Reliability of categorization of the shape of the lateral edge of the first metatarsal head in hallux valgus

Hallux valgus is a triplanar deformity with rotation being a significant contributing factor. Several methods exist for evaluating rotational deformity of the first ray, one of which involves assessing the shape of the lateral edge of the first metatarsal head. This study aimed to investigate the interobserver and intraobserver reliabilities of circle measurements on weight-bearing anteroposterior radiographs to categorize the shape of the lateral edge of the first metatarsal head. A power analysis determined that a minimum of 128 radiographs was required to assess observer agreement for categorical data (alpha = 0.05, beta = 0.2, power = 80%). Patients diagnosed with hallux valgus at our institution between January 2015 and December 2019 were retrospectively reviewed and 128 radiographs were randomly selected. Two foot and ankle surgeons and 2 radiologists evaluated the radiographs on 2 separate occasions, classifying the first metatarsal head shape into 3 categories (angular, intermediate, and round) using circular measurements. Interobserver reliability was assessed using Fleiss' kappa, whereas intraobserver reliability was evaluated using Cohen kappa. Interobserver reliability for classifying the shape of the lateral edge of the first metatarsal head was found to be very good in both assessments (Fleiss' kappa = 0.83 and 0.84, respectively). The subcategory analysis revealed the highest agreement for the angular type, followed by the round type, with the lowest agreement observed for the intermediate type. The intraobserver Cohen kappa values ranged from 0.78 to 0.91. The level of interobserver agreement was found to be very good for the classification of the shape of the first metatarsal, and the intraobserver agreements were substantial to very good. In our opinion, this method is reliable and may be useful in determining the necessity for rotation-correcting surgery and advanced imaging with weight-bearing computed tomography.