► [Frontal schisis of the vertebral bodies]
As mentioned above, two primary ossification centers, instead of a single center, occasionally appear in the vertebral body. In these cases, the two ossification centers expand progressively during embryonic life, until complete fusion is attained at approximately 16 weeks of gestation (Epstein 1976). The unossified cartilaginous bridge between the two ossification centers corresponds to the 'cleft' seen radiographical-ly. Depending on whether the primary ossification centers are ventral and dorsal or left and right, coronal clefts or sagittal clefts, respectively, are observed. In a series of 157 spontaneously aborted human embryos and fetuses, 9 complete and 16 incomplete coronal clefts were found, representing variations of the normal enchondral ossification. It was concluded that the notochord plays no role in the pathogenesis of vertebral coronal clefts (Tanaka and Uhthoff 1983). In another reported case, however, the coronal cleft represented persistence of an axial rod of noto-chord (Wollin and Elliot 1961). Transient coronal clefts at birth that resolve spontaneously within a few weeks are normal development variations, representing delayed bone maturation (Cohen et al. 1956). Clefts persisting beyond 4 months of age are likely to be related to structural abnormalities of the vertebral bodies. The nature of such abnormalities is not known. It has been suggested that the primary defect involves formation of the vertebral cartilaginous anlage, possibly as a result of the abnormal distribution of the intersegmental arteries (Tanaka and Uhthoff 1981). Overproduction and/or reduced removal of cartilage in the area of the cleft have also been suggested (Wells et al. 1992). Coronal clefts are more frequent in males than in females, especially in children in the 1st year of life. The lower thoracic and lumbar vertebral bodies are the most commonly involved.
Many vertebrae are often affected, sometimes interpolated with vertebral bodies without clefts.
Sagittal clefts probably represent a localized splitting of the notochord due to developmental errors involving the ectoderm and the entoderm in embryonic life (Macpherson and Genez 1992). Sagittal clefts involving one vertebra may be of no clinical relevance, whereas those extending through many levels are often associated with malformation syndromes.
According to the cases recorded in the database of the International Skeletal Dysplasia Registry, vertebral clefts are most frequent and are of major diagnostic value in atelosteogenesis (88%), chondrodysplasia punctata (79%), dyssegmental dysplasia (73%), Kniest dysplasia (63%), and short rib-poly-dactyly syndrome (53%) (Westvik and Lachman 1998). The coronal-to-sagittal cleft ratio is 4:1, with an equal male-to-female distribution and a tendency for vertebral clefts to disappear with increasing age. In atelosteogenesis type I (OMIM 108720), a lethal short-limbed chondrodysplasia characterized by deficient ossification of various bones including the humerus, femur, and thoracic vertebrae, coronal thoracolumbar clefts are seen virtually in all cases (Sillence et al. 1997). Additional radiographic features include hypoplasia of the upper thoracic vertebral bodies and distally tapering, club-shaped humeri and femurs. Ossification of the tubular bones in the hand is anarchic, with well-ossified distal phalanges and hypoplasia or absence of the remainder of the tubular bones. Clinically, the patients have short limbs with bowed legs, dislocated major joints (especially the elbows), clubfeet and, in rare cases, cleft palate. The fibulas may be absent. Histologically, clusters of degenerated chondrocytes are encapsulated in fibrous tissue (Maroteaux et al. 1982). Multinuclear giant cells occur as a nonspecific finding in the growth plate (Yang et al. 1983). Atelosteogenesis type I and boomerang dysplasia (OMIM 112310) share similarities and probably belong to the same group of conditions, having in common a defect in cartilage and bone formation (Greally et al. 1993). Vertebral clefts also occur in atelosteogenesis type II (de la Chapelle dysplasia, OMIM 256050) and in atelosteogenesis type III (OMIM 108721) (Fig. 3.36 a,b), a disorder sharing features with oto-palato-digital syndrome type II (OMIM 304120) (Stern et al. 1990). In chondrodysplasia punctata, rhizomelic type (OMIM 215100), coronal clefts are almost an obligatory finding (Fig. 3.37), while in chondrodysplasia punctata, Conradi-Hunermann type (OMIM 118650), sagittal clefts are characteristic (Westvik and Lachman 1998) (Fig. 3.38 a,b). Both sagittal and coronal clefts are observed in dyssegmental dysplasia, either the mildest form (Rolland-Desbuquois syndrome, OMIM 224400) or the severe form (Silverman-Handmakersyndrome,
OMIM 224410). Other characteristic radiographic features in dyssegmental dysplasia include marked variation in the size and shape of all vertebrae (anisospondyly), anterior wedging, and lack of normal cranio caudal interpediculate widening in the lumbar spine (Handmaker et al. 1977). In Kniest dysplasia (OMIM 156550) the vertebral bodies are flat and elongated, with anterior tapering and end-plate irregularities. Coronal clefts are frequently seen in infants at the lumbar level (Fig. 3.39). Later in life, kyphoscoliosis develops. Coronal cleft vertebrae have been described in Larsen syndrome (OMIM 150250, 245600) (Weisenbach and Melegh 1996), in the association referred to as platyspondyly, pelvic dysostosis and metaphyseal chondrodysplasia (Cur-rarino 1986), and in combination with humeral bifurcation, elbow subluxation, and congenital heart disease (Cortina et al. 1979). Weissenbacher-Zwey-muller syndrome (OMIM 277610), the heterozygous form of OSMED (non-ocular type III Stickler syndrome, OMIM 184840) caused by mutations in COL11A2 gene mapped to 6p21.3 (Pihlajamaa et al. 1998), is characterized by midface hypoplasia, mi-crognathia, cleft palate, rhizomelic limb shortening with dumbbell-shaped femurs and humeri, and vertebral coronal clefts. Regression of bone changes with normal growth in later years is a striking feature (Weissenbacher and Zweymuller 1964).
Coronal cleft vertebrae also occur on an acquired basis, after trauma leading to vertical fracture lines through the vertebral body (Wilson et al. 1989).
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