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Multifocal Designs
Slab Off




Anisometropia is that condition where the refractive error of one eye differs significantly from that of the other and where each eye contains the same sign. An anisometropic condition exists above where the right eye is + 1.00 sphere and the left eye is +4.00 sphere. Through the distance optical centers no prism exists. However if the gaze is directed away from the optical centers unwanted prism is induced. In this example, the patient is forced to look 10 mm away from the optical centers in order to utilize the bifocals. In doing so vertical prism is induced in each eye resulting in a total of 3.0 D of vertical prismatic imbalance. Excessive vertical prismatic imbalance may result in an inability for the patient to fuse the two images. Bicentric grinding or “slab off” is one way to correct for vertical imbalance.



How to Calculate Slab Off
  1. Determine power of each lens in 90° meridian

  2. Determine distance from optical center of carrier lens to reading level.

  3. Apply Prentice's Rule to calculate amount of vertical prism induced in each eye at the reading level.

  4. The difference in amount of vertical prism in each eye is approximately equivalent to amount of prism to be “slabbed off.”

  5. Conventional Slab Off: Is always ground base up on the lens with the most minus or least plus power in the 90° meridian.

  6. Reverse Slab Off: A relatively new lens which is available to the laboratory from the lens manufacturer in which the prism has already been molded into the semi-finished lens. As the name suggests, the prism base is oriented base down, or in the reverse direction of the conventional base up orientation. In this case the slab off is placed in the lens with the least minus or most plus. To simplify matters it is useful to calculate all slab off as if it were to be ground in the standard or conventional manner. Then, to use a reverse slab, simply order the lens base down in the opposite eye.

Simply stated, slab off, also called bicentric grinding, is base up prism ground in the reading portion only of a lens, generally a multifocal lens. It is used for the purpose of minimizing excessive vertical prismatic imbalance which may be induced through the reading portion, usually as a result an anisometropia.

Grinding conventional base up slab off requires the skills of an expert optical journeyman. It is both costly and time consuming to produce. The reverse slab off described above is molded in the factory, allowing the laboratory to grind the lens in a manner very similar to any other single vision or multifocal lens. Both styles of slab off serve the patient equally well.


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