In proliferative diabetic retinopathy (PDR), abnormal blood vessels and scar tissue grow on the surface of the retina and also attach firmly to the back surface of the vitreous. The vitreous then pulls on the scar tissue and can cause the blood vessels to bleed into the vitreous cavity (called vitreous hemorrhage). This can cause immediate and severe visual loss. Often, the hemorrhage will clear by itself. If it doesn't clear, a vitrectomy can be performed to remove the blood-filled vitreous.

When the vitreous pulls on the scar tissue, it can detach the retina. This is called a "traction retinal detachment". When the detachment involves the macula, central vision is lost. Also, scar tissue may wrinkle the retina and cause visual loss.

The patient can regain some vision only if the scar tissue is removed from the surface of the retina and the retina is reattached. This is accomplished by vitrectomy. The surgeon removes the vitreous and scar tissue from the surface of the retina so that they stop pulling on the retina, thereby releasing the traction. Removal of the scar tissue also reduces or eliminates wrinkling of the retina.

The surgeon may use laser inside the eye to prevent later development of abnormal new blood vessels on the iris, called "rubeosis". Preventing rubeosis is important since rubeosis may casue bleeding and blinding glaucoma. Laser is used to seal off any tears of the retina. If there are tears of the retina, a large air bubble may be placed in the eye to press the retina completely against the back wall of the eye while the laser treatment takes hold. In time, the air bubble will disappear and be replaced by the eye's own fluid.