► [Deviation of a finger in the mediolateral plane]
Clinodactyly is a common finger defect, often occurring as an isolated anomaly devoid of clinical significance. The incidence has not been established precisely because of the difficulty of definition, but it is thought to be in the range of 0.1-1.0% of the general white population. The incidence is higher in non-whites. Although any finger can be involved, and the curvature can be to either the radial or the ulnar side of the hand (Burke and Flatt 1979), radial deviation of the 5th finger is by far the most common form (Fig. 6.34). Thus, it is this finger anomaly that is usually meant when the term clinodactyly is used with no further specification. Mechanisms of finger deviation include phalangeal asymmetry resulting from unequal growth, either congenital or acquired in origin and the presence of an extra bone within the digit (Caouette-Laberge et al. 2002). In this section, examples of clinodactyly resulting from each of the aforementioned mechanisms will be provided. By contrast, finger deviation caused by focal destruction of well-developed joints, as in rheumatoid arthritis and other arthritides, is not included here. Soft tissue contracture and abnormal tendon insertion can also lead to various types of finger deviation, including clinodactyly. However, these factors most typically cause crooked fingers and camptodactyly, a subject discussed in the next section.
Clinodactyly of the 5th finger owing to asymmetrical hypoplasia of the middle phalanx occurs in brachydactyly type A3 (brachymesophalangy V,OMIM 112700), a common hand anomaly usually inherited as an autosomal dominant trait with incomplete pen-etrance. A cone-shaped epiphysis, or a milder degree of epiphyseal dysplasia only involving the radial margin of the epiphysis, is often associated, contributing to clinodactyly (Wetherington 1983). Most frequently,type A3 brachydactyly occurs as an isolated defect in otherwise normal individuals (Buschang and Malina 1980). However, it is also sometimes seen in association with a number of syndromes. As in brachydactyly type A3, it is asymmetrical hypoplasia of the 2nd middle phalanx, resulting in its rhomboid or triangular appearance, that forms the basis of the development of radial clinodactyly of this digit in brachydactyly type A2 (OMIM 112600) (Freire-Maia et al. 1980). Another example of clinodactyly due to a misshapen phalanx is provided by the 'delta phalanx,' or longitudinal bracket epiphysis, in which a trapezoid phalanx results in angular deviation of the finger (Martinot-Duquenoy et al. 1990). The phalangeal shape is reminiscent of the Greek letter D (delta), which explains the designation. This bizarre configu ration results from a 90° tilt of a diaphysis-metaphy-seal osseous unit that is longitudinally bracketed by an abnormal, C-shaped epiphysis (Olason and Dohler 1988; Ogden et al. 1981). The proximal phalanx of the 1st digit and the middle phalanx of the 5th digit are the most commonly affected. The delta phalanx can be an isolated deformity but is commonly associated with other digital anomalies, including polydactyly, hyperphalangy, symphalangism, clinodactyly, and brachydactyly type A2 (Sobel et al. 1996). Furthermore, it can occur in the context of well-defined syndromes, including Down syndrome, acrocephalosyn-dactyly, and Rubinstein-Taybi syndrome. On rare occasions, a duplicated delta phalanx (kissing delta phalanx) is seen (Wood and Rubinstein 1999).
There are several situations in which clinodactyly is caused by acquired diseases that injure a developing epiphysis. Infections and trauma, for example, can cause growth arrest on one side of the epiphysis, thereby leading to angular deviation of the finger. The clinical history, and whether involvement is unilateral or random and finger deviation is radial or ul-nar are clues to the diagnosis. Thermal and electrical burns typically cause distal clinodactyly, presumably by inducing unilateral necrosis of the epiphysis (Fingerhut et al. 1982). As discussed later in this chapter, macrodactyly is often associated to clinodactyly, as a result of the side-to-side variations in the accelerated rate of growth that are characteristic of this condition (Goldman and Kaye 1977). Angular deformities in the sagittal plane, especially in the toes, are not uncommon (Chen et al. 1997).
Clinodactyly caused by a supernumerary bone in the digit,specifically in the index and middle fingers,is a characteristic feature of type C brachydactyly (OMIM 113100). The overall malformation pattern in the index finger (an extra-ossicle, with ulnar deviation) is distinctively different from that seen in type A2 brachydactyly (a rhomboid phalanx, with radial deviation). Bilateral ulnar deviation of the index fingers at the metacarpophalangeal joint due to the presence of an extra-ossicle is also seen in Catel-Manzke syndrome (OMIM 302380), a familial, possibly X-linked recessive disorder. Here, the index finger anomaly occurs in association with Pierre-Robin sequence (micrognathia, glossoptosis, and cleft palate) (Catel 1961; Manzke 1966; Silengo et al. 1977; Sundaram et al. 1982; Brude 1984). Since the extra-ossicle presumably represents a duplicated proximal phalanx, the digital anomaly in Catel-Manzke syndrome can be best described as hyperphalangy of the index fingers with clinodactyly (Thompson et al. 1986). Bilateral 5th finger clinodactyly can also occur (Wilson et al. 1993).
Kirner's deformity (dystelephalangy, OMIM 128000) is the designation for palmar bending of the tip of the 5th finger due to bowing of the distal phalanx. Although this peculiar form of'crooked' finger is distinctively different from clinodactyly, it is briefly described here because a variable proportion of radial deviation may be observed concurrently with the most typical ventral bending. Kirner's deformity is extremely rare and can be uni- or bilateral, the latter being more common in females than in males (Blank and Girdany 1965). The etiology is still uncertain; the combination of a primary epiphyseal disorder and tendon traction has been stressed (Rasmussen 1981; Dubrana et al. 1995). When isolated, the anomaly is usually transmitted as an autosomal dominant trait with incomplete penetrance (David and Burwood 1972). A 'polytopic' distribution of the defect, with several fingers involved on both hands, has been described (Sugiura 1989). Kirner's deformity also occurs in the context of well-defined syndromes, notably Silver-Russell syndrome (OMIM 180860) and de Lange syndrome (OMIM 122470).
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