VISCOELASTIC BEHAVIOR OF ACRYLIC BONE CEMENTS

Local contact stresses at the bone-cement interface are thought to pla y an important role in the initiation of component loosening. A reduce d-modulus bone cement can lower these local contact stresses. The visc oelastic properties of such a cement raised the question of long term subsidence of the implant system. In this study, the viscoelastic prop erties of a reduced-modulus bone cement were compared with standard po lymethylmethacrylate, PMMA, bone cement using stress relaxation tests. Unconstrained stress relaxation tests were performed at 37 degrees 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 i nitial rapid stress relaxation occurring over the first hour and the s teady state stress relaxation occurring between 15 and 100 h were anal yzed. 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 e ndosteal bone surface. Steady state stress relaxation experiments show ed a significant difference in 2.5% and 5% stress relaxation values (P < 0.05) between PMMA and PBMMA specimens, but not at the 1% stress va lues. Length measurements indicated that the viscoelastic PBMMA specim ens demonstrated little recovery after 100 h of imposed strain whereas the elastic PMMA specimens showed substantial recovery. This seems to indicate relatively larger subsidence rates in unconstrained PBMMA sp ecimens compared to PMMA specimens. In vivo, the cement is surrounded by endosteal bone at the outer side and by an implant on the inner sid e. Therefore, constrained creep tests are necessary to obtain the data required for an assessment of in vivo subsidence. (C) 1998 Elsevier S cience Ltd. All rights reserved.