintaining the required strength. Composite materials show improved fatigue properties and can have enhanced biocompatibility. Several manufacturing techniques for composite materials used in medical applications such has hand lay-up or injection molding have been investigated. This study deals with the use of resin transfer molding for the manufacture of composite hip prostheses.
MATERIALS AND METHODS: Resin transfer molding (RTM) is a composite manufacturing process where a fabric preform made out of fibers, generally glass, kevlar or carbon, is placed in a mold cavity. The cavity is closed and a mixture of thermoset resin, accelerator and curing agent is injected under pressure. The curing reaction takes place with or without applying heat. When the curing reaction is completed, the part can be removed from the mold cavity and is ready to use. The preform for the hip prosthesis is made by draping six layers of braided, high strength carbon fiber socks around a balsa wood insert. Different fiber architectures with various fiber angles are used in order to obtain various mechanical properties while keeping the thickness at constant value of 6 mm. The preform is placed in a aluminum mold and the resin mixture containing vinyl ester resin, cobalt naphthalene and cumene peroxide is injected from a pressure pot with a pressure of 18 psi. The curing reaction takes less than an hour and the implant can be removed from the mold. After that, the prosthesis is placed in an oven for one hour at 100 C for post-curing. Theoretical investigations included structural analysis in order to predict the mechanical behavior of the implant. The deformation behavior and stress distribution were evaluated under several loading conditions, using various material properties. Further, the manufacturing process was modeled using a numerical simulation to derive optimized processing conditions. The mechanical performance of the prostheses was evaluated experimentally through static, dynamic and impact testing. Static and dynamic testing were performed using a hydraulic testing system.
The impact tests were conducted with the aid of a drop tower. In all cases, the implants were fixed in a resin environment in order to simulate the support of the surrounding bone.
RESULTS: Resin Transfer Molding allows one to manufacture hollow composite hip prostheses reinforced by a three dimensional continuous preform. By using RTM an average fiber volume fraction of 38 % could be obtained. The results obtained from the structural analysis overpredict the experimental deformation behavior by up to 3 mm. The stress distribution enables the designer to predict the region and mode of failure qualitatively. Hence, the structural analysis is performed conservative in this study and can be used in the design stage for composite hip prostheses. Static tests show an ultimate load of 7.5 kN (10 times bodyweight of a 75 kg person) for a preform architecture with a 20 degree fiber angle. Dynamic tests of the implants show that, with a load of 5 kN applied, 2 million cycles can be applied at a frequency of 5 Hz without any visible damage. Impact tests prove the feasibility of using the investigated type of prosthesis for press fit surgeries.
DISCUSSION: The deviation of the predicted results from the experimental results can be explained due to the inaccuracy of modelling the boundary conditions and material properties in the structural analysis. The actual material properties of the implant are likely to be higher than the average properties used in the analysis due to the higher local fiber volume fractions. The use of carbon fiber socks allows one to tailor the material properties of the prosthesis by changing the fiber architecture or the volume fraction but the architecture of the fabric changes during it over the balsa wood core. The vinyl ester resin used for resin transfer molding is not for use in the human body but thermosets such as EPON are available. The use of vinyl ester to simulate the support by the surrounding bone during testing is not representative due to the higher stiffness of the cortical bone. The static and dynamic tests are performed without torsion.
CONCLUSION: The use of composite materials manufactured by resin transfer molding for orthopaedic implants can be beneficial because of the tailorable mechanical properties and the improved fatigue behavior compared to metals. The geometry of the prosthesis has to be optimized for both process and performance. The use of biocompatible thermosets for resin transfer molding needs to be investigated before in-vivo testing is attempted.
The authors would like to thank Dr. F.K. Ko and Mr. J.M. McKelvie from the Department of Materials Engineering, Drexel University, Philadelphia and Mr. B.J. Osborne for their support.
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