Reliability study of the Phillips Biometer™ for the measurement of subtalar joint range of motion

BackgroundReproducible accurate clinical measurements of the subtalar joint are one of the essential components of a biomechanical exam in order to effectively aid in the diagnosis and treatment of foot pathology.ObjectivesThe objective of this study was to evaluate the intra and interrater reliability of the Phillips Biometer™ measuring subtalar joint range of motion.MethodTwelve volunteer subjects (six females, and six males, ages 20–40 years) participated. Two novice testers measured subtalar joint range of motion on the right foot of each subject during two testing sessions. Intrarater and interrater reliability coefficients (ICC) were determined using intraclass correlation coefficients. ICC values ranged from 0 to 1, where 0.0 indicated that there is no agreement and 1.0 indicated perfect agreement. The patterns of interrater reliability were also investigated using a paired t-test and the Bland-Altman technique of assessing agreement. The patterns of intrarater reliability were also investigated using a repeated-measures ANOVA and analysis of coefficient of variation.ResultsInterpreter reliability for all trials averaged 0.99 for inversion and 0.94 for eversion of the subtalar joint. The paired t-test revealed no statistically significant difference between the testers when measuring inversion (t = −1.10, p = 0.30) or eversion (t = 1.62, p = 0.13). The Bland-Altman analysis showed a small bias (−0.53°) for tester-1 relative to tester-2 for eversion. The 95% confidence interval for the measurement was between +1.61° and −2.67°. There was no systematic trend in bias as a function of the size of the measured angle.Intrarater reliability averaged 0.99 for inversion and 0.93 for eversion, indicating a high degree of agreement between trials. The only statistically significant difference on replicate measurements, based on ANOVA, was found on tester-1's measurement of inversion. The coefficient of variation was approximately 5% for inversion and approximately 10% for eversion with no systematic trend as a function of the size of the measured angle.ConclusionThe results indicated a high degree of reliability of the measurements between testers. The study demonstrated that interrater and intrarater reliability were high for both subtalar joint inversion and eversion. This study showed that the Phillips Biometer™ had a high degree of reliability and was a more accurate way to obtain subtalar joint measurements in the biomechanical exam versus the goniometer, the K-square, the gravity angle finder, the digital goniometer, the Polhemus Isotrak II™ tracking system, and a computer-assisted video analysis system.