The answer is D. The posterior optical zone will directly affect the lens to cornea relationship of a rigid gas permeable lens. If the posterior optical zone is steepened, it will increase the sagittal depth. If it is flattened, it will decrease the sagittal depth. Changing the power, which is accomplished by changing the anterior optical zone, will not affect the sagittal depth of the lens, nor will changing its edge design. 1. The sagittal depth of a rigid gas permeable contact lens can be altered without changing the diameter by: A. Changing the power B. Changing the edge design C. Changing the radius of curvature of the anterior optical zone D. Changing the radius of curvature of the posterior optical zone The answer is A. A lens is truncated by removing a segment of the lower edge of a contact lens. This creates a straight edge that interacts with the lower lid and aids in lens positioning. Of the lens designs listed in the question, the only one that requires precise positioning of its optical portions is a translating bifocal lens. 2. A truncation is used on a rigid gas permeable contact lens to help enhance the performance of which type of design? A. Bifocal B. Aphakic C. Bitoric D. Keratoconus The answer is B. A lenticular or myoflange design will decrease the mass of an aphakic lens as well as increase the edge profile, which allows it to interact more with the upper lid and aid in positioning of the lens. 3. The mass of an aphakic rigid gas permeable contact lens can be reduced by making the lens design: A. Spherical B. Lenticular C. Tricurve D. Aspheric The answer is B. Increasing the optical zone diameter and increasing the overall lens diameter will both tighten and steepen the relationship of the lens to the cornea. Decreasing the