Causes / typical injury mechanism: Thumb hypoplasia is a congenital defect with a spectrum of presentations ranging from a slight decrease in thumb size to complete thumb absence. Thumb hypoplasia belongs to the radial longitudinal deficiency spectrum of diseases.

Prevalence:

• Age – Present from birth.
• Sex / gender – There is no difference in prevalence due to sex.

Risk factors: Approximately 80% of cases of hypoplastic thumb have an associated condition.

Syndromes associated with hypoplastic thumb include:

• VACTERL association – vertebral defects, anal atresia, cardiac defects, tracheoesophageal anomalies, and renal and limb anomalies
• Holt-Oram syndrome
• Fanconi anemia
• Thrombocytopenia with absent radius (TAR) syndrome
• CHARGE syndrome

Pathophysiology: Embryologic origin remains unclear.

Grade / classification system:
Blauth Classification

• Type I: Minor generalized hypoplasia.
• Type II: Absence of the intrinsic thenar muscles. First web space narrowing. Ulnar collateral ligament (UCL) insufficiency.
• Type III: Musculotendinous and osseous deficiencies.
  • A – stable carpometacarpal (CMC) joint
  • B – unstable CMC joint
  • C – remnant head of metacarpal only
• Type IV: Pouce flottant, or floating thumb.
• Type V: Aplasia.