Syndactyly

Syndactyly is failure of digital separation during embryological development, representing the most common congenital hand anomaly.

Epidemiology

• Incidence: 1 in 2,000 live births
• Most Common Location: 3rd web space (long-ring fingers)
• Bilaterality: 50% are bilateral
• Male:Female: 2:1 male predominance
• Inheritance: Can be sporadic or autosomal dominant

Embryology

• Digital rays form at 4-5 weeks gestation
• Interdigital separation (apoptosis) occurs at 7-8 weeks
• Syndactyly results from failure of programmed cell death
• Sonic Hedgehog and WNT signalling pathways involved

Associated Syndromes

The single most important operative-anatomy point (and a stock viva follow-up) is that the neurovascular bifurcation in a syndactylised web sits more DISTALLY than normal, and this dictates how deep a commissure you can safely create.

In complete syndactyly always assume the supply is aberrant: identify the bifurcation early, and if it is too distal, choose between accepting a slightly shallower commissure, ligating one branch, or staging - never blindly dissect proximally through the artery.

Embryological Development

Digital separation occurs through programmed cell death (apoptosis) of interdigital tissue between weeks 6-8 of gestation. Syndactyly results from failure of this normal apoptotic process.

Pathophysiology by Type

Third Web Space Predilection

The 3rd web space (long-ring) is most commonly affected because these digital rays separate latest in embryological development (week 7-8), providing longer window for developmental disruption.

History

• Family history: Autosomal dominant inheritance in some forms
• Pregnancy history: Teratogen exposure, maternal diabetes
• Syndromic features: Head shape, chest wall, other anomalies
• Functional concerns: Grip, fine motor development

Physical Examination

Key Examination Points

1. Complete vs Incomplete: Does web reach fingertip?
2. Simple vs Complex: Palpate for bony connection
3. Nail Assessment: Shared nail suggests complex type
4. Border Digit Involvement: Assess for angular deformity
5. Other Hand Anomalies: Polydactyly, camptodactyly
6. Syndromic Features: Head, chest, feet

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Imaging

• X-ray: Essential for surgical planning
  • Assess bony anatomy
  • Identify shared/fused bones
  • Evaluate joint anatomy
  • Detect accessory phalanges

When to Consider Further Investigation

• Genetic Testing: If syndromic features (Apert, Poland)
• Cardiac Echo: Associated cardiac anomalies in syndromes
• CT/MRI: Rarely needed, complex cases only

X-ray Findings by Type

Differential Diagnosis

Not every pair of digits held together is true syndactyly. Distinguish these on examination and radiograph.

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A favourite "explain the principle" question: why can a conjoined web almost never simply be sewn back as two digits?

• The geometry: two separated digits present a far greater combined skin circumference than the flat, fused mass they came from. Separating them therefore creates an obligatory skin shortfall - there is simply not enough skin to wrap both digits and line the new commissure.
• What the flaps do: the dorsal commissure (rectangular or hourglass) flap resurfaces the new web at the correct level, and the interdigitating zigzag flaps resurface part of each digit while breaking up the scar line.
• What is left over: the residual raw areas - typically on the mid-lateral sides of the digits - are covered with full-thickness skin graft.

This is also why the graft-free debate hinges on web extent: a narrow incomplete web has a small deficit that local and dorsal flaps can cover alone, whereas a complete (and especially complex) web has a large deficit that still generally needs FTSG for tension-free closure.

Early Complications

• Vascular compromise: From releasing both sides of digit
• Flap necrosis: Tension, poor design
• Graft failure: Infection, haematoma, poor bed
• Infection: Standard surgical risk

Late Complications

Revision Surgery Indications

• Significant web creep
• Scar contracture limiting function
• Angular deformity
• Nail deformity
• Growth-related changes

Global Epidemiology

• Syndactyly is among the most common congenital hand anomalies worldwide, with widely quoted figures around 1 in 2,000-3,000 live births (population estimates vary with ascertainment).
• Registry data place overall congenital hand/upper-limb malformation incidence at roughly 1-3 per 10,000 live births, with simple syndactyly consistently among the top anomalies after radial polydactyly.
• Male predominance and a slight right-side preponderance are reported across populations.
• Around 10-20% of hand-anomaly cohorts carry a syndromic association, so every case warrants a screen for dysmorphic, craniofacial, chest-wall and lower-limb features.

Society Guidance Compared

Syndactyly is managed by surgical principle rather than by a single binding national guideline; the table below summarises how the major bodies frame care.

Registry & Classification Notes

• Modern reporting uses the IFSSH-endorsed Oberg-Manske-Tonkin (OMT) classification, which frames syndactyly within malformations of hand-plate formation/differentiation, replacing purely descriptive systems for research and audit.
• The Withey score is the most widely used outcome instrument for grading post-release web creep, scar and digital deformity, allowing cross-centre comparison.
• No implant is involved, so there is no arthroplasty-style registry; epidemiology relies on birth-defect surveillance programmes (e.g. EUROCAT in Europe) and national hand-surgery audits.

High- vs Limited-Resource Practice Variation

• High-resource settings: elective staged release at specialist paediatric hand units, full-thickness grafting or graft-sparing dorsal flaps, formal hand therapy, silicone and night web-spacers, and longitudinal follow-up to skeletal maturity.
• Limited-resource settings: later presentation is common, raising rates of fixed angular deformity in border digits and reducing the window for graft-free techniques; single-stage, graft-minimising approaches and visiting/charitable surgical teams play a larger role.
• Universal principles (resource-independent): zigzag interdigitating incisions, a dorsal flap to recreate the commissure at the correct level, never releasing both borders of one digit simultaneously, and tension-free coverage to limit web creep.

The traditional teaching is well established, but several points remain genuinely debated and are favourite viva discriminators.