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
- Kilian and Neimkin(1) reported a 22% deficiency of skin dependent on the design of the flaps for surgical correction
- 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
Ref: (1) Kilian JT, Neimkin RJ (1985). South Med J 78: 414-418.