Correlation of OPLL with spinal instability

To evaluate the relationship between spinal instability and ossification of the posterior longitudinal ligament(OPLL). Materials and Methods: In 70 patients(M:F=45:25, mean age=53 years) diagnosed as OPLL on the bosis of with surgical operation field findings and radiological evaluation[ plain film(n=70), CT(n=64),MRI(n=55) ], involved levels were the cervical spine(n=32), lumbar spine(n=23), and both the cervical and lumbar spine(n=15). Spinal instability was radiologically diagnosed as horizontal displacement of one vertebra by another of more than 3.5 mm, or a difference in rotation from either adjacent vertebra by more than 11 degree in lateral cervical spine and a difference of more than 1.5 mm from the posterior body margins to the point of intersection of two lines drawn parallel to the opposing segmental endplate in extension lateral lumbar spine. We divided OPLL into group I(continuous, segmental, mixed) and group II(retrodiscal), and compared spinal instability in these two groups. Results: In cervical OPLL, group I comprised 33 cases and group II 14. In group I, spinal instability was noted in 8/33 cases(24%)) or 10/123 segments(8.1%). Spinal instability in group II, on the other hand, was found in 13/14 cases(93%) or 17/26 segments(65%). Ossification occurred at the retrodiscal level in 37 cases, byt in case was continuous. In group II, spinal instability was found in 25 of 37 cases(69%), oe in 29 of 55 segments(53%). Conclusion: Compared to other types of OPLL, the frequency of retrodiscal OPLL in association with spinal instability was high. Spinal instability may thus be the most important cause of retrodiscal OPLL