There is a wide range of variation in the shape and size of the carpal bones among individuals. These variations are sometimes only apparent, reflecting differences in position of the wrist during film exposure (Poznanski 1984). The scaphoid, lunate, and trapezium are particularly subject to variation, specifically in the onset of ossification, pattern of ossification, and progress of maturation (Gupta and Al-Moosawi 2002). Marginal notches, small bone cysts, and bone partitions are common anatomical variants in the human carpus (Sherwin et al. 1971). It is beyond the scope of this book to discuss these aspects at length.
Even greater variability in the shape and size of the carpal bones is seen in several pathologic conditions. Although ossification from multiple centers may be devoid of clinical significance, it might also suggest severely delayed maturation such as occurs in hypothyroidism. Unusual shapes of the scaphoid are common in Holt-Oram and in hand-foot-genital syndromes. Gross alteration in the structure and shape of the carpals,with severe carpal irregularities, occurs in disorders with impaired ossification, notably the epiphyseal dysplasias and mucopolysaccharidoses, and in such acquired disorders as rheumatoid arthritis and infection. Small carpal bones of bizarre shape are found in diastrophic dysplasia and in Seckel syndrome (Poznanski et al. 1983). Scalloping deformities sometimes occur in association with neurofibromatosis and soft tissue hemangiomas. In Kienböck disease, an osteonecrotic process involving the lunate, progressive disruption of this carpal bone is observed, with sclerosis, compression fracture, and collapse (Almquist 1987). Because of the predilection for the right hand of young men engaged in manual labor, the role of trauma in triggering avascular necrosis has long been emphasized (Allan et al. 2001). External compression forces may be accentuated by the presence of a short ulna, a finding ascertained in 75 % of cases of Kienböck disease (Gelber-man et al. 1975). Secondary arthritic changes are common. Treatment in the early stages aims at restoring revascularization of the lunate by relieving the compression forces. This is accomplished by lengthening the ulna and/or shortening the radius, with capitate-hamate fusion. In more advanced cases, treatment includes excision of the lunate and various types of intercarpal fusion (Almquist 1986; Tamai et al.1993).
In the following pages, the discussion will focus on the variations in the relative position of proximal carpals as expressed by carpal angle abnormalities, on the extra carpal bones, and on carpal synostosis. Congenital hypoplasia and aplasia of the carpals (Tada et al. 1977; Kuz and Smith 1997) are discussed together with the longitudinal defects of the forearm. The disorders involving disappearance of the carpus are outlined in the section entitled "Acro-osteolyses."
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