New radiographic parameters assessing forefoot abduction in the adult acquired flatfoot deformity

Stage II flatfoot secondary to posterior tibial tendon insufficiency may be subclassified into mild (IIa) and severe (IIb) deformity based on the degree of talonavicular abduction. Current assessment of this abduction is difficult. We hypothesized that two new anteroposterior radiographic parameters, the lateral talonavicular incongruency angle (IA) and incongruency distance (ID) would demonstrate good reliability, correlate with current abduction parameters, and differ in IIb deformity, IIa deformity, and controls.

Preoperative radiographs for consecutive patients undergoing flatfoot reconstruction were reviewed and subdivided into those with a Stage IIb (n = 32) or Stage IIa (n = 8) deformity. A third group of patients without flatfoot served as control (n = 30). Radiographs were measured blindly by two investigators. Reliability was assessed with intraclass correlation coefficients (ICC), correlation with existing parameters with Pearson coefficients, and comparison between groups with analysis of variance.

The mean intrarater and interrater ICC's for the IA (0.88 and 0.81, respectively) were high. The IA correlated well with the coverage angle (r = 0.86) and uncoverage percent (r = 0.76). The IA was higher in the IIb versus IIa patients (p = 0.007) and in the IIb group versus control (p < 0.001). The ID demonstrated excellent reliability (ICC's of 0.83 and 0.83), but correlated poorly with the two other abduction parameters (r = -0.59 and -0.49) and failed to differentiate between the three groups (p = 0.0528).

This data suggests that the IA is reliable and may help subclassify Stage II flatfoot deformity.