Biomechanical and clinical aspects of total hip dislocation

Recurrent dislocation ranks second as a cause for revision of total hip replacement (THR). The probability of dislocation ranges from 2% to 5% in primary THR and gains even more importance after revision or tumor surgery where dislocation rates up to 20% are reported. Because of the possible damage to the implants and the soft tissue, dislocation of the artificial hip joint has to be noticed as a serious complication. Avoiding THR dislocation has to be aimed for because of clinical, social and economical reasons. Prevention of dislocation requires the knowledge of the common risk factors as insufficient pseudo-capsular tissue, muscle weakness and malpositioning of the implant components. Recurrent impingement of the prosthetic neck on the acetabular cup can induce cup loosening, excessive wear of polyethylene liners and brittle fracture of ceramic components with material failure. Several studies have been conducted to evaluate the dislocation stability of different implant designs and orientations. Our research group established an experimental testing device to analyse the range of motion of THR until impingement and dislocation as well as the resisting moments during subluxation. Furthermore, we performed computer-based range of motion studies and developed a three-dimensional finite element (FE) model capable of predicting dislocation stability of various designs of the prosthetic head, neck and liner. The aim to prevent dislocation of THR demands an adequate design and correct positioning of the components. This strategy comprises an intraoperative monitoring of the ROM and stability of the artificial hip joint as well as sufficient postoperative care.