Prism ballast and truncation of a toric lens are used to:

• A. Increase oxygen permeability
• B. Stabilize the lens
• C. Change lens power
• D. Reduce lens diameter

Answer: (B)

Toric lenses must stay oriented on the eye because their cylindrical power only corrects astigmatism when the axis is in the right position. If the lens rotates, the correcting power shifts and vision becomes blurred. Prism ballast and truncation are stabilization features added to help the lens maintain a predictable orientation.

Prism ballast adds a small vertical prism to the lower part of the lens, making that portion effectively heavier. Gravity and lid interaction help the lens settle with the prism base at the bottom, reducing rotation and keeping the axis aligned.

Truncation involves cutting away part of the lens edge to create a defined reference area. This edge alteration interacts with the eyelids and tear film to resist unwanted rotation, helping the lens settle in a stable orientation.

These design tweaks are about keeping the toric lens steady rather than changing how much oxygen passes through the lens or altering its corrective power or diameter.