Syndactyly is the fusion of soft tissue and bone of the finger. This article details the aetiology, diagnosis, and treatment of syndactyly.

Summary Card

A failure of differentiation fuses soft tissue ± bone of the finger

This can be classified as incomplete or complex, simple or complex.

A clinical diagnosis. Bony involvement is confirmed on the X-Ray.

Before the age of 2. Earlier if length discrepancy or border digits.

Evidence-based flashcards based on active recall and spaced repetition.

Definition of Syndactyly

Syndactyly is a congenital hand malformation defined as a "failure of differentiation" by Swanson or a "malformation of unspecific axis" according to OMT classification.

It can be classified as simple (soft tissue), complex (soft tissue and bone), complete (fusion of adjacent nails) or incomplete.

Definition of Syndactyly

A failure in apoptosis results in the fusion of interdigital soft tissue and bones during weeks 6-8 of gestation. This is the result of activation, suppression and overexpression.  

  1. WNT is activated
  2. BMP and Retinoic acid suppression
  3. FGF-8 is overexpression in the AER

Fun Fact: Isolated syndactyly is an autosomal dominant condition with variable penetrance.

Classification of Syndactyly

Key Point

Syndactyly is classified as complete or incomplete, simple or complex. This is related to the extent of fusion and structures involved.

Syndactyly is classified by the extent of the fusion (entire finger or partial) and the structures (bone or soft tissue) involved.

  • Simple: no synostosis (soft tissue only)
  • Incomplete: fusion does not involve distal phalanx or overlying soft tissue.
  • Complete: fusion involves the distal phalanx and/or overlying soft tissue.
  • Complex: soft tissue with adjacent phalangeal fusion (synostosis).
  • Complicated: accessory digits or phalanges, additional cleft hand
  • Acrosyndactyly: shortened digits fused distally with proximal fenestration.

This classification is illustrated in the diagram below.

Classification of Syndactyly

Fun Fact: Acrosyndactyly is a re-fusion of the distal tips that is thought to occur due to constriction ring syndrome.

Several phenotypic subtypes can result in synopolydactyly, a fusion of border digits and even polydactyly of all digits.

Diagnosis of Syndactyly

Key Point

Syndactyly is a clinical diagnosis that can occur in isolation or with an association syndrome. A plain X-ray will confirm synostosis.

Clinical Findings

Syndactyly is a clinical diagnosis. It most commonly occurs in the middle-ring fingers and can be associated with other structural abnormalities.

It typically occurs in caucasian males with family history and can be bilateral in ~50% of cases.

Features of Syndactyly

The important pathological features of syndactyly include:  

  • Bone: degrees of synostosis, joints usually preserved unless symphalangism.
  • Ligaments: Cleland's and Grayson's ligaments thickened and coalesced.
  • Tendons: duplicated or shared flexor and extensor tendons, sheaths
  • Neurovascular: duplicate or shared vessels and nerves
  • Skin: short of skin

Tip: Synonychia (nail fusion) is a clinical sign suggestive of distal phalanx fusion.

Radiological Findings

Associated Features

Syndactyly can be associated with other structural and craniofacial conditions. This includes:

  • Poland Syndrome: chest wall abnormalities and symbrachydactyly
  • Apert Syndrome: craniosynostosis and complex, bilateral syndactyly with symphalangism.
  • Carpenter syndrome
  • Acrocephalopolysyndactyly
  • Constriction ring syndrome (distal digital fusion, proximal digit fenestrations)
  • Familial syndactyly is associated with syndactyly of the second and third toes
  • Timothy Syndrome: long Q-T can lead to sudden cardiac arrest

Management of Syndactyly

Key Point

Syndactyly is generally treated in the first 24 months. This is influenced by the finger(s), webspace, and length discrepancies

Patient care should be individualised. In the immediate setting, patient education and family counselling are essential. This should all be managed by a multi-disciplinary team.

Timing of surgery

Syndactyly release should be performed by after the age of 1 and ideally before age 2. The exact timing is influenced by the type of finger and webspace involvement.  

  • Earlier surgery if border digits are involved or a length discrepancy (~6 months)
  • Later surgery if "similar" digits are involved (e.g. ring-long, long-index) as a larger hand enables a technically more straightforward operation with more predictable results

Fun Fact: Tethering from a length discrepancy leads to flexion contractures and rotational deformities of the longer digit

Principles of Surgery  Techniques

Surgery aims to separate digits, create minimal scarring and respect the web space. This is performed through a combination of flaps and grafts adhering to the following principles:


  1. Age: between 1-2 years or ~6 months if fusion of unequal digits.
  2. Length: release digits with length discrepancy first.
  3. Location: release border digits first (ring-little, thumb-index).
  4. Multiple digits: staged release to avoid vascular issues ~3-6 months apart.
  5. Bilateral: simultaneously if < 18 months (less active) or staged if older.

Tip: If syndactyly of index-middle-ring-small digits, release index-middle and ring-small first, and do central syndactyly (middle-ring) 6 months later.


  1. Incisions: dorsal and volar interdigitating zigzag 60° flaps to the midline whilst preserving the paratenon.
  2. Flaps: dorsal and volar random-pattern proximal flaps for webspace.
  3. Graft: FTSG from wrist crease for colour, no hair, less contracture. Incomplete syndactyly can often be closed with local flaps only.
  4. Nailfold: "stilleto flap" is a hyponychial interdigitating flap described by Buck-Gramcko or Hentz's "pulp plasty" with a composite graft.
  5. Sutures: use absorbable sutures
  6. Defatting: reduce tension and graft requirements, increase vascular injury

The management plan is similar for complex syndactyly except that flaps not grafts should cover any bone or joints. In patients with syndromic syndactyly, for example Aperts Syndrome, symphalangism can also be present. As a result, a zig-zag incision can be replaced with a straight-line incision.

Tip: A skin graft is required because the circumference of two separated digits measured separately is nearly 20% greater than the circumference of two conjoined digits.

Commisure Flap Design

There a large number of flap design for syndactyly reconstruction. This is one technique:


  • A "dorsal commisure flap"
  • Aim: reconstruct the commisure.
  • Design: metacarpal head to distal 2/3 proximal phalanx.


  • A "volar resurfacing rectangular flap"
  • Aim: resurface proximal end of the digit that is adjacent to the commissure.
  • Design: a proximal transverse incision that represents the level of commissure reconstruction, and the distal transverse edge equals the length of the dorsal commissure flap.
  • This can also be performed as a "T-shaped" flap.

Tip: The width of these flaps does not cross the midline (similar to the zig-zag incisions) 

In the thumb-index webspace:

  • Mild: four-flap Z-plasty (120 degrees)  to access to adductor & first dorsal interosseous muscles (may require a partial release for adequate web-space breadth)
  • Complete: modified dorsal rotation advancement flap by Abdel Ghani

Tip:  Dorsal skin is thinner and easier to mobilize than the glabrous palmar skin. The dorsal flap also recreates the normal dorsal-to-palmar commissure slope.

Contraindications for Syndactyly Release

Each patient should be managed as an individual. Generally speaking, relative contradindications includes:

  • Systemic comorbidities that make surgery unsafe
  • A severe complex or complicated syndactyly that could result in a poor functional status
  • Abnormal vasculatur that could compromise finger viability
  • Mild syndactyly that has no functional deficit.
  • A hypoplastic digit that would become functionless if seperated from a functional digit it is fused to.


Complications of syndactyly release can be intra-operative or post-operative and relate to the surgery or the anaesthetic.

  • Peri-operative: nerve, tendon, artery or vein injury; haematoma
  • Wound: infection, dehiscence
  • Scarring: hypertrophy, contracture, web creep (commissure pulled distally)
  • Bone: joint instability due to insufficient collateral ligaments
  • Nail: nail deformities

Venous complications are more common and often related to too tight a closure.

Fun Fact: Early web creep is a wound issue (flap or graft loss). Late creep is caused by discrepant growth between scar/skin graft and surrounding tissue during the growth spurt. Mild migration does not require treatment, whereas substantial creep requires revision surgery.


Evidence-based flashcards are designed for active recall & spaced repetition using the Feynman Technique.

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