Abnormalities of the vertebral column

Embryology of the spine

Interpretation of congenital and acquired anomalies of the vertebral column is aided by an understanding of normal development. In early fetal life the ectodermal germ layer gives rise to the primitive neural tube. This normally closes by the end of the fourth intrauterine week; failure of this primary neurulation results in fusion defects such as anencephaly or spina bifida. By this time the primary brain vesicles are present, representing forebrain, midbrain and hindbrain. Mesoderm lies around the neural tube and by the end of the fifth intrauterine week will have completed segmentation into 42–44 recognizable somite pairs (occipital to coccygeal). Once established, the epithelioid cells of these somites rapidly transform and migrate towards the notochord where they differentiate into three distinct cell lines: sclerotomes (from which connective tissue, cartilage and bone are derived), myotomes (providing segmental muscle) and dermatomes (providing segmental skin). Chondrification of the sclerotomes leads to the development of ossification centres, with an anterior and posterior centre for each vertebral body and a pair for each arch. The process is largely complete by the end of the third month of fetal development. Disruption during these early stages accounts for many of the vertebral and craniocervical anomalies. There is increasing interest in the possible role of abnormal notochord signalling and Pax-1 gene expression in these segmentation defects (see for example, David et al., 1997). After the third month of gestation the vertebral column and dura lengthen more rapidly than the spinal cord resulting in regression of the cord tip, leaving the filum terminale below. By term the cord tip typically lies at the L2–3 interspace. Failure of normal cord ascent may lead to tethering of the spinal cord.

Idiopathic scoliosis

Scoliosis refers to a lateral deviation of the spine and is always abnormal. It may be classified on the basis of clinical examination into structural and non-structural forms. In a structural scoliosis there is a rotational component to the curve which is best seen on forward flexion when prominence of rib or loin musculature becomes apparent. This is not the case in non-structural scoliosis, where there is no rotational element. Non-structural scoliosis may be a marker of other pathology such as leg length discrepancy or muscle spasm but is rarely of clinical significance in itself.