Variations in acetabular offset between preoperative planning (mediCAD®) and postoperative radiographic measurement: a neglected source of error influencing global offset restitution after THA

The restoration of global offset after total hip arthroplasty (THA) is correlated with functional outcomes and patient satisfaction. While preoperative planning is widely used to guide implant size and positioning, most studies focus on femoral offset, with limited attention paid to acetabular offset despite anatomical variability and the absence of reliable intraoperative landmarks during acetabular reaming. We hypothesised that variations in global offset observed after THA could be primarily linked to variations in acetabular offset relative to its initial planning.

Methods: This retrospective study analysed prospectively collected data from patients who underwent primary THA performed by a single surgeon between January 2023 and November 2024, with 2D preoperative planning using mediCAD® software. After application of strict radiographic quality criteria, 43 patients were included. All received a Hueter-Gaine approach, an uncemented short stem, and a press-fit cup with a ceramic-on-ceramic bearing. Acetabular, femoral, and global offsets were measured on preoperative plans and postoperative radiographs by two independent senior surgeons, using two validated methods adapted to the radiographic field available. Intra- and inter-observer reproducibility was assessed using intraclass correlation coefficients, and relationships between offset variations were explored by bivariate analyses and multiple linear regression, including Shapley value analysis.

Results: Intraclass correlation coefficients were good to excellent, confirming the reliability of the measurement methods. Significant correlations were observed between planned and postoperative global (r = 0.62), femoral (r = 0.60), and acetabular (r = 0.35) offsets. Variation in acetabular offset correlated strongly with variation in global offset (r = 0.74, p < 0.0001), as did variation in femoral offset (r = 0.68, p < 0.0001). Multiple linear regression showed that acetabular and femoral offset variations contributed equally (50% each) to global offset variation, with no correlation between them.

Conclusion: Acetabular implant positioning influences postoperative global offset to an equivalent degree as the femoral stem. This finding should encourage surgeons to pay increased attention to acetabular implant placement and its preoperative planning during THA.