Hip stem fatigue: : The implications of increasing patient mass

General trends of increasing body mass index have been observed in many western countries along with an increasing demand for joint replacement. Standards have been developed for testing the fatigue properties of femoral stems; however, the loads that these apply are based on a historic patient weight and may not be valid in the current patient population. Several fatigue tests were conducted using distally fixed titanium alloy stems positioned according to the ISO standard but with a cyclic load based on a current 75th percentile patient sample. Smaller sized stems (currently not weight restricted) fractured in; 30,000 cycles, while larger sized stems were found to have excellent durability under loads simulating walking and stumbling. The results suggest that while the fatigue properties of medical grade titanium are very good, the ISO pre-clinical durability testing standard does not represent the influence of femoral offset or stem size sufficiently to reflect safe design practice.