Normative Values for Femoral Length, Tibial Length, and the Femorotibial Ratio in Adults Using Standing Full-Length Radiography

Abstract

Knowledge of the normal length and skeletal proportions of the lower limb is required as part of the evaluation of limb length discrepancy. When measuring limb length, modern standing full-length digital radiographs confer a level of clinical accuracy interchangeable with that of CT imaging. This study reports a set of normative values for lower limb length using the standing full-length radiographs of 753 patients (61% male). Lower limb length, femoral length, tibial length, and the femorotibial ratio were measured in 1077 limbs. The reliability of the measurement method was tested using the intra-class correlation (ICC) of agreement between three observers. The mean length of 1077 lower limbs was 89.0 cm (range 70.2 to 103.9 cm). Mean femoral length was 50.0 cm (39.3 to 58.4 cm) and tibial length was 39.0 cm (30.8 to 46.5 cm). The median side-to-side difference was 0.4 cm (0.2 to 0.7, max 1.8 cm) between 324 paired limbs. The mean ratio of femoral length to tibial length for the study population was 1.28:1 (range 1.16 to 1.39). A moderately strong inverse linear relationship (r = −0.35, p < 0.001, Pearson’s) was identified between tibial length and the corresponding femorotibial ratio. The PACS-based length measurement method used in this study displayed excellent inter-observer reliability (ICC of 0.99). This study presents a normal range of values for lower limb length in adults and is the first to identify a linear relationship between tibial length and the femorotibial ratio.