Vitrectomy is a surgical procedure in which the vitreous humor (the jelly-like substance in the middle of the eye) is partially or completely removed or replaced. 23-gauge vitrectomy is performed with 23-gauge instruments (rather than 20 or 25). In this surgery, the doctor will access the vitreous humor, suction it out, and perform any other necessary eye procedures such as repairing the retina. Then, the doctor will fill the empty space with an air bubble or oil. This procedure might be done in the case of vitreous hemorrhage or retinal detachment.