Key Wordsthumb duplication, triphalangeal thumb, Wassel classification, radial polydactyly, central polydactyly, ulnar polydactyly, mirror hand, ulnar dimelia
Polydactyly of the hand refers to the presence of an extra digit or part of a digit. It can be defined as radial, central, or ulnar, referring to the region of the extra digit. The condition can occur in isolation or as part of a syndrome. Radial polydactyly, or thumb duplication, is classified by the Wassel classification. Ulnar polydactyly is classified into two or three types.
Polydactyly (poly = more; dactylos = finger) refers to the presence of more than five digits on at least one extremity. It is one of the most common congenital upper extremity differences, together with camptodactyly and syndactyly. The disorder is easily detected at birth, and cosmetic concerns usually lead the parents to seek surgical intervention at an early age. Depending on the type and severity of duplication, there may also be functional impairment.
The classification of polydactyly of the hand can be divided into radial, central, and ulnar polydactyly, referring to the anatomical location of the extra digit or part of a digit. The incidence of polydactyly varies depending on the type of polydactyly and the studied population. Ulnar polydactyly is 10 times more common in those of African descent and twice as common in males, whereas radial polydactyly is more common in Asians. Central polydactyly is extremely rare in all published studies.
A separate entity is the mirror hand or ulnar dimelia. In the forearm, two ulna bones may exist without a radius. Because the medial side is a mirror image of the lateral side, the term mirror hand became popular. It is extremely rare. The classic mirror hand is unilateral, usually with 7 to 8 fingers without a thumb, and is not combined with other anomalies. The etiology is unknown. An inheritance pattern has not been described. Geneticists believe it is due to a spontaneous mutation.
The incidence of radial polydactyly is 0.08 to 1.4 per 1000 live births, and 20% of cases are bilateral. With respect to ulnar polydactyly, in a prospective screening study of 11,161 newborns, the prevalence rate of type B ulnar polydactyly was 1 in 143 live births of African American infants and 1 in 1339 live births of Caucasian infants. Bilateral involvement was present in 76% of patients, and 86% had a positive family history. Central polydactyly is less common than radial and ulnar polydactyly. In a review of 144 cases of polydactyly at the University of Iowa, 22 cases of central polydactyly were reported. Central polydactyly is often seen with associated syndactyly and cleft hand. It can occur in isolation or as part of a syndrome, such as Grebes’ chondrodysplasia. The incidence of mirror hand is very rare, with only approximately 70 cases reported since its original description in the sixteenth century.
Radial polydactyly occurs most commonly as an isolated problem, whereas ulnar polydactyly is more commonly associated with a systemic syndrome.
Radial polydactyly, also referred to as thumb duplication, is usually classified according to the Wassel system ( Fig. 5.2.1 ). It is based on radiographs and refers to the skeletal level of thumb duplication : type I at the distal phalanx; type II at the interphalangeal (IP) joint ( Fig. 5.2.2A and B ); type III at the proximal phalanx; type IV at the metacarpophalangeal (MP) joint ( Fig. 5.2.3A and B ); type V at the metacarpal; and type VI at the carpometacarpal (CMC) joint. When one of the thumbs is triphalangeal, it is classified as type VII. The most common Wassel types are type IV (50%), type II (20%), and type VII (12%). Wassel type I thumb duplication is the least common type. Some types of radial polydactyly are not accounted for by the Wassel classification. These include the triplicated thumb, the pedunculated type or a duplicated thumb with no bony attachment to the main thumb, the radially deviated type, and the radial polydactyly associated with thenar muscle hypoplasia.
Several authors do not agree with the term “duplicated thumb” since both thumbs are usually smaller than a normal thumb and have less mobility. For this reason, the term “split thumb” can be encountered instead.
Radial polydactyly usually occurs sporadically. Genetic counseling is therefore not necessary. An exception to this includes cases of radial polydactyly where one thumb is triphalangeal (Wassel type VII). This phenotype has been linked to a mutation on chromosome 7q36, and it demonstrates an autosomal dominant inheritance pattern. The affected gene is involved in the regulation of the sonic hedgehog protein. Radial polydactyly can occur in the setting of a syndrome, such as Holt-Oram syndrome, Fanconi’s anemia, and Rubinstein-Taybi syndrome.
In the absence of surgical intervention, most children with radial polydactyly can function without any serious functional limitation. However, some patients have an associated pollex abductus that restricts normal flexion and extension of the associated joints. Additionally, patients with uncorrected deformity may face the stigma associated with abnormal thumb appearance as they integrate into society.
The first step in treating a patient with radial polydactyly includes obtaining a complete medical history and performing a full physical examination. Even in the setting of apparent unilateral involvement, both upper extremities should be examined closely. First, ensure that the other digits (index through little fingers) are normal and have flexion and extension creases and that the hypothenar eminence is well developed. After ensuring that the non-thumb structures are normal, attention may be turned to the radial side of the hand.
In patients with unilateral radial polydactyly, it should be stressed to the parents that the duplicated thumbs are each smaller and thinner than the contralateral unaffected thumb. They should be reminded that a difference will persist even after reconstructive procedures.
Examination proceeds systematically from proximal to distal, starting with the thenar muscles. While they are usually well developed, especially in Wassel type I and II thumb duplications, they may demonstrate hypoplasia in more severe forms of thumb duplication.
Next, range of motion of the thumbs is assessed. Because this can be challenging in a young child, attention should be focused on joint passive range of motion and stability, palpability of flexor and extensor tendons, and presence of skin creases. Thumb motion is primarily dependent on the integrity of the CMC joint, which is usually normal in Wassel I–IV thumb duplications but can have varying degrees of hypoplasia in Wassel type V and VI thumb duplications. Although both MCP and IP joints can have decreased range of motion, they are not as important for thumb mobility.
Radiographic examination of the duplicated thumbs should be obtained to determine the Wassel type of the duplication and to evaluate the anatomy of the articular surfaces. However, in a very young child with an immature skeleton, there can be a cartilaginous connection not revealed on x-ray, becoming evident only at the time of surgery, thereby downgrading a Wassel type II into a type I, or a type IV into a type III.
Correction of radial polydactyly is recommended at around 12 months of age, before development of thumb index finger pinch. Additionally, general anesthesia is safer at this age, and dissection of important anatomical structures is easier when the child is older. However, timing of surgery can vary in patients with multiple associated anomalies, and correction of radial polydactyly can be postponed in patients with significant medical problems.
Surgical reconstruction of radial polydactyly aims to achieve a straight, mobile, and stable thumb of good size and shape.
Pedunculated-type radial polydactyly can be treated with ligation or simple excision of the floating thumb. Ligation is performed at the base of the floating thumb with a 5-0 nylon suture with or without local anesthesia in the nursery or outpatient setting. However, ligation can result in a residual nubbin or painful neuroma. Alternatively, elliptical excision of the floating thumb can be undertaken under general anesthesia. The vessels present in the soft tissue pedicle are cauterized before transection, and the nerve is sharply divided and buried within the soft tissue to prevent subsequent painful neuroma.
Type I (Distal Phalanx) and Type II (Distal Interphalangeal Joint)
Treatment of these less severe forms of thumb duplication depends on the extent of the duplication and on whether the duplication is symmetrical or asymmetrical. When the duplicated thumbs share a nail and are well aligned, surgical correction is optional.
In the case of an asymmetrical duplication, the smaller thumb is ablated, its collateral ligament is transferred to the dominant thumb, and the tendons are centralized. The skin incision must be planned to allow sufficient coverage of the retained dominant thumb. It can be placed dorsally or, less conspicuously, laterally. Proximal exposure follows a zig-zag pattern. First, the nail and nail bed of the lesser thumb are excised, and the skin is dissected superficial to the extensor tendon. The collateral ligament of the minor thumb is elevated distally with an osteoperiosteal sleeve ( Fig. 5.2.2C ). If the IP joint has a separate facet for the minor thumb, the collateral ligament is also detached from its proximal insertion in continuity with the periosteum, which is raised as a longitudinal flap. This forms a proximally based ligament-periosteal sleeve. Chondroplasty is then performed, and the extra articular facet is trimmed with a scalpel blade. If malalignment of the retained thumb persists, a closing wedge osteotomy can be performed in the future. The slips of extensor and flexor tendons to the lesser thumb may be used to augment their counterparts on the retained/dominant thumb. The collateral ligament is sutured to the periosteum of the base of the retained distal phalanx using a nonabsorbable braided suture. A percutaneous 0.045-inch Kirschner wire (K-wire) may be driven from the thumb tip across the IP joint to maintain proper alignment. The skin incision is closed with absorbable sutures ( Fig. 5.2.2D ), and the upper limb is placed in a long arm thumb spica cast.
In patients with nearly symmetrical thumb duplication, the Bilhaut-Cloquet procedure may be considered. This procedure involves resection of central portions of bone and nail of the duplicated segments longitudinally and reapproximation of the remaining structures with either sutures or a transverse K-wire. This procedure, however, has been criticized for resulting in post-operative nail deformity, physeal growth disturbance, and IP joint stiffness. Baek et al. have proposed a modified extra-articular technique in which the IP joint is reconstructed with one thumb and the second thumb is used to reconstruct the distal phalanx. Nonetheless, results in both cases seem to demonstrate obvious nail deformity, and it is the authors’ preference to avoid these techniques.