Full-Body Radiographic Imaging-Based Thigh Muscle Measurement for Sarcopenia: Association with Functional Assessments and Sagittal Alignment in Adult Spinal Deformity Patients.

OBJECTIVE: Evaluate the impact of EOS-derived thigh muscle measurements as indicators of sarcopenia and their effect on compensatory mechanisms in adult spinal deformity (ASD) patients.

SUMMARY OF BACKGROUND DATA: ASD patients frequently present with sarcopenia, the progressive loss of muscle strength and mass associated with worse postoperative outcomes. Routine EOS full-body radiographs allow opportunistic thigh muscle measurement without added cost or radiation. This study evaluated EOS-derived thigh and quadriceps thickness against clinical indicators of sarcopenia and their impact on compensatory mechanisms in ASD.

METHODS: We retrospectively analyzed prospectively collected data from 24 U.S. and Canadian spine centers(2019-2024). Sarcopenia was defined using validated sex-specific EOS cutoffs. Patients were classified as sarcopenic only when both AP thigh and LAT quadriceps measurements fell below threshold. Clinical frailty scores, grip strength, 3-meter timed up and go(TUG), and epigenetic age were compared between sarcopenic(SARCO) and non-sarcopenic(NON-SARCO) patients. Multivariate regressions assessed associations between thigh measurements, sarcopenia status, and compensatory radiographic parameters.

RESULTS: Among 540 ASD patients (mean age 60, 71% female), 61 (11.3%) were SARCO. SARCO patients had lower BMI(23.6 vs. 27.3 kg/m²), higher clinical frailty scores (3.4 vs. 3.0), and slower TUG (12.2 vs. 10.5s) (all P<0.05). Multivariate analyses showed smaller thigh and quadriceps thickness and sarcopenia status correlated with higher frailty, weaker grip, slower TUG, and older epigenetic age (all P<0.05). Sarcopenia was also associated with greater thoracic kyphosis (β=6.87, P<0.01), cervical lordosis (β=5.84, P=0.01), sagittal vertical axis (β=13.17, P=0.04), and knee flexion angle (β=2.29, P=0.04), but not pelvic tilt, shift, or sacro-femoral angle (all P>0.05).

CONCLUSIONS: Full-body radiographic derived thigh measurements significantly correlate with frailty, grip strength, TUG, and epigenetic age. Sarcopenic ASD patients demonstrate impaired proximal and increased distal compensations. Incorporating thigh and quadriceps muscle thickness measurements into preoperative assessment may improve surgical planning and patient management in ASD.

STUDY DESIGN: Multicenter retrospective cohort study of prospectively collected data.

LEVEL OF EVIDENCE: Prognostic Level III.