Range of rotation of thoracolumbar vertebrae in Japanese macaques

In humans, the range of thoracic vertebral rotation is known to be greater than that of the lumbar vertebrae due to their zygapophyseal orientation and soft tissue structure. However, little is known regarding vertebral movements in non-human primate species, which are primarily quadrupedal walkers. To understand the evolutionary background of human vertebral movements, this study estimated the range of axial rotation of the thoracolumbar spine in macaque monkeys. First, computed tomography (CT) was performed while passively rotating the trunk of whole-body cadavers of Japanese macaques, after which the motion of each thoracolumbar vertebra was estimated. Second, to evaluate the influence of the shoulder girdle and surrounding soft tissues, specimens with only bones and ligaments were prepared, after which the rotation of each vertebra was estimated using an optical motion tracking system. In both conditions, the three-dimensional coordinates of each vertebra were digitized, and the axial rotational angles between adjacent vertebrae were calculated. In the whole-body condition, the lower thoracic vertebrae had a greater range of rotation than did the other regions, similar to that observed in humans. In addition, absolute values for the range of rotation were similar between humans and macaques. However, in the bone-ligament preparation condition, the upper thoracic vertebrae had a range of rotation similar to that of the lower thoracic vertebrae. Contrary to previous speculations, our results showed that the mechanical restrictions by the ribs were not as significant; rather, the shoulder girdle largely restricted the rotation of the upper thoracic vertebrae, at least, in macaques.