Faulty design increases the risk of neck fracture in a hip prosthesis.

From October 1993 to May 1994, we performed 35 total hip arthroplasties (20 women) using a Charnley type modular 28 head manufactured by Medical Tee Brazil. 9 fractures through the neck of the stem prostheses occurred, all but 1 in men, after mean 4 (0.3-6) years. All fractures were due to an inadequate confluent radius causing abnormal transmission forces through the neck. The higher the load (men, heavy body weight), the greater the risk of fracture. The metallographic, chemical and microhardness analyses showed no abnormalities. We ascribe these fractures to faulty design of the stem prosthesis.