The foot drawing method: reliability of measuring foot length and outward rotation in children with clubfoot.

Background: The Ponseti method is the gold standard for clubfoot treatment. However, relapse and residual gait deviations are common, and follow-up until 7 years of age is recommended. We evaluated the reliability of the foot drawing method, a new instrument for the follow-up of clubfoot. The method uses drawings of the foot in the neutral position and external rotation to measure foot length and outward rotation.

Methods: Nineteen children aged 2.5-7 years who were treated with the Ponseti method for congenital clubfoot were included. Two raters made the drawings twice (D1 and D2). Each rater measured foot length, foot rotation, and foot-tibial rotation independently (D1). Later, the raters repeated the measurements (D2). Interrater reliability was assessed using the D1 from each rater. Intrarater reliability was assessed using the measurements from each rater's D1 and D2. Bland-Altman plots were used to visualize the limits of agreement (LoA). The mean, 95% confidence interval, and one standard deviation of the differences in all measurements were calculated.

Results: The mean differences between and within raters were: foot length < 1 mm, foot rotation < 1°, and foot-tibia rotation < 2°, which indicated no systematic differences. The LoA for foot length were: 4.5 mm and 5.9 mm between raters for D1, - 4.8 mm and 5.9 mm for rater 1 (D1-D2), and - 5.1 mm and 5 mm for rater 2 (D1-D2). The LoA for foot rotation: were - 12° and 10.6° between raters (D1), - 8.4° and 6.6° for rater 1 (D1-D2), and - 14° and 14.1° for rater 2 (D1-D2). The LoA for foot-tibia rotation were: - 17.8° and 14.3° between raters (D1), - 12° and 12.2° for rater 1 (D1-D2), and - 12.7° and 13.6° for rater 2 (D1- D2).

Conclusions: The absence of systematic differences between and within raters, and LoA observed indicate that the foot drawing method is applicable in clinical practice and research. However, the results of the foot and foot-tibia rotation analyses imply that caution is needed when interpreting changes in foot rotation in feet with higher degrees of rotation.