Reliability of PMMA bone cement fixation: fracture and fatigue crack-growth behaviour.

Fracture mechanics tests were performed to characterize the fracture toughness and fatigue crack-growth behaviour of polymethylmethacrylate (PMMA) bone cement, commonly used in joint replacement surgery. Compact tension specimens of various thicknesses were prepared and tested in both air and Ringer's solution. Contrary to previous reports citing toughness as a single valued parameter, the PMMA was found to exhibit resistance-curve behaviour with a plateau toughness of approximately 0.6 MPa m1/2 in air, and approximately 2.0 MPa m1/2 in Ringer's solution. The increased toughness in Ringer's solution is thought to arise from the plasticizing effect of the environment. Under cyclic loads, the material displayed true mechanical fatigue failure in both environments; fatigue crack-growth rates, da/dN, were measured over the range approximately 10(-10) to 10(-6) m/cycle and found to display a power-law dependence on the stress intensity range, DeltaK. The cement was found to be more resistant to fatigue-crack propagation in Ringer's solution than in air. Wear debris was observed on the fatigue fracture surfaces, particularly those produced in air. These findings and the validity of using a linear-elastic fracture mechanics approach for viscoelastic materials are discussed in the context of providing more reliable and fracture-resistant cemented joints.