Back injuries affect eight of ten people, with direct and indirect costs approximating over $20 billion annually in the United States. Poor control of costs stems partially from inadequate understanding of normal spine function and difficulty in accurately diagnosing spinal disorders. Many diagnosis and intervention strategies used in manual medicine depend on a model of spinal behavior described by Harrison Fryette (1918). Presently, there is no consensus regarding thoracic three-dimensional motion characteristics, and no one has systematically examined Fryette’s laws of vertebral movement. Fryette stated the following:​ (1) When sidebending is introduced with the thoracic spine in a neutral posture, axial rotation will always be coupled to the opposite side, (2) When sidebending is introduced with the thoracic spine in a non-neutral posture (flexed or hyperextended) axial rotation will always be coupled to the same side, and (3) When spinal motion is introduced in one plane, mobility will be reduced in secondary planes. The purpose of this investigation was to examine Fryette’s laws using a video-based motion analysis system to parameterize (with Cardan and helical axis analysis) the three-dimensional kinematics of the thoracic spine/cage in healthy subjects with normal spinal alignment.

When subjects were motion tested with their spine in a neutral position only 22% exhibited contralateral coupling of sidebending and axial rotation, thus there are insufficient data to support Fryette’s law of neutral spine mechanics for the thoracic spine. In non-neutral postures (either flexed or hyperextended) subjects demonstrated ipsilateral coupling of sidebending and axial rotation 79% of the time, although with axial rotation in a slouched position 46% of the subjects showed contralateral coupling. Overall, data support Fryette’s second law. There was borderline support for Fryette’s third law that says that motion is reduced in secondary planes of motion. The finite helical axis unit vectors were found to correspond with Cardan parameterizations of three-dimensional thoracic spine movements suggesting some clinical relevance for the use of helical paramters.

Results suggest manual medicine practitioners, who base their diagnostic and intervention strategies on Fryette’s laws of spinal behavior, may need to re-examine their working assumptions. It appears also that helical axis analysis of the spine might have a more important role in the clinical setting than previously thought. Further research on normal subjects and application to clinical cases will continue to add to our understanding of dysfunction of the thoracic spine.