Which modification to a gas permeable lens cannot be made in the office?

The modification that cannot be made in the office is changing the base curve of a gas permeable lens. The base curve is a critical component of the lens design, determining how the lens aligns with the cornea and its overall fit on the eye. Changing the base curve generally requires the lens to be re-manufactured, as it involves reshaping the entire back surface of the lens to achieve the desired curvature.

In-office modifications typically include adjustments to more superficial aspects of the lens, such as edge design or surface polishing, which can be done using specific tools and techniques without needing to remake the lens entirely. Applying a new coating, such as anti-reflective or UV-blocking treatments, can also be completed in the office since it does not require structural changes to the lens itself. Thus, while these modifications may be made in the office, base curve changes are substantial enough that they necessitate new lens fabrication.