Functional navigation in hip resurfacing

Background: Acetabular malpositioning in total hip arthroplasty (THA) can result in edge loading, leading to increased wear, pseudotumours and bearing failure. Historically Lewinnek's safe zone has been the goal for optimal THA cup positioning; however, research indicates that one generic safe zone is not ideal. In hip resurfacing (HR) there is a lack of data regarding optimal position, and studies indicate that edge loading is related to increased inclination. Methods: We used a functional positioning protocol to produce patient-specific (PS) dynamic analyses, identifying individual functional ideal acetabular zones. We hypothesized that identifying and implanting into PS functional ideal zones would result in decreased edge loading and improved clinical outcomes. Fifteen adept HRs were implanted using a PS functional positioning protocol. As plans were created for standard THA acetabular position, adjustments were made intra-operatively. Results: Dynamic post-operative analyses showed 80% of implants were appropriately positioned. The remaining 20% had edge loading, where implants had inclination ≤5° of planned positioning and 10° of planned anteversion. 60% were ≤10° of planned inclination and anteversion. No pre-operative planned positions were within ‘Lewinneks's zone’, indicating its questionable relevance, especially in HR. A total of 20% of patients developed pain; however, at 1 year post-operatively all patients recorded improved Oxford hip scores. Conclusion: This study confirms that functional positioning in HR consistently results in improved precision and accuracy in achieving the functional acetabular ideal zone, leading to improved clinical outcomes and prevention of edge loading.