Tripod Index: diagnostic accuracy in symptomatic flatfoot and cavovarus foot: part 2.

BACKGROUND

The Tripod Index (TI) has been created to allow assessment of complex foot deformities. It utilizes tripod relationship between center of the heel, medial/lateral borders of the forefoot, and compare it to the center of the talar head. This study aimed to verify diagnostic accuracy of the TI in symptomatic flatfoot and cavovarus foot.

METHODS

Weightbearing radiographs including foot AP with a hemispherical marker around the heel, lateral, and hindfoot alignment views were obtained on 91 patients (110 feet) presenting with medial foot and ankle pain and on 89 patients (90 feet) presenting with lateral foot and ankle pain between June 2010 and May 2011. Radiographs were evaluated blindly for the TI, AP talonavicular coverage angle, lateral talo-first metatarsal angle, calcaneal pitch angle, medial cuneiform-fifth metatarsal height, and coronal plane hindfoot alignment. The sensitivity, specificity, likelihood ratios, and predictive values were calculated. Clinically diagnosed flatfoot and cavovarus foot deformity indicated for surgical reconstruction by one of our foot and ankle orthopaedic surgeons was used as the accepted standard for diagnosis.

RESULTS

In flatfoot, sensitivity of the TI was 100%, comparable with lateral talo-first metatarsal angle (100%), and medial cuneiform-fifth metatarsal height (100%). Specificity of the TI was 93%, comparable with coronal plane hindfoot alignment (98%), but superior to other parameters. Positive likelihood ratio of the TI was 14.29, which was less than coronal plane hindfoot alignment (47.5), but more than other parameters. In cavovarus foot, sensitivity of the TI was 96%, comparable with coronal plane hindfoot alignment (100%), but superior to other parameters. Specificity of the TI was 95%, comparable with lateral talo-first metatarsal angle (94%), but superior to other parameters. Positive likelihood ratio of the TI was 19.2, which was more than other parameters.

CONCLUSION

The Tripod Index showed high accuracy as a quantitative assessment in diagnosis of a symptomatic flatfoot and cavovarus foot.