By Rebecca Soto, ABOC, NCLEC

Bicentric grinding, also known as slab-off, is used to correct vertical imbalance. Typically, slab- off can be used to correct vertical imbalance over 1.5 diopters. Bicentric grinding means we are creating two optical centers on a lens. ‘Bi’ meaning two, and ‘centric’ meaning center. Bicentric grinding or slab-off is used to neutralize an unwanted prismatic effect caused by unequal refractive errors. Slab-off can be either conventional or reverse. Conventional slab-off is added to the front surface, lower part of the lens, and reverse slab-off is added to the rear surface, lower part of the lens.

What causes a vertical imbalance?
Antimetropia and anisometropia are two refractive errors that can potentially cause a vertical imbalance in multifocal wearing patients. Antimetropia is a condition in which a patient can have one eye myopic (minus) and the other eye hyperopic (plus).
For example:
OD: -2.50 Sphere
OS: +2.75 Sphere


Anisometropia is a condition in which a patient can have a significant difference in refractive error between the two eyes. Typically, a difference of two diopters or more would be considered anisometropia.
For Example:
OD: -1.50 Sphere
OS: -4.75 Sphere


Image courtesy of 20/20 Magazine
How to identify Vertical Imbalance:
1. Calculate power in the 90th meridian
2. Determine reading drop/depth
3. Use Prentice’s Rule

Bicentric grinding Tips:
Can be verified with a lens clock
Imbalance can be calculated by finding the total power at 90 degrees

Types of Slab-off:

Conventional Slab-off: Base Up (on the lens that is Most Minus or Least Plus)
Reverse Slab-off: Base Down (on the lens that is Least Minus or Most Plus)

Vertical Imbalance Alternatives:

• Dissimilar segs (for example: one seg that is Round 22 on the lens with the Most Plus or Least Minus, with the other seg that is FT28 or FT35 on the lens with the Most Minus or Least Plus)

• Compensated segs (ribbon style bifocal segments which have been modified so that the segment optical center for one lens is high in one segment, and low in the other)

• Two pairs (single vision distance and single vision near)

Bicentric Grinding Example:
OD: -1.25
OS: -0.75 +2.50 X 180 (transpose to +1.75 -2.50 X 90)
Add +2.00 Reading Depth 10 mm

Calculate the Vertical Imbalance:
1. Calculate power in the 90th meridian
OD: -1.25
OS: +1.75

2. Determine reading drop/depth
10 mm

3. Use Prentice’s Rule
Power X Decentration (in this case, the drop) /10

OD:
1.25 X 10 /10 = 1.25 Base down (minus)

OS:
1.75 X 10 /10 = 1.75 Base up (plus)

Think of slab off as an extension of the prism. You can calculate prism and understand canceling and compounding prism. It essentially is the ability to neutralize excess prism.
Therefore, 1.25 base down and 1.75 base up create a compounding prism of 3.0. You can either order conventional slab-off, 3.0 base up in the right lens, or order reverse slab-off, 3.00 base down in the left eye, to neutralize the vertical imbalance.

You can learn more about prism with our CE, "Eyes Right!" An Overview of Prism Use in Eyewear, at 2020mag.com/ce. And don’t forget about our CE BOGO Summer Sale! Enter CESummerSale at checkout by July 7.