Two methods of calculating thorax kinematics in children with myelomeningocele.

OBJECTIVE: The purpose of this study was to determine the importance of model and cardan angle sequence for calculating thorax kinematics.

BACKGROUND: Patients with myelomeningocele often show a significant lateral bending and large trunk rotation over the gait cycle. The temporal pattern and magnitudes of these rotations are important for clinical decision-making. Two common thorax models and two orders of rotation in calculating the thoracic kinematics are compared.

METHODS: Thoracic kinematics in eight myelomeningocele patients were studied retrospectively. Four markers were placed on the trunk in addition to the lower limb markers. Each child walked at a self-selected speed over the walkway in bare feet. The spatial position and orientation of each segment were computed from the marker position at 2% of the gait cycle. Cardan angles for two thorax models were calculated using two different orders of rotation with respect to both the laboratory and the pelvic reference systems.

RESULTS: The results showed that differences between models in the coronal and transverse planes were greater than in the sagittal plane. The changes in the modelling technique yielded the greatest differences when comparing between models.

CONCLUSIONS: Clinical significant differences were found between changes in different thorax models and between the sequences of rotation for subjects with pathological thoracic motion. This clearly indicates that existing thorax models based on indirect marker placement may give misleading kinematic measurements. Different rotation sequences also lead to different results and an analysis of these suggests that the conventional sequence (flexion, lateral bending, axial rotation), is preferable for the thorax.

RELEVANCE: Accurate measurement of thoracic motion will enhance our understanding of both the pathological and compensatory mechanism in the gait pattern of children with myelomeningocele. Furthermore, the current investigation on thorax kinematics will lead to calculation of the motion of the center of mass and future methodology to calculate thoracic kinetics.