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.

Definition of Syndactyly

Syndactyly is a failure of differentiation that results in the fusion of interdigital soft tissue and bones. It is classified as simple or complex, complete or incomplete.

Definition of Syndactyly

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 or accessory bones/
  • 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.

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: Synostosis (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 syndactyly
  • Carpenter syndrome
  • Acrocephalopolysyndactyly
  • Constriction ring syndrome (distal digital fusion, proximal digit fenestrations)
  • Familial syndactyly is associated with syndactyly of the second and third toes

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 school-age 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: do all releases before school age
  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.
  5. Bilateral: simultaneously if < 18 months (less active) or staged if older.


  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, consistency, no hair, less contracture.
  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

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.

Flap Design

For the commissure flap,

  • Dorsal: begins at the level of the metacarpal heads and ends two-thirds of the length of the proximal phalanx.
  • Volar: a rectangular flap is fashioned to resurface the proximal area of a digit adjacent to the commissure. The proximal transverse incision represents the level of commissure reconstruction, and the distal transverse edge equals the length of the dorsal commissure flap.
  • Subsequently, interdigitating zigzag dorsal and palmar flaps are constructed distal to the dorsal commissure and palmar rectangular flap (Fig 7). The dorsal zigzag incision begins at one distal corner of the commissure flap, whereas the palmar starts at the opposite corner of the rectangular flap.

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.


The web space can be a challenging anatomical area in syndactyly. Here are some general principles:

  • Reconstruct with dorsal fasciocutaneous flaps
  • Location defined by the level of the bifurcating neurovascular bundles
  • It can be deepened with multiple z-plasty
  • If all the web spaces are involved, the release of the first and third web spaces first

In the thumb-index webspace:

  • Mild: four-flap Z-plasty 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


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.

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