INSTANTANEOUS HELICAL AXIS ESTIMATION FROM 3-D VIDEO DATA IN NECK KINEMATICS FOR WHIPLASH DIAGNOSTICS

To date, the diagnosis of whiplash injuries has been very difficult an d largely based on subjective, clinical assessment. The work by Winter s and Peles Multiple Muscle Systems-Biomechanics and Movement Organiza tion, Springer, New York (1990) suggests that the use of finite helica l axes (FHAs) in the neck may provide an objective assessment tool for neck mobility. Thus, the position of the FHA describing head-trunk mo tion may allow discrimination between normal and pathological cases su ch as decreased mobility in particular cervical joints. For noisy, uns moothed data, the FHAs must be taken over rather large angular interva ls if the FHAs are to be reconstructed with sufficient accuracy; in th e Winters and Peles study, these intervals were approximately 10 degre es. In order to study the movements' microstructure, the present inves tigation uses instantaneous helical axes (IHAs) estimated from low-pas s smoothed video data. Here, the small-step noise sensitivity of the F HA no longer applies, and proper low-pass filtering allows estimation of the IHA even for small rotation velocity omega of the moving neck. For marker clusters mounted on the head and trunk, technical system va lidation showed that the IHAs direction dispersions were on the order of one degree, while their position dispersions were on the order of 1 mm, for low-pass cut-off frequencies of a few Hz (the dispersions wer e calculated from omega-weighted errors, in order to account for the a dverse effects of vanishing omega). Various simple, planar models rela ting the instantaneous, 2-D centre of rotation with the geometry and k inematics of a multi-joint neck model are derived, in order to gauge t he utility of the FHA and IHA approaches. Some preliminary results on asymptomatic and pathological subjects are provided, in terms of the ' ruled surface' formed by sampled IHAs and of their piercing points thr ough the mid-sagittal plane during a prescribed flexion-extension move ment of the neck.