OBJECTIVE:The goal of this study was to establish the criteria for sufficient decompression of the cervical spinal cord in laminoplastic surgery.METHODS:Radiological examinations and neurological evaluations were conducted for 63 patients with cervical spondylotic myelopathy (CSM) and 31 patients with ossification of the posterior longitudinal ligament (OPLL) of the cervical spine who underwent suspension laminotomy.RESULTS:The dural configuration in computed tomographic myelograms was evaluated in comparison with the critical value for the dural configuration established from 36 control subjects. Seventy-six of 94 patients were judged to have achieved full decompression. The neurological improvement of patients with full decompression was significantly better than that of patients with insufficient decompression (P< 0.01). The postoperative sagittal diameter of the spinal canal was more than 14 mm in most of the sufficiently decompressed cases of CSM and more than 17 mm in OPLL cases. The laminotomy width was more than 70% of the transverse diameter of the spinal canal in both CSM and OPLL cases. There were four OPLL cases with insufficiently decompressed dura mater in the well-enlarged spinal canal after surgery. In these cases, the ossified posterior longitudinal ligament continuously occupied more than 25% of the preoperative spinal canal area over three or more segments.CONCLUSION:The following conditions were considered critical for sufficient decompression of the spinal cord in laminoplastic operations: postoperative sagittal diameter of the spinal canal of more than 14 mm in CSM cases and more than 17 mm in OPLL cases and laminotomy width of more than 70% of the transverse diameter of the spinal canal in both CSM and OPLL cases. However, in cases of extensive OPLL, sufficient decompression could not be obtained through posterior enlargement of the spinal canal.