Viscoelastic behaviour of acrylic bone cements

Local contact stresses at the bone–cement interface are thought to play an important role in the initiation of component loosening. A reduced-modulus bone cement can lower these local contact stresses. The viscoelastic properties of such a cement raised the question of long term subsidence of the implant system. In this study, the viscoelastic properties of a reduced-modulus bone cement were compared with standard polymethylmethacrylate, PMMA, bone cement using stress relaxation tests. Unconstrained stress relaxation tests were performed at 37′C in an aqueous environment by applying 1%, 2.5%, and 5% strains on bone cement specimens and monitoring the diminishing load for 100 h. The initial rapid stress relaxation occurring over the first hour and the steady state stress relaxation occurring between 15 and 100 h were analyzed. A fast stress diminution occurred in PBMMA specimens indicating that, in a total hip arthroplasty application, PBMMA bone cement would transfer the stress quickly and distribute it over a larger area of endosteal bone surface. Steady state stress relaxation experiments showed a significant difference in 2.5% and 5% stress relaxation values ( P