Smartphone Photogrammetry as a Tool for Pes Planus Assessment: Reliability and Agreement with Radiographic Measurements

Background/Objectives: The purpose of this study was to evaluate the reliability and diagnostic accuracy of smartphone-based photogrammetry for the assessment of pes planus and to determine its agreement with standard radiographic measurements. Methods: This prospective diagnostic study included 100 skeletally mature patients (50 males, 50 females; mean age 43.4 years) who underwent standardized lateral weight-bearing foot radiographs and smartphone-based foot photography. The calcaneal pitch angle (CPA) was measured on radiographs, and a corresponding photographic arch pitch angle (P-APA) was measured from standardized smartphone photographs using digital software (Angle Meter iOS v1.9.8). Three independent observers performed each measurement twice. Inter- and intra-observer reliability was assessed using intraclass correlation coefficients (ICC). Agreement between methods was evaluated with Pearson correlation, Lin’s concordance correlation coefficient (CCC), Bland–Altman analysis, and Deming regression. Receiver operating characteristic (ROC) analysis was performed to determine the diagnostic accuracy of calibrated P-APA, with the radiographic threshold of 18° serving as the reference standard for pes planus classification. Results: All measurements demonstrated excellent intra- and inter-observer reliability (ICC ≥ 0.900). P-APA values were systematically higher than radiographic values (31.8° ± 4.3 vs. 21.8° ± 5.5; p < 0.001). A strong correlation was observed between the two methods (r = 0.799, p < 0.001), but concordance was poor (CCC = 0.222). Bland–Altman analysis revealed a mean bias of +10.1° with wide limits of agreement (3.8° to 16.4°). Deming regression yielded the calibration equation Radiographic CPA = (P-APA × 1.371) − 21.883. ROC analysis of calibrated values yielded an AUC of 0.885 (95% CI, 0.820–0.951), with an optimal cutoff of 22.8° (sensitivity, 100%; specificity, 61.1%), corresponding to 32.6° on the uncalibrated photographic scale. Conclusions: Conventional weight-bearing radiography remains the reference standard for diagnosis and clinical decision-making in pes planus. The smartphone-derived photographic arch pitch angle is a non-equivalent surrogate measure that shows substantial systematic bias and limited agreement with radiographic calcaneal pitch, and therefore cannot replace weight-bearing radiographs. Smartphone photogrammetry may be used only as a complementary tool for preliminary screening or telemedicine support; any positive or equivocal findings require radiographic confirmation.