CORR Insights®: Small Random Angular Variations in Pelvic Tilt and Lower Extremity Can Cause Error In Static Image-based Preoperative Hip Arthroplasty Planning: A Computer Modeling Study

THA is a remarkably successful operation. Some of the large problems that were evident in the early years of THA, such as wear and osteolysis, have largely been solved. The Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) reported that for THAs implanted between 2013 and 2018, the most common reasons for revision were loosening, dislocation, infection, and fracture [¹]. The 5-year incidence of THA revision for dislocation dropped from 1% for implants placed between 1999 and 2005 to 0.6% for implants placed between 2013 and 2018. This reduction is likely because of improvements in implant design, such as head modularity, offset options, neck geometry improvements, and increased head sizes.

The etiology of dislocation is certainly multifactorial. Soft tissue management is related to the risk of dislocation [⁷], and clinical experience supports a role for muscle weakness and patient selection. Implant position also influences the risk of THA dislocation, but the best femoral and acetabular component position remains elusive and may not be the same for all patients.

Recent research has identified a relationship between spine mechanics and THA dislocation, and studies have been directed toward understanding this relationship [^{2, 3, 6}]. Ultimately, the goal of such research is to make it easy for surgeons to customize the implant position for each patient. Surgeons interested in doing this currently obtain functional radiographs such as standing and sitting radiographs of the pelvis to understand spine-pelvis mechanics [⁴], but Eslam Pour et al. [⁵] have identified a problem with this approach. The surgeon may be able to position implants to avoid impingement based on static images of the pelvis in various functional positions, but dynamic activities of daily living will not exactly reproduce these static positions. Deviations in pelvic position from the static images of as little as 2° to 3° may cause a prosthesis (positioned to avoid impingement in a static set of radiographs) to impinge during dynamic real-life situations. The current study by Eslam Pour et al. [⁵] thus raises the question of whether a simple static radiographic assessment of hip or spine motion is enough to define a personalized “safe zone” for THA component placement. The authors suggest that a more sophisticated analysis is needed, possibly with dynamic motion analysis or dynamic computer simulation models. I believe the findings of this study are important, but I am uncertain regarding the need for this additional analysis and am concerned that the authors’ suggested path forward would add much complexity and cost to an otherwise fairly routine, common, and successful operation.