Excerpt from Hand, Congenital Hand Deformities

Synonyms, Key Words, and Related Terms: congenital hand deformity, limb anomaly, apical ectodermal ridge, AER, radial dysplasia, constriction bands, Holt-Oram syndrome, VATER syndrome, Fanconi syndrome, TAR syndrome, Aase syndrome, Nager syndrome, Treacher Collins syndrome, Roberts syndrome, intercalated phocomelia, amelia, phocomelia, acheiria, adactyly, absence of hand, absence of fingers, preaxial deficiency, cleft hand, syndactyly, Apert syndrome, Poland syndrome, acrosyndactyly, symphalangism, radioulnar synostosis, flexion deformities, camptodactyly, arthrogryposis, clinodactyly, delta phalanx, duplication, mirror hand, ulnar dimelia, macrodactyly, arthrogryposis multiplex congenita

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Embryology

Limb development takes place during the third to eighth weeks of gestation. The limb buds (ie, Wolff crest) appear as swellings on the ventrolateral aspect of the embryo and are condensations of mesenchyme covered with a thick layer of ectoderm termed the apical ectodermal ridge (AER). The AER is a transient structure that is believed to elaborate morphogens that modulate growth patterns. The underlying mesoderm is organized into a posteriorly located zone of polarizing activity (ZPA) and a progress zone (PZ) more anteriorly. Vessels and then nerves subsequently grow into the limb. Mesodermal differentiation into cartilage and muscle begins to occur. The limb begins to pronate, the elbow flexes, and the hand is flexed and the ulnar deviated.

Recent work with retinoic acid demonstrates that limb formation can be directed chemically. Inclusion of the ZPA in chick abdomens results in the formation of a normal wing. Apparently, the important factors in the genesis of congenital limb anomalies are the duration of the AER inductive influence, the amount of mesoderm available, and the number of circulating morphogens. Thalidomide results in the formation of abnormal cells within the PZ; if these cells are excised, a normal limb results. Condensation of mesoderm gives rise to cartilage, and ossification follows by the endochondral route. Joint capsules are present prior to the formation of joints. At the point where joint formation is to take place, areas of flattened chondrocytes form dense plates termed interzones, the precursors of articular cartilage. Motion must occur if cavitation is to take place, thus forming the joint.

Ossification of the phalanges takes place antenatally, while the carpal bones ossify postnatally. Ossification of the congenitally deformed limb is delayed. By the third week, vessels begin to penetrate the mesodermal mass. Myogenic regions of the limb become vascular, and chondrogenic regions become avascular. Large interosseous and median arteries later give way to the radial and ulnar arteries.

Muscle formation is derived from a dorsal (extensor) and ventral (flexor) muscle blastoma. Tendons develop independently of their muscle bellies, only linking later in development. If the adjacent bone is not developed, the tendon attaches to the nearest adjacent structure. The specificity of the nerve-muscle relationship is not known: is it random or specific? Mixed motor and sensory nerves enter the limb as a pioneer growth cone. In general, the anterior division of these nerves supplies the flexors, and the posterior division supplies the extensors. The period of hand differentiation is short, taking place during the fourth to seventh weeks.

Incidence

A 20-year study in New York gave the incidence of congenital hand deformities as 1 in 626 (Conway, 1956). Regional and ethnic differences exist. Camptodactyly, syndactyly, and polydactyly are the most common abnormalities. Other studies give the incidence as 0.7%