Malformed vertebrae: a clinical and imaging review
A variety of structural developmental anomalies affect the vertebral column. Malformed vertebrae can arise secondary to errors of vertebral formation, fusion and/or segmentation and developmental variation. Malformations can be simple with little or no clinical consequence, or complex with serious structural and neurologic implications. These anomalies can occasionally mimic acute trauma (bipartite atlas versus Jefferson fracture, butterfly vertebra versus burst fracture), or predispose the affected individual to myelopathy. Accurate imaging interpretation of vertebral malformations requires knowledge of ageappropriate normal, variant and abnormal vertebral morphology and the clinical implications of each entity. This knowledge will improve diagnostic confidence in acute situations and confounding clinical scenarios.
This review article seeks to familiarize the reader with the embryology, normal and variant anatomy of the vertebral column and the imaging appearance and clinical impact of the spectrum of vertebral malformations arising as a consequence of disordered embryological development.
• Some vertebral malformations predispose the affected individual to trauma or myelopathy.
• On imaging, malformed vertebrae can be indistinguishable from acute trauma.
• Abnormalities in spinal cord development may be associated and must be searched for.
• Accurate interpretation requires knowledge of normal, variant and abnormal vertebral morphology.
KeywordsVertebral malformations Children Spine Magnetic resonance imaging Developmental
The vertebral column and spinal cord develop early in gestation in a fine-tuned, sequential manner . Any disruption of this normal sequence of events can lead to variations in the structural anatomy of the spine and spinal cord . Structural malformations of the spine are often simple and may either go undetected or be discovered fortuitously. Occasionally, these may be complex with serious structural or neurological implications [2, 3]. These may occur sporadically, in isolation or as an accompaniment to multiorgan developmental malformations . When symptomatic, these abnormalities can predispose the affected individual to biomechanical instability, spinal canal narrowing and myelopathy and can even be life-threatening. Developmental defects of cardiovascular, neurological, urinary and reproductive systems may be associated .
This article overviews the embryology of the vertebral column and imaging appearance and clinical impact of the spectrum of abnormalities arising as a consequence of disordered development. It is difficult to separate discussion of the spine from that of the enclosed spinal cord, so associated spinal cord/brain anomalies will be mentioned where applicable but will not be elaborated.
Since the notochord is created during gastrulation, multisystem abnormalities typically coexist with notochordal developmental anomalies . After the notochord interacts with the overlying ectoderm to form the neuroectoderm, neural tube closure occurs to complete the process of primary neurulation. The sacrum and coccyx develop last at about 31 days of gestation  from a large aggregate of undifferentiated cells (caudal cell mass) representing remnants of the primitive streak. Underlying embryological processes include canalisation (secondary neurulation) and retrogressive differentiation.
The craniovertebral junction (CVJ) is embryologically unique and complex. Four occipital sclerotomes contribute to formation of the occipital bone, clivus and occipital condyles, anterior arch of the atlas and the apical, cruciate and alar ligaments. The posterior arch of the atlas is derived from both the first occipital and first cervical sclerotomes. The axis is derived from both the fourth occipital and first and second cervical sclerotomes. The ventral portion of the first cervical sclerotome forms most of the odontoid process [3, 5].
The final steps in vertebral formation, chondrification and ossification occur after 6 and 9 weeks respectively. Chondrification of the CVJ begins at 45 days and chondrification of the C1 anterior arch begins at 50 to 55 days of gestation [5, 8].
At the sacrococcygeal levels, the first three sacral elements contain an additional pair of ossification centres. Fusion of sacral vertebrae begins early in puberty and is complete in the middle part of the 3rd decade. Ossification of the first coccygeal segment begins between 1 to 4 years of age; remaining coccygeal segments ossify craniocaudally between 5 to 20 years of age .
Vertebral malformations have been classified based on the underlying embryopathy [5, 12] (ESM_1). Developmental variances leading to transitional vertebrae at the thoracolumbar and lumbosacral junctions and developmentally short/absent pedicles have been identified as a separate category. The subsequent paragraphs outline the embryology, clinical and radiological manifestations of commonly encountered vertebral malformations (ESM_1).
The embryological processes underlying this spectrum of malformations involve disorders resulting from abnormal development of the notochord during gastrulation. These typically manifest with malformation of the neuraxis and axial skeleton involving tissues derived from all three primary germ cell layers . Broadly, these include disorders of midline integration of the notochord (split notochord syndrome, split cord) and disorders of notochordal formation (caudal agenesis and segmental spinal dysgenesis) (ESM_2).
Split notochord syndrome
This results from splitting of the notochord leading to a persistent connection between the ventral endoderm and the dorsal ectoderm. The most severe manifestation is a dorsal enteric fistula through which the intestinal cavity communicates with a dorsal skin surface to the midline, traversing all the intervening structures . Variants on this theme include a dorsal bowel hernia and diverticuli, duplications, cysts or sinuses along this anomalous tract .
Split cord/diastematomyelia (Fig. 3a,b)
Erroneous specification of the rostrocaudal positional encoding of prospective notochordal cells can result in inadvertent apoptosis of notochordal precursors and subsequent interference with primary and secondary neurulation, leading to disordered formation of the notochord; these can manifest as caudal/sacral agenesis or segmental spinal dysgenesis.
Caudal/sacral agenesis (Fig. 4a-e)
Segmental spinal dysgenesis (Fig. 7a-e)
Abnormal alignment of sclerotomal rests
These result from complete failure of formation of one of the two paired cartilaginous centres of the developing vertebra, secondary to tardy development of a somite on one side resulting in a caudal metameric segmental shift of one somatic column relative to another, leading to an unpaired sclerotome and resultant hemivertebra . The contralateral vertebral centrum and corresponding dorsal vertebral arch are characteristically absent. The ipsilateral posterior arch, though present, is incorporated into the vertebral arch above or below.
Hemivertebrae can occur sporadically or in association with spinal dysraphisms, skeletal, cardiac, genitourinary and gastrointestinal anomalies; the latter category of infants is predisposed to increased perinatal mortality .
Disordered vertebral formation from sclerotomal precursors
Embryologically, these occur as a consequence of disruption of the somitic mesoderm during gastrulation, somites during segmentation or sclerotomal precursors during the membranous phase. Examples include wedge vertebrae and less than 10% of hemivertebrae.
Defective vertebral segmentation
These abnormalities occur as a result of failed segmentation of somites. These include block vertebrae, unsegmented bars and congenital cervical spine fusions as seen with Klippel-Feil syndrome .
Klippel Feil syndrome
On radiographs, vertebral bodies typically have narrow, tall configurations and decreased anteroposterior dimensions with absent or small intervening disks. Associated fusion of posterior elements, occipitalisation of the atlas, congenital scoliosis, kyphosis and Sprengel deformity (see below) may be present. Flexion and extension radiographs should be routinely performed in these patients to assess for potential instability . Patients should be counselled to protect themselves against injury.
This refers to a dysmorphic, high-positioned scapula at birth that results from lack of normal caudal migration of the scapula during embryogenesis. The scapula often has a convex medial margin, concave lateral margin, decreased height to width ratio and associated hypoplasia of the scapular muscle . CT can identify associated congenital scoliosis and omovertebral bars and help in surgical planning .
Disordered vertebral alignment
This entity results from simple mechanical buckling of the embryo between the 4th and 6th embryonic weeks, after neurulation but before chondrification . This commonly occurs at or near the thoracolumbar junction and manifests as congenital vertebral dislocation. The spinal canal at the affected level is typically widened, pedicles of the more cephalad vertebra are elongated, and the dorsal vertebral arches are dysraphic. The spinal cord is frequently low-lying, although intact across the lesion. Usually, patients are neurologically intact or present with subtle neurological deficits. Associated tracheoesophageal fistula and unilateral renal agenesis may be present.
Disordered fusion of the sclerotome, chondrification or ossification centres
Vertebral malformations attributed to disordered assimilation or fusion include butterfly vertebra and dysplastic spondylolysis .
Butterfly vertebra (Fig. 12)
Localised failure of fusion of ventral and dorsal ossification centres can result in malformed vertebral pedicles or facets (dysplastic spondylolysis). When this process occurs dorsally, spina bifida occulta can result.
Failure of development of the vertebral centrum during late stages of gestation manifests as isolated hypoplasia or aplasia of the vertebral centrum, without corresponding alterations in the dorsal vertebral arch. On imaging, part or all of the vertebral centrum may be absent with intact pedicles and posterior body up to the neurocentral synchondrosis. Also included in the spectrum are dorsal hemivertebra with isolated absence or wedging of the ventral portion of the centrum .
Craniovertebral junction (CVJ) malformations
Persistent ossiculum terminale (Fig. 15a-b)
This category includes vertebral malformations that do not strictly fall under any of the above categories but arise secondary to variances in development. Entities discussed under this category include transitional vertebrae  and congenitally short/absent vertebral pedicles.
Thoracolumbar and lumbosacral junctional variances
Thoracolumbar junctional variances manifest either as an increase or decrease in the number of rib-bearing thoracic vertebrae or altered appearances of the costal processes of the most caudal thoracic or the uppermost lumbar vertebra. Lumbosacral transitional vertebrae have been described as either sacralisation of the lowest lumbar segment or lumbarisation of the most superior sacral segment of the spine . On imaging, these are best seen with CT. Clinically, these can be associated with back pain/“Bertolotti syndrome”  and can lead to nomenclature and/or surgical errors . Castellvi et al. proposed a radiographic classification system based on the morphological characteristics of these vertebrae . Treatment may be conservative or surgical.
Congenital/developmental spinal stenosis secondary to short pedicles (Fig. 17a–b)
Pedicular agenesis 
This unusual disorder manifests with isolated agenesis of a vertebral pedicle, most commonly C5 or C6. Imaging findings include misleading appearance of enlarged ipsilateral neural foramen, dysplastic dorsally displaced ipsilateral lamina and a dysplastic ipsilateral transverse process. Although a stable congenital anomaly, this can be mistaken for acute trauma .
Structural abnormalities of the spine can occur at multiple levels and have a variety of imaging and clinical manifestations. The radiologist plays an important role in assessing these abnormalities and can alert the clinician to the likelihood of a serious complication arising secondary to such abnormalities and assist in the pre-operative workup and postoperative follow-up.
The authors are grateful to Margaret Kowaluk, Nadezhda Kiriyak and Gwen Mack from the Graphics section, Department of Imaging Sciences, University of Rochester Medical Center, Rochester, New York, for help with radiographic images and original artwork in this article.
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