We developed a new computerized program for analyzing strain distribution of the intervertebral discs, and used it to clarify the mechanism underlying progressin of ossification of posterior longitudinal ligaments (OPLL). Biomechanical analysis was performed in 101 patients who had OPLL by putting their dynamic lateral X-ray films of the cervical spine into a microcomputer. The progression of OPLL was investigated after a minimum 5 years follow up. Correlation between progression of OPLL and the concentration of strain of intervertebral discs existed in 79% of patients with continuous type OPLL, 78% with mixed type, 65% with segmental type. Biomechanical factors play an important role in progression of OPLL.
