Functional endoscopic sinus surgery (FESS) is a minimally invasive surgical procedure that opens up sinus air cells and sinus ostia (openings) with an endoscope.

The use of FESS as a sinus surgical method has now become widely accepted; and the term "functional" is meant to distinguish this type of endoscopic surgery from nonendoscopic, more conventional sinus surgery procedures.

Purpose

The purpose of FESS is to restore normal drainage of the sinuses. Normal function of the sinuses requires ventilation through the ostia (mouth-like opening) and is facilitated by a mucociliary transport process that maintains a constant flow of mucus out of the sinuses. All sinuses need ventilation to prevent infection and inflammation, a condition known as sinusitis. In healthy individuals, sinus ventilation occurs through the ostia into the nose. The sinuses open into the middle meatus (curved passage in each nasal cavity) under the middle turbinate (thin, bony process that is the lower portion of the ethmoid bone in each nasal cavity), which together are known as the osteomeatal complex, the key area of the nose. The hair-like cilia direct the flow of mucus toward the ostia.

From http://www.answers.com/topic/endoscopic-sinus-surgery