Boomerang deformity of cervical spinal cord migrating between split laminae after laminoplasty

Patients with cervical compression myelopathy were studied to elucidate the mechanism underlying boomerang deformity, which results from the migration of the cervical spinal cord between split laminae after laminoplasty with median splitting of the spinous processes (boomerang sign). Thirty-nine cas es, comprising 25 patients with cervical spondylotic myelopathy, 8 patients with ossification of the posterior longitudinal ligament, and 6 patients w ith cervical disc herniation with developmental canal stenosis, were examin ed. The clinical and radiological findings were retrospectively compared be tween patients with (B group, 8 cases) and without (C group, 31 cases) boom erang sign. Moderate increase of the grade of this deformity resulted in no clinical recovery, although there was no difference in clinical recovery b etween the two groups. Most boomerang signs developed at the C4/5 and/or C5 /6 level, where maximal posterior movement of the spinal cord was achieved. Widths between lateral hinges and between split laminae in the B group wer e smaller than in the C group. Flatness of the spinal cord in the B group w as more severe than in the C group. In conclusion, the boomerang sign was c aused by posterior movement of the spinal cord, narrower enlargement of the spinal canal and flatness of the spinal cord.