The GP lens edge apex should be located relative to the posterior surface within which range?

• A. 1/2 to 2/3 posterior
• B. 1/4 to 1/3 anterior
• C. 3/4 to 1 anterior
• D. at the equator

Answer: (A)

The key idea is how the edge of a gas permeable lens sits relative to the back (posterior) surface, because that position governs how the lens edge interacts with the corneal surface and eyelids. The edge apex is the highest point of the edge in cross-section, and its location tells you how much edge lies toward the front of the lens versus toward the back.

If the apex is placed about halfway to two-thirds of the distance from the posterior surface toward the anterior surface, the edge sits in a sweet spot: it remains behind the immediate front of the cornea enough to avoid excessive edge contact, yet it isn’t so far back that there’s inadequate tear flow or stability. This promotes a smooth, stable peripheral fit with comfortable lid interaction and proper centration.

Placing the apex too posteriorly can cause the edge to sit too close to the cornea or scleral conjunctiva, leading to edge irritation or instability. If it’s too anterior, the edge becomes too prominent, increasing lid interaction and discomfort. Therefore, the best range is halfway to two-thirds from the posterior surface toward the anterior.