title: Effectiveness of posterior decompression for patients with ossification of the posterior longitudinal ligament in the thoracic spine: usefulness of the ossification-kyphosis angle on MRI.
abstract: METHODS
  A reliability study was conducted.
  
  
  OBJECTIVE
  To 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.
  
  
  BACKGROUND
  In 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.
  
  
  METHODS
  The 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.
  
  
  RESULTS
  Thirteen 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).
  
  
  CONCLUSIONS
  The 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.
authors.affiliation: Department of Orthopaedic Surgery, Nihon University School of Medicine, Tokyo, Japan. ytoku@med.nihon-u.ac.jp
authors.forename: Yasuaki
authors.initials: Y
authors.lastname: Tokuhashi
authors.forename: Hiromi
authors.initials: H
authors.lastname: Matsuzaki
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Oda
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Uei
mesh_terms: D000328:Adult; D000368:Aged; D019299:Decompression, Surgical / Q000379:methods*; D005260:Female; D006801:Humans; D007738:Kyphosis / Q000175:diagnosis*; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D017887:Ossification of Posterior Longitudinal Ligament / Q000150:complications / Q000473:pathology / Q000601:surgery*; D013117:Spinal Cord Compression / Q000209:etiology / Q000473:pathology / Q000601:surgery*; D013904:Thoracic Vertebrae / Q000473:pathology / Q000601:surgery*; D016896:Treatment Outcome