Assessment of the 3-dimensional Fastrak measurement system in measuring range of motion in ankylosing spondylitis.

Objective: To assess the repeatability and validity of the electromagnetic 3-dimensional tracking system, Fastrak, in measuring cervical spine and shoulder movement in patients with ankylosing spondylitis (AS).

Methods: Fifty patients with AS had their cervical spine and shoulder movements measured on up to 3 occasions with the Fastrak. Patients also completed disease-specific and generic patient assessed health instruments, and their spinal mobility was assessed by tape measure methods. Repeatability over 2 weeks was assessed using intraclass correlation coefficients (ICC). Fastrak measurements were compared between patients with different self-ratings of AS related health. Comparisons between the Fastrak measurements and patient assessed health instruments and tape measurements were made using Spearman correlations and multilevel modeling.

Results: Patients with AS tended to be limited in both cervical spine and shoulder movements. ICC were all > 0.80 (except shoulder extension, 0.75), indicating substantial reliability. Fastrak was able to differentiate between patients with a high self-rating of AS related health and those with a poorer rating. Cervical spine flexion and shoulder flexion and abduction were most strongly related to the patient assessed health instruments, although the shoulder movements had limited relationships with the tape measurements of spinal mobility.

Conclusion: The Fastrak appears to be reliable and valid in an AS population. Shoulder movements tended to have a stronger relationship with the patient assessed health instruments than cervical spine movements. Shoulder movement may be more related to everyday function measured by these instruments, which indicates the importance of this joint in assessment of AS.