Polydactyly

Polydactyly is the congenital duplication of digits and one of the most common congenital hand anomalies. It is classified by position: preaxial (radial, the thumb), postaxial (ulnar, the small finger) or central.

How common. The overall incidence is 1-2 per 1,000 live births, and the distribution varies by type:

• Preaxial - more common in Caucasians and Asians
• Postaxial - 10 times more common in people of African descent, at 1 in 300
• Central - rare, about 1 in 100,000

The thumb. A normal thumb has two phalanges, against three in the fingers, so the three phalanges of a Wassel VII thumb are abnormal. FPL inserts on the distal phalanx and is critical to thumb function. The thenar muscles, APB, FPB and OP, provide opposition, which is unique to the thumb and enables pinch grip.

The MCP joint. The UCL and RCL provide MCP joint stability. Asymmetric ligament support produces angular deformity, and unequal growth can cause progressive deformity.

The duplicated thumb. The metacarpal may be single (Wassel I-V) or duplicated (Wassel VI), and each thumb has its own digital nerves and arteries. The physes are preserved at surgery to maintain growth.

The small finger. Its UCL is critical for power grip, and the hypothenar muscles, ADM, FDM and ODM, provide small-finger function.

Polydactyly is a disorder of anterior-posterior (radial-ulnar) axis patterning in the developing limb bud. It is not a mechanical splitting of a normal digit. It arises during limb-bud formation, at 4-8 weeks of gestation, from abnormal signalling within the bud.

The SHH-ZPA axis. The zone of polarising activity (ZPA), at the posterior (ulnar) margin of the limb bud, secretes Sonic Hedgehog (SHH). SHH establishes the radial-ulnar gradient that specifies digit identity and number.

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The genes. Each maps to a pattern of duplication:

• ZRS - SHH expression is controlled by a distant cis-regulatory element, the ZPA regulatory sequence, within intron 5 of LMBR1 at 7q36.3. Point mutations or copy-number gains of the ZRS cause ectopic anterior (radial) SHH activity, producing preaxial polydactyly and triphalangeal thumb.
• GLI3 - a downstream SHH effector and repressor. Loss of function underlies postaxial and central polydactyly, including Greig and Pallister-Hall syndromes.
• HOXD13 - expansion or mutation produces synpolydactyly, a central duplication fused with syndactyly.

From signal to phenotype. Ectopic or excess SHH activity expands the digit field and generates a supernumerary ray. The level at which the extra skeletal element separates, from distal phalanx to metacarpal, determines the Wassel type of a thumb duplication. Whether the elements are shared (bifid) or separate (duplicated) reflects how early in ray condensation the duplication occurred.

Why neither thumb is normal. The duplicated ray shares tendons, ligaments and neurovascular structures with the dominant ray, so neither thumb is fully normal. That is why simple ablation without soft-tissue rebalancing fails.

Inheritance. Postaxial polydactyly is often autosomal dominant with variable penetrance, and isolated postaxial Type B typically is, often GLI3-related. Preaxial duplication is usually sporadic, but ZRS-related forms such as triphalangeal thumb-polysyndactyly are autosomal dominant.

Wassel (preaxial). Thumb duplication is classified by Wassel into types I-VII by the level of bifurcation, from distal to proximal: the higher the number, the more proximal the duplication. Odd numbers are bifid, sharing an element; even numbers are duplicated, with separate elements. Type VII is the triphalangeal thumb component.

Wassel IV is the most common type, at 43%. It is a duplicated proximal phalanx: two complete proximal phalanges articulating with a single metacarpal.

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PreaxialPostaxialCentral

I

Level of Bifurcation

Bifid distal phalanx (shared)

Frequency

Rare (2%)

II

Level of Bifurcation

Duplicated distal phalanx

Frequency

15%

III

Level of Bifurcation

Bifid proximal phalanx (shared)

Frequency

Rare (6%)

IV

Level of Bifurcation

Duplicated proximal phalanx

Frequency

MOST COMMON (43%)

V

Level of Bifurcation

Bifid metacarpal (shared)

Frequency

Uncommon (10%)

VI

Level of Bifurcation

Duplicated metacarpal

Frequency

Rare (4%)

VII

Level of Bifurcation

Triphalangeal thumb component

Frequency

20%

Wassel Classification - Thumb Duplication

Type	Level of Bifurcation	Frequency
I	Bifid distal phalanx (shared)	Rare (2%)
II	Duplicated distal phalanx	15%
III	Bifid proximal phalanx (shared)	Rare (6%)
IV	Duplicated proximal phalanx	MOST COMMON (43%)
V	Bifid metacarpal (shared)	Uncommon (10%)
VI	Duplicated metacarpal	Rare (4%)
VII	Triphalangeal thumb component	20%

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Postaxial duplication is classified by Stelling and Turek.

Type A is a well-formed extra digit containing bone and cartilage, with a joint articulating with the fifth metacarpal. It requires formal surgical excision and is more common in Caucasians.

Type B is rudimentary and pedunculated, attached by a narrow skin bridge only, with minimal or no bone. It is more common in people of African descent and can be ligated in the nursery, which is controversial.

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Central polydactyly duplicates the index, middle or ring finger. It is rare, often associated with syndactyly, and may be syndromic (Bardet-Biedl, Joubert).

It often involves shared tendons and neurovascular structures and requires complex reconstruction, and it represents the most challenging surgical cases.

Triphalangeal thumb and the delta phalanx. A triphalangeal thumb has an extra (middle) phalanx in the thumb ray. It is the component of Wassel VII duplication, and it also occurs in isolation and in the five-fingered (non-opposable) hand. It is strongly linked to ZRS/SHH dysregulation and is autosomal dominant in many families.

The delta phalanx. The extra phalanx is frequently a delta phalanx, a wedge or trapezoid-shaped bone with a longitudinally bracketed (C-shaped) epiphysis. The bracket tethers one side and drives progressive angulation (clinodactyly).

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Treating it. Management depends on the phalanx:

• A small delta phalanx is excised in infancy
• A larger rectangular extra phalanx needs a reduction or closing-wedge osteotomy, or fusion to an adjacent phalanx, to shorten and realign the over-long thumb
• A non-opposable, finger-like thumb (the five-fingered hand) requires pollicisation or opponensplasty, not simple excision

Recognising a triphalangeal component is essential, because ablating the duplicate alone leaves a long, deviated, poorly opposing thumb.

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History. The questions that matter:

• Family history - especially for postaxial (autosomal dominant)
• Pregnancy - maternal diabetes, teratogens
• Syndromic features - cardiac and renal anomalies
• The parents' goals - functional and cosmetic concerns

Examination. Inspection establishes the location (preaxial, postaxial or central), whether the digit is well formed or rudimentary, the width of the skin bridge in a Type B digit, and any associated syndactyly. Function is judged from active movement of each thumb, thenar bulk, stability and grip pattern. Look beyond the hand for a cardiac murmur (Holt-Oram), syndactyly (central), other limb anomalies and facial dysmorphism.

Which thumb to keep. The dominant thumb is usually the ulnar one, and the radial thumb is often hypoplastic. The features that decide it are gathered below; assess MCP and IP joint stability alongside them.

Radiographs are essential for surgical planning. They define:

• The level of bifurcation or duplication
• The bony anatomy of each digit
• Joint articulation
• Metacarpal morphology
• Any delta phalanx, whose bracketed physis is identified before choosing excision, osteotomy or fusion

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Further investigation. Consider:

• Cardiac echo - Holt-Oram syndrome (radial anomalies with ASD/VSD)
• Renal ultrasound - associated genitourinary anomalies
• Genetic testing - if there are syndromic features

Differential diagnosis. Not every extra or broad digit is a simple duplication. These entities must be distinguished, because they change the operation and the counselling.

Algorithm

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Timing. Preaxial duplication and postaxial Type A are operated on at 12-18 months. By then the hand is large enough for surgical manipulation, functional hand patterns are not yet established, and the child will not remember the operation. A postaxial Type B digit is either ligated in the nursery or formally excised at 6-12 months.

Planning. For a thumb duplication, decide which digit to retain, design the incision, and anticipate the need for ligament or tendon reconstruction. Consider Bilhaut-Cloquet if both thumbs are equally hypoplastic.

Non-operative management is rarely appropriate; most cases benefit from surgery. It is kept for families who decline surgery and children with significant medical comorbidities. The child is then monitored for functional limitation, any progression of angular deformity is documented, and the hand is reassessed at regular intervals.

Principles of thumb reconstruction. The aim is a functional, stable, cosmetically acceptable digit:

1. Retain the more functional thumb, usually the ulnar one with the better FPL insertion
2. Reconstruct the collateral ligaments, which are critical for stability
3. Rebalance the tendons, transferring the intrinsics from the excised thumb
4. Correct angular deformity, which may need an osteotomy

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The zigzag deformity. The reason a duplicate thumb is never normal, and recurs after naive excision, is the zigzag (Z) collapse deformity of Wassel IV. The shared metacarpal head is broad and its articular surface often obliquely inclined, the FPL and EPL insert eccentrically and are bowstrung toward one side, and thenar (intrinsic) pull is imbalanced. Together these angulate the MCP joint in one direction and the IP joint in the opposite direction.

Correcting it. Excision plus collateral ligament reconstruction is not enough for a durable result. It also requires:

• Re-centralisation of the bowstrung FPL/EPL over the joint axis
• Rebalancing of the intrinsics
• A corrective (closing-wedge) osteotomy of the metacarpal or proximal phalanx to square an oblique articular surface

Failing to address the tendon eccentricity and bony obliquity is the commonest cause of recurrent deformity and the 15-20% revision rate.

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Early. The risks in the first weeks:

• Wound infection - 1-2% incidence
• Flap necrosis - avoid excessive tension
• Nerve injury - the digital nerve is at risk
• Vascular compromise - ensure perfusion intraoperatively

Joint instability is the most common complication. It comes from inadequate ligament reconstruction and is associated with progression of angular deformity. It requires revision surgery, and meticulous technique is the prevention.

Angular deformity is progressive malalignment, related to growth and to asymmetric growth-plate injury. It is monitored during growth and may need corrective osteotomy.

Nail deformity is usually cosmetic only: the central ridge after Bilhaut-Cloquet, or a small hypoplastic nail. It may need nail-bed reconstruction.

Revision. 15-20% may need secondary procedures. Collateral ligament reconstruction is the most common; the others are corrective osteotomy for angular deformity, tendon rebalancing and web-space deepening.

The first two weeks. A thumb spica or volar splint, the wound kept clean and dry, and elevation to reduce swelling. Paracetamol is typically sufficient for pain.

Two to six weeks. Sutures are removed at 2 weeks, the splint is weaned gradually, and gentle range of motion begins, passive then active. Monitor for infection and stiffness.

Six weeks to six months. Full range-of-motion exercises and normal use are encouraged, with age-appropriate strengthening. Growth is monitored for angular deformity, and occupational therapy is added if there are functional concerns.

Long term. The child is monitored through growth, with annual assessment until skeletal maturity of ligament integrity, stability, and pinch and grip strength. Revision is considered for progressive deformity or instability.

Thumb duplication. Functional outcomes are good in most cases. Some residual instability is common, and the thumb may be slightly smaller than the normal side.

Postaxial polydactyly. Outcomes are excellent with proper technique. Type A excision has a low complication rate, while Type B ligation carries a risk of neuroma and a residual bump.

Satisfaction is high overall, and cosmetic concerns are more common than functional ones. Realistic expectations are important, including the possibility that secondary procedures may be needed.

Polydactyly is a clinical, not registry-tracked, condition, so practice is guided by national society reviews and IFSSH consensus rather than arthroplasty-style registries. The descriptive framework is converging worldwide on the Oberg-Manske-Tonkin (OMT) classification, endorsed by the IFSSH, with the Wassel (Wassel-Flatt) scheme retained for the thumb.

Global Epidemiology

• Overall: one of the most common congenital hand anomalies, roughly 1-2 per 1,000 live births worldwide.
• Postaxial (ulnar): markedly more frequent in populations of African ancestry (around 1 in 150-300), where it is often isolated and autosomal dominant; far less common (around 1 in 1,500-3,000) and more often syndromic in those of European ancestry.
• Preaxial (radial/thumb): more common in East Asian and white populations; usually sporadic and unilateral.
• Central: rare in all populations and frequently associated with syndactyly (synpolydactyly).

Side-by-Side Society Guidance

High- vs Limited-Resource Practice Variation

• High-resource settings: subspecialist paediatric hand surgery, day-case excision at 12-18 months, microsurgical capability for Bilhaut-Cloquet and on-top plasty, formal outcome scoring (JSSH/Tada), genetic counselling and cardiac/renal screening for syndromic cases.
• Limited-resource settings: nursery suture ligation or surgical-clip application remains widely used for narrow-based Type B digits because it avoids general anaesthesia and theatre access; families may present late, so reconstruction is performed at an older age. Awareness of the higher residual-bump and neuroma rate with ligation should still inform consent.
• Universal principle: regardless of resources, palpable bone or a broad base reclassifies a digit as Type A (or wide Type B) and mandates formal excision rather than ligation.

These are common viva escalation points. Take a position but acknowledge the evidence is limited.

Nursery suture ligation or formal excision for Type B postaxial. Ligation is convenient and avoids anaesthesia, but the systematic-review signal favours excision, with reported complication rates as high as 23.5% (residual bump, neuroma) for ligation versus around 3% for excision. Surgical-clip application is a middle ground for narrow-based digits. There is no high-level randomised evidence, so practice varies by resource setting and surgeon preference.

Near-symmetric Wassel IV thumbs. When the two thumbs are nearly equal in size, simple excision-plus-reconstruction yields worse JSSH outcomes. Whether Bilhaut-Cloquet, combination (on-top plasty) procedures, or accepting a smaller thumb is best remains debated; the symmetry index is an emerging predictive tool rather than a validated decision rule.

Bilhaut-Cloquet and its modifications. The classic procedure reliably creates a stable, wide thumb but is criticised for IP joint stiffness (it crosses the physis), a central nail ridge and a broad appearance. Modified Bilhaut, neurovascular island-flap pulp/nail augmentation and combination techniques aim to mitigate these, but comparative evidence is sparse.

Limits of the Wassel classification. It poorly captures the immature epiphysis, triphalangeal components and soft-tissue anatomy. The OMT framework addresses categorisation but the thumb-specific Wassel scheme persists for surgical planning, and intra- and inter-observer reliability of subtypes is imperfect.

Optimal timing. 12-18 months is conventional but not evidence-mandated. Some centres operate slightly earlier for technical convenience or later where access is constrained, with no clear functional penalty within the first few years; delay beyond 24 months nonetheless appears among the poor prognostic factors above.