What modification cannot be performed in-office on RGP lenses?

The correct answer is that changing the base curve cannot be performed in-office on RGP (rigid gas permeable) lenses. The base curve is a critical parameter of a contact lens that affects its fit, optical performance, and comfort on the ocular surface. Modifying the base curve typically requires re-manufacturing the lens because it involves reshaping the lens surface to alter how it sits on the cornea. This adjustment demands specialized equipment and skills beyond the capabilities of standard in-office adjustments.

In contrast, the other modifications—changing the material, adding plus power, and adjusting lens diameter—can be performed in-office to varying extents. For example, adding plus power involves making changes to the lens prescription using existing technology, and adjusting the lens diameter can often be done with minor alterations to the peripheral lens design. Changing the material, while not always a simple process, can also be accomplished when a new blank of a different RGP material is used and fabricated. Therefore, while some adjustments can be made in-office, changing the base curve requires a more extensive manufacturing process that cannot be executed in an office setting.