Etiology of Cervical Myelopathy Induced by Ossification of the Posterior Longitudinal Ligament Determining the Responsible Level of OPLL Myelopathy by Correlating Static Compression and Dynamic Factors

Study Design: Retrospective study. Objective: To determine the responsible level of cervical myelopathy induced by ossification of the posterior longitudinal ligament (OPLL). This was achieved by correlating the intervertebral range of motion (ROM) as the dynamic factor with the space available for spinal cord (SAC) as the static compression factor. Summary of Background Data: The association between spinal canal stenosis and the occurrence of the myelopathy has previously been reported for OPLL patients, but not the detailed relationship between SAC, ROM, and myelopathy. Methods: We investigated OPLL type, SAC, and ROM in relation to the responsible level of cervical OPLL myelopathy in 27 cases. SAC and ROM were measured at each vertebral and intervertebral levels. The responsible level was diagnosed using spinal cord-evoked potentials and classified as group A, whereas the nonresponsible level was classified as group B. Results: Spinal cord-evoked potentials revealed 21 cases with a single responsible level and 6 cases with 2 responsible levels. The mean ROM of group A (8.9 degrees) was significantly higher (P < 0.01) than that of group B (5.7 degrees). The mean SAC of group A (8.2 mm) was significantly lower (P < 0.01) than that of group B (12.4 mm). Using discriminate analysis, significant differences for both SAC and ROM were observed between groups A and B [Box's M test: chi(2) = 3.31 <chi(2/3) (0.05)]. The discriminate formula for the borderline of symptomatic spinal compression can be described as: Z = -0.21ROM + 0.47SAC - 2.76. Conclusions: Cervical OPLL myelopathy is induced by static factors, dynamic factors, or a combination of both. The discriminate formula for symptomatic cervical OPLL myelopathy contains both ROM and SAC.