On damage accumulation in cemented hip replacements.

The process of failure of cemented hip replacements due to mechanical loading is little understood. The objective of this thesis was to investigate the fundamental characteristics of damage accumulation in the complete implant structure, to advance the state of knowledge regarding the gradual breakdown of its mechanical integrity under fatigue cycling.

The porosity of the bone cement is in the range 7-20%. A fracture mechanics analysis showed that pores in the cement mantle give rise to the highest stresses and that under normal loading conditions, these stresses can lead to crack initiation. It was shown that a crack emanating from a pore can propagate to critical dimensions after only four years of normal walking. The analysis also predicted that stress intensity factors greater than the fatigue threshold occur for interface cracks under expected combinations of normal and shear stresses.

Experimental models were developed to monitor the continuous accumulation of damage under (i) flexural and (ii) torsional fatigue loading conditions. Damage accumulation was evident in the increase in the number of cracks and in the total crack length present in specimens, as measured at several stages up to 5 million cycles of loading. Under torsion, most of the cracks, by far, occurred at the cement/stem and the cement/bone interfaces while, under flexure, cracks occurred predominantly from pores within the bulk cement. There are significantly more cracks on the lateral side as compared to the medial side of the flexural specimens, which is in agreement with the predictions from a finite element stress analysis carried out of the experimental model. The average crack growth rate is 20 <IMG WIDTH=8 HEIGHT=14 ALIGN=MIDDLE SRC="/maths/mu.gif">m per million cycles of loading.

The theoretical analysis and experimental observations support the hypothesis that failure of cement hip replacements occurs by a continuous process of damage accumulation in the cement mantle.