Classification

The established scheme for categorising thumb hypoplasia was first described by Blauth in 1967(1). Manske(2) subsequently went on to subdivide the third category into IIIa and IIIb:

• type I:
  • all structures present
  • generalised hypoplasia - clinically minimally shortened and narrowed
  • mild hypoplasia of:
    • bones
    • muscles: opponens pollicis, abductor pollicis brevis
• type II:
  • more hypoplasia
  • bones smaller:
    • phalanges
    • trapezium
    • scaphoid
    • metacarpal
  • intrinsic thenar muscles of thumb hypoplastic or absent
  • narrowing of first web space
  • may be only one neurovascular bundle
  • possible extrinsic muscle absence or hypoplasia eg FPL, EPL
• type III:
  • all the features of a class II anomaly but with a more severe phenotype eg severe first web space contracture, severe hypoplasia of intrinsic and extrinsic muscles, significant reduction in thumb size
  • subclassified by Manske into IIIa and IIIb with important implications for treatment:
    • IIIa: carpometacarpal joint present and stable; usually hypoplastic extrinsic muscles
    • IIIb: carpometacarpal joint absent and instability at this level; absent extrinsic muscles
• type IV:
  • also known as pouce flottant or floating thumb
  • rudimentary phalanges, no metacarpal
  • small soft tissue bridge between thumb and side of hand
  • often single residual neurovascular bundle
• type V:
  • complete absence of thumb

Ref: (1) Blauth W(1967) Arch Orthop Unfall Chir 62: 225-246. (2) Manske PR, McCarroll HR Jr, James M. (1995). J Hand Surg 20: 246-253.