• Yasuaki Tokuhashi, Hiromi Matsuzaki, Hiroshi Oda, and Hiroshi Uei.
• Department of Orthopaedic Surgery, Nihon University School of Medicine, Tokyo, Japan. ytoku@med.nihon-u.ac.jp
• Spine. 2006 Jan 1; 31 (1): E26-30.

Study DesignA reliability study was conducted.ObjectiveTo report the utility of the ossification- kyphosis angle of a decompression site in the sagittal view of MRI in the preoperative evaluation of the effectiveness of posterior decompression for patients with ossification of the posterior longitudinal ligament in the thoracic spine.Summary Of Background DataIn patients with ossification of the posterior longitudinal ligament, in the thoracic spine, posterior decompression may not always be effective for those with physiologic kyphosis; however, posterior decompression is sometimes useful for patients with multilevel lesions or with ossification of the ligamentum flavum. Therefore, it is necessary to evaluate the indications or limitations of posterior decompression before surgery.MethodsThe indications or limitations of posterior decompression for this disease were reviewed from the findings of intraoperative ultrasonography and various parameters on preoperative imaging in 22 patients with thoracic myelopathy.ResultsThirteen patients had echo-free space and 9 patients had no echo-free space in intraoperative ultrasonography after posterior decompression. In the kyphosis angle of the decompression site and the configuration of the maximal prominent OPLL, there was no significant difference between the group with echo-free space and the group without echo-free space (P = 0.49, P = 0.55). On the other hand, the ossification-kyphosis angle of the decompression site was more than 23 degrees in all patients with no echo-free space (23 degrees -34 degrees , 28.2 degrees +/- 3.6 degrees ), and it was less than 23 degrees in all patients with echo-free space (10 degrees -23 degrees , 17.4 degrees +/- 4.1 degrees ). There was also significant difference between the groups statistically (P < 0.01, P = 1.50789E-06). In addition, in the occupation rate of the greatest prominence of the OPLL, there was statistically significant difference between the groups (P = 0.032).ConclusionThe ossification-kyphosis angle in the sagittal view of MRI has potential as an indicator of the effectiveness of posterior decompression in this disease. It is thought that there is a critical point of posterior decompression at nearly 23 degrees of the ossification-kyphosis angle of the decompression site. When the ossification-kyphosis angle is more than 20 degrees , the presence of echo free space should be carefully confirmed in intraoperative ultrasonography.

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