Syndactyly

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.

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

Classification of Syndactyly

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.

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

Diagnosis of Syndactyly

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.

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

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

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

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:

Timings

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.

Technique  

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.

Commisure Flap Design

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

Dorsal:

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

Volar:

• 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.

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

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

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.

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