Increasing patients' body mass. Are the criteria for testing stemmed femoral components in total hip arthroplasty still valid?

The increasing incidence of obesity in the population is a topic of current interest. This trend is reflected in patients undergoing primary Charnley hip replacement. Over a 21 year period from January 1986 to October 2006, during which 6910 primary operations were performed, the body mass of patients increased at a mean rate of 0.48 kg/year. The findings suggest that the test criteria for the endurance of stemmed femoral components be modified. The maximum load should be increased from 2.3 kN to 2.5 kN to reflect patients' increasing weight. The duration of the test should be increased from 5 x 10(6) cycles during which the 'femoral component shall not fracture' to 10 x 10(6) cycles in order to reflect not only the patients' activity level but also the 4-11 year period at risk for stem fracture, and so 'reflect developing clinical experience'.