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Supernumerary Digit

  • Author: Luke Lennox, MD; Chief Editor: Dirk M Elston, MD  more...
Updated: Mar 29, 2016


Polydactyly is the most common congenital digital anomaly of the hand and foot. It may appear in isolation or in association with other birth defects. Isolated polydactyly is often autosomal dominant, while syndromic polydactyly is often autosomal recessive.[1]  See the image below.

Supernumerary digit, no magnification. Supernumerary digit, no magnification.


Early theories for polydactyly concerned disorders in the programmed cell death cycle of fetal limb development. Current theories focus on mutations in specific genetic locations that cause limb development to go awry.[2]

Many specific mutations have been linked to polydactyly; however, a molecular etiology has not been found in a third of human disorders associated with polydactyly.[3]

Mammals have been shown to have genetic clusters identified as homeobox or Hox genes corresponding to five domains across the limb bud. According to Muragaki et al,[4] mutations in the HOXD13 gene are associated with synpolydactyly.

The steps in limb development and outgrowth are controlled by at least two described signal centers, as follows[3] :

  1. Zone of polarizing activity (ZPA) - Sonic Hedgehog is a molecule found to mediate ZPA activity
  2. Apical ectodermal ridge (AER) - Expresses fibroblast growth factors

As limb growth in utero progresses along a preset timeline, elongation of the limb, development of soft tissue, and differentiation of digits progresses.

The Medscape Genomic Medicine Research Center may be of interest.




United States

Despite being a common malformation, the true incidence of polydactyly is not fully known. The epidemiologic data on polydactyly is limited because most birth defect registries do not include the entity. One study by Finley et al[5] combined data from Jefferson County, Alabama and Uppsala County, Sweden. This study showed incidence of all types of polydactyly to be 2.3 per 1000 in white males, 0.6 per 1000 in white females, 13.5 per 1000 in black males, and 11.1 per 1000 in black females. The Swedish data alone showed polydactyly of all types to have an incidence of 1.0 per 1000, equally distributed between males and females.


Genetic and ethnic factors seem to influence the nature of polydactyly in the world population. Unfortunately, the knowledge of polydactyly is limited to select studies of certain subpopulations. Preaxial polydactyly is very common in various Asian populations, accounting for 90% of cases in South China, Hong Kong, and Japan. Duplicated thumbs at the metacarpophalangeal level are the usual anomaly.[6, 7]


Postaxial hand polydactyly is a common isolated disorder in black children, and autosomal dominant transmission is suspected. In contrast, postaxial polydactyly seen in white children is usually syndromic and associated with an autosomal recessive transmission. Postaxial polydactyly is approximately 10 times more frequent in blacks than in whites and is more frequent in male children. 

Other factors associated with postaxial hand polydactyly include twinning, low maternal education, parental consanguinity, occurrence in first-degree relatives, and maternal bleeding in the first trimester.

Contributor Information and Disclosures

Luke Lennox, MD Resident Physician, Department of Medicine, University of Buffalo, State University of New York School of Medicine and Biomedical Sciences

Disclosure: Nothing to disclose.


Thomas N Helm, MD Clinical Professor of Dermatology and Pathology, University of Buffalo, State University of New York School of Medicine and Biomedical Sciences; Director, Buffalo Medical Group Dermatopathology Laboratory

Thomas N Helm, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Dermatology, American Society for Dermatologic Surgery, American Society of Dermatopathology

Disclosure: Nothing to disclose.

Specialty Editor Board

Richard P Vinson, MD Assistant Clinical Professor, Department of Dermatology, Texas Tech University Health Sciences Center, Paul L Foster School of Medicine; Consulting Staff, Mountain View Dermatology, PA

Richard P Vinson, MD is a member of the following medical societies: American Academy of Dermatology, Texas Medical Association, Association of Military Dermatologists, Texas Dermatological Society

Disclosure: Nothing to disclose.

Robert A Schwartz, MD, MPH Professor and Head of Dermatology, Professor of Pathology, Pediatrics, Medicine, and Preventive Medicine and Community Health, Rutgers New Jersey Medical School; Visiting Professor, Rutgers University School of Public Affairs and Administration

Robert A Schwartz, MD, MPH is a member of the following medical societies: Alpha Omega Alpha, New York Academy of Medicine, American Academy of Dermatology, American College of Physicians, Sigma Xi

Disclosure: Nothing to disclose.

Chief Editor

Dirk M Elston, MD Professor and Chairman, Department of Dermatology and Dermatologic Surgery, Medical University of South Carolina College of Medicine

Dirk M Elston, MD is a member of the following medical societies: American Academy of Dermatology

Disclosure: Nothing to disclose.

Additional Contributors

Albert C Yan, MD Section Chief, Associate Professor, Department of Pediatrics, Section of Dermatology, Children's Hospital of Philadelphia and University of Pennsylvania School of Medicine

Albert C Yan, MD is a member of the following medical societies: American Academy of Dermatology, Society for Investigative Dermatology, Society for Pediatric Dermatology, American Academy of Pediatrics

Disclosure: Nothing to disclose.

Carter G Abel, MD Clinical Assistant Attending, Department of Dermatology, New York Presbyterian Hospital

Carter G Abel, MD is a member of the following medical societies: American Academy of Dermatology, American College of Mohs Surgery, American Society for Dermatologic Surgery, Medical Society of New Jersey

Disclosure: Nothing to disclose.

Denise M McCarthy, MD Associate Professor, Department of Radiology, Morristown Memorial Hospital

Denise M McCarthy, MD is a member of the following medical societies: American Association for Women Radiologists, American Roentgen Ray Society, Association of University Radiologists, Radiological Society of North America

Disclosure: Nothing to disclose.

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  16. Ban M, Kitajima Y. The number and distribution of Merkel cells in rudimentary polydactyly. Dermatology. 2001. 202(1):31-4. [Medline].

  17. Eskandari MM, Oztuna V, Demirkan F. Late psychosocial effects of congenital hand anomaly. Hand Surg. 2004 Dec. 9(2):257-9. [Medline].

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Twenty-eight-day-old supernumerary digit, rudimentary nail unit cartilage (2x).
Twenty-eight-day-old supernumerary digit, nerve bundle base (40x).
Twenty-eight-day-old supernumerary digit, connective tissue and epidermis (40x).
Supernumerary digit, no magnification.
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