Polydactyly is the radial, ulnar or central duplication of digits. This article details the classification, treatment and aetiology of this congenital condition.

Summary Card

Congenital duplication of digits that can be radial, ulnar or central.

Radial Polydactyly
A pre-axial duplication of the thumb classified by Wassell.

Ulnar Polydactyly
Post-axial duplication of little finger classified by Stelling.

Definition of Polydactyly

Polydactyly is a congenital duplication of a finger. It can be pre-axial (radial), central and post-axial (ulnar) and is caused by disruption at the ZPE and AER embryological signalling zones.

Definition of Polydactyly

Tip: Polydactyly is classified by Swanson's congenital hand anomalies as "duplication".

Overview of Radial Polydactyly

Key Point

A pre-axial duplication of the thumb classified by Wassell.

Radial (pre-axial) polydactyly is an isolated thumb duplication that is generally unilateral and non-syndromic. From a demographic perspective, it is different to ulnar polyadactyly becuase:

  • It is usually sporadic
  • It primarily affects Caucasions
  • Patient's should be screened for Fanconi anaemia

Radial Polydactyly

Classification of Radial Polydactyly

Radial polydactyly is a duplication of the thumb. It is classified using by Wassell into 7 groups based on the degree of bony duplication. Type IV is the most common radial polydactyly.

  • Type I: Bifid Distal Phalanx
  • Type II: Duplicate Distal Phalanx
  • Type III: Bifid Proximal Phalanx + Duplicate Distal Phalanx
  • Type IV: Duplicate Distal Phalanx + Duplicate Distal Phalanx
  • Type V: Bifid 1st metacarpal + Duplicate Distal and Proximal Phalanx
  • Type VI: Duplicate 1st metacarpal + Distal Phalanx + Proximal Phalanx
  • Type VII: Triphalangeal thumb (triangular or rectangular extra middle phalanx) + normal thumb

Diagnose triphalangeal thumb by excluding a five-finger hand. Look at the nail position (not in normal pronation) and x-ray (extra finger metacarpal). Surgery includes pollicisation and web space creation.

Tip: Radial duplicate usually receives the thenar muscles and is median nerve innervated. Ulnar duplicate receives the adductor pollicis, 1st dorsal intraosseous muscle and ulnar innervated.

This Wassell classification of radial polydactyly is illustrated below.

Wassell Classification for Radial Polydactyly

Fun Fact: There is often a degree of hypoplasia in both digits. A subtle type I or II duplications may show signs of a widened nail plate.

Management of Radial Polydactyly

Key Principles

Surgery aims to create an anatomically aligned thumb with joint stability that enables function. Ideally, it is performed prior to developing the thumb-pinch index at age 1.

A decision should be made on which duplicate to retain. This can be guided by:

  • Size: one duplicate is usually hypoplastic or rudimentary
  • Function: ulnar collateral ligament is important for pinch
  • Movement: passive and active range of movement
  • Radiology: location of duplication

The Wassell classification can "equal parts" or "spare parts" surgery.  

"Equal parts surgery"

The Bilhaut-Cloquet Technique This is indicated in Types I + II  or convergent and symmetrical type IV polydactyly. Excise adjacent inner halves of each duplicate and fuse the remaining outer segments.

  • Standard Technique: simple wedge excision of structures
  • Modified Technique: preservation of one nail complex and physis of one of the duplicates.

Nail problems, epiphysiodesis and reduced range of movement limit this technique.  

This is illustrated in the image below

"Spare Parts Surgery"

This is indicated in types III and IV or asymmetrical I and II. Excise one duplicate and reconstruct the other using "spare parts surgery" in a single operation. This should be done before the patient turns 1 year of age before they develop the thumb-index pinch.

The key steps to my operation are:

  • Incision: avoid straight lines to prevent full extension/flexion, and the nail bed to reduce the risk of nail plate ridging.
  • Excision: excise radial duplicate (proximal and smaller than ulnar digit)
  • Ligaments: reconstruct radial collateral ligament of IPJ with a periosteal slave from the excised digit (Manske), preserve RCL of thumb MCPJ.
  • Tendons: realign EPL insertion to form a single tendon, centralise FPL by reconstructing the pulley, transfer APB insertion from radial duplicate
  • Bone: excise asymmetric joint surfaces, closed wedge osteotomy of proximal phalanx, trans-articular 0.6 or 0.7mm K-wire for 4 weeks.

These patients can be discharged same day if the patient is well and followed up in a specialist hand therapy clinic.

Additional Techniques

  • Types V + VI also requires web space reconstruction, intrinsic reconstruction, basal joints and metacarpal osteotomies.
  • Type VII requires treatment of the extra interphalangeal joint. If a delta phalanx is present, it is excised, and soft tissue is reconstructed. If the extra phalanx is rectangular, chrondresis of the least mobile joint.

Tip: Ideally, the ulnar thumb should be preserved as the ulnar collateral ligament is required for pinch stability. 

The surgical correction of radial polydactyly is shown in the video below.

Fun Fact: The radial duplicate is usually more hypoplastic and smaller than the ulnar. 

Ulnar Polydactyly

Key Point

Post-axial duplication of little finger classified by Stelling.

Ulnar post-axial polydactyly is a congenital duplication of the little finger digits. Unlike radial polydactyly, it is commoner in African American males and is more often bilateral. It is usually autosomal dominant with variable penetrance.

Fun Fact: Polydactyly is linked to a HOX and Gli3 gene mutation resulting in the disruption of the AER and APZ signalling pathways. This is discussed in more detail in the upper limb embryology topic.

Classification of Ulnar Polydactyly

Ulnar polydactyly can be described using Stelling's classification. It also guides management.

  • I: "Nubbin" - soft tissue without bone excised or ligated as a neonate.  
  • II: extra digit is structurally complete and articulates with normal or bifid metacarpal or phalanx.
  • III: extra digit is structurally complete and has its metacarpal.

Types II and III patients require resection and reconstruction. This involves the abductor digiti minimi and ulnar collateral ligament attachments to be transferred to the adjacent finger.

An alternative classification can be Type A (well-developed supernumerary digit) or Type B (digit is rudimentary and pedunculated).

Ulnar Poydactyly X-Ray

Fun Fact: Central poylydactyly does exist, but it is rare. It usually involves the ring finger and can occur with syndactyly to create a 'synopolydactyly'. It has been linked to a gene mutation (HOXD13 gene) on Chromosome 2.

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