Which statement about prism ballast and truncation is true?

Which statement about prism ballast and truncation is true?

• A. They stabilize the lens
• B. They improve oxygen permeability
• C. They increase lens diameter
• D. They change sphere power

Answer: (A)

Prism ballast and truncation are design features that keep a contact lens from rotating on the eye by altering how the lens sits. The extra weight at the bottom (ballast) or the trimmed edge (truncation) creates a bottom-heavy lens that settles upright, improving orientation stability—especially for toric or prism-corrected designs. This stabilization is the primary benefit, and it doesn’t change the lens’s central refractive power or increase the overall diameter. Oxygen permeability is determined by the material (and overall thickness), not by ballast or truncation, so these features aren’t used to improve oxygen flow.

Prism ballast and truncation are design features that keep a contact lens from rotating on the eye by altering how the lens sits. The extra weight at the bottom (ballast) or the trimmed edge (truncation) creates a bottom-heavy lens that settles upright, improving orientation stability—especially for toric or prism-corrected designs. This stabilization is the primary benefit, and it doesn’t change the lens’s central refractive power or increase the overall diameter. Oxygen permeability is determined by the material (and overall thickness), not by ballast or truncation, so these features aren’t used to improve oxygen flow.