Spinal balance failure: a potential cause of spinal ligament ossification.

Ossification of the posterior longitudinal ligament (OPLL) and of the ligamentum flavum (OLF), as well as calcification of ligamentum nuchae in the cervical spine and ossification of ligamentum flavum in the thoracic spine, are common types of spinal ligament ossification. As is known, the pathogenesis of OPLL and OLF has been unclear until recently, and mechanical stress assumably plays an important role. In this paper, we try to make a hypothesis that spinal balance failure (characterized by the gravity line, spinal curvature and flexion/extension in the sagittal plane), may play more important roles than any other factors in the development of ligament ossified diseases. Moreover, different stress thresholds and flexion/extension frequency may induce various ossification regions. It is supported by the conclusions that drawn from literature review: (1) OPLL and OLF seldom occur in lumbar spine which is of stiffer and lower-frequency motion than cervical spine, although its range of motion (ROM) was relatively great. (2) Spinal ligament ossification often appears in the cervical spine, which is significantly flexible with a larger motion range. (3) OLF is often seen in the thoracic spine, which is farthest away from the gravity line for thoracic kyphosis. (4) Many OPLL or OLF patients are often found with a local kyphosis or high segmental ROM. (5) The prevalence of ossification is higher in the larger stress regions, as described above, indicating that there may be a stress threshold for the spinal ligaments.