What is the most common manufacturing method used for a GP lens?

• A. Biconvex lens
• B. Tricurve lens
• C. Spherical lens
• D. Aspheric lens

Answer: (B)

The key idea here is that gas-permeable lenses get their proper fit from the shape of the back surface. A single-radius (spherical) back surface or a simple front-surface shape often can’t match the complex curvature of the cornea across its entire surface, which leads to poor alignment or unstable wear. The most common manufacturing approach uses a back surface with three distinct curvatures—central, intermediate, and peripheral—called a tricurve design. This three-curve back surface lets the lens sit more precisely on the cornea, provides better edge control, and allows the tear layer to settle evenly, resulting in stable centration and comfortable wear. Lathe-cut processes typically produce this tri-curve back geometry, making it the standard method in GP lens production.

A spherical back surface is simpler but doesn’t adapt as well to the cornea’s shape. Aspheric back surfaces are used in some designs to tailor fit or optics, but they aren’t the most common approach for GP lenses. Biconvex isn’t the usual description for GP lenses, which focus on the back-surface geometry rather than both surfaces being equally curved.