Triphalangeal thumb

Triphalangeal thumb

Description, Causes and Risk Factors:

ICD-10: Q74.0

A triphalangeal thumb is a thumb with three phalanges. The thumb often appears long and fingerlike, and can sometimes be in the same plane as the other fingers. Anatomically, the extra phalanx can have different shapes.

Triphalangeal thumb is a rare and interesting anomaly with a strong hereditary tendency. It occurs in about one per 25,000 births, constituting the second most common type of thumb duplication. It can occur as an isolated defect, in association with other abnormalities of the hands and feet, or as a part of a syndrome. Sporadic cases have been described, but it is usually inherited as an autosomal dominant trait.

The underlying genetic defect is unknown, but presumably involves genes that regulate the differentiation of the developing forelimb. In two large kindreds with triphalangeal thumb, evidence for linkage to the long arm of chromosome 7 was obtained with a maximum LOD score of 12.61. Multipoint linkage and haplotype analysis placed the gene close to the telomere of the long arm. To our knowledge this is the first time that a human gene involved solely in the pathologic morphogenesis of the hand and feet has been localized.

Clinical presentation is variable within families, ranging from mild to severe. Malformations of the feet are usually less severe than those of the hands. Triphalangeal thumb-polysyndactyly syndrome is caused by duplication encompassing the limb-specific regulatory element (ZRS) of sonic hedgehog SHH, which lies in intron 5 of the `limb region 1 homolog' gene, LMBR1 (7q36).

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.


The list of signs and symptoms mentioned in various sources for Triphalangeal thumb, non opposable includes the 3 symptoms listed below:

    Abnormal thumb placement.

  • Finger-like thumb.

  • Three bones in thumb.

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.


Diagnosis is based on clinical examination, X-ray of the affected skeletal segments,and hematologicaltests.In accordance with autosomal dominantinheritance, recurrence risk for this condition is50%. Genetic counselling should be offered infamilial and sporadic cases, emphasizing thewide expressivity of the syndrome.


Surgery is necessary to create a single, functioning thumb. Typically this is performed around one year of age, before the development of pinch and fine motor function. Simply removing one of the two thumbs will not suffice, as each of the split thumbs has elements that need to be combined to recreate the best possible thumb. As a result, surgical treatment usually entails removing the bony elements of the smaller thumb and reconstructing the remaining skin, tendons, ligaments, joints, and fingernails to form a new thumb. Even in the best of situations, the resulting thumb may smaller than the child's other, normal thumb. Furthermore, given the possibility of recurrent deformity in a small percentage of patients, additional operations may be needed later in life.

NOTE: The above information is educational purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.

DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.


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