Depending on the orientation
of the vertebrae, shear forces can
act (A) forward or (B) backward

In humans the individual vertebrae in the lower and middle thoracic spine lean backwards when they are upright. These vertebrae/bones are therefore subjected to ‘dorsal shear forces’ [13]. It means that when the vertical spine is loaded with gravitational forces, the individual bones/vertebrae are stressed and tend to shear backwards (Fig 1).

These dorsal shear loads are something that is unique to humans (please refer to our post, The Human Spine and Idiopathic Scoliosis)

Castelein, a scientist, co-wrote a paper with his colleagues on the proposed hypothesis [13] that dorsal shear forces play a crucial part in the development of idiopathic scoliosis.

In their opinion, the vertebrae are not well designed to resist dorsal shear loads because the facet joints detach and are effectively rendered inoperative under this condition. They also pointed out that “the predominant posterior location and longitudinal alignment of major muscles and ligaments is not well suited to counteract dorsal shear forces.” Arising from these opinions, they questioned the kind of effect the forces that are directed backwards, has on the rotational stability in the affected parts of the spine.

The idea of dorsal shear forces and later the understanding of its influence on rotational stability of the spine has had a big ripple effect in the scientific community as it paved the way to a better understanding of idiopathic scoliosis.

I personally consider it as a major step forward and will therefore post the findings of the study on rotational stability in the near coming posts.

13. Castelein RM, van Dieën JH, Smit TH.
The role of dorsal shear forces in the pathogenesis of adolescent idiopathic scoliosis–a hypothesis.
Med Hypotheses. 2005;65(3):501-8.