Thumb hypoplasia is a 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.
Thumb hypoplasia has an incidence of 1 in 100,000 live births. Both males and females are equally affected. About 60% of cases occur in isolation whereas others are associated with defects of the 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.
- VACTERL or VATER Syndrome
- Holt-Oramthrombocytopenia-absent radius (TAR)
- Fanconi anemia
- Thumb duplication
- Transverse deficiencies
- Cleft hand complex
- Ulnar longitudinal deficiency
- Congenital constriction ring syndrome
- Apert syndrome
- Rubinstein–Taybi syndrome
Classification of Thumb Hypoplasia [Blauth]
This type is mildest form and thumb only has minor hypoplasia. All musculoskeletal and neurovascular components of the digit are present though small in size.
All of the bony structures are present though may be small. The metacarpophalangeal joint has ulnar collateral ligament instability. There is thenar hypoplasia.
There are musculotendinous and osseous deficiencies and the carpometacarpal joint is intact. There is an absence of active motion at the metacarpophalangeal joint or interphalangeal joint.
This is similar to type IIA with an addition of aplasia of the base of metacarpal and there is deficient carpometacarpal joint.
It is also called a floating thumb. The remnant of thumb is only attached to hand by the skin and digital neurovascular structures
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. The metacarpophalangeal joint may be in the abduction. There may be laxity of ulnar collateral ligament and the web space [between index finger and thumb], may be tight. The patient should be examined for radial and ulnar abnormalities as well as for cardiac and abdominal abnormalities.
Grasp and prehension of the thumb are hampered.
Peripheral blood smear and complete blood count should be done. It is important to rule out Fanconi anemia
Apart from routine x-rays of 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 the thumb.
Type II and IIIA thumb hypoplasia need the 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.
The timing of surgery is usually between 6 and 12 months of age.