The optimum diameter should stabilize the lens along which meridians, and should allow for 1 mm of movement in which direction?

• A. Horizontal and vertical; movement horizontally
• B. Horizontal and vertical; movement vertically
• C. Horizontal only; movement vertically
• D. Vertical only; movement horizontally

Answer: (D)

Stability of a GP lens comes from choosing a diameter that keeps the lens oriented with the vertical meridians while allowing a small, lateral movement. When the diameter is appropriate, lid interaction tends to lock the lens in a vertical orientation, so it doesn’t rotate around the visual axis as the eye moves. A small amount of movement is still needed for tear exchange and comfort, and about 1 mm of movement in the horizontal direction provides that exchange without causing decentration or rotation. Movement that would occur vertically would reflect poor centration or excessive lid influence, whereas the described pattern—vertical stabilization with a modest horizontal shift—is typical for a well-fitting GP lens.