IN VIVO EVALUATION OF A REDUCED MODULUS BONE CEMENT

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The most frequent postoperative complication of cemented total joint replacement procedures is loosening of one or both components. The loosening process begins with resorption of bone at the bone-cement interface. Excessive local stresses and the presence of submicron debris can each trigger the resorption of bone and start the loosening cascade. A bone cement with a lower modulus and greater toughness than polymethylmethacrylate (PMMA) will decrease local contact stresses and produce less particulate debris. Any irregular interface, such as the bone-cement interface, will have peaks in contact stress at geometrical and material discontinuities. According to quantitative models for materials contact, both analytical and numerical, the stresses will be reduced if the elastic moduli of the cement are reduced. A low-modulus acrylic bone cement, formulated with polybutylmethacrylate beads in a PMMA matrix, has been developed and its static and viscoelastic material properties have been characterized. Its modulus at body temperature is significantly lower than that of PMMA and its toughness is much greater. Ongoing in vitro experimentation is verifying the local contact stress hypothesis. The modulus of the cement has the expected effect of the shear strain of the adjacent bone; reducing the cement modulus reduces the bone strain and will thus reduce microdamage which triggers the loosening cascade. Preliminary in vivo investigations support the conclusion that loosening rates can be decreased by diminution of the interface contact stresses. The loosening rate, assessed radiographically and mechanically, in sheep total hip arthroplasties using a reduced-modulus acrylic bone cement was significantly lower than that in sheep where standard PMMA bone cement was used. The logical next step is a larger in vivo investigation of loosening rates to statistically validate the findings of the pilot study. The loosening rates of components implanted using standard PMMA and reduced-modulus acrylic bone cements will be compared in a sheep total hip model.