Polydactyly means extra digits – in the hand or foot. Broadly speaking, there are three types of polydactyly
- Postaxial or Ulnar
- Preaxial or Radial
Postaxial or Ulnar
It is the most common type of polydactyly where the extra digit is on the ulnar side of the hand [side of the little finger].
Most commonly, the extra finger is rudimentary, consisting of an end phalanx with a nail, and connected to the hand with a small skin pedicle.
In case of a fully developed extra finger, the duplication usually presents itself at the level of the metacarpophalangeal joint. Triplication of the little finger is very rare.
In about 14% of the cases, this type of polydactyly is hereditary.
It is further of three types.
- Type I includes nubbins or floating little fingers
- Type II includes duplications at the metacarpophalangeal joint.
- Type III includes duplications of the entire ray
Preaxial or Radial
Preaxial polydactyly or extra thumbs are the common, occurring incidence of the deformity is 0.08 per 1000.
It ranges from a small fleshly webbing on the radial border of the hand to triplication. There are seven types of duplicate thumbs.
Type I – The distal phalanx is bifid
Type II– The distal phalanx is duplicated
Type III – The proximal phalanx is bifid but the distal phalanx is duplicated
Type IV – Both the proximal and distal phalanges of the thumb are duplicated
Type V – The metacarpal of the thumb is bifid, the proximal and distal phalanges are duplicated
Type VI – The thumb metacarpal and both proximal and distal phalanges are duplicated (4 percent) and
Type VII – The thumb is triphalangeal
Preaxial polydactyly may be associated with syndactyly, it is inherited as an autosomal dominant trait and is termed as polysyndactyly.
In thumb duplication, circulation may be anomalous, being supplied by one artery instead of two. Often one of the thumbs is dominant while the other is underdeveloped; sometimes, however, both thumbs are of equal size, posing a problem as to which one to ablate.
Thumb polydactyly is usually sporadic, though if associated with triphalangeal thumb it may be familial.
Preaxial polydactyly may be associated with vertebral anomalies, the absence of the tibia, cleft palate, and imperforate anus. It may also be seen in a number of syndromes.
They may be of special clinical importance are Down’s syndrome, Fanconi pancytopenia, and acrocephalosyndactyly.
Duplication of the index, long, and ring fingers is referred to as axial or central polydactyly. Often the extra digit of a long or ring finger is concealed in a web between adjoining normal digits.
The tendons, nerves, and vessels of the supernumerary digits are usually abnormal, as are the epiphyses of the extra digit.
The epiphysis of the extra digit usually does not lie in the normal line of growth. As a result, its phalanges diverge from the longitudinal axis into ulnar or radial deviation and distort adjacent digits.
Duplication of the index finger is very rare, representing about 3.5 percent of all cases of polydactyly.
This anomaly should not be confused with the triphalangeal thumb. The extra index finger may consist of a pedunculated soft-tissue mass or it may have normal bony and soft-tissue components. It may be associated with congenital radioulnar synostosis, and occasionally the duplicated index finger may be fused to the long finger.
Treatment of Polydactyly
If the duplicated small finger is hypoplastic, rudimentary, and attached to the hand by a narrow pedicle it can be removed in the newborn.
The fully developed and complete extra little finger needs to be excised and can be done at a later date.
Operative treatment for preaxial type is more complicated.
The important considerations are
- To maintain size and stability
- To provide adequate motor control and strength
- To establish anatomic alignment.
The type of treatment varies according to the type of thumb duplication.
If one of the two thumbs is significantly smaller or if only one deviates from the anatomic axis, the smaller accessory thumb is ablated.
However, at skeletal maturity, the thumb would be smaller than the opposite thumb.