6 Congenital Deformities of the Spine
The spectrum of congenital deformities of the spine includes a range of conditions that blend gradually from scoliosis through kyphoscoliosis to pure kyphosis. These deformities occur when an asymmetric failure of development of one or more vertebrae results in a localized imbalance in the longitudinal growth of the spine and an increasing curvature affecting the coronal and/or sagittal plane, with a risk for progression during skeletal growth. The consequence of unbalanced growth of the spine can be the development of a benign curve with slow or no progression, in which case observation may be the only treatment required. In contrast, there are types of vertebral abnormalities that can produce considerable asymmetry in spinal growth and the development of very aggressive deformities with consequent functional, cosmetic, respiratory, and neurologic complications. Understanding the anatomical features of the individual vertebral anomalies and their relation to the remainder of the spine makes it possible to predict those abnormalities that are likely to produce a severe curve. Recognizing the natural history of the deformity at an early stage can in turn allow appropriate surgical treatment, with the aim of preventing the development of severe spinal curvature and trunk decompensation that would require much more complex and dangerous treatment with a suboptimal clinical outcome.
6.1 Incidence
Congenital scoliosis is the most common type of deformity; it had a prevalence of nearly 80% among more than 1,000 patients with congenital deformities of the spine followed as part of the Scottish National Spine Deformity Service. Congenital kyphoscoliosis is the second most common type of deformity, affecting 14% of patients, while congenital kyphosis has developed in 6% of our patients. Congenital scoliosis has a prevalence of 1 in 1,000 live births, with a reported familial incidence of 1 to 5%. Girls are affected more often than boys.
6.2 Etiology
The causes of congenital vertebral anomalies are likely to be genetic factors, including defects in the Notch signaling pathways. The Notch 1 gene has been shown to coordinate the process of somitogenesis by regulating the development of vertebral precursors in mice. Chromosome 13 and 17 translocations have been associated with the development of hemivertebrae. Genetic theories are supported by molecular, animal, and twin population studies, including several demonstrations of abnormal HOX gene expression. Environmental factors have also been suggested, and these include exposure to toxins including carbon monoxide, the use of antiepileptic medication, and maternal diabetes.
6.3 Embryologic Development
The embryologic development of the spine and that of the ribs are closely interrelated, and the complete anatomical mold is formed in mesenchyme during the first 6 weeks of intrauterine life. Mesodermally derived adjacent somites coalesce during this period of development and yield vertebral bodies ventrally and neural arches dorsally; these ossify during organogenesis through a single primary center in the centrum for the body and two centers dorsally for the arches. Anomalies of the spine and ribs may occur during this period of fetal development, and once the mesenchymal mold is established, the cartilaginous and bony stages follow that pattern. Five secondary ossification centers ossify the vertebral end plates, as well as the spinous and transverse processes after birth.
Vertebral anomalies occurring during the mesenchymal stage may be due either to a unilateral defect of formation or to segmentation of the primitive vertebrae, resulting in a unilateral imbalance in the longitudinal growth of the spine that produces a congenital scoliosis. Vertebral anomalies may also occur during the subsequent chondrification stage and are thought to be due to a localized failure of vascularization of the developing cartilaginous centrum. This results in varying degrees of failure of vertebral formation, producing a congenital kyphosis or kyphoscoliosis. In the late chondrification and ossification stages, bony metaplasia may occur in the anterior part of the anulus fibrosus and ring apophysis, producing an anterior or anterolateral unsegmented bar that can also result in a congenital kyphosis or kyphoscoliosis.
The ribs form from costal processes, which are small, lateral mesenchymal condensations of the developing thoracic somites and contribute cells to all parts of the developing ribs. The distal tips of the costal processes elongate to form ribs only in the thoracic spine. The ribs develop from cartilaginous precursors that ossify during the fetal period. Rib anomalies (fused ribs, bifid ribs, chest wall defects) probably form during the process of segmentation and resegmentation of the developing somites, after which the ribs come to articulate between the definitive thoracic vertebrae.
The scapula develops along with the arm. The arm bud appears in the third week of embryonic life as a small swelling opposite the vertebral segments from C5 to T1. The scapula appears in mesenchyme during the fifth week and gradually migrates caudally. By the end of the third fetal month, it reaches its final anatomical position lateral to the spine and extending from T2 to T7-T8. Occasionally, the scapula may fail to descend fully to its normal position and remains in a permanently elevated location, a condition known as a Sprengel deformity.
6.4 Classification
Congenital deformities of the spine are classified by their anatomical location, as well as by the pathologic anatomy of the anomalous vertebrae (Fig. 6.1). Congenital vertebral abnormalities producing a scoliosis, kyphosis, or kyphoscoliosis can be due mainly to a failure of vertebral formation or a failure of vertebral segmentation. Mixed anomalies are less common, and these are often unclassifiable and difficult to define at birth, when the spine is only 30% ossified.
6.4.1 Congenital Scoliosis
Failures of vertebral formation causing a scoliosis can be complete or incomplete. Complete failures include four types of hemivertebra, in which one-half of the vertebral body has failed to form. A hemivertebra can be fully segmented, semisegmented, unsegmented, or incarcerated. A fully segmented hemivertebra has cephalic and caudal end plates and is separated from the vertebrae above and below by normal disk spaces. A semisegmented hemivertebra has only one end plate and is congenitally fused to either the vertebra above or the vertebra below, with one remaining disk space. The space contralateral to the hemivertebra where the vertebral body has not formed is occupied by disk. An unsegmented hemivertebra is fused to both the cephalic and caudal vertebrae, with no contralateral disk space and no growth potential. An incarcerated hemivertebra is a small, ovoid vertebral segment that sits within a niche formed between the vertebrae above and below. It also has very limited growth potential and limited ability to produce a deformity. The presence of a fully segmented or semisegmented hemivertebra with normal growth plates causes an asymmetric development of the spine because the opposite side is deficient and lacks growth; therefore, a scoliosis develops. Incomplete failures of vertebral formation include a wedge vertebra and an asymmetric butterfly vertebra. A wedge vertebra has bilateral pedicles, but the concave pedicle is hypoplastic. This produces height asymmetry on the concave side and the development of a curvature. An asymmetric butterfly vertebra can cause a scoliosis if there is a localized imbalance of vertebral body growth.
Failures of vertebral segmentation can be unilateral or bilateral. Unilateral failures of segmentation include an unsegmented bar with or without contralateral hemivertebrae at the same level. The bar can extend across two or more segments and constitutes a unilateral tether of vertebral growth, which in the presence of normal contralateral end plates (as demonstrated by open disk spaces opposite the bar on plain radiographs) produces a rapidly progressive scoliosis. Scoliosis develops as a consequence of a combination of inhibition of growth caused by the unsegmented bar and the preservation of contralateral growth, which is more accelerated in the presence of one or more hemivertebrae at the same level. A bilateral failure of segmentation produces a block vertebral segment, which does not have growth potential and does not carry a risk for the development of a significant deformity.
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