Reliability and concurrent validity of smartphone-based inclinometer applications for sagittal spinal curvature assessment

To evaluate the same-day intra-rater reliability, 24-48-hour test-retest reliability, concurrent validity, and inter-device agreement of sagittal spinal alignment measurements obtained using iOS- and Android-based smartphone inclinometer applications and a conventional bubble inclinometer.

Sixty-three healthy young adults (32 males, 31 females; mean age, 20.3 ± 1.2 years) were assessed at five spinal regions using the three measurement methods. Measurements were repeated within the same session and 24-48 h later. Reliability was evaluated using intraclass correlation coefficients (ICC), standard error of measurement, and minimum detectable change (MDC). Concurrent validity and inter-device agreement were assessed using ICC(2,1), Pearson's correlation, Lin's concordance correlation coefficient (CCC), and Bland-Altman analysis.

Same-day ICC values ranged from 0.758 to 0.884 for iOS, 0.541-0.921 for Android, and 0.727-0.871 for the bubble inclinometer. Test-retest reliability was lower, ranging from 0.469 to 0.702, 0.097-0.607, and 0.392-0.745, respectively. The lowest ICC occurred for Android at the sacrum (0.097). MDC₉₀ ranged from 7.11° to 14.46°, and CCC values from 0.512 to 0.888. Bland-Altman analysis showed systematic bias in several device-landmark comparisons, with the largest absolute bias at C7 for iOS versus bubble inclinometry (- 5.08°).

Measurement performance varied by device, anatomical landmark, and assessment interval. Same-day repeatability was higher than 24-48-hour test-retest reliability, and the MDC values indicate that small longitudinal changes should be interpreted cautiously. Smartphone-based inclinometry may be useful for same-session assessment, but the observed inter-device bias does not support routine interchangeability. Serial measurements should preferably use the same device, application, landmarks, and standardized protocol.