A MODEL OF SPINE, RIBCAGE AND PELVIC RESPONSES TO A SPECIFIC LUMBAR MANIPULATIVE FORCE IN RELAXED SUBJECTS

One class of manipulative techniques commonly used during assessment a nd treatment of spinal disorders involves the patient lying face down while the therapist slowly applies a posteroanterior force to a select ed vertebra, The aim of this investigation was to develop a model whic h was capable of predicting the vertebral displacements resulting from such a manipulative force, applied to the lumbar spine. A linear thre e-dimensional finite element model was generated using both previously published and original data to define the geometry and material prope rties. The complete model included the ribcage, thoraco-lumbar spine a nd pelvis with their associated soft tissues. The model simulated the relaxed state in a normal subject so the muscle forces were assumed to be negligible. Sensitivity analysis suggested that if the model was t o be used to simulate the behaviour of individual subjects, then the m odel dimensions and pelvic constraints should be matched to the partic ular subject. The model validity was studied by comparing the predicte d responses with those that have been observed in living human subject s. The model predictions were found to be in good agreement with the m ean observed human responses, with predicted displacements being withi n one standard deviation of the mean observed values. This agreement s uggests that the model is useful for predicting the linear region resp onses to slowly applied lumbar posteroanterior forces. The simulations predicted that appreciable global vertebral displacements (up to 1.5 mm) and rotations (up to 1 degrees) occurred as far away as the middle and lower thoracic spine during low lumbar loading. Intervertebral tr anslations were predicted to be 1 mm or more at up to four interverteb ral jonts away from the point of load application. Variation of direct ion of applied force was predicted to have small effects on displaceme nt responses compared with loading at different lumbar levels.