New radiographic parameter assessing hindfoot alignment in stage II adult-acquired flatfoot deformity.

BACKGROUND The hindfoot moment arm is a reliable measurement of hindfoot valgus deformity in stage II adult-acquired flatfoot deformity (AAFD) and can be used to guide intraoperative correction of the hindfoot. There is currently little understanding of how the hindfoot moment arm relates to angular measurements of hindfoot alignment. The purpose of this study was to develop a new hindfoot alignment angle that can reliably quantify hindfoot valgus in patients with AAFD and to establish the relationship of this angle with the hindfoot moment arm. METHODS Preoperative hindfoot alignment radiographs were reviewed for 10 consecutive patients (10 feet) who were indicated for reconstruction for stage II AAFD. A second group of 10 patients (10 feet) without flatfoot were identified to serve as normal controls. The hindfoot moment arm and the new hindfoot alignment angle were measured in blinded fashion by 2 readers. Reliability was assessed using intraclass correlation coefficients (ICCs). The difference in angle between normal and flatfoot patients was assessed with a Mann-Whitney U test. A linear regression model was used to assess the relationship between hindfoot moment arm and the new hindfoot alignment angle. RESULTS Intra- and interrater reliability for the hindfoot alignment angle was excellent (ICC = 0.979 and 0.965, respectively). Flatfoot patients had greater mean angles than did normal patients (22.5 ± 4.9 vs 5.6 ± 5.4 degrees, P < .001). The hindfoot moment arm was correlated significantly with the hindfoot alignment angle (P < .001), increasing by 0.81 mm for every degree increase in angle (adjusted R (2) = 0.9046). CONCLUSION These results indicate that the new hindfoot alignment angle is a reliable measure of hindfoot valgus and can differentiate between flatfoot and normal patients. In addition, the strong linear relationship between the hindfoot alignment angle and moment arm may allow for the use of this angle in the intraoperative correction of hindfoot valgus. LEVEL OF EVIDENCE Level III, retrospective case control study.