The one bone spine: a failure of notochord/sclerotome signalling?

Three cases of extensive vertebral fusions with absent clivo-axial angle are presented. The 'bone-within-bone' appearance in two patients with almost complete fusion of the spine suggested ossification of the notochord and perinotochordal sheath. On the basis of the radiological appearances and the results of recent molecular genetic studies on vertebrate embryos, the suggested time of segmentation failure along the axis of the craniovertebral junction and between vertebrae is the third to fifth week of gestation. The possible roles of the Pax-1 gene and of signalling between notochord and sclerotome are discussed, concluding that an early defect of the notochord may be responsible for this type of failure of segmentation. Indications for surgery in these cases included cord compression with brachialgia and 'chin-on-chest' deformity causing severely restricted visual fields. A critical review of clinical lessons learned in the operative treatment is presented.