Thumb Hypoplasia

Thumb hypoplasia is spectrum of the defect varies from a working small thumb to total absence of the thumb.

In the normal hand, the tip of the thumb reaches to the middle of the proximal phalanx of the index finger. When the thumb is shorter than this it can be classified as hypoplastic.

Epidemiology

Thumb hypoplasia has an incidence of 1 in 100,000 live births. Both males and females are equally affected. About 60% cases occur in isolation whereas others are associated with defects of radius bone.

About 60% of the cases have bilateral involvement and in unilateral cases right side is affected more than left.

It is crucial to rule out associated abnormalities because often this malformation is a manifestation of a syndrome.

There could be associated abnormalities of the heart, spine, and gastrointestinal tract.

Associated Conditions

• VACTERL or VATER Syndrome
• Holt-Oramthrombocytopenia-absent radius (TAR)
• Fanconi anemia
• Thumb duplication
• Transverse deficiencies
• Symbrachydactyly
• Brachydactyly
• Cleft hand complex
• Ulnar longitudinal deficiency
• Congenital constriction ring syndrome
• Apert syndrome
• Rubinstein–Taybi syndrome

Classification of Thumb Hypoplasia [Blauth]

Type I 

This type is mildest forma and thumb only has minor hypoplasia.   All musculoskeletal and neurovascular components of the digit are present though small in size.

Type II 

All of the bonystructures are present though may be small. Metacarpophalangeal joint has ulnar collateral ligament instability. There is thenar hypoplasia.

Type IIIA

There are musculotendinous and osseous deficiencies and carpometacarpal joint is intact. There is absence of active motion at metacarpophalangeal joint or interphalangeal joint.

Type IIIB 

This is similar to type IIA with an addition of aplasia of base of metacarpal and  there is deficient carpometacarpal joint.

Type IV

It is also called floating thumb. The remnant of thumb is only attached to hand by the skin and digital neurovascular structures

Type V

Complete absence of the thumb

 

Presentation of Thumb Hypoplasia

Apart from deformed or shortened thumb, there could be hypoplasia of thenar musculature and absence of skin creases indicating muscle or tendon abnormalities. Metacarpophalangeal joint may be in abduction. There may be laxity of  ulnar collateral ligament and the web space [between index finger and thumb], may be tight. Patient should be examined for radial and ulnar abnormalitiesas well as for cardiac and abdominal abnormalities.

Grasp and prehension  of the thumb are hampered.

Lab Studies

Peripheral blood smear and complete blood count should be done. It is important to rule out  Fanconi anemia

Imaging

Apart from routine x-rays of the both hands, x-rays of both forearms should be done to rule out any abnormality.

Treatment of Thumb Hypoplasia

The treatment of  hypoplastic thumb is guided by stage of thumb hypoplasia and presence of carpometacarpal joint stability.

In type I hypoplasia where augmentation of thumb abduction is not necessary and these patients generally do not need surgical treatment.

Patients with  type I thumb hypoplasia with insufficient thumb abduction would need opponensplasty a procedure of tendon transfer  for opposition movement of thumb.

Type II and IIIA thumb hypoplasia  needs release of first web space, opposition transfer, stabilization of MCP joint.

Type IIIB, IV, V thumb hypoplasia would require ablation of the existing remnant and pollicization [creation of new thumb by transferring index finger.

Timing of surgery is usually between 6 and 12 months of age.

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