How much overlap should occur for the appropriate lens diameter (OAD)?

How much overlap should occur for the appropriate lens diameter (OAD)?

• A. 1 mm
• B. 0.5 mm
• C. 2 mm
• D. 0 mm

Answer: (A)

The idea is that the overall diameter of the lens should extend beyond the corneal edge by about 1 mm to achieve a stable, well-centered fit. This small overlap ensures the lens covers the cornea adequately, allows smooth lid interaction during blinking, and provides good tear exchange under the lens. If the overlap is smaller (no or minimal extension), the edge may lift or decenter; if it’s larger (much more than 1 mm), the edge can overhang onto the conjunctiva/sclera, causing irritation and instability. So about 1 mm of overlap is the best balance for an appropriate lens diameter.

The idea is that the overall diameter of the lens should extend beyond the corneal edge by about 1 mm to achieve a stable, well-centered fit. This small overlap ensures the lens covers the cornea adequately, allows smooth lid interaction during blinking, and provides good tear exchange under the lens. If the overlap is smaller (no or minimal extension), the edge may lift or decenter; if it’s larger (much more than 1 mm), the edge can overhang onto the conjunctiva/sclera, causing irritation and instability. So about 1 mm of overlap is the best balance for an appropriate lens diameter.