Polyethylene Wear is Related to Patient-specific Contact Stress in THA

Background General numerical models of polyethylene wear and THA simulators suggest contact stresses influence wear. These models do not account for some patient-specific factors. Whether the relationship between patient-specific contact stress and wear apply in vivo is unclear.

Questions/purposes We therefore determined whether (1) contact stress distribution at the prosthesis-cup interface and (2) hip geometry and cup inclination are related to wear in vivo.

Methods We retrospectively reviewed the radiographs of 80 patients who had aseptic loosening of their THAs as determined by radiographic criteria. We determined linear penetration and volumetric wear using postoperative and last followup radiographs. Contact stress distribution was determined by the HIPSTRESS method. The biomechanical model was scaled to fit the patient’s musculoskeletal geometry of the pelvis, trochanteric position, and cup inclination using the standard postoperative radiograph.

Results Linear penetration and volumetric wear correlated with peak contact stress. Polyethylene wear was greater in THAs with a medial position of the greater trochanter and smaller inclination of the acetabular cup.

Conclusions Our observations suggest wear is specific to contact stresses in vivo.

Clinical Relevance Long-term wear in a THA can be estimated using contact stress analysis based on analysis of the postoperative AP radiograph.