Polydactyly

Polydactyly is congenital duplication of digits, classified as preaxial (thumb/radial), postaxial (small finger/ulnar), or central. Preaxial is more common in Caucasians; postaxial is common in African descent (1:300). For thumb duplication, the Wassel classification (I-VII) is essential: odd numbers indicate bifid elements, even numbers indicate duplicated elements. Wassel IV (duplicated proximal phalanx) is most common. Surgery at 12-18 months: retain the more functional digit (usually ulnar thumb) and reconstruct collateral ligaments. Postaxial Type B (rudimentary) can be ligated in nursery; Type A requires formal excision.

Polydactyly is the congenital duplication of digits, one of the most common congenital hand anomalies.

Epidemiology

• Overall Incidence: 1-2 per 1,000 live births
• Preaxial (Thumb): More common in Caucasians and Asians
• Postaxial (Small Finger): 10x more common in African descent (1:300)
• Central: Rare (1:100,000)

Genetics and Inheritance

• Postaxial: Often autosomal dominant with variable penetrance
• Preaxial: Usually sporadic, can be autosomal dominant
• Associated Syndromes: Holt-Oram, Ellis-van Creveld, trisomy 13

Embryology

• Develops during limb bud formation (4-8 weeks gestation)
• Sonic Hedgehog (SHH) signalling pathway involved
• Zone of Polarising Activity (ZPA) regulates digit formation
• Duplication from abnormal signalling in limb bud

Polydactyly is a disorder of anterior-posterior (radial-ulnar) axis patterning in the developing limb bud, not a mechanical splitting of a normal digit.

Molecular Basis

• SHH-ZPA axis: The zone of polarising activity at the posterior (ulnar) limb-bud margin secretes Sonic Hedgehog (SHH), establishing the radial-ulnar gradient that specifies digit identity and number.
• ZRS dysregulation: SHH expression is controlled by a distant cis-regulatory element, the ZPA regulatory sequence (ZRS) 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 and the SHH-GLI axis: Loss-of-function in GLI3 (a downstream SHH effector and repressor) underlies postaxial and central polydactyly, including Greig and Pallister-Hall syndromes.
• HOXD13: Expansion/mutation produces synpolydactyly (central duplication fused with syndactyly).

From Signal to Phenotype

• Ectopic or excess SHH activity expands the digit field, generating a supernumerary ray.
• The level at which the supernumerary skeletal element separates (distal phalanx to metacarpal) determines the Wassel type for the thumb.
• Shared versus separate elements (bifid vs duplicated) reflect how early in ray condensation the duplication occurred.
• Because the duplicated ray shares tendons, ligaments and neurovascular structures with the dominant ray, neither thumb is fully normal, which is why simple ablation without soft-tissue rebalancing fails.

Inheritance Correlate

• Isolated postaxial Type B is typically autosomal dominant with variable penetrance (often GLI3-related).
• Preaxial duplication is usually sporadic, but ZRS-related forms (triphalangeal thumb-polysyndactyly) are autosomal dominant.

Thumb Anatomy

• Unique Features: Thumb has 2 phalanges (vs 3 in fingers)
• Triphalangeal Thumb: Wassel VII has 3 phalanges - abnormal
• FPL Insertion: Critical for thumb function - inserts on distal phalanx
• Thenar Muscles: APB, FPB, OP provide thumb opposition

Key Anatomical Considerations in Thumb Duplication

• Metacarpal: May be single (Wassel I-V) or duplicated (Wassel VI)
• Growth Plates: Preserve physis during surgery to maintain growth
• Collateral Ligaments: Essential for joint stability - MUST reconstruct
• Neurovascular Bundle: Each thumb has its own digital nerves/arteries

Biomechanics of the Thumb

• Opposition: Unique to thumb - enables pinch grip
• Stability: UCL and RCL provide MCP joint stability
• Angular Deformity: Occurs if asymmetric ligament support
• Growth: Unequal growth can cause progressive deformity

Postaxial Anatomy

• Small Finger UCL: Critical for power grip
• Hypothenar Muscles: ADM, FDM, ODM provide small finger function
• Type A: Has articulation with 5th metacarpal
• Type B: Minimal bone - mainly skin bridge

History

• Family history: Especially for postaxial (autosomal dominant)
• Pregnancy history: Maternal diabetes, teratogens
• Syndromic features: Cardiac, renal anomalies
• Functional concerns: Parents' goals, cosmetic concerns

Physical Examination

Key Examination Points for Thumb Duplication

1. Which thumb is dominant? (usually ulnar)
2. FPL insertion: Which thumb has better flexor function?
3. Thenar muscles: Which side has better bulk?
4. Stability: MCP and IP joint stability
5. Size comparison: Often radial thumb is hypoplastic

Imaging

• X-ray: Essential for surgical planning
  • Level of bifurcation/duplication
  • Bony anatomy of each digit
  • Joint articulation
  • Metacarpal morphology

When to Consider Further Investigation

• Cardiac echo: Holt-Oram syndrome (radial anomalies + ASD/VSD)
• Renal ultrasound: Associated GU anomalies
• Genetic testing: If syndromic features

Associated Syndromes

Differential Diagnosis

Not every extra or broad digit is a simple duplication. The following entities must be distinguished because they change the operation and the counselling.

Early Complications

• Wound infection: 1-2% incidence
• Flap necrosis: Avoid excessive tension
• Nerve injury: Digital nerve at risk
• Vascular compromise: Ensure perfusion intraoperatively

Late Complications

Revision Surgery Requirements

• 15-20% may need secondary procedures
• Collateral ligament reconstruction most common
• Corrective osteotomy for angular deformity
• Tendon rebalancing
• Web space deepening

Immediate Postoperative (0-2 weeks)

• Splinting: Thumb spica or volar splint
• Wound care: Keep clean and dry
• Elevation: Reduce swelling
• Pain management: Paracetamol typically sufficient

Early Phase (2-6 weeks)

• Suture removal: 2 weeks postoperatively
• Splint weaning: Gradual reduction
• Gentle ROM: Passive then active
• Monitor for: Infection, stiffness

Late Phase (6 weeks to 6 months)

• Full ROM exercises: Encourage normal use
• Strengthening: Age-appropriate activities
• Monitor growth: Assess for angular deformity
• Occupational therapy: If functional concerns

Long-term Follow-up

• Monitor through growth: Annual assessments until skeletal maturity
• Assess stability: Check ligament integrity
• Functional assessment: Pinch, grip strength
• Consider revision: If progressive deformity or instability

Expected Outcomes

Thumb Duplication:

• Good functional outcomes in most cases
• Some residual instability common
• May have slightly smaller thumb than normal side
• May need secondary procedures (15-20%)

Postaxial Polydactyly:

• Excellent outcomes with proper technique
• Type B ligation: Risk of neuroma, bump
• Type A excision: Low complication rate

Prognostic Factors

Patient Satisfaction

• High satisfaction rates overall
• Cosmetic concerns more common than functional
• Realistic expectations important
• Secondary procedures may be needed

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

• Nursery suture ligation vs formal excision (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.
• Management of 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 vs 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.
• Limitations 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; 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.

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.