[Effect of a constructive change in the Charnley total hip endoprosthesis on its permanent stability in the bone]

Because of long experiences with cemented hip joint prostheses and the simple geometric form of this joint shape and material of those prostheses are nearly established. Therefore, success of different systems is measured by duration of stabile fixation in the bone. A minor modification of the acetabular cup is able to improve this duration of stability. The cup has a additional central cone which will be wedged firmly in a centering hole, drilled in the acetabulum. In a longtime follow-up study of 1004 operated patients - medium observation time 7.8 years - the cup loosening rates are only 2.6%. The biomechanical basis of the value of the central cone is discussed.