Strength testing of ceramic hip joint heads: criticism of the ISO- and FDA-procedures

For the certification (premarket notification) of ceramic ball hip joint systems test guidelines have been developed by the FDA and by the ISO committees. Applying the procedures which are suggested by the FDA, different types of ball-stem-combinations were tested with respect to their mechanical integrity. None of the tested systems met the requirements, although the same types have been successfully implanted for many years. The reason for these obviously contradictory results could be the different stress distribution which is found for high and low loading heights as in the test and in the physiological situation, respectively. This argument is based on Finite Element calculations and the analysis of the fracture pattern under different loads.