► [Duplicate/triplicate calcaneus, or stippled calcaneus]
As anticipated, ossification in the calcaneus begins at approximately the 24th week of gestation with the appearance of one or, more commonly, two primary centers that rapidly coalesce into a single center: a lateral center, appearing slightly earlier and located between the retrotrochlear eminence and the lateral process of the tuber, and a central ossific center situated in the anterior third of the cartilaginous calca-neus in relation to the sustentaculum tali (Meyer and O'Rahilly 1976). Occasionally, these two centers fail to fuse and persist as separate centers for some months after birth, thus resulting in 'duplication' or, more rarely, triplication of the calcaneus. When fusion between the original ossification centers is incomplete a bifid calcaneus results (Kohler and Zimmer 1970).Al-though a bipartite (duplicated) calcaneus can occur as a variant in otherwise normal children, it is generally associated with distinct skeletal dysplasias and syndromes, including the thanatophoric dysplasia and the short-rib dysplasia groups (Cormier-Daire et al. 2001), and also Larsen syndrome (Latta et al. 1971). Interestingly, calcaneal duplication seems to occur in thanatophoric dysplasia type I (OMIM 187600) (Fig. 7.16) but not in type II (OMIM 187601) (Cormi-er-Daire et al. 2001). These two thanatophoric dysplasia disorders are characterized by platyspondyly, short limbs, and a small characteristically formed pelvis. However, curved femurs are predominantly observed in type I, while straight femurs and severe cloverleaf skull are more typical of type II. Duplication of the calcaneus can be regarded as an additional distinctive radiographic feature that can be helpful in the differential diagnosis (Cormier-Daire et al. 2001). The short-rib (with or without polydactyly) dysplasia group encompasses several disorders with narrow thorax and short ribs, micromelia, multivis-ceral involvement, and autosomal recessive inheritance (Lachman 1997). These disorders are differentiated on the basis of the radiologic and histological findings, as discussed elsewhere in this book. Duplication or triplication of the calcaneus has been observed in short-rib polydactyly syndrome type I (Saldino-Noonan, OMIM 263530) and type III (Ver-ma-Naumoff, OMIM 263510), chondroectodermal dysplasia (Ellis-van Creveld,OMIM 225500), and asphyxiating thoracic dysplasia (Jeune syndrome, OMIM 208500) (Fig. 7.17), but not in short-rib polydactyly syndrome type II (Majewski, OMIM 263520) or type
Fig. 7.17. Asphyxiating thoracic dysplasia in a newborn. Close-up lateral view of the ankles and feet showing the ossification center for the talus lying above a duplicated calcaneus. (Cormier-Daire et al. 2001)
IV (Beemer-Langer, OMIM 269860) (Cormier-Daire et al. 2001). Therefore, duplicated calcaneus could be an additional feature allowing discrimination among the different entities in this dysplasia group. Multiple calcaneal ossification centers,in combination with accessory carpal bones, have been reported as a specific feature of Larsen syndrome (OMIM 150250, 245600) (Latta et al. 1971). A duplicate calcaneus resulting from delayed coalescence of the primary ossification centers is also a feature in children with brachydacty-ly, type A6 (Osebold-Remondini syndrome, OMIM 112910), a condition combining hypoplasia/aplasia of middle phalanges, mesomelic limb shortening, mild short stature, capitate-hamate fusion, and normal intelligence (Osebold et al. 1985).
Another mechanism by which multiple ossification centers may appear in the calcaneus is through evolution from epiphyseal stippling. This abnormal ossification pattern, which is typical of the heterogeneous group of chondrodysplasia punctata disorders (OMIM 118650, 118651, 215100, 215105, 222765, 302950,302960,600121,602497), has been reviewed in Chapter 5 (section "Stippled Epiphyses"). In contrast to the aforementioned disorders, in which duplication/triplication of the calcaneus results from delayed coalescence of the primary ossification centers, in the disorders subsumed under the general name of chon-drodysplasia punctata the radiographic pattern consists in multiple punctate calcifications in the cal-caneal area, which eventually coalesce into calcified clods and subsequently disappear in the early years of life (Fig. 7.18). At microscopic level, stippling corresponds to calcification of aberrant cysts resulting from mucoid degeneration of the cartilaginous matrix (Rasmussen and Reimann 1973). Stippling in the cal-caneus similar to that of chondrodysplasia punctata also occurs in several apparently unrelated disorders, including Zellweger syndrome (cerebro-hepato-renal syndrome, OMIM 214100), various embryopathies (warfarin, phenytoin, alcohol), rubella infection, vitamin K epoxide reductase deficiency,GM1 gangliosidosis, and Down syndrome (Silverman 1993).
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