Restoring femoral medial offset could reduce pelvic obliquity following primary total hip arthroplasty, an observational study

Abstract

Background: Since femoral and horizontal offsets may be contributing factors to hip and pelvic balance, this study seeks to determine whether there is a correlation between pelvic obliquity (PO) after unilateral total hip arthroplasty (THA) and horizontal/vertical offset differences of the replaced and contralateral natural joints.

Methods: A cross-sectional study was performed on adult patients who underwent unilateral THA between 2017 and 2020. An expert orthopaedic resident measured PO angles and offset parameters. "Delta medial offset" is considered medial offset of the replaced hip minus the medial offset of the contralateral side. "Absolute delta medial offset" is considered the absolute value of the "Delta medial offset."

Results: Finally, 133 patients were included in the study with a mean (SD) age of 45.3 ± 14.8 years and 57.9% female. The PO values (median, IQR) changed from 3.2 (1.7-5.7) before THA to 3.0 (1.50-5.6) after THA, not significantly decreased (P = 0.31). The PO after THA is significantly correlated with PO before THA (correlation coefficient of 0.457, P < 0.001), the delta medial offset after THA (correlation coefficient of - 0.24, P = 0.006), and the absolute delta medial offset after THA (correlation coefficient of 0.284, P = 0.001). The amount of changes of delta medial offset, before and after surgery, was not significantly correlated to PO or PO changes after surgery.

Conclusion: PO before the THA and medial offset discrepancy after THA are two important contributing factors for post-operative PO. Restoring the medial offset of the affected side and lowering the delta medial offset between the two sides can significantly decrease post-operative PO.

Keywords: Arthroplasty; Hip joint; Hip prosthesis; Obliquity; Offset; Pelvic bones; Pelvis.