Shoulder Defects. Congenital elevation of the scapula (see Plate 4-28) and absence of the pectoral muscles are the two types of failure of differentiation in the shoulder. Skeletal involvement at this level can result in congenital humerus varus.

Elbow and Forearm Defects. Soft tissue involvement may be manifested by aberrations of the long flexor, extensor, or intrinsic muscles in the upper limb. Failure of skeletal differentiation can result in either dislocation or synostosis of the humeroradial, humeroulnar, proximal, or distal radioulnar joint. Synostosis of the proximal radioulnar joint, the most severe elbow deformity in this category, is genetically determined and often associated with synostosis elsewhere in the body. Surgical correction may be indicated if flexion/extension or pronation/supination deformities that interfere with function are present.

Wrist and Hand Defects. Failure of differentiation can occur in either the skeletal or soft tissue elements of the carpus, metacarpals, or fingers.

In symphalangism, an intermediary joint in the digit is missing, most commonly the proximal interphalangeal joint. This bilateral malformation most frequently involves the ring and little fingers. Symphalangism of the distal interphalangeal joint is rare and almost never seen in the thumb. The affected joint is immobile, and its flexion and extension folds are absent. Radiographs taken after closure of the epiphysis show bony ankylosis. If ankylosis is established, the deformity can be treated with implant arthroplasty or with osteotomy and fusion of the joint in a functional position.

Syndactyly, one of the two most common malformations in the hand, is often bilateral and can involve two or more digits, usually the middle and ring fingers. In some patients, only the soft tissues are fused (simple syndactyly); in other patients, the nails and bones are joined as well (complex syndactyly). Syndactyly often occurs in association with webbing of the toes (usually between the second and third toes) and is frequently associated with other deformities in the same hand or elsewhere in the body, such as Poland syndrome, Apert syndrome, or craniofacial dysostosis. Syndactyly is occasionally hereditary, and this type affects males more often than females and is rare in African-American children. It is believed to arise during the fetal period and must be differentiated from acrosyndactyly secondary to congenital constriction band syndrome (see Plate 4-50).

If the syndactyly does not interfere with alignment of the digits, growth, or hand function, surgical repair can be postponed until the child is 2 or 3 years of age. However, syndactyly in digits of unequal length (e.g., ring and little fingers or, more commonly, the thumb and index finger) requires early surgical correction to avoid permanent deformity. In complex syndactyly, the nails of the joined digits are usually fused and the nail and bony bridge must be divided and resurfaced with a graft. If more than two digits are affected, adjacent pairs are separated at different times to avoid compromising the blood supply. Pairs of unequal length are divided first.