Describing three-dimensional cervical spine movement in a diseased and a non-diseased group using multilevel modelling.

Clinicians often request patients to perform movements of joints as an aid to assessment and diagnosis of many diseases. However, patients may perform abnormal secondary movements where their primary movement is limited. A new measuring system, the FASTRAK, has the potential to measure movement in three dimensions, and to measure movements of the body away from the primary movement. This paper explores the use of multilevel modelling to describe the patterns of movement where subjects are measured on several occasions for both a non-diseased group and for a group of subjects suffering from ankylosing spondylitis. Three-level multilevel models are described with level 3 being the subjects, level 2 being the measurement occasions (up to three) and level 1 being the measurements (three) on each occasion. The models highlight which are the most important secondary movements during the primary movement and show clinically interpretable patterns of movement that can highlight differences between diseased and non-diseased groups.