Talar Anatomy and Subtalar Joint Alignment on Weight-Bearing Computed Tomography

Abstract

Category: Hindfoot Introduction/Purpose: Underlying bony deformities may contribute to the development of adult acquired flatfoot deformity (AAFD), although the exact pathogenesis remains unknown. Subtalar valgus tilt and underlying talar deformity can both be accurately assessed using multiplanar weightbearing computed tomography (MP-WB). On coronal MP-WB images, two angles have been used to reliably evaluate the subtalar joint axis in AAFD: (1) the angle between the inferior facet of the talus and the horizontal (inftal-hor), and (2) the angle between the inferior and superior facets of the talus (inftal-suptal). While previous work showed that these angles differ significantly between AAFD patients and controls, it is not known how they correlate with radiographic measures of flatfoot. We hypothesized that they would correlate strongly with commonly used radiographic measures of AAFD. Methods: 45 patients with stage II AAFD and 17 control patients underwent MP-WB imaging as well as standard weightbearing radiographs. Inftal-hor and inftal-suptal were measured on the coronal MP-WB images of all patients. These measurements were then correlated with standard radiographic measurements used to assess AAFD: talar-first metatarsal angle on antero-posterior (AP) and lateral views, talocalcaneal angle on AP and lateral views, talonavicular coverage angle, calcaneal pitch, medial column height, and hindfoot alignment. Basic demographic data including age, sex, and body mass index (BMI) were also collected. Differences between AAFD and control patients were assessed using independent samples t-tests and Mann-Whitney U tests. To examine the correlation between each MP-WB measurement and each radiographic measurement, a factorial generalized linear model (GLM) was constructed with presence of flatfoot, the radiographic measurement, and their interaction as covariates. Results: The flatfoot group was older than the control group (p=0.049); the two groups did not differ in terms of sex or BMI. The patients with AAFD differed significantly from the controls in all measured angles (p≤0.001 for each). Inftal-hor and inftal- suptal correlated with radiographic measures of flatfoot to the same degree in patients with and without AAFD. After accounting for differences between flatfoot and control patients, inftal-hor did not significantly correlate with any of the radiographic angles. Inftal-suptal, however, did significantly correlate with AP coverage angle, AP talar-first metatarsal angle, calcaneal pitch, Meary’s angle, medial column height, and hindfoot alignment (Figure). Conclusion: As measured on coronal MP-WB images, patients with stage II AAFD had more innate valgus in their talar anatomy as well as more valgus alignment of their subtalar joints than did control patients. The inftal-suptal angle correlated significantly with other measures of flatfoot deformity including arch collapse and forefoot abduction. Though causation cannot be established directly from this study, the data does suggest that AAFD may develop in patients with a valgus inclined subtalar joint axis. This information can be used to identify those patients likely to develop progressive flatfoot deformity and may ultimately guide the approach to surgical reconstruction.