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.