Effect of stem preheating and precooling on residual stress formation at stem/cement interface for cemented hip implants.

PMMA polymerization is an exothermic phenomenon during which stresses and porosity are observed. An experimental model is devised to directly measure radial forces, to be converted to radial stresses, at the stem/cement interface, and temperatures at both interfaces during cement curing. The effects of stem and bone cement initial temperatures (preheating or precooling vs. room temperature) as well as mixing method (hand vs. vacuum mixing) and cement type (Simplex P vs. Palacos R) on radial stress and temperatures are investigated. Compressive radial residual stresses at the stem/cement interface are measured for hand mixed PMMA with preheated stem, with a maximum magnitude of 1.0 MPa. No radial residual stresses are observed when the stem is initially at room temperature or precooled, suggesting that during curing, bone cement can polymerize away from the stem/cement interface generating radial stress in tension or gaps. The results demonstrate the reverse direction of polymerization for preheated stems. Stem preheating significantly increases transient temperatures at the bone/cement interface and also the risk of bone thermal necrosis, because the exposure time to high temperature is prolonged. The results provide interfacial characteristics for accurate modeling of bone cement polymerization to better understand the debonding process of cemented hip prostheses.