Biomechanical study of the spinal cord in thoracic ossification of the posterior longitudinal ligament

Background: Ossification of the posterior longitudinal ligament (OPLL) in the thoracic spine produces myelopathy. This is often progressive and is not affected by conservative treatment. Therefore, decompressive surgery is usually chosen. Objective: To conduct a stress analysis of the thoracic OPLL. Methods: The three-dimensional finite element spinal cord model was established. We used local ossification angle (LOA) for the degree of compression of spinal cord. LOA was the medial angle at the intersection between a line from the superior posterior margin at the cranial vertebral body of maximum OPLL to the top of OPLL with beak type, and a line from the lower posterior margin at the caudal vertebral body of the maximum OPLL to the top of OPLL with beak type. LOA 20 degrees, LOA 25 degrees, and LOA 30 degrees compression was applied to the spinal cord in a preoperative model, the posterior decompressive model, and a model for the development of kyphosis. Results: In a preoperative model, at more than LOA 20 degrees compression, high stress distributions in the spinal cord were observed. In a posterior decompressive model, the stresses were lower than in the preoperative model. In the model for development of kyphosis, high-stress distributions were observed in the spinal cord at more than LOA 20 degrees compression. Conclusions: Posterior decompression was an effective operative method. However, when the preoperative LOA is more than 20 degrees, it is very likely that symptoms will worsen. If operation is performed at greater than LOA 20 degrees, then correction of kyphosis by fixation of instruments or by forward decompression should be considered.