OBJECTIVEThe range of decompression in posterior decompression and fixation for ossification of the posterior longitudinal ligament in the thoracic spine (T-OPLL) can be established using an index of spinal cord decompression based on the ossification-kyphosis angle (OKA) measured in the sagittal view on MRI. However, an appropriate OKA cannot be achieved in some cases, and posterior fixation is applied in cases with insufficient decompression. Moreover, it is unclear whether spinal cord decompression of the ventral side is essential for the treatment of OPLL. In this retrospective analysis, the efficacy of posterior decompression and fixation performed for T-OPLL was investigated after the range of posterior decompression had been set using the OKA.METHODSThe MRI-based OKA is the angle from the superior margin at the cranial vertebral body of the decompression site and from the lower posterior margin at the caudal vertebral body of the decompression site to the prominence of the maximum OPLL. Posterior decompression and fixation were performed in 20 patients. The decompression range was set so that the OKA was ≤ 23° or the minimum if this value could not be achieved. Cases in which an OKA ≤ 23° could and could not be achieved were designated as groups U (13 patients) and O (7 patients), respectively. The mean patient ages were 50.5 and 62.1 years (p = 0.03) and the mean preoperative Japanese Orthopaedic Association (JOA) scores were 5.9 and 6.0 (p = 0.9) in groups U and O, respectively. The postoperative JOA score, rate of improvement of the JOA score, number of levels fused, number of decompression levels, presence of an echo-free space during surgery, operative time, intraoperative blood loss, and perioperative complications were examined.RESULTSIn groups U and O, the mean rates of improvement in the JOA score were 50.0% and 45.6% (p = 0.3), the numbers of levels fused were 6.7 and 6.4 (p = 0.8), the numbers of decompression levels were 5.9 and 7.4 (p = 0.3), an echo-free space was noted during surgery in 92.3% and 42.9% of cases (p = 0.03), the operative times were 292 and 238 minutes (p = 0.3), and the intraoperative blood losses were 422 and 649 ml (p = 0.7), and transient aggravation of paralysis occurred as a perioperative complication in 2 and 1 patient, respectively.CONCLUSIONSThere was no significant difference with regard to the recovery rate of the JOA score between patients with (group U) and without (group O) sufficient spinal cord decompression. The first-line surgical procedure of posterior decompression and fixation with the range of posterior decompression set as an OKA ≤ 23° before surgery involves less risk of postoperative aggravation of paralysis and may result in a better outcome.
Morphometric analysis of the relationships between intervertebral disc and vertebral body heights: an anatomical and radiographic study of the human thoracic spine