Subtalar Joint’s Middle Facet Yields Insight into Flatfoot

Many foot and ankle surgeons would relish a simple measurement made from a readily available imaging modality to help detect whether patients with adult acquired flatfoot deformity (AAFD) are at high risk for progressive collapse—and to help them with surgical planning. According to the findings from a case-control study by de Cesar Netto et al. in the October 16, 2019 issue of The Journal of Bone & Joint Surgery, that wish may soon be realized.

The authors made standing, weight-bearing computed tomography (CT) scans of 30 patients with stage-II AAFD (mean age of 57.4 years) and 30 matched controls (mean age of 51.8 years). From those images, 2 fellowship-trained surgeons, who were blinded regarding the patient cohorts, measured the amount of subluxation (percentage of uncoverage) and the incongruence angle of the middle facet of the subtalar joint in the coronal plane. The authors found substantial to almost perfect intraobserver and interobserver reliability for both measurements.

Based on these middle-facet measurements, the mean value for joint uncoverage in patients with AAFD was 45.3% compared with 4.8% in controls. Similarly, the mean incongruence angle in the AAFD group was 17.3° in the AAFD group and 0.3° in controls. Further analysis led the authors to conclude that “an incongruence angle of >8.4° and an uncoverage percentage of 17.9% were found to be highly diagnostic for symptomatic stage-II AAFD.”

De Cesar Netto et al. say the biomechanics of the subtalar joint made focusing on the middle facet a sensible approach, and they attributed the high reliability of the measurements to the relatively simple anatomy of the middle facet. Still, because clinical outcomes were not assessed in this study, the role of the middle facet as a marker of peritalar subluxation and a tool for deformity correction in AAFD patients needs further investigation in prospective, longitudinal studies.

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