Three-dimensional measurement of intervertebral range of motion in ossification of the posterior longitudinal ligament: are there mobile segments in the continuous type?

Object. In this paper, the authors' goals were to determine the extent of the effect of continuous-type ossification of the posterior longitudinal ligament (OPLL) of the cervical spine on intervertebral range of motion (ROM) and to examine the relationship between the 3D morphology of OPLL and intervertebral ROM. Methods. The authors evaluated 5 intervertebral segments in each of 20 patients (11 men and 9 women) with continuous-type OPLL, for a total of 100 intervertebral segments, using functional CT in anteroposterior (AP) flexion and right and left axial rotation. Three-dimensional kinematics were evaluated using the voxel-based registration method. Ossification was classified on the basis of 3D kinematics and morphology. Results. The authors found 49 ossifications that were obviously of the continuous type. They were divided into 2 types: 1) bridging (13 instances), with thick, continuous ossification of the anterior or posterior longitudinal ligament bridging intervertebral segments and with an ROM of 0.3 degrees in AP flexion and 0.2 degrees in rotation; and 2) nonbridging (36 instances), with a minute gap in the ossification itself or between the ossification and vertebra and with an ROM of 4.9 degrees in AP flexion and 4.0 degrees in rotation. There were 8 stalagmite-type ossifications in the nonbridging group that had the unique kinematics of restricted AP flexion and normal axial rotation. Conclusions. The authors' findings indicate that most continuous-type ossifications that are categorized using the conventional radiographic classification system have mobile segments. The discrimination between bridging and nonbridging on CT scans can be a useful predictive index for dynamic factors. (http://thejns.org/doi/abs/10.3171/2012.3.SPINE111083)