Validity and reliability of hallux valgus angle measurement on smartphone digital photographs

BACKGROUND: This prospective study aimed to test the reliability and validity of hallux valgus angle (HVA) measurement on smartphone digital photographs compared with the standard radiographic evaluation. METHODS: Twenty Seven female patients (45 feet) with forefoot deformity were evaluated with weight-bearing anteroposterior foot radiographs and smartphone photographs. Radiographic hallux valgus angle (rHVA) was measured on digital radiographs. Two different photographic HVA measurement methods were used. In the first, the longitudinal axes of the first metatarsal and proximal phalanx were determined, and the angle between these axes was measured (pHVA), similar to the radiographic method. In the other method, the angle of the margo medialis pedis was measured on the photograph (pMMP). Two independent observers performed all measurements twice on two different occasions. Reliability analysis was performed using the interclass correlation coefficient. Agreement between the measurements was tested using Bland-Altman analysis. RESULTS: The repeated rHVA, pHVA and pMMP measurements showed excellent intra and inter-observer reliability, with ICC values above 0.900. The mean rHVA, pHVA, and pMMP were statistically similar (p:0.929, 27.03°±8.7°, 27.11°±8.8° and 26.5°±9.0° respectively). The mean difference between the rHVA and pHVA was − 0.07°±5.1° (range, --9.67 to 9.56°), and the mean difference between the rHVA and pMMP was 0.53°±4.4° (range, -9.76° to 8.22°). There was a strong positive correlation between both photographic methods and radiographic measurements (rho = 0.809, p = 0.001 and rho = 0.872, p = 0.001). In the Bland Altman plot, the upper and lower LOAs (95%CI) ranged from − 10.11° to 9.93° for rHVA and pHVA, and from − 8.26° to 9.33° for rHVA and pMMP. Linear regression analysis showed a proportional bias for pHVA but not for the pMMP (p:0.010 versus p:0.633, respectively). The range of the mean difference (prediction interval) between the pMMP and rHVA was 17.59° and 20° for pHVA and rHVA. Simple linear regression showed that the rHVA was predicted by the following equation: rHVA = 4.73 + 0.84 × pMMP (r2 = 0.761, p < 0.001). CONCLUSIONS: Although measuring HVA through smartphone photographs is reliable, it is not a valid prediction method. LEVEL OF EVIDENCE: Level II, diagnostic assessment.