Thumb Hypoplasia

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.

Diagnosis of Thumb Hypoplasia

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.

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

Classification of Thumb Hypoplasia

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

Staged Reconstruction of Thumb Hypoplasia

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

• This can be performed through a Z-plasty, e.g. a four-flap Z-plasty.

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.

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

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.  

Incisions

• 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.

Dissection

• 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

Resections

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

Stability

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

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