Vertically Challenged Part 2

By Andrew S. Bruce, LDO, ABOM

Part 2 (visit Part 1 at 2020mag.com/CE) continues the discussion and explores a third option to correct for vertical imbalance: slab-off. It also covers how to calculate power in oblique meridians and demonstrate the use of the power cross when calculating vertical imbalance.

SLAB-OFF/REVERSE SLAB- OFF/BI-CENTRIC GRINDING

The most common way of correcting vertical imbalance is to induce a vertical prismatic effect in the lower half of one lens. This type of correction is referred to as bi-centric grinding or slab-off. It can be used to correct for imbalance amounts ranging from 1.5D to 6D. Slab-off provides base up (BU) prism and is applied to the most minus, or least plus lens in the vertical meridian to offset excessive base down (BD) induced by the opposite lens. Slab-off can be incorporated into either a glass or plastic lens, although each is manufactured in a different way. The main difference is that a glass lens has the slab-off ground on the front surface since the bifocal is fused into the lens, and a plastic lens has it ground on the back surface due to the presence of the molded bifocal segment on the front.

Reverse slab-off lenses are molded, or cast with base down prism in the lower segment area, rather than having base up prism generated using bi-centric grinding. The advantage here is that reverse slab-off lenses can be kept in inventory by the lab as semi-finished lenses facilitating normal surfacing techniques, resulting in faster delivery time. Because reverse slab-off provides BD prism instead of BU, it is always used on the most plus or least minus lens in the vertical meridian to offset excessive BU induced by its partner.

In a situation where large amounts of vertical imbalance are present, for example, greater than 6D, consider using slab-off for one eye and reverse slab for the other.

RECOMMENDED STEPS FOR ORDERING

Calculate how much vertical imbalance is induced by the distance lenses and determine if it will create visual discomfort for the patient.

Using Table 1, apply slab-off or reverse slab to the appropriate lens.

Order the slab line placement at the appropriate position based on the multi-focal style being used as indicated in Table 2.

Occasionally, the prescribing doctor will indicate the need for slab off. However, the optician must always be on the alert for its need since ultimately it is the optician's responsibility.

LENS DETERMINATION FOR SLAB-OFF/REVERSE SLAB

The following table illustrates on which lens to place the slab-off/reverse slab depending on the lens combination being utilized. A simple way to remember the rule is to visualize the slab for what it is and what it provides to the optical system:

Slab-off provides base up prism so it will always be applied to the lower part of the lens that is naturally inducing the most base down, or least base up prism when viewing through a position below the distance OC. Hence, the most minus or least plus.

THE SLAB-OFF MANUFACTURING PROCESS (Modified from "Clinical Optics" by Fannin and Grosvenor) A slab-off lens is made using a procedure called bi-centric grinding. After the front surface of the lens is finished in the usual manner, a dummy or cover lens manufactured to match the base curve of the required lens is then cemented onto the front surface. The front surface is then reground using the tool originally used for that surface, but ground in a way that the dummy is ground away in the upper portion while leaving it attached to the lower portion. The back surface is then finished with the remaining dummy considered as an integral part of the blank. The blank is now an equal thickness at the top and bottom unless the prescription calls for prism in the distance. When the lens is finished, the remaining dummy on the lower portion is removed. The dummy is base down prism resulting in the addition of base up prism in the lower portion of the lens.     This procedure also results in an upward displacement of the center of curvature of the front surface of the lens in the lower portion, resulting in the front surface having two centers of curvature, one for the upper portion and for the lower, but both having the same curvature. This produces a unique optical axis for each of the two portions of the lens.     Bi-centric grinding can be done on either the front surface in the case of a fused lens such as a glass flat top bifocal, or on the back surface in the case of a lens where the multifocal segment results in a wedge on the front surface such as a plastic bifocal.

Conversely, reverse slab provides base down prism so it will always be applied to the lower part of the lens that is naturally inducing the most base up or least base down prism when viewing through a position below the distance OC. Therefore, the most plus or least minus.

It should be noted that although slab-off can be used on any lens, cosmetically it works best on a flat top bifocal due to the slab line forming a continuation of the top of the segment. In addition, the wider the bifocal used, the less noticeable the slab line will be.

Note: When working with a progressive lens, your lab can often offer the best recommendation for the location of the slab placement depending on the progressive being used. Some patients wearing progressive lenses will experience less vertical imbalance than with a lined multi-focal due to the moving optical center in a progressive from distance to near. For this reason, it may not always be necessary to correct for moderate amounts of vertical imbalance.

EVALUATING FOR VERTICAL IMBALANCE IN A SPHERO-CYLINDER LENS

Determining lens power in the vertical meridian of a spherical lens is very straightforward since it is the same in both meridians. However, with a sphero-cylinder lens, this is not the case; the sphere and cylinder powers are ground at 90 degrees to one another. All the cylinder power is effective at 90 degrees to the cylinder axis, and only the sphere power is effective along the cylinder axis. When evaluating for lateral or horizontal prism, the effective power in the horizontal or 180-degree meridian is the one to use for calculation purposes. When determining vertical prism or vertical imbalance, the effective power in the vertical or 90-degree meridian is the one to be considered. The following examples will demonstrate how to calculate vertical imbalance when presented with a variety of different prescriptions.

• Example 1:
  OD: PL -2.00 x 090   ADD +2.50
  OD: PL SPH             ADD +2.50

At first glance, the prescription for both eyes appears notably different. Assuming this prescription would need correcting for vertical imbalance based on the numbers alone would be an easy mistake to make. However, before jumping to any conclusions, it is important to closely dissect the prescriptions. The simplest way to do this is using a power cross. A power cross is used to illustrate the effective power in each meridian separated by 90 degrees. It is beneficial for the optician to become familiar with using a power cross since the process is also used when determining vertexed power modifications with toric contact lenses.

Remember, for vertical imbalance we are concerned only with the lens power in the vertical or 90-degree meridian. Also, remember all the cylinder power is present in a lens at 90 degrees to the axis, and only the sphere power is effective along the meridian in line with the cylinder axis.

For the right eye, the axis is 090 so all the cylinder power is present in the hori zontal or 180-degree meridian. In the vertical or 90-degree meridian, the total power is plano. For the left eye, the lens is plano in both horizontal and vertical meridians. Let's plot this out:

The effective power in the vertical meridians for both eyes is plano. Therefore, regardless of the reading position, because there is effectively no power in the vertical meridian, using the Prentice rule, we can determine there will be no induced prism and consequently no vertical imbalance at the reading level in this scenario.

• Example 2:
  OD: +2.50 - 1.00 x 045   ADD +2.00
  OS: +4.00 - 0.50 x 180   ADD +2.00

Once again, let's transfer the numbers to a power cross to aid with visualization:

The right eye is a little more complex due to the oblique axis. The table below illustrates how to compute power in the horizontal and vertical meridian when dealing with oblique axes.

This can often be a source of confusion, so here's a simplified way to view it. It can be seen that axis 45 is halfway between the horizontal and vertical meridian. If 100 percent of the cylinder power is present 90 degrees from the cylinder axis, it stands to reason that 50 percent of the cylinder power is present 45 degrees from the cylinder axis. Thus, in this case, in both the horizontal and vertical meridian, 50 percent of the cylinder power is present. Therefore, the power in the vertical meridian for the right eye is: +2.50 plus (50 percent of -1.00) = +2.00. And the power in the vertical meridian for the left eye is: +4.00 plus (100% of -0.50) = +3.50.

Now, consider a reading position 10 mm below the OC. Induced prismatic effect can again be calculated using the Prentice rule:

• Prism = Distance from OC (cm) x Lens Power
  
  OD: = 1 x +2.00 = 2D and the direction is BU (Because the lens is "+")
  
  OS: = 1 x +3.50 = 3.5D and the direction is BU (Because the lens is "+")

Both lenses are base up which results in 1.5D of vertical imbalance (the difference between the two). The lenses are both plus, and slab-off is always applied to the most minus or least plus lens. Therefore, this can easily be neutralized by using 1.5D slab-off on the right lens. 1.5D reverse slab could just have easily been used on the left lens.

What if the cylinder axis differs from those included in the table? The cylinder axis almost always falls somewhere between the listed values. In such cases approximations are accurate enough for evaluation purposes. Let's explore how to approximate.

Approximation Example: Presented with the following prescription, the objective is to determine the effective power in the 90-degree meridian.

• Rx: -1.50 -1.00 x 050
  
• Axis is 40 degrees from the vertical, or 90-degree meridian.

Based on the above table, approximately 45 percent of the cylinder power is effective in the 90-degree meridian = -0.45D which can be rounded to -0.50D.

Therefore, the approximate effective power in the 90-degree/vertical meridian = -2.00D.

Using this approximation technique provides a relatively efficient way to determine if vertical prism might present itself as a problem. With some practice, this will become second nature and a very useful tool for the optician.

VERIFICATION OF SLAB-OFF

There are two basic ways to verify slab-off:

1. Comparing the vertical prismatic effects of the two lenses through a lensometer and checking for actual image displacement at the reading level. The amount of the slab-off is the difference between the calculated amount of vertical imbalance and the amount found using lensometry. ("Clinical Optics," Fannin and Grosvenor)
   
   For Example:
   • Calculated vertical amount from the Rx = 3D
   • Image displacement at near point observed using lensometry = 1D
   • Slab-off prism applied = 3 - 1 = 2D
   
   
2. Using a lens clock is a much easier method. First, position the lens clock horizontally across the lens center in the distance portion paralleling the slab line and record this base curve. Second, rotate the lens clock through 90 degrees with the pins perpendicular to the slab line with the central pin directly on the line and record this base curve. The difference between these two base curves indicates the amount of slab-off prism applied.

The goal of this discussion is to help dispel the belief that dealing with such topics is overwhelming and intimidating. Using tools such as the power cross and being able to visualize lens meridian powers can significantly simplify matters. Having knowledge of the techniques and options covered in this program empowers the optician to confidently handle complex prescriptions such as those presented here and provide the patient with superior eyecare. Our role as eyecare professionals involves using our skills and knowledge to predict the outcome of the written prescription so there are no surprises. It is our responsibility to complete the eyecare chain by "filling" the written prescription, watching for any foreseeable complications and intercepting them with preventative mea- sures. Only by doing so, we can provide our patients with the best and most comfortable vision possible.