Carpal Synostosis

► [Congenital fusion between two or more adjacent carpal bones]

This section offers an overview of the various types of congenital carpal coalition, from the isolated forms to those in which the anomaly occurs as a part, either essential or marginal, of an inherited disorder. Acquired causes of carpal bone fusion, including infection, various arthritides (Maldonado-Cocco et al. 1980), trauma, and surgical fixation (Jebson and Adams 2001), are not detailed. Given that the term 'carpal fusion' is used worldwide as a synonym for synostosis, this usage is maintained in this work. However, 'fusion' refers to intervening coalition of two originally separated bones and is therefore best reserved for the acquired cases, whereas 'synostosis' indicates a prenatal defect of differentiation. Indeed, congenital carpal synostosis results from a failure in segmentation of the primitive cartilaginous model, with lack of formation of the joint at the interface of two contiguous carpal bones (Cockshott 1963).With-in this unsegmented cartilaginous model two ossification centers appear and eventually coalesce in late childhood or adolescence.

Isolated fusion often involves two contiguous bones within the same row, either the proximal or the distal. In the majority of cases these fusions cause no symptoms. However, pain has been recorded in some cases, mainly as an effect of early-onset arthritis (Ganos and Imriglia 1991; Simmons and McKenzie 1985). The most common form of carpal fusion is between triquetrum and lunate (Fig. 6.54), with a reported frequency ranging from 0.1% in white Americans to 1.6% in black Americans (Garn et al. 1971; Szaboky et al. 1969). It is twice as common in males as in females, and is bilateral in about 60% of cases (Sy et al. 1996). The second most common type is that between capitate and hamate (Fig. 6.55). As in the case of triquetrum-lunate fusion, capitate-hamate fusion is more common in people of African ancestry (Smitham 1948). The next most common isolated fusion is trapezium-trapezoid, while capitate-trapezoid (Geutjens 1994) and pisiform-hamate (Cockshott 1969) are rare. Isolated fusions involving multiple bones across rows are exceedingly rare (Knezevich and Gottesman 1990; Ingram et al. 1997). Thus, with fusion occurring across rows, as well as with massive fusion or fusion between carpals and radius/ulna, the presence of additional malformations must be sought (Hughes and Tannes 1966; Forney et al. 1966; O'Rahilly 1953; Cope 1974; Gurkan et al. 1998).

Carpal Synostosis
Fig. 6.54. Holt-Oram syndrome in an adult male patient. Observe absence of trapezium, hypoplastic scaphoid, and synos-tosis between triquetrum and lunate. The pisiform is displaced downward. There was a severely hypoplastic, distally placed (floating) thumb (not shown)
Holt Oram Syndrome
Fig. 6.55. Crouzon craniofacial dysostosis in a 6-year-old boy. There is bilateral capitate-hamate synostosis. (From Anderson et al. 1997)

A number of familial disorders displaying carpal fusion are recognized. Multiple progressive joint fusions commencing at the proximal interphalangeal joints (proximal symphalangism), and progressing to the carpal, tibiotarsal, and tarsal articulations are typical of multiple synostosis syndrome (facio-audio-symphalangism syndrome, OMIM 186500), a condition caused by mutation in the gene encoding Noggin, and allelic to proximal symphalangism syndrome (OMIM 185800). Progressive ankylosis of other joints, including the elbow, the spine, the hips, and the stapes, is characteristic (Maroteaux et al. 1972). A form of familial tarsal, carpal and digital synostosis

(OMIM 186400) has been described in a mother and daughter (Pearlman et al. 1964). Manifestations in this condition include carpotarsal fusion, synostosis of the metacarpophalangeal joints, aplasia/hypoplasia of the middle phalanges, and radial head subluxation. In the feet, both calcaneonavicular (Wray and Herndon 1963) and talocalcaneal (Diamond 1974) coalition have been described. Whether this is the same syndrome as facio-audio-symphalangism syndrome or a distinct entity is not known. Another disorder caused by mutation in the NOG gene, on a chromosomal region overlapping the critical interval of the multiple synostosis syndrome, is tarsal-carpal coalition syndrome (OMIM 186570). This autosomal dominant disorder is characterized by carpal and tarsal synostosis, proximal symphalangism, short 1st metacarpals, and humeroradial fusion (Gregersen and Petersen 1977; Drawbert et al. 1985). Interpha-langeal fusion is present to varying degrees at birth, whereas humeroradial synostosis usually appears in childhood. Normal conductive hearing unambiguously differentiates this disorder from the multiple synostosis syndrome (Dixon et al. 2001). Spondylo-carpo-tarsal synostosis syndrome (congenital syn-spondylism, OMIM 272460) is a familial, autosomal recessive condition featuring short-trunk dwarfism of postnatal onset, unilateral unsegmented bars resulting in severe scoliosis, carpal synostosis (usually capitate-hamate and triquetrum-lunate), and tarsal synostosis (Wiles et al. 1992; Langer and Moe 1975). Clubfoot, dental enamel hypoplasia, renal anomalies, sensorineural hearing loss, ocular abnormalities, and odontoid hypoplasia with cervical spine instability are variable manifestations (Ventruto and Catani 1986; Seaver and Boyd 2000; Steiner et al. 2000). The Liebenberg syndrome (OMIM 186550) is an association of elbow dysplasia causing flexion deformity, carpal fusion (triquetropisiform), and brachydactyly type B (Liebenberg 1973). Additional features include variations in the size of the carpals (small capitate, trapezium and trapezoid; enlarged triquetrum and hamate), radial deviation of the hand, and 5th finger camptodactyly. The inheritance pattern is most probably autosomal dominant, with NOG as the candidate gene (Tiberio et a. 2000). The absence of other bony fusions clearly differentiates this entity from other forms of multiple synostoses. Banki syndrome (OMIM 109300), which involves lunatotriquetral fusion, clinodactyly and short metacarpals with thin diaphyses, is distinguished from Liebenberg syndrome by normal elbows (Banki 1965).A dominantly inherited form of mesomelic dwarfism with severe skeletal abnormalities involving the ankles, knees, and elbows has been described (Verloes and David 1995). This disorder goes under the designation of 'mesomelia-synostoses syndrome' (OMIM 600383). Skeletal changes include short metacarpals and metatarsals, carpometacarpal and tarsometatarsal synostoses, fusions in the proximal row of carpal bones, and mild vertebral anomalies. Affected patients also have down-slanting palpebral fissures, beaked nose, hypertelorism, ptosis, microretro-gnathia, and transverse agenesis of the soft palate (Pfeiffer et al. 1995). Progressive curvature of the forearm is typical. Overlap with the Kantaputra type of mesomelic dysplasia (OMIM 156232), which also involves mesomelia and synostoses, is recognized.

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