Effect of cement modulus on the shear properties of the bone–cement interface

Shear tests of the bone–cement interface were performed in vitro using two types of bone cement, standard poly(methyl methacrylate) (PMMA) and a reduced-modulus formulation with poly(butyl methacrylate) beads in a methyl methacrylate matrix (PBMMA). Tests of shear properties were also calculated on cancellous bone and on each cement alone. The tests were done using the Iosipescu shear test method which generates a pure shear force in a zero-moment section of the specimen. With this method, shear properties can be determined at specified locations throughout the specimen. Tests were performed across the entire interface region, specifically in the middle of the region of cement–bone interdigitation and at both the bone and cement ends of that region. Ultimate shear strengths and shear moduli were calculated. The shear modulus of the PBMMA is less than 3% that of PMMA. The strength and modulus of cancellous bone had a direct relation to the apparent density of the bone, as did the strength and modulus of the bone–composite interface and the composite region. Strength and modulus were dictated by the bone at the bone–composite interface, and by the cement at the cement–composite interface. Through the composite region, the stiffer of the two materials in the composite determined the shear properties. Reduced-modulus bone coment substantially decreases the interfacial shear stresses at the bone–cement interface which should decrease the rate of resorptive bone remodelling at this interface.