Variations in Acetabular Offset Between the Preoperative Plan (mediCAD®) and Postoperative Radiographic Measurements: an overlooked source of error that affects restoration of global offset after THA.

INTRODUCTION: Restoring the offset after THA is positively correlated with good functional outcomes and patient satisfaction. Preoperative planning is now an essential tool for optimizing this restoration. Nevertheless, recent studies have focused mainly on the femoral offset, at the expense of the acetabular offset. Our hypothesis was that the variations in global offset after THA are mainly due to variations in acetabular offset relative to its initially planned value. METHODS: This retrospective study involved 104 patients who had undergone primary anterior THA with two-dimensional preoperative planning between January 2023 and November 2024. The planned preoperative radiographs and the postoperative radiographs were analyzed independently by two attending orthopeadic surgeons who measured the global, acetabular, and femoral offsets. The intra-rater and inter-rater agreement was determined. The differences between the planned implant sizes and the size actually implanted were also determined. RESULTS: In the end, 43 patients met all the pre and postoperative radiographic criteria and were included in the analysis. There was a correlation between the variation in acetabular offset and the variations in global offset (r = 0.74, p < 0.0001) and between the variation in femoral offset and the variations in global offset (r = 0.68, p < 0.0001). Using multiple linear regression, the coefficients of regression for the variation in the acetabular offset and femoral offset were 1.00, indicating an equal contribution (50%) to variations in global offset. CONCLUSION: Variations in the positioning of the acetabular cup are an important factor in the variations in global offset after THA. The postoperative variations in acetabular and femoral offsets contribute equally to changes in the global offset. Clinically, this means that practitioners should focus more on implanting the acetabular component. REGISTRATION: IRB approval N° 22727513. LEVEL OF EVIDENCE: III.