Background: It is well recognized that the alignment of the lower limb changes during early childhood. The hip-knee-ankle angle is often referred to as the mechanical femoral-tibial angle and is measured on a full-length radiograph of the lower extremity. While several authors have independently reported consistent reference values for the hip-knee-ankle angle in adults, such values have not been well documented for children. The purpose of our study was to establish reference values for the hip-knee-ankle angle and assess the relationship between it and the anatomic femoral-tibial angle in children.

Methods: A database at a single institution was searched for patients who were between one and less than eighteen years old at the time that a standing full-length radiograph of the lower extremities was made. Radiographs of the uninvolved extremity (the limb without any radiographic abnormalities or documented clinical concerns) were analyzed. The angle between a line connecting the center of the ossified femoral head and the center of the distal femoral epiphysis and another line connecting the center of the distal femoral epiphysis and the center of the talar dome was measured. Simple regression analyses were performed to determine the relationship between this angle and the anatomic femoral-tibial angle.

Results: A total of 354 unaffected lower extremities of 253 children were analyzed. The mean hip-knee-ankle angle was +3.6 degrees (varus) in children between one and two years old and -2.5 degrees (valgus) in those between two and three years old. After the age of seven years, the mean value was +0.3 degrees (varus), which was within 1 degrees of the reference values available for the adult population (mean, +1.2 degrees [varus]). There was a linear relationship between the hip-knee-ankle and anatomic femoral-tibial angles in the children (r = 0.87, p < 0.0001). Despite varying hip-knee-ankle angles at different ages, the mean absolute difference between that angle and the anatomic femoral-tibial angle remained relatively constant (mean, 6.7 degrees ) and was not associated with changing age (r = -0.09).

Conclusions: In our study sample, reference values for the hip-knee-ankle angle in children older than seven years of age approached those reported for adults in North America. Although this angle and the anatomic femoral-tibial angle in children younger than seven were distinct from those reported for the adult population, the difference between the two angular measurements remained essentially unaffected. The use of age-specific reference values for both the hip-knee-ankle and the anatomic femoral-tibial angle is recommended for children younger than seven years old.