Composite materials are being investigated for use as replacements for metals in the femoral component of total hip arthroplasty with current interest being focused primarily on carbon fiber reinforced thermoplastics. Since a composite hip prosthesis will be a primary load bearing component in an aggressive in vivo environment for the lifetime of the patient, a time-dependent effect such as creep must be thoroughly characterized and is the focus of this paper.
Environmental flexural creep testing of two laminate designs of carbon fiber reinforced polyetheretherketone (C/PEEK) coupons at 207 MPa for 3 months resulted in a maximum creep strain of 226 μtstrain. After 3000 h at an equivalent in vivo load of 4000 N, two prototype C/PEEK hips with the same laminate designs as previously mentioned displayed 93 μm and 250 μm of creep.
In both the coupon and prototype hip study, the measured and predicted creep strains are an order of magnitude below the elastic strains that are themselves significantly lower than the strain to failure of this material. The results clearly indicate that the viscoelastic behavior of the composite hip is fiber dominated and is not expected to be a limiting factor in the long-term performance of a properly designed composite hip.