Hindfoot realignment after coronal plane correction: a novel radiographic angle on long-leg standing radiograph

Accurate assessment of hindfoot alignment is important when evaluating distal compensation after correction of varus knee osteoarthritis. Dedicated hindfoot imaging is not routinely obtained with standard long-leg radiographs. This study developed and validated a novel hindfoot alignment angle measurable on routine patella-facing long-leg standing radiographs. This retrospective cohort study included 170 operated lower limbs from adult patients who underwent high tibial osteotomy or total knee arthroplasty for varus knee osteoarthritis. Preoperative and postoperative patella-facing long-leg standing radiographs were analyzed. Interobserver reliability and construct validity were assessed in a 36-case subset with complete long-leg, foot, and Saltzman-view radiographs. Intraclass correlation coefficients were used for reliability. Pearson correlations, Shapiro–Wilk normality tests, Spearman sensitivity analyses, and Bland–Altman agreement analysis were performed. The novel hindfoot alignment angle demonstrated excellent average-measure interobserver reliability (ICC = 0.905; 95% CI, 0.835–0.948). It showed a moderate correlation with the Saltzman-view hindfoot alignment angle ( r = 0.443, P = 0.007) and a weak correlation with the lateral talus–first metatarsal angle ( r = 0.331, P = 0.048). Bland–Altman analysis showed a systematic positive bias of 7.96° for the novel angle relative to the Saltzman-view angle, with 95% limits of agreement from − 1.05° to 16.98°. In the overall cohort, the hip–knee–ankle angle changed from 8.02 ± 3.08° to − 0.74 ± 2.95° ( P < 0.001), and the novel hindfoot alignment angle changed from 9.53 ± 3.85° to 14.09 ± 3.83° ( P < 0.001). With change defined as postoperative minus preoperative value, change in hip–knee–ankle angle correlated mildly with change in the novel hindfoot alignment angle ( r = − 0.301, P < 0.001). The novel hindfoot alignment angle is a reproducible radiographic parameter that can be measured on patella-facing long-leg standing radiographs and is moderately associated with Saltzman-view hindfoot alignment. The Bland–Altman findings indicate that the novel angle should not be used interchangeably with the Saltzman-view angle, but it may serve as a practical adjunct for assessing hindfoot alignment when long-leg radiographs are already being obtained. Level III, retrospective cohort study.