Background: Flatfoot is practically diagnosed by physical examination, radiographs, or footprint. Talar first-metatarsal
angle on a weight-bearing lateral radiograph provides an accurate measurement for the diagnosis of flatfoot and is frequently used by foot and ankle specialists. Staheli Index is also considered as a reliable method. However, there is no information of the sensitivity and specificity of this index compared to the talar-first metatarsal angle for the diagnosis of flatfoot.
Objective: To evaluate the usefulness of the Staheli Index on Harris mat footprint for the diagnosis of flatfoot.
Material and Method: The weight-bearing lateral radiographs were obtained from 157 patients (314 feet). The radiographs were examined for the talar-first metatarsal angle. Harris mat footprint was also obtained from each participant for the measurement of the Staheli Index. The sensitivity and specificity of the Staheli Index was calculated using the talar-first metatarsal angle as a gold standard. ROC curve was also performed to determine the cut-off point of the Staheli Index. Interobserver and intra-observer reliability was also tested.
Results: The cut-off point of the Staheli Index at 0.77 revealed the sensitivity of 70.2% and specificity of 73 %, and the accuracy value was 72% for the detection of flatfoot compared to the talar-first metatarsal angle. There was no significant difference of the area under the ROC curves performed by two physicians was found. The area under the ROC curves showed no difference when performed at two different times by the same physician.
Conclusion: The Staheli Index obtained from the Harris mat footprint could be considered as the screening or diagnostic
method for flatfoot.

Keywords: Flatfoot, Harris mat footprint, Staheli index, Talar-first metatarsal angle, Sensitivity, Specificity