What is the goal of the appropriate lens diameter (OAD)?

• A. minimum of 0.5 mm beyond the limbus
• B. maximum of 0.5 mm beyond the limbus
• C. at the limbus exactly
• D. 0.5 mm inside the limbus

Answer: (C)

The goal is to have the edge of the lens sit exactly at the limbus. This positioning places the central cornea well covered by the lens, while the peripheral edge rests on the sclera where blinking and lid movement are better tolerated. The limbus marks the border between cornea and sclera, so aligning the edge there helps with stable centration and minimizes edge-related irritation. If the edge sits beyond the limbus, it can overhang onto sclera and cause mechanical rubbing or instability; if it sits inside the limbus, the edge can rub on the limbal/conjunctival area and cause discomfort. So, placing the edge at the limbus provides a balanced, comfortable fit.