Design aspects involved in a cemented THA stem failure case

Fatigue failure of total hip arthroplasty stem is today a rare case due to improvements of the technique along many decades. However, any case of stem fracture should be treated with attention due to the problems that can result in the revision for the patient. The most common cause of stem fracture is the proximal aseptic loosening. This scenario is reproduced in endurance test protocol ISO 7206-4 and should be applied for all design changes to prevent fracture of the stem even in the event of mechanical or biological problem that result in the aseptic loosening. This paper presents a cemented hip stem failure analysis. The study consisted of fracture analysis, material characterization, numerical simulation of the stem under the conditions of the endurance test, as well as fatigue tests of the ASTM F 745 stainless steel. The failure, in the mechanical aspect, is associated to the use of the cast stainless steel and could have been predicted through testing prior commercialization. Some design aspects are discussed. The use of cylindrical distal portion, matt surface and collar can reduce the stability in a loosened cemented hip stem.