The human spine is mainly loaded with axial compression when we sit, stand, walk and even more so when we carry something heavy. This axial compression subject part of our spine to dorsal shear forces, more specifically the vertebras that are tilted backwards in the lower and middle parts of our chest. This clearly indicates that dorsal shear loads are naturally present in the human spine but what is more interesting to know is the effect that these loads have on the rotational stability.

Illustrating the effect of dorsal shear load on rotational
instability on a thoracic vertebrae.

The idea that facet joints in the thoracic spine would be ineffective in providing rotational stability during dorsal shear forces as claimed by Castelein (see Dorsal Shear Forces and Idiopathic Scoliosis), was tested in a study by a group of scientists.

A study was set up to evaluate the effects of dorsal versus ventral shear loads on the rotational stability of the thoracic spine. In this study, parts of both human and animal thoracic spine were subjected to ventral (forward) and dorsal (backward) shear loads and when these were applied to the center of the spine, no significant difference in rotation was observed.

It was only when eccentric dorsal shear loads were applied that a significant difference in rotation occurred and the study, therefore, concluded that “the thoracic spine is less rotationally stable under dorsal shear loads than under ventral shear loads”.

With this understanding and its potential implications in the cause of idiopathic scoliosis, we should read what influence the sagittal (side) profile of the spine has progression of the scoliosis curve.

14. Kouwenhoven JW, Smit TH, van der Veen AJ, Kingma I, van Dieën JH, Castelein RM.
Effects of dorsal versus ventral shear loads on the rotational stability of the thoracic spine: a biomechanical porcine and human cadaveric study.
Spine. 2007 Nov 1;32(23):2545-50.