Thumb Hypoplasia

Thumb Hypoplasia

Thumb hypoplasia is congenital undergrowth of the thumb. This article details its diagnosis, classification and treatment.

Summary Card

Congenital thumb "undergrowth" due to a radial longitudinal deficiency

A clinical diagnosis supported by radiology and specific investigations for associated anomalies.

Blath Classification from I (functional small thumb) to V (absent thumb). This guides management.

Stage Reconstruction
Indicated in Blauth II and IUIa for webspace release, UCL reconstruction, opponensplasty and tendon reconstruction

Indicated for Blauth IIIb, IV or V.

Evidence-based flashcards based on spaced repetition and active recall.

Definition of Thumb Hypoplasia

Thumb Hypoplasia is the congenital underdevelopment of the thumb due to a radial longitudinal deficiency. It is classified as "undergrowth" as per the Swanson classification.

Definition of Thumb Hypoplasia

Diagnosis of Thumb Hypoplasia

Key Point

Thumb hypoplasia is a clinical diagnosis supported by radiological investigations. Further investigations may be required for associated conditions.

Clinical Diagnosis

Thumb hypoplasia is a clinical diagnosis based on findings during clinical inspection and assessment of the range of motion.  It can be supported by radiological findings.

  • Skin: absent skin creases, web-space tightness.
  • Ligaments: Excessive abduction of MCP joint in UCL insufficiency
  • Muscles: Hypoplasia of thenar musculature
  • Tendons: Pollex abductus (FPL has extra insertion onto extensor tendon)
  • Movement: UCL laxity, absence of motion at joints (Blauth III)

These clinical features of thumb hypoplasia are illustrated below.

Clinical Features of Thumb Hypoplasia

Radiological Findings

It is recommended to request bilateral films of the hand, wrist and forearm.

  • Can help determine musculotendinous versus osseous deficiencies.
  • Type III B Basal metacarpal aplasia with deficient CMC joint
  • Any associated hypoplastic radius.

Associated Conditions

Thumb hypoplasia can be associated with a diverse group of genetic conditions and syndromes. These include:  

  • VACTERL: Vertebral anomalies, Anal atresia, Cardiac defects, Tracheal anomalies, Esophageal atresia, Renal and radial abnormalities, Limb abnormalities.
  • Holt-Oram: autosomal dominant, upper-limb defects, congenital heart malformation, cardiac conduction disease. Common cardiac defects include atrial or ventricular septal defects and conduction abnormalities or blocks
  • Thrombocytopenia-absent radius: bilateral absence of the radii with the presence of both thumbs and thrombocytopenia.  It can be distinguished from Fanconi anemia by the neonatal onset of thrombocytopenia, which is typically seen later in life with Fanconi anemia, and the absence of pancytopenia.
  • Fanconi anaemia: autosomal recessive condiciton characterised by bone marrow failure and radial limb abnormalities. Other common anomalies include café au lait spots, short stature, genital abnormalities, and microcephaly. Patients usually die of complications from pancytopenia, with a life expectancy of 20-30 years. It can be diagnosed on peripheral blood smear,  complete blood count, chromosomal challenge test

Fun Fact: In the slight majority of patients, thumb hypoplasia is bilateral. 

Classification of Thumb Hypoplasia

Key Point

The Blauth classification is used in thumb hypoplasia to describe the severity of the condition and structures involved. It can be used as a template to guide management.

The Blauth Classification is used in thumb hypoplasia. It stratifies patients based on the degree of hypoplasia, collateral ligament, muscle and tendon pathology.

  1. Blauth I: functional hypoplastic thumb, normal neurovascular bundles,
  2. Blauth II: UCL insufficiency, webspace contracted, thenar hypoplasia.
  3. Blauth IIIa: type II + abnormal extrinsic tendons + stable MCPJ
  4. Blauth IIIb: type II + abnormal extrinsic tendons + unstable MCPJ
  5. Blauth IV: floating thumb (pouce flottant) attached by a soft tissue bridge.
  6. Blauth V: absent Thumb

This classification subsequently guides the reconstructive pathway for conservative, staged procedures or index pollicisation. Here are some general principles

  1. Blauth I: if good function then does not require surgery
  2. Blauth II + IIIA: staged procedure for each abnormality
  3. Blauth IIIb: Pollicisation

Fun Fact: Thumb hypoplasia can be associated with partial or complete absence of the radius, and patients should be screened for Fanconi's anaemia.

Staged Reconstruction of Thumb Hypoplasia

Key Point

Staged reconstruction is indicated in patients with a stable MCPJ (Blauth II, IIIa). It involves first webspace release, UCL reconstruction, opponensplasty, tendon reconstruction.

Thumb hypoplasia is usually surgically corrected by age 2 when indicated. A staged correction is indicated for Type II and IIIB thumb hypoplasia. This involves reconstruction of webspace, MCPJ UCL insufficiency, hypothenar hypoplasia and extrinsic tendons.

First Webspace Release

UCL Reconstruction

UCL deficiency may be primary or secondary to a pollex abductus (FPL inserts into the distal phalanx and the EPL). This can be treated by

  • Fusion
  • UCL reconstruction with a tendon graft (if UCL remnant present)
  • UCL reconstruction with FDS opponensplasty.


The aim of this stage is to augment the hypoplastic hypothenar muscles. Options for this opponensplasty include

  • Ring Finger FDS transfer (can also help reconstruct the UCL)
  • Huber transfer (ADM to APB)

Tendon Reconstruction

Type III thumb hypoplasia has abnormal extrinsic tendons with a stable MCPJ. These patients can receive

  • EIP to EPL tendon transfer
  • Pulley reconstruction

Index Pollicisation for Thumb Hypoplasia

Key Point

Index pollicisation is indicated in Blauth IIIb, IV and V thumb hypoplasia.

Index pollicisation is indicated in Blauth IIIB, IV and V thumb hypoplasia. The fundamental goal is to recreate a functional thumb by utilising a shortened and rotated index finger.  


  • Radial palmer incision to create skin flaps at the base of the index finger that can be used to create a first webspace.  
  • Dorsal incisions should preserve dorsal draining veins.


  • Ulnar digital nerve is mobilisation by interfasciular dissection of the common digital nerve to the index-middle finger web.
  • Radial digital artery to the middle finger is ligated and divided.
  • Interossei are elevated from the index finger metacarpal


  1. Index metacarpal is removed with it's head preserved to form trapezium
  2. Epiphysis excised to prevent subsequent growth


  1. Metacarpal head is rotated and secure with K-wires
  2. 120-160° rotation, 40° palmar abduction with the MCPJ hyperextended.

Intrinsic and Extrinsic Rebalance:

  • Extrinsic shortened: FPL and EPL are trimmed to the length of the metacrpal
  • First dorsal interosseous to radial lateral band =  APB
  • First palmar interosseous to ulnar lateral band = Adductor pollicis
  • EIP acts as EPL
  • EDC to index acts as APL


Evidence-based flashcards are designed for active recall & spaced repetition using the Feynman Technique.

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