Ulnar or post-axial polydactyly is the most common form of polydactyly. It has a racial predilection for African and African Americans boys with an approximate incidence of 1 in 300; this group tends to have a small, poorly formed digit with an autosomal dominant inheritance. The Stelling Classification describes a categorisation for the extra digit or parts of digit in the upper limb:
- Type 1: incomplete digit comprising soft tissue alone:
- attached to the neighbouring digit by a soft tissue pedicle
- most frequent form
- more common in black population
- tends to occur in association with conditions like Ellis-van Creveld syndrome and chondroectodermal dysplasia in white children
- treated by clip or suture ligation, or more favourably by formal excision with an ellipse of skin and burying of the neurovascular bundle
- usually have full longterm hand function
- complications include neuroma formation and a residual nubbins of soft tissue, particularly with clip or suture ligation
- Type 2: complete or incomplete extra digit:
- articulates with a metacarpal or phalanx
- excised with use of a flap of skin from the digit to cover point of articulation
- Type 3: complete extra digit and metacarpal:
- extra digit and metacarpal are excised
- a corrective osteotomy may occasionally be necessary
- may require reinsertion of muscles from the excised digit onto the residual ulnar sided digit eg abductor digiti minimi
- ulnar collateral ligament may need reconstruction
- tend to have good longterm hand function but little finger may exhibit a reduced range of motion
Alternatively, another classification scheme divides ulnar polydactyly into:
- type A: a well-developed supernumerary digit
- type B: a small nubbins connected by a soft tissue bridge
Ref: Temtamy SA, McKusick VA. Birth Defects Orig Art Ser (1978); 14: 1-619.