Effects of dorsal versus ventral shear loads on the rotational stability of the thoracic spine: a biomechanical porcine and human cadaveric study.

STUDY DESIGN A biomechanical in vitro study on porcine and human spinal segments. OBJECTIVE To investigate axial rotational stability of the thoracic spine under dorsal and ventral shear loads. SUMMARY OF BACKGROUND DATA Idiopathic scoliosis is a condition restricted exclusively to humans. An important difference between humans and other vertebrates is the fact that humans ambulate in a fully erect position. It has been demonstrated that certain parts of the human spine, more specifically the dorsally inclined lower thoracic and high lumbar parts, are subject to dorsally directed shear loads. It has been hypothesized that these dorsal shear loads reduce the rotational stability of the spine, thereby increasing the risk to initiate idiopathic scoliosis. METHODS Fourteen porcine and 14 human thoracic functional spinal units (FSUs) with intact costotransverse and costovertebral articulations were used for biomechanical testing. In both dorsal and ventral directions, shear loads were applied to the upper vertebra of the FSU in the midsagittal plane (centrally), and at 1 cm to the right and to the left (eccentrically), resulting in a rotary moment. Vertebral rotation was measured at 3 incremental loads by an automated optoelectronic 3-dimensional (3D) movement registration system. RESULTS The results of this study showed that eccentrically applied shear loads induce vertebral rotation in human as well as in porcine spinal segments. At the mid-thoracic and lower thoracic levels, significantly more vertebral rotation occurred under dorsal shear loads than under ventral shear loads. CONCLUSION These data show that, in humans and in quadrupeds, the thoracic spine is less rotationally stable under dorsal shear loads than under ventral shear loads.