Radiographic Mid-Thigh Girth, a Proxy for Body Habitus, Correlates With Patient-Reported Outcomes More Strongly Than Limb Alignment Parameters in Children With Lower Limb Differences

BACKGROUND: The relationship between standard radiographic parameters and condition-specific and generic PROMs in pediatric patients with lower-limb malalignment, with mid-thigh girth as a radiographic proxy for body habitus, remains largely unknown. METHODS: We retrospectively reviewed the records of patients aged 8 to 18 years or younger who presented for surgical reconstruction of lower-limb differences at a single institution between 2019 and 2025. All patients, with caregiver assistance or by caregiver proxy, completed 2 validated PROMs preoperatively: the Limb Deformity-Scoliosis Research Society (LD-SRS) and the Patient-Reported Outcomes Measurement Information System-25 (PROMIS). Radiographic parameters, including limb length discrepancy, limb alignment (mechanical axis deviation), joint orientation angles (lateral distal femoral angle and medial proximal tibial angle), and mid-thigh girth, a new radiographic parameter, were measured from preoperative bone length studies by 2 independent evaluators. Intraclass correlation coefficients assessed interrater reliability. Spearman correlations were calculated between all radiographic parameters and the LD-SRS and PROMIS domains for the total cohort and within the varus- and valgus-malalignment subgroups. Age- and sex-standardized BMI z-scores were calculated for BMI analyses, and multivariable linear regression including mid-thigh girth, BMI, age, and sex was performed within each subgroup. RESULTS: One hundred ten patients were included (mean age: 12.6 y (8.0 to 17.0 y); 55% male). All radiographic parameters demonstrated excellent interrater reliability (0.84-0.99). Across the entire cohort, most standard radiographic parameters showed no significant correlations with PROMs. Mid-thigh girth strongly correlated with BMI (ρ=0.83, P<0.01) and demonstrated significant negative correlations with LD-SRS Overall (ρ=-0.30), Pain (ρ=-0.23), Function (ρ=-0.37), and Self-Image (ρ=-0.19), and with PROMIS Pain Interference (ρ=0.23) and Functional Mobility (ρ=-0.24). Significant mid-thigh girth correlations were observed only in the valgus subgroup (LD-SRS Overall: ρ=-0.31 and Function: ρ=-0.35), with none in the varus subgroup. In the total cohort, BMI demonstrated a similar pattern of correlations, while in subgroup analyses, BMI correlations were observed primarily in the varus subgroup (LD-SRS Overall: ρ=-0.38; Pain: ρ=-0.39; and Function: ρ=-0.40). On multivariable analysis, neither mid-thigh girth nor BMI was consistently independently associated with PROMs. CONCLUSIONS: Standard radiographic alignment parameters show limited correlation with PROMs in pediatric patients with lower-limb differences. Higher BMI and greater mid-thigh girth, a strong radiographic surrogate of BMI, were both associated with worse PROMs. Body habitus should be considered in the comprehensive evaluation and counseling of patients with lower limb malalignment. Mid-thigh girth, a new radiographic parameter, can be reliably assessed on standard bone length radiographs and serves as a radiographic proxy for the patient's BMI. LEVEL OF EVIDENCE: Level III.