Shrinkage stresses in bone cement

Abstract

Shrinkage of bone cement is reported primarily as a consequence of polymerisation, however thermal shrinkage also occurs as a result of its exothermic reaction. It is proposed that the latter effect is important, since it occurs late in the curing cycle at a time when the cement has attained its mechanical properties as a solid, and that residual stresses result. Observations from experiments and literature reports suggest that residual stresses may be sufficient to initiate cracks at the interface between hip replacement stems and cement. A theoretical model has been developed to calculate interference stresses, using thick-walled cylinder theory, on the basis of thermal and total shrinkages. Thermal shrinkage values were calculated using the coefficient of linear thermal expansion of bone cement, while total shrinkages were measured. Moduli of elasticity values were measured for acrylic bone cements ranging from 2.1 to 2.7 GPa, as were Poisson's ratio values ranging from 0.38 to 0.46. Theoretical calculation of stresses in a cement mantle, based on assumptions of thermal shrinkage alone, predicted circumferential stresses of 8.4–25.2 MPa for cement curing temperatures in the range 60–140°C. It is concluded that cracks observed around hip prosthesis stems in laboratory specimens of bone cement are due to shrinkage and that residual stresses are sufficient to cause crack initiation prior to functional loading.