Subluxation of the Middle Facet of the Subtalar Joint as a Marker of Peritalar Subluxation in Adult Acquired Flatfoot Deformity: A Case-Control Study

J Bone Joint Surg Am. 2019 Oct 16;101(20):1838-1844. doi: 10.2106/JBJS.19.00073.


Background: Progressive peritalar subluxation (PTS) is part of adult acquired flatfoot deformity (AAFD). We investigated the use of the middle facet as an indicator of PTS using standing, weight-bearing computed tomography (CT) images. We hypothesized that weight-bearing CT would be an accurate method of measuring increased subluxation ("uncoverage") and incongruence of the middle-facet among patients with AAFD.

Methods: We included 30 patients with stage-II AAFD (20 female and 10 male; mean age, 57.4 years [range, 24 to 78 years]) and 30 matched controls (20 female and 10 male; mean age, 51.8 years [range, 19 to 81 years]) who underwent standing, weight-bearing CT. Two independent and blinded fellowship-trained foot and ankle surgeons measured the amount of subluxation (percentage of uncoverage) and the incongruence angle of the middle facet at the midpoint of its longitudinal length, using coronal-plane, weight-bearing, cone-beam CT images. Intraobserver and interobserver reliabilities were assessed using intraclass correlation coefficients (ICCs). Comparisons were performed using independent t tests or Wilcoxon tests. P values of <0.05 were considered significant.

Results: Substantial to almost perfect intraobserver and interobserver reliability was observed for both measurements. We found that the middle facet demonstrated significantly increased PTS in patients with AAFD, with a mean value for joint uncoverage of 45.3% (95% confidence interval [CI], 38.5% to 52.1%) compared with 4.8% (95% CI, 3.2% to 6.4%) in controls (p < 0.0001). A significant difference was also found for the incongruence angle, with a mean value of 17.3° (95% CI, 14.7° to 19.9°) in the AAFD group and 0.3° (95% CI, 0.1° to 0.5°) in controls (p < 0.0001). A joint incongruence angle of >8.4° was found to be diagnostic for symptomatic stage-II AAFD.

Conclusions: We investigated the use of the middle facet of the subtalar joint as a marker for PTS in patients with AAFD. We confirmed that standing, weight-bearing CT images allowed accurate measurements and that significant differences were found in the percentage of joint uncoverage and the incongruence angle compared with controls.

Clinical relevance: The assessment of the amount of subluxation and incongruence of the middle facet of the subtalar joint represents an accurate diagnostic tool for symptomatic adult acquired flatfoot deformity.

MeSH terms

  • Adult
  • Aged
  • Aged, 80 and over
  • Case-Control Studies
  • Female
  • Flatfoot / diagnostic imaging
  • Flatfoot / etiology*
  • Humans
  • Joint Dislocations / complications*
  • Joint Dislocations / diagnostic imaging
  • Male
  • Middle Aged
  • Observer Variation
  • Subtalar Joint / diagnostic imaging
  • Subtalar Joint / injuries*
  • Tomography, X-Ray Computed
  • Weight-Bearing / physiology
  • Young Adult