Do Sex and BMI Predict or Does Stem Design Prevent Muscle Damage in Anterior Supine Minimally Invasive THA?

Background Cadaveric and clinical studies have suggested that, despite being touted as muscle-sparing, the direct anterior approach is still associated with muscle damage, particularly to the tensor fascia lata (TFL). Patient body mass index (BMI) and/or sex may also influence this parameter.

Questions/purposes The purposes of the study were to determine if using a shorter femoral component reduces TFL damage or if patient sex or increasing BMI increases intraoperative TFL damage in direct anterior THA.

Methods Over a 1-year period, 599 direct anterior THAs were performed by three experienced anterior hip surgeons; of those, 421 direct anterior hips had complete data (70%) and were included in the study. The amount of visible damage to the TFL was recorded before closure. Two stem types were used, a standard-length flat-wedge taper (standard) or a 3-cm shortened version of the same stem (short). Stem selection was based on timeframe of the surgery, surgeon preference, or matching a previous implant type. During the study period, the three surgeons performed an additional 225 primary THAs with other approaches such that the direct anterior approach represented 73% of the THAs performed. A member of the operating team, either a fellow or physician assistant, graded the extent of damage based on a 0 to 3 scale. On this scale, 0 represented no muscle fiber damage, 1 superficial tearing, 2 deep tearing or maceration, and 3 complete tear or severe damage. Patient sex and BMI were recorded and compared with stem type and muscle damage scores. An ordinal logistic regression model was used for statistical analysis.

Results After controlling for relevant confounding variables using logistic regression, we found that mean muscle damage was associated with male sex (0.93, SD 0.76 versus 0.70, SD 0.68; p < 0.001) and increasing BMI levels (p < 0.001). As BMI increased, more muscle damage also was found in men compared with women (p = 0.05; odds ratio [OR], 1.029; 95% confidence interval [CI], 1.000-1.060). There was no overall difference in mean muscle damage between short and standard-length stems (0.78, SD 0.77 versus 0.85, SD 0.69, p = 0.32); however, as BMI increased, less damage was seen with a short stem (p = 0.04; OR, 0.968; 95% CI, 0.931-0.997).

Conclusions Visible muscle damage occurred in most hips during anterior supine intermuscular hip arthroplasty. The clinical importance of this muscle damage requires further study, because some evidence suggests earlier restoration of gait and cessation of walking aids with direct anterior THA despite this damage; however, this was not studied here. Surgeons performing this approach can expect more difficulty and as a result possibly more damage to the TFL in patients with male sex and increased BMI. The use of a short stem can be considered for patients with increased BMI to limit damage to the TFL.

Level of Evidence Level III, therapeutic study.