The size selection of the outer metal shell to fit the acetabulum is a dilemma to the surgeons for the bipolar hemiarthroplasty. However, no body ever mentioned the final results of motion behaviour of bipolar hip endoprostheses by different size selections of outer metal shell. The purpose of this study was to evaluate the motion behaviour of bipolar hip endoprostheses under different sizes of outer metal shell.

A fresh cadaver of size of 50 mm and three different sizes of bipolar hip endoprostheses were used to evaluate the motion behaviour of bipolar hip endoprostheses under the MTS machine. Each size had six sets of bipolar hip endoprostheses to get statistical data. The single axial load scaled from 300 N to 3000 N with increment of 300 N was applied on both the inner bearing and outer bearing to obtain the frictional torque of two bearings randomly. The axis was rotated from 0° to 90° under the speed of 1 deg/sec. The motion behaviour of bipolar hip endoprostheses was predicted and verified by the comparison of the frictional torque of both bearings and repeat the experiments again.

The dynamic frictional torque of 48mm was larger than the dynamic frictional torque of the 50mm at the inner bearing. The size of 52mm had the largest value of dynamic frictional torque for the outer bearing under any load condition. The prosthesis that had larger outer metal shell than the acetabulum had the same relative motion behaviour as the design hypotheses, because the difference of frictional torque was much higher.

In conclusion, the motion of the bipolar hip endoprostheses is influenced by the frictional behaviour of the both bearings. The relative motion of bipolar hip endoprostheses with a larger outer shell size will have an idea relative motion behaviour, because the larger difference of frictional torque. The thickness of the UHMWPE has a positive effect to reduce the friction between the UHMWPE liner and metal head.