The vitreous cavity may be invaded by cells penetrating through dehiscences in the inner limiting lamina of the retina, and exaggerated vitreo-retinal adhesions may also develop at such sites. The vitreous often separates from the inner limiting lamina except at these anomalous adhesions, and retinal traction thus results from excessive movement of the otherwise detached gel; traction on the retina may also follow proliferation and contraction of invading cells on the retinal surface, along the posterior hyaloid interface, or traversing the gel. Traction complications are liable to occur at sites of vitreo-retinal adhesion during surgical removal of opaque gel, while division of transgel strands and epiretinal or posterior hyaloid membranes may be necessary in order to relieve retinal traction.