The sex specificity of hip-joint muscles offers an explanation for better results in men after total hip arthroplasty.

PURPOSE: Choosing a surgical approach for total hip arthroplasty (THA) has a patient-specific impact on peri-operative muscle damage as well as postoperative functional outcome. Women and aged patients increasingly benefit from minimally invasive surgical procedures. For this reason, and due to the distinctly different bony anatomy of men and women, the hypothesis of this study is that muscle distribution around the hip joint is dependent on sex and age. The goal of this study was to analyse hip musculature in men and women and to correlate total muscle volume distribution.

METHODS: From 93 computed tomography (CT) scans of the pelvis (45 men, 48 women) volumes of gluteus medius (GMV), gluteus maximus (GXV) and tensor faciae latae (TFL) muscles were measured on both sides of the pelvis. The distribution of muscle volumes was normalised to patient weight and then correlated with sex and age.

RESULTS: The measured muscle volumes featured no major differences between the left and the right side. The absolute total volume of the hip-encompassing muscular system (TMV) is bigger in men than in women. Correlations between TMV and collected data were observed in both sexes in relation to body weight and size (men p < .00001; women p 0.001). With increased body weight, the TMV of the male patients increased progressively (women 11.2 cm TMV/kg KG vs. men 17.4 cm TMV/kg KG) (p 0.04). The relative distribution of each muscle volume (GMV, GXV, TFL) around the hip joint showed no major differences with respect to sex and/or age (p 0.986 and 0.996, respectively).

CONCLUSIONS: The equal relative muscle distribution in men and women around the hip joint reflects neither sex-related differences observed in clinical outcomes after THA nor bony anatomy. Yet men exhibited more muscle reserves (muscle volume; absolute and in relation to body mass) , which could explain the better outcome in men after THA. Furthermore, this suggests the extraordinary importance of muscle-sparing surgical approaches in women. The results represent the rationale for designing and analysing future studies of sex-specific therapies with regard to hip-joint muscles.