Extrinsic - the pressure at the edges of the amnion on the limbs when the limbs burst out from the amnion, cause the constriction by accretion of the amnion and the fetus due to the inflammation of the amnion. The fingers are already formed and, due to the injury sustained by the amniotic bands, the fingers then heal together at the site of injury, causing postinjury syndactyly.
intrinsic (Streeter) - postulated that the ring is a developmental and hematogenous deficiency of the subcutis. Bleeding inside the hand plate might cause the necrosis of the subcutaneous cells, which was associated with the abnormal shrinking of the wound, causing the skin to slough and healing leads to annular constriction band
presence of epidermal inclusion cysts in some of these fingers and more common involvement of longer digits give support to the extrinsic theory
3 patterns of limb involvement
amputation (most common)
finger distal to the constriction ring is usually atrophic, or it has been amputated in utero due to ischemia.
typically, syndactyly is distal with proximal separation of digits (reverse of true syndactyly)in the form of fistulas/sinuses or fenestrations
associated findings include encephalocele, cleft lip or palate, renal abnormalities, cardiac defects, hemihypertrophy, anterolateral bowing of the tibia, tibial pseudarthrosis, and leg-length discrepancy.