In the slab-off technique used to correct vertical retinal imbalance, which prism orientation is added to the more minus lens?

• A. BU prism to the more minus lens
• B. BU prism to the more plus lens
• C. BI prism to the more minus lens
• D. BI prism to the more plus lens

Answer: (A)

Slab-off works by creating a small vertical prism effect that differentially shifts the retinal image between the two eyes, specifically to reduce vertical imbalance caused by anisometropia when using bifocals. The modification is applied to the eye with the greater minus power, and the prism is oriented with the base up. This base-up wedge in the more minus lens shifts the image position in that eye in a direction that helps align the near images with the other eye, minimizing vertical diplopia during near viewing. Placing the prism on the more plus eye or using the opposite base direction would not produce the needed directional shift and would fail to correct the vertical disparity.