Individual recruited patient will be randomly assigned to one of the two groups: group 1 will receive intravitreal injection of 1.25 mg of avastin (0.05 ml) 7 to 9 days before vitrectomy; group 2 will not receive avastin pretreatment. Standard 3 port pars plana vitrectomy will be performed followed by silicone oil (5000 CS) infusion. A total of 30 cases (15 in each group) will be recruited.