Musculoskeletal: polydactylism in Horses (Equis) | Vetlexicon
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Musculoskeletal: polydactylism

ISSN 2398-2977

Contributor(s) :

Synonym(s): Supernumerary digits


  • Rare but the most commonly reported congenital phalangeal anomaly in the horse.
  • 80% of cases occur in the forelimb.
  • Cause: teratological or ? heredity. Transmission to offspring not proven.
  • Signs: partial or complete duplication of the digit, self-evident, interference injury to opposite limb; may be of more cosmetic than functional importance.
  • Diagnosis: signs, radiography.
  • Treatment: none; surgical removal.
  • Prognosis: teratological: poor; atavistic: good.

Presenting signs

  • Polydactylism is defined as the occurrence of more digits than typical for a certain species.
  • Diagnosis is self-evident.

Age predisposition

  • Neonate.



  • Teratological: unknown cause.
  • Atavistic: mode of inheritance unknown: ? autosomal dominant gene with incomplete penetrance (humans).


  • Congenital defect. 
  • Usually an isolated defect in the horse, cf humans, other domestic species.
  • Teratological:
    • Duplication distal to fetlock.
    • 2 separate digits articulate with MC3/MT3.
    • Distal MC3/MT3 may or may not be divided.
    • Occasionally fusion between digits (P1).
  • Atavistic:
    • Most commonly on medial aspect of forelimb.
    • Most commonly the third metacarpal is duplicated, complete with functional metacarpal joint and phalanges, vascular and nerve supply, flexor/extensor tendons, sesamoid bones, navicular bone and hoof + second metacarpal bone is atrophied or present over entire length of duplicated digit.
    • Usually no direct connection between the two fetlock joints.
    • Second metacarpal bone is less commonly duplicated. It may be duplicated along its entire length terminating in either a complete or an atrophied digit.
  • Self-induced trauma is common due to interference from the supernumerary digit with the opposite limb.


Client history

  • Present from birth.

Diagnostic investigation


Initial symptomatic treatment

  • Atavistic form:
    • No treatment may be required.
    • Surgical removal if needed for functional or cosmetic reasons.
  • Teratological form: no treatment possible.
  • Treatment of interference injuries.


Take care to conserve vital neurological and vascular structures of the normal digit during amputation of the supernumerary digit.

  • Incise the skin at base of supernumerary digit.
  • Identify the vascular and nervous supply to the supernumerary digit and carefully dissect back to their origins with the main supply vessels/nerves.
  • Ligate close to the main supply vessels/nerves and transect.
  • Identify and dissect out the extensor/flexor tendons to the supernumerary digit.
  • Sever close to their origins at the tendons of the normal digit.
  • Incise and elevate the periosteum on the supernumerary digit.
  • Remove supernumerary digit close to the normal digit using an osteotome.
  • Smooth remaining sharp edges of cortex using a curette.
  • Suture periosteum over the exposed bone and marrow cavity.
  • Close skin and subcutaneous tissues routinely.




  • No proven heredity but it would seem advisable to avoid breeding from affected animals.



  • Teratological form: poor for extensive deformity, euthanasia recommended.
  • Atavistic form: depends on number and extent of involvement of extra digits with the limb. Extensive resection through bone or simply soft tissue dissection may be required.
  • If a normal limb is achieved the outlook is good and the cosmetic result can be excellent.
  • Lameness following surgical amputation is rare.

Reasons for treatment failure

  • Wound breakdown or infection problems.
  • Overly heroic surgery in very severely affected animals.
  • Cosmetic appearance may be improved but abnormality in the area may still be evident.

Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Stanek C & Hantak E (1986) Bilateral atavistic polydactyly in a colt and its dam. Equine Vet J 18, 76-79 PubMed.

Other sources of information

  • Welsh R D (1999) Polydactylism. In: Equine Medicine and Surgery. 5th edn. Eds: Colahan P T, Merritt A M, Moore J N & Mayhew I G. Mosby, USA. pp 1586.