Radiographic Measurement of Lower Limb Length Discrepancy: A Comparison of Three Measurement Techniques for Assessing Total and Segmental Limb Lengths

BACKGROUND: Full-length standing lower limb radiographs are commonly used to measure limb length and femoral and tibial segmental lengths. The purpose of the study was to compare the differences in length measurements using 3 methods. METHODS: We retrospectively reviewed preoperative full-length standing radiographs of skeletally immature patients with limb length discrepancy (LLD) scheduled for isolated epiphysiodesis of the proximal tibia and/or distal femur between 2012 and 2024 at a single center. Method 1 (True length) measured the longitudinal length from the top of the femoral head to the distal edge of the medial femoral condyle for the femoral segment, and from the distal edge of the medial femoral condyle to the center of the distal tibial plafond for the tibial segment. In method 2 (Apparent length), segmental height was measured as the vertical distance between horizontal reference lines drawn tangential to the top of the femoral head, the bottom of the medial condyle, and the distal tibial plafond. In method 3, functional LLD was measured based on the difference in femoral head height. Bland-Altman plots were used for measurement agreement analysis. RESULTS: Thirty-seven patients (16 boys, 43%) with a mean age of 13±1.5 years underwent epiphysiodesis for LLD. The mean absolute difference between methods 1 and 2 in the measurement of total limb length, femoral and tibial segmental lengths was 0.5 mm, 1.5 mm, and 1 mm, respectively (P<0.0001). True Length (Method 1) slightly overestimated the length compared with Apparent Length (Method 2). LLD measured with methods 1 and 2 was similar to method 3 (P>0.05). The subtle differences between the methods correlated with coronal plan limb alignment. CONCLUSION: The 3 radiographic LLD measurement methods were comparable. The subtle variability in measurement values correlated with lower limb angular deformity, which highlights the importance of considering coronal plane deformity when interpreting radiographic measurements for LLD. Consistent use of a single measurement technique is important to avoid subtle variability in LLD estimation, especially in the presence of angular deformities. LEVEL OF EVIDENCE: Level III.