A triphalangeal thumb (TPT) is a congenital malformation where the thumb has three phalanges instead of two. Besides the three phalanges, there can also be other malformations. The extra phalanx can vary from a very small phalanx to a full extra phalanx in a more finger-like thumb. It was first described by Columbi in 1559. [1] The true incidence is unknown, but is estimated at 1:25,000 live births. [2] In about two-thirds of the patients with triphalangeal thumbs, there is a hereditary component. [3]
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The triphalangeal thumb has a different appearance than normal thumbs. The appearance can differ widely; the thumb can be a longer thumb, it can be deviated in the radio-ulnar plane (clinodactyly), thumb strength can be diminished. In the case of a five fingered-hand it has a finger-like appearance, with the position in the plane of the four fingers, thenar muscle deficiency, and additional length. There is often a combination with radial polydactyly.
An opposable thumb can be placed ‘opposite’ the other four digits or fingers. This function of the thumb is very important for using the hand. It makes precision grips (e.g. key grip, tripod grip) possible. For opposition and therefore precision grips, several factors are necessary: adequate thumb position, adequate length of the thumb, stabile joints and adequate thenar muscle or thumb strength. [4] With a non-opposable thumb, the hand can be less effective in use and therefore be problematic in daily life.
Malformations of the upper extremities can occur In the third to seventh embryonic week. [5] In some cases the TPT is hereditary. In these cases, there is a mutation on chromosome 7q36. [6] If the TPT is hereditary, it is mostly inherited as an autosomal dominant trait,[7] non-opposable and bilateral.[3] The sporadic cases are mostly opposable and unilateral.[8]
Triphalangeal thumb can occur in syndromes but it can also be isolated. The triphalangeal thumb can appear in combination with other malformations or syndromes.[5]
Syndromes include:
Malformations include:[5]
There are multiple classifications for the triphalangeal thumb. The reason for these different classifications is the heterogeneity in appearance of the TPT. The classification according to Wood [9] describes the shape of the extra phalanx: delta (Fig. 4), rectangular or full phalanx (Table 1). With the classification made by Buck-Gramcko a surgical treatment can be chosen (Table 1). Buck-Gramcko differentiates between six different shapes of the extra phalanx and associated malformations.[10]
Table 1: Classifications of Wood [9] and Buck-Gramcko [10]
| Classification according to Wood by shape of the extra phalanx | Shape | Classification according to Buck-Gramcko by shape/size of extra phalanx and associated malformations | Shape |
|---|---|---|---|
| I | Delta | I | Rudimentary triphalangism |
| II | Short triangular middle phalanx (brachymesophalangeal) | ||
| II | Rectangular | III | Trapezoidal middle phalanx (intermediate) |
| IV | Long rectangular middle phalanx (dolichophalangeal) | ||
| III | Full | V | Hypoplastic triphalangeal thumb |
| VI | Triphalangeal thumb associated with polydactyly |
The goals of surgical treatment are: reducing length of the thumb, creating a good functioning, a stable and non deviated joint and improving the position of the thumb if necessary. Hereby improving function of the hand and thumb.
In general the surgical treatment is done for improvement of the thumb function. However, an extra advantage of the surgery is the improvement in appearance of the thumb. In the past, surgical treatment of the triphalangeal thumb was not indicated [11], but now it is generally agreed that operative treatment improves function and appearance. Because an operation was not indicated in the past, there’s still a population with an untreated triphalangeal thumb. The majority of this population doesn’t want surgery, because the daily functioning of the hand is good. [11] The main obstacle for the untreated patients might not be the diminished function, but the appearance of the triphalangeal thumb. [11] The timing of surgery differs between Wood and Buck-Gramcko. Wood advises operation between the age of six months and two years, [12], while Buck-Gramcko advises to operate for all indications before the age of six years. [13]