Which two methods are used for stabilization of rigid gas permeable (RGP) lenses?

Truncation and prism ballast are indeed effective methods used to stabilize rigid gas permeable (RGP) lenses.

Truncation involves modifying the shape of the lens by cutting off a small section at the bottom, which helps anchor the lens in place during gaze shifts. This modification can help maintain the position and orientation of the lens on the eye, reducing movement that might cause discomfort or interfere with visual clarity.

Prism ballast, on the other hand, utilizes the principle of adding a small amount of prism to the bottom portion of the lens. This creates a weighted effect that encourages the lens to settle in a stable position on the cornea. The added weight from the prism helps keep the lens oriented correctly, enhancing comfort and ensuring that the optical center remains aligned with the pupil.

These two methods are integral to the design and fitting of RGP lenses, as they address the common concern of lens stability during various head movements and blinking.