During vitreoretinal surgery, the surgeon is required to precisely manipulate multiple tools in a confined intraocular environment, while the tool tip to retina contact forces are at the limit of human sensation limits. During typical vitrectomy procedures, the surgeon inserts various tools through small incisions performed on the sclera of the eye (sclerotomies), and manipulates them to perform surgical tasks. During intraocular procedures, tool-tissue interactions occur at the sclerotomy ports and at the tool-tip when it contacts retina. Measuring such interactions may be valuable for providing force feedback necessary for robotic guidance. In this paper, we measure and analyze force measurements at the sclerotomy ports. To the best of our knowledge, this is the first time that the scleral forces are measured in an in vivo eye model. A force sensing instrument utilizing Fiber Bragg Grating (FBG) strain sensors was used to measure the scleral forces while two retinal surgeons performed intraocular tool manipulation (ITM) task in rabbit eyes as well as a dry phantom. The mean of the measured sclera forces were 129.11 mN and 80.45 mN in in vivo and dry phantom experiments, respectively.