Agreement and Screening Performance of an App-Based Cranial Shape Assessment Tool Compared With a 3D Scanner in Municipal Child Health Checkups in Japan: Instrument Validation Study.

Background: Positional cranial deformities are common in infancy. Smartphone-based apps are increasingly used to assess infant head shape, but evidence comparing these tools with 3D scanners across age groups in community-based settings remains limited. Objective: This study evaluated age-stratified agreement and screening performance of a mobile app for estimating Cranial Vault Asymmetry Index (CVAI) and Cephalic Index, compared with a 3D scanner, when used by a trained examiner during routine municipal health checkups in Japan. Methods: We conducted a cross-sectional study in Goto City, Japan, between May 2023 and March 2025. Independent, age-specific cohorts of children attending municipal health checkups at 4, 18, and 42 months were included. Paired cranial measurements were obtained by a trained examiner using a VECTRA H2 3D scanner (Canfield Scientific Inc) and the mobile app. Agreement was assessed using mean differences, 95% limits of agreement (LoA), and intraclass correlation coefficients (ICCs), with device-stratified sensitivity analyses. Screening performance was evaluated using sensitivity and specificity based on prespecified clinical thresholds. Receiver operating characteristic (ROC) analyses were secondary exploratory analyses. The primary outcome was app-based CVAI for detecting scanner-defined moderate-or-worse plagiocephaly (CVAI≥7%). Secondary outcomes were app-based Cephalic Index for detecting brachycephaly (Cephalic Index>94%) and dolichocephaly (Cephalic Index<79%). Results: The final analytic samples comprised 262 children at 4 months, 171 at 18 months, and 216 at 42 months. Agreement for continuous CVAI was highest at 4 months (mean difference 0.14, 95% LoA -4.18 to 4.46; ICC 0.76, 95% CI 0.71-0.81) and lower at 18 months (ICC 0.42, 95% CI 0.29-0.54) and 42 months (ICC 0.49, 95% CI 0.38-0.59). For Cephalic Index, the app consistently overestimated values relative to the scanner, with mean differences of 3.83 SD 3.42, 3.24 SD 3.07, and 2.61 SD 3.33 points at 4, 18, and 42 months, respectively. For detecting scanner-defined moderate-or-worse plagiocephaly, app sensitivity and specificity were 84.8% and 88.0% at 4 months, 50.0% and 88.4% at 18 months, and 53.3% and 89.6% at 42 months, respectively. Device-stratified analyses showed some variation in measurement agreement and screening performance across devices. Conclusions: Under municipal health checkup conditions with a trained examiner, app-based CVAI showed the strongest performance at 4 months and may be useful for early screening. However, performance was weaker at 18 and 42 months. App-based Cephalic Index was systematically higher than scanner-based Cephalic Index, potentially increasing false-positive classifications for brachycephaly when identical thresholds are applied. Performance varied somewhat between devices, although device effects could not be separated from study period and accumulated examiner experience. Positive findings, particularly at later ages or for Cephalic Index-based measures, should be interpreted in the context of the child's age and broader clinical findings, with further evaluation or referral considered only when clinically indicated.