An extra thumb occurs in eight out of every 10,000 births. What is often called a double thumb is in fact a split thumb and is generally unilateral. The most accepted classification is that of Wassel (Fig. 8.1). This classification is based on the level of the split, starting at the distal phalanx. Splitting of the thumb tends to be sporadic. Only with a triphalangeal thumb may there be an autosomal dominant anomaly that may be hereditary. Other abnormalities that can be present are congenital anomalies of the spinal column, tibial hypoplasia or aplasia, cleft palate or anal atresia. A split thumb is also present in several syndromes (Table 8.1).

An extra little finger is the most common type of polydactyly (Fig. 8.2). According to the classification of Stelling and Turek there are three types: type A presents with a rudimentary little finger without phalanges. In type B there is a complete duplication of the little finger. In type C not only the little finger but also the fifth metacarpal is duplicated (Fig. 8.3). Type C is very rare.

In a central polydactyly there is an extra index, middle or ring finger (3.5 % of all polydactylies). Tendons, nerves and blood vessels of the extra finger tend to be abnormal. The same applies to growth plates of the extra finger, often leading to ulnar or radial deviation.

Ulnar dimelia is also known as mirror hand because the ulnar part of the forearm and hand are present as a mirror image. A mirror hand is a very rare and a mostly unilateral anomaly. The entire literature describes fewer than 100 cases. There is a double ulna and ulnar side of the hand, while the radius and the radial part of the hand are absent. The thumb is always absent. Both parts of a mirror hand are usually not entirely symmetrical, and seven fingers are more common than eight (Fig. 8.4). The function of the hand is very limited. The elbow and the wrist are usually widened, and the forearm and upper arm tend to be shorter. There may be irregularities in the shoulder blade, clavicle, humerus and shoulder joint. The capitulum is absent and the trochlea is duplicated in the elbow. The elbow is generally stiff. The biceps and brachialis muscles do not insert into the forearm but into the distal part of the humerus. The musculature and vascular systems of the forearm and hand are also abnormal. Another irregularity that often occurs on its own is fibular dimelia, with a duplication of the fibula and outer toes and absence of the tibia and the tibial part of the foot and the big toe and second toes.

One part of the split thumb tends to be clearly larger than the other part. In such cases, the smallest part is excised (Fig. 8.5). In types I, II and III (Fig. 8.1) the central part is resected and the remaining parts are coapted (Bilhaut-Cloquet procedure). Sometimes it makes sense to limit resection of the central part to the soft parts and nails (Fig. 8.6).

Type A: resection of the rudiment. Type B: ablation of the extra little finger. Type C: resection of the extra little finger and metacarpal.

Resection of the extra finger. This is technically very difficult and it is a tall order to obtain an acceptable appearance and function in the hand in such cases.

This is extremely difficult, if not impossible to treat, and the procedure can only be performed by a highly experienced hand surgeon specialized in congenital hand abnormalities.

During growth the thumb and/or finger(s) remain proportional to the normal digits.

The macrodactyly becomes disproportionately larger than the rest of the hand during childhood.

This is a rare, progressive palmar and radial curvature of the distal phalanx usually involving the little finger (Fig. 8.9). The cause is unknown. It typically affects preadolescent females, usually bilaterally. This anomaly doesn’t usually cause many problems, and treatment is generally not required.

In a trigger finger there is snapping or locking of the digit at the A1 pulley. A trigger finger is usually due to a congenital anomaly of the flexor mechanism.