Assessing cervical rotation in ankylosing spondylitis: iPhone Compass versus traditional tools

The aim of the study was to assess the reliability of using the iPhone's built-in Compass application for measuring cervical rotation (CR) degrees in patients with ankylosing spondylitis (AS), compared to traditional methods using an inclinometer and a universal goniometer.

Patients diagnosed with AS and receiving care at our rheumatology outpatient clinic were enrolled in this study. Two examiners measured CR using the iPhone 4 Compass application, an inclinometer, and a universal goniometer. Each measurement was performed twice. Intrarater and interrater reliability were assessed using intraclass correlation coefficients (ICCs), while agreement between methods was analysed by the Bland-Altman method.

The study sample included 30 AS patients (73% male). Excellent intrarater and interrater reliability were observed across all three measurement methods in the entire sample. Bland-Altman analysis showed good agreement between the iPhone and inclinometer, with a mean difference (bias) of -5.6 for Examiner 1 [95% confidence interval (CI) -7.6 to -3.6] and -6.3 for Examiner 2 (95% CI -8.8 to -3.8). The mean differences between the iPhone and universal goniometer measurements were 2.3 for Examiner 1 (95% CI -0.4 to 5.2) and 4.1 for Examiner 2. Similarly, between the universal goniometer and inclinometer, mean differences were -8.0 for Examiner 1 (95% CI -11.2 to -4.8) and -10.4 for Examiner 2.

Through the integration of everyday smartphone technology for clinical use, the iPhone Compass application can be considered a practical and accessible tool for measuring CR in patients with AS, offering potential benefits for clinical practice.