Residual Equinus After the Ponseti Method: An MRI-based 3-Dimensional Analysis

BACKGROUND: Residual equinus deformity is present in up to 20% of clubfeet treated by the Ponseti method. These patients may require surgical release to restore dorsiflexion. Despite complete posterior release; persistent intraoperative equinus may be present and suggest concurrent joint incongruity. The purpose of this study was to characterize differences in ankle morphology in toddlers with residual equinus following the Ponseti method. METHODS: Preoperative magnetic resonance imaging (MRI) data from 10 patients who underwent reconstruction (17 feet; 7 bilateral, 3 unilateral clubfeet) for persistent equinus were compared with 16 age-matched controls. Through reverse engineering software, MRI data were used to generate 3-dimensional (3D) models. Four talus-based measures were performed on both MRI data and 3D models-neck depth, neck angle, width, and length. Models were also used to calculate talus volume and arc of curvature (plafond and talar dome). Standard statistical analyses were performed. RESULTS: Talus volumes, width, and length were less in clubfeet then in control feet. Although some measures were significant there was no mismatch with the ankle mortise dimensions or arc curvature that could account for any decrease in dorsiflexion. We found that from MRI measures the clubfoot neck depth was 2.3 versus 3.6 mm in controls (P<0.001) and from 3D modeling the clubfoot neck depth was 2.3 and 3.5 mm in controls (P=0.003). With 3D modeling talus clubfoot neck angle was 153.7 versus 140.4 degrees in controls (P=0.01). The clubfoot neck angle obtained from MRI measures were also different yet not significant [126.6 in clubfeet versus 122.5 degrees in controls (P=0.12)]. CONCLUSIONS: In comparison to age-matched feet; we have noted a decrease in talar neck depth and an obtuse talar neck angle in clubfeet treated in the manner of Ponseti. This may result in anterior ankle impingement and be the cause of residual equinus despite posterior release. In these procedures, the surgeon should recognize this possibility when the amount of dorsiflexion is less than expected. LEVEL OF EVIDENCE: Level III-case control study.