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 and treatment of spinal disorders involves the patient lying face down while the therapist slowly applies a posteroanterior force to a selected vertebra, The aim of this investigation was to develop a model which was capable of predicting the vertebral displacements resulting from such a manipulative force, applied to the lumbar spine. A linear three-dimensional finite element model was generated using both previously published and original data to define the geometry and material properties. The complete model included the ribcage, thoraco-lumbar spine and 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 to be used to simulate the behaviour of individual subjects, then the model dimensions and pelvic constraints should be matched to the particular subject. The model validity was studied by comparing the predicted responses with those that have been observed in living human subjects. The model predictions were found to be in good agreement with the mean observed human responses, with predicted displacements being within one standard deviation of the mean observed values. This agreement suggests that the model is useful for predicting the linear region responses 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 translations were predicted to be 1 mm or more at up to four intervertebral jonts away from the point of load application. Variation of direction of applied force was predicted to have small effects on displacement responses compared with loading at different lumbar levels.