Effects of strain distribution in the intervertebral discs on the progression of ossification of the posterior longitudinal ligaments

Study Design. Strain distribution in the intervertebral discs was evaluated biomechanically using an engineering true strain calculation formula. Objectives. This study was performed to clarify the involvement of dynamic factors in the progression of ossification of the posterior longitudinal ligament. Summary of Background Data. In patients with ossification of the posterior longitudinal ligament of the cervical spine, ossification frequently progresses after laminectomy. This suggests the involvement of dynamic factors in the progression of ossification. However, these factors have not yet been clarified. Methods. The analysis was performed on 101 patients with ossification of the posterior longitudinal ligament by employing dynamic lateral x-ray films of the cervical spine, The x-ray films were digitized and used as computer data for calculating the strain distribution. x-ray films were obtained again 5 years later, and the strain distribution and the presence or absence of progression of ossification were evaluated. Results. The progression of ossification of the posterior longitudinal ligament was highly correlated with abnormal strain distribution in the intervertebral discs. Progression of ossification was frequently observed in areas having disc distortion in tension and extension on the posterior longitudinal ligament. Conclusions. In this study, the area of progression of ossification corresponded to the area showing uneven strain distribution and resultant concentration of dynamic stress. These results suggest an important role for dynamic factors in the progression of ossification of the posterior longitudinal ligament.