In the normal human hand the thumb has two phalanges; the triphalangeal thumb contains three palanages instead of the normal two.
The triphalangeal thumb may be an isolated anomaly without duplication.
It occurs in about one per 25,000 births, constituting the second most common type of thumb duplication. It is inherited as an autosomal dominant trait.
There is no sex predilection. In about 87 percent of the cases involvement is bilateral.
The thumb of the contralateral hand may be normal, absent, or duplicated.
Triphalangeal thumb may be part of a generalized syndrome .
It may be associated with other anomalies such as
- Duplication of the hallux or the little toe
- Lobster feet and hands
- Absence of the tibia
- Absent carpal bones
- Absent pectoral muscles.
Other associated findings may be hypoplasia of nails, tooth anomalies, deafness, and mental retardation. The anomaly may occur in diastrophic dwarfism.
In all forms of life form the amphibians and up, the preaxial (or first) digit is composed of three phalnges.
The triphalangeal thumb with full-length normal second phalanx and without duplication resembles a finger; when it does not oppose because the thenar muscles are absent, the hand is referred to as a “thumbless” five finger hand.
In the adult, the triphalangeal thumb is extra long, cosmetically unattractive, and not normal.
Functionally, fine motor activities such as precision handling are defective. The extra phalanx may be delta-shaped, rectangular, or normal. A small trapezoid second phalanx will cause ulnar angulation.
Treatment
In thumb duplication, when a triphalangeal thumb accompanies a normal thumb, the three pahalanxed thumb is ablated.
When the accompanying two-phalanx thumb is hypoplastic but functional, again it is best to ablate the triphalangeal thumb.
When both thumbs are triphalanxed and hypoplastic, the more functional of the duplicated thumbs is retained.
Excision of the accessory phalanx of a triphalangeal thumb results in instability at the site of excision. Surgery should be performed early in life, preferably prior to one year of age.
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