Pathology

The pathology of congenital syndactyly goes beyond simple fusion of skin with or without bone. Other structures may be hypoplastic or abnormally formed, creating problems for surgical correction:

• absence of normal commissure which is difficult to construct given that it is made of non-hair-bearing dorsal skin that slopes at an angle of 45 degrees
• digital arteries and nerves:
  • often shared between the two digits
  • the digital artery and nerve may bifurcate distally
  • the digital nerve can loop around the artery
• connections between tendons eg pollex abductus between the flexor and extensor pollicis longus tendons
• fascial bands may run between the two digits
  • composed of existing fascial structures such as Cleland's ligament, superficial palmar fascia or the intermetacarpal ligament
  • can attach to dermis
  • tend to constrict the growth of any longer digit involved in syndactyly causing flexion and deviation with growth
• skin:
  • there is a shortage as the overall circumference of the two fused digits is less than the combined circumference of both digits in isolation
• bones:
  • phalanges may be malformed, eg a triangular bone termed the delta phalanx
  • can be osseous fusion of distal phalanges or whole digit, seen in complicated syndactyly
  • bony deformity worsens with uncorrected growth due to the tethering effect of fascial interconnections
  • symphalangism of joints

Reference

1. Buck-Gramcko D. Congenital malformations of the hand and forearm. Chir Main. 2002 Mar;21(2):70-101