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Exams and Signs

Historical Overview

  • Thumb hypoplasia is a term used to describe a spectrum of clinical abnormalities that range from a slightly small digit to complete aplasia of the thumb.1
  • Müller proposed a classification system for thumb hypoplasia in 1937 as a “teratologic sequence,” which was later refined and detailed in 1989 by Buck-Gramcko to include five reasonably distinct categories for the condition.2
  • Type IV thumb hypoplasia is often referred to as pouce flottant, or “floating thumb.” These cases are characterized by a short, unstable thumb that contains a hypoplastic proximal and distal phalanx, and total or subtotal aplasia of the metacarpal.3,4


  • An observation of floating thumb indicates that patient has type IV thumb hypoplasia, in which the thumb only has a few rudimentary bony or soft tissue structures and typically lacks intrinsic and/or extrinsic muscles.2,4


  • In the floating thumb patient, the thumb usually contains two rudimentary phalanges and is attached to the hand only by a narrow soft tissue skin pedicle, which has aneurovascular bundle coursing through it. The remainder of the thumb osteoarticular column is absent, and there are no musculotendinous structures or osseous elements joining it to the hand. As a result, there is no voluntary muscle control of the thumb.2-5
  • The exact embryologic insult that leads to the clinical manifestation of thumb hypoplasia remains unclear, but the underlying mechanism may be related to an insult of mesenchymal cell populations in the limb bud. Thumb hypoplasia is seen in association with radial longitudinal deficiency, and some congenital anomalies with radial deficiencies can occur with the deletion of chromosome 22q11, but no single gene or enzymatic defect has been shown to entirely account for its development.1


  1. Obtain an accurate and complete patient history.
  2. Perform an examination and evaluation of the affected thumb.
  3. Take note of the thumb’s size relative to the little finger.
  4. Observe the thumb’s resting rotational posture.
  5. Assess the thumb’s range of motion (ROM), grasp, and opposition, if possible. 

Related Signs and Tests1,3

  • Conditions related to thumb hypoplasia:
    • Radial longitudinal deficiency
    • Holt-Oram syndrome
    • Thrombocytopenia-absent radius syndrome
  • Genetic testing
  • X-ray
    • Radiographs of both hands, wrists, and forearms should be obtained to look for other associated upper extremity anomalies and to demonstrate the degree of metacarpal and phalangeal hypoplasia.

Diagnostic Performance Characteristics

  • The affected hand may also be devoid of a trapezium and scaphoid, and the proximal and distal phalanges may be only cartilage remnants. A thumbnail is usually present and developed to an extent similar to the general development of the entire distal thumb unit.2
Presentation Photos and Related Diagrams
Pouce Flottant
  • Right Pouce Flottant
    Right Pouce Flottant
  • Left Pouce Flottant
    Left Pouce Flottant
Definition of Positive Result
  • A positive result occurs when the clinician observes that the affected thumb is attached to the hand only through a thin skin pedicle that lacks musculotendinous structures and consequently prevents the patient from voluntarily moving the thumb.
Definition of Negative Result
  • A negative result occurs when the clinician observes that the thumb is connected to the hand with normal attachments.
Comments and Pearls
  • Thumb hypoplasia is a component of radial dysplasia, and is commonly seen either alone or in conjunction with other conditions associated with radial longitudinal deficiency.1
  • The origin of thumb hypoplasia is along the radial midaxial line of the hand, and quite distal. The resultant appendages are variable in size.2
Diagnoses Associated with Exams and Signs
  1. Riley, SA and Burgess, RC. Thumb hypoplasia. J Hand Surg Am 2009;34(8):1564-73. PMID: 19801112
  2. Kleinman, WB. Management of thumb hypoplasia. Hand Clin 1990;6(4):617-41. PMID: 2269676
  3. Light, TR and Gaffey, JL. Reconstruction of the hypoplastic thumb. J Hand Surg Am 2010;35(3):474-9. PMID: 20138713
  4. Ashbaugh, H and Gellman, H. Congenital thumb deformities and associated syndromes. J Craniofac Surg 2009;20(4):1039-44. PMID: 19625838
  5. Manske, PR and Goldfarb, CA. Congenital failure of formation of the upper limb. Hand Clin 2009;25(2):157-70. PMID: 19380058
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