Polydactyly and Syndactyly
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Direct answer
Polydactyly is the presence of an extra digit, and syndactyly is the abnormal union of two or more digits, both arising from disturbances in limb bud development. Preaxial polydactyly, on the thumb side, is often complex and needs reconstruction, whereas postaxial polydactyly is commonly managed by ligation or excision. Syndactyly is separated surgically, usually in early childhood, most often between the middle and ring fingers.
What you must remember
- Polydactyly: an extra digit, classified as preaxial when it lies on the radial or thumb side and postaxial when on the ulnar or little finger side.
- Postaxial form: the more common form in most textbook accounts, often a small rudimentary digit that can be removed by ligation at its base.
- Preaxial form: thumb duplication, often with a bony component, which usually needs reconstruction of one functional thumb.
- Wassel classification: grades thumb duplication by the level of bifurcation, from a bifid distal phalanx in type I to a duplicated metacarpal in type VI, with type IV (duplicated proximal phalanx) the commonest.
- Syndactyly: a union of adjacent digits, simple when only skin and soft tissue are involved and complex when bone or nail is also fused.
- Commonest web: the middle and ring finger web, which is the third web space.
- Timing: release is usually planned in infancy or early childhood, before school age, with staged surgery for bilateral or complex cases.
Planning reconstruction by the Wassel level
A newborn has a duplicated thumb with a well-formed second digit on the radial side. The surgeon must decide which digit to keep and how to reconstruct it, guided by the level of duplication.
| Wassel type | Level of duplication | Bony pattern | Typical planning |
|---|---|---|---|
| Type I | Bifid distal phalanx | One bone with a split tip | Tip and nail reconstruction may suffice |
| Type II | Duplicated distal phalanx | Two distal phalanges on one proximal phalanx | Reconstruction of one thumb, keeping the better-formed tip |
| Type III | Bifid proximal phalanx | Split proximal phalanx, two distal phalanges | Reconstruction of one thumb, keeping the better-formed side |
| Type IV | Duplicated proximal phalanx | Two proximal phalanges on one metacarpal; the commonest type | Usually one thumb rebuilt and the lesser digit excised |
| Type V | Bifid metacarpal | Split metacarpal head, two phalangeal chains | More complex, needing bone and joint realignment |
| Type VI | Duplicated metacarpal | Two metacarpals, each with its own phalanges | Often the most complex, with one functional thumb rebuilt |
The higher the level of bifurcation, the more the reconstruction depends on bones, joints and tendons. Surgeons therefore grade the duplication before choosing between excision of the lesser thumb and reconstruction of a single functional thumb.
Where the exam tries to catch you
Examiners test the classification, the surgical principles and the timing of syndactyly release.
- Trap: treating all extra digits as equally simple. Correct: postaxial digits are often removed by ligation or excision, while preaxial thumb duplication usually needs a more complex reconstruction.
- Trap: releasing a syndactyly without planning skin cover. Correct: the raw surfaces must be covered, often with flaps or grafts, because a narrow or unlined web can recur.
- Trap: missing a syndrome in a child with syndactyly. Correct: bilateral, mitten-like syndactyly raises the question of Apert syndrome, which needs craniofacial and genetic assessment.
Frequently asked questions
What is the difference between preaxial and postaxial polydactyly?
Preaxial polydactyly involves the radial or thumb side of the hand or foot, while postaxial polydactyly involves the ulnar or little finger side. Postaxial polydactyly is the more common form overall, whereas preaxial involvement is usually more complex to treat.
What does the Wassel classification grade in thumb duplication?
The Wassel classification grades preaxial thumb duplication by the level of bifurcation, from a bifid distal phalanx in type I to a duplicated metacarpal in type VI, with type IV (duplicated proximal phalanx) the commonest. Surgeons use it to plan which thumb to keep and how to reconstruct it.
When is surgery for syndactyly usually performed?
Syndactyly surgery is usually performed in infancy or early childhood, before school age, to support normal hand function and growth. Timing is planned case by case, and bilateral or complex syndactyly may need staged operations.
Why does bilateral syndactyly raise the question of Apert syndrome?
Apert syndrome is classically associated with complex, mitten-like syndactyly of the hands and feet along with craniofacial anomalies. A child with bilateral syndactyly therefore needs examination for associated syndromes, not only a hand assessment.