The mechanical properties of the human spine are best described by load-displacement curves which include coupling effects. Three-dimensional load-displacement curves were obtained for all levels of the human thoracic spine using fresh cadaver spines in an atmosphere containing 100 per cent humidity at 22 degrees centigrade to stimulate a physiological environment. Six forces and six moments were applied, one at a time, to the center of the upper vertebra while its subadjacent fellow was fixed. Assuming sagittal plane symmetry, vertebral displacement was measured in three-dimensional space and load-displacement curves were plotted for the main as well as the coupled motions. The thirty-six curves necessary to define the mechanical characteristics of each motion segment completely were determined for all eleven thoracic levels. The curves showed that all the thoracic spine is a complex three-dimensional structure with coupled motion characteristics. Axial forces (compression/tension) resulted in significant horizontal displacements. Spine motion segments were more flexible in flexion than in extension. The spine was found to be least flexible during axial compression.
Copyright 1976 by The Journal of Bone and Joint Surgery, Incorporated