The macula is the small part of the retina responsible for central vision. A tear or hole in the macula impairs a person’s ability to see. Many activities, including reading and writing, may become nearly impossible.
This condition, most prevalent among the elderly, arises when the vitreous (the jelly-like substance which fills the inner eye) becomes condensed and fibrous. The fibers often bond to the macula, and when they shrink they exert a pulling force. A hole is opened and the surrounding tissue peels away from the eye, causing a loss of central vision.
To repair the retinal tear, the surgeon removes some of the vitreous and replaces it with a bubble of gas or some other liquid. The bubble rises and applies pressure to the area in need of healing.
Since the macula is at the back of the eye, the bubble will only work if the patient’s head is positioned face down. Pressure against the hole flattens the macula against the wall of the eye and the impaired visual area is minimized.
During the ensuing weeks, the afflicted eye begins to heal, and the macula re-bonds to the eye wall.
Article from: http://www.myvitrectomy.com/why.shtm