Association Between Spinal Alignment and Trunk and Lower Extremity Muscle Mass in Preoperative Total Hip Arthroplasty: A Cross-Sectional Analysis

Background The sagittal vertical axis (SVA) reflects trunk balance. This study investigated the relationship between trunk balance and muscle mass, as well as the prevalence of sarcopenia in preoperative total hip arthroplasty (THA) patients using SVA. Materials and methods This retrospective cohort study included 126 patients who underwent THA over a one-year period and met the eligibility criteria. Data collected included body mass index, duration of illness, number of falls, sarcopenia, sagittal alignment, pelvic tilt, and skeletal muscle mass. Patients were divided into two groups based on SVA: the unbalanced (U) group (SVA ≥50 mm) and the balanced (N) group (SVA <50 mm). Results The U group and the N group had n=61 and n=65, respectively. There were no significant differences in disease duration (9.3 ± 13 vs. 10.8 ± 13.3 months) or prevalence of sarcopenia (14 (23%) vs. 14 (22%)) between the U and N groups. However, the nonoperative lower extremity muscle mass was significantly lower in the U group (5.7 ± 1.2 kg) compared to the N group (6.1 ± 1.3 kg; <i>p</i> = 0.04). Trunk muscle mass was also lower in the U group (17.1 ± 3.2 kg vs. 18.4 ± 3.4 kg; <i>p</i> = 0.026). Pelvic-spine alignment angles (sacral slope (SS)) were significantly smaller in the U group in the standing (35.4° ± 13.1° vs. 40.6° ± 10.6°), supine (7.4° ± 15.7° vs. 14.3° ± 13.3°), and seated (36.2° ± 12° vs. 41.3° ± 10°) positions. Conclusion SVA is associated with trunk balance in preoperative THA patients. Those with poor trunk balance (SVA ≥50 mm) exhibited decreased muscle mass in the trunk and lower limbs on the nonoperative side, highlighting the importance of assessing trunk balance using SVA in this population.