A clinical, radiological and computational analysis of the thrust plate prosthesis in young patients.

Background and purpose: A thrust plate prosthesis can be used as an alternative to a conventional stem prosthesis, preserving the diaphyseal bone stock. Recent findings however predict a higher rate of aseptic loosening than with intramedullary devices. The purpose of our investigation was to compare the clinical outcome and radiological findings with a finite element analysis of bone remodeling. The hypothesis was that aseptic loosening after thrust plate prosthesis of the hip is inherent to the design.

Methods: From 1997 to 2001, 58 thrust plates were implanted in 52 patients. Average age at the time of surgery was 40.9 years. Ninety four percent returned for follow up at an average of 26 months. A finite element model of the thrust plate within the femur was developed and stress shielding as well as bone remodeling were analyzed.

Results: A total of 4 patients required revision surgery (6.9%). Data from the finite element analysis revealed an inherent failure mechanism to the implant, facilitating stress shielding and loosening.

Interpretation: Lacking the ideal total hip prosthesis in young patients, the thrust plate can still be regarded as a feasible implant. However, surgeons and patients should be aware of possible mechanical problems regarding the design of the thrust plate. There is evidence that thrust plate prostheses are prone to early aseptic loosening. Clinical and radiological observations are in agreement with the results from the numerical simulations. Stress concentrations computed at the leash are interpreted as an explanation for leash pain. The authors regard computational methods as an aid to improve existing prosthesis design and future developments.