A formula to predict patients' gluteus medius muscle volume from hip joint geometry.

The volume of the gluteus medius muscle (GMV) has been shown to be closely related to hip joint function. A defined relation between joint geometry and GMV would allow a calculation of the patient-specific GMV providing reference values to individually determine the goals for rehabilitation programs after total hip arthroplasty (THA). The aim of this study was to investigate correlations between hip geometry and GMV. One hundred and two (50 female, age: 58.53 (18-86)) pelvic computed tomography (CT) scans were analyzed to determine femoral offset (FO), body weight lever arm (BWLA) and the GMV. Relationships between demographic data, FO and GMV were analyzed using correlation and regression analysis. The mean GMV was found to be 289 ± 72 cm(3), the mean FO measured was 4.14 ± 0.55 cm; and the mean value for BWLA measured was 8.88 ± 0.4 cm. A formula to calculate the GMV with a good coefficient of determination (R(2) = 0.681) (p < 0.0001) was derived. In conclusion, the formula obtained predicts individual GMV with good model fit and could be used to individually determine rehabilitation goals. Moreover, the correlation found could account for a hand in hand development of FO and GMV during growth and a continuous functional relationship thereafter.