Aug
2004

Rxpertise

Take a careful look at your next lens formula. Then really look at your patient. You have all the starting points for picking the best lens material, for guiding the frame selection, for making the correct eye size and bridge size choices, and even for making vertex-dictated power adjustments. By making the right choices you will turn that lens formula into comfortable, attractive and well-performing eyewear. The lens formula gives you the light bending information, but it also contains a lot of other clues that are easy to overlook. Each of the clues deals with a factor that, by itself, may be minor. In optics however, factors tend to be additive. Here are the clues top-notch dispensers look at in every lens formula and how each clue bears on the final eyewear design.

Specified Power vs. What You Should Order
Should the lenses you order have the same power as the lens formula the doctor has written? Usually… but not always. That lens formula the doctor wrote for the patient is based on the refracting distance. The specified power is the power needed at the sometimes unspecified refracting distance, yet it may not be exactly the power you should order from your lab. Lenses fitted at the refracting distance in the specified powers is a “best case scenario,” but…

• Watch out for the guy with the prominent nose that may have slid way out between the phoropter cells during testing. His test distance may have been very short when the position of the forehead rest determined the refracting distance. In frames, a longer vertex may exist simply because his nose keeps his spectacles 16 or more millimeters from his corneas.

• For low powers, it doesn’t matter much if the spectacle vertex distance is not exactly the same as the refracting distance. Over about 10 diopters, a difference between the refracting distance and the vertex distance for the selected frame may require compensation in the power ordered. A patient who refracts -12.00D at a distance of 12mm will require more minus power if his glasses are fitted at 16mm. Both minus and plus lenses gain plus power as they move away from the eye and lose plus power as they move toward the eye. An approximate calculation for this is F2/1000 = the power change per millimeter. In this case: 12.00D x -12.00D divided by 1,000 and then multiplied by 4mm (the difference between the refracting distance and the vertex distance). This gives a change of .576 diopters and the power to order (rounding up by .044 diopters) becomes
-12.62D. For cylinders this calculation should be done for both major meridians and may change both the sphere and cylinder powers that should be ordered.

Axis and Eyewear Selection
The lens axis tells you where the major meridians are located. If you know the location of the meridian of greatest power you have information that relates strongly to the ultimate edge thickness of minus lenses and to the center thickness of plus lenses. Thicker lens edges in minus lenses mean more prominent “bulls eye rings” and a less appealing appearance from the side.

Increased center thickness in plus lenses means greater magnification. In strong plus powers this means a reduced field of view for the patient and an undesirable “magnified eyes” effect. Added thickness means added weight in all lenses.
The determination of the strength and location of the strongest meridian of a lens is a quick and easy way to focus your thinking on the lenses you are ordering.
To find the power of the strongest meridian in any cylinder lens, simply combine the sphere power and the cylinder power. The power of the strongest meridian will be either the sphere power or the combination of sphere and cylinder power, which ever is greater.  For -3.00D -2.00D x 180, combining -3.00D with -2.00D gives -5.00D, which would be the power of strongest meridian in this lens. For +3.00D -2.00D x 180, combining +3.00D with -2.00D gives +1.00D; so in this case the power in the strongest meridian would be the sphere power, +3.00D.

The strongest meridian is 90 degrees from the axis if the sphere and cylinder powers combined are greater than the sphere power. The strongest meridian is in the axis meridian if the sphere power is greater than the sphere and axis powers combined. For -3.00D -2.00D x 180, the strongest meridian is at 090, and for +3.00D -2.00D x 180 the strongest meridian is at 180.

The boxing system is the industry’s accepted system for eyewear measurement and is the foundation for understanding the lens/frame relationship for any power.


•If the strongest meridian falls close to the 180° meridian the “minimum possible” lens thickness will be more affected by decentration, so it will be more important to select a frame that has a frame PD that equals, as closely as possible, the anatomical PD.

•For strongest meridians that fall close to 90° or straight up and down, your lab can achieve a thinner, more attractive lens by not using a deep (i.e. large B measurement) frame.

•Beware of the strongest meridians that fall close to the meridian of the effective diameter, or ED (see section below on “Understanding the ED”) This affects the “minimum possible” lens thickness. For strongest meridians in minus powers the thickest edge is at or near, depending on decentration, the meridian of the ED. For strongest meridians in plus powers, the center must always be thick enough to allow sufficient material at the edge of the lens at, or near, the meridian of the ED.
Defining Details
BD:
BD is a notation that indicates a “base down” prism component is needed with the thickest part (base) of the prism directed vertically downward.
BI: BI is a notation that indicates a “base in” prism component is needed with the thickest part (base) of the prism directed horizontally inward toward the patient’s nose.
BO: BO is a notation that indicates a “base out” prism component is needed with the thickest part (base) of the prism directed horizontally outward away from the patient’s nose.
BU: BU is a notation that indicates a “base up” prism component is needed with the thickest part (base) of the prism directed vertically upward.
Combining Powers: Combining lens powers is an algebraic combination. For example +1 combined with +1 is +2, and +1 combined with +3 is +4. And +1 combined with -1 is 0, while +2 combined with -3 is -1. With a little practice and perhaps a look your old algebra book, you’ll catch onto it in no time.
Decentration: This is the distance in millimeters that the distance MRPs must be displaced from the eyewire centers.
Frame PD: This is the distance between the centers of the eyewires. To calculate the frame PD, simply add the eye size and the bridge size. A 4820 frame has a 68 frame PD.
Major Meridians: These are the two meridians made up of the meridian of the axis power and the meridian that is 90 degrees from the axis.
Locating the Meridian of Greatest Power: The easiest way to visualize this is to draw an optical power cross with one line in the axis meridian and the other at a right angle to it. Note the sphere power on the axis meridian and the combined sphere and cylinder power on the other axis. The meridian with the larger number is the meridian of greatest power.
Meridian: Ophthalmic lens power is specified according to a pattern similar to the face of a clock. The horizontal (180) meridian would be a line passing through the three and the nine. The vertical (090) meridian would be a line passing through six and 12. The one to seven line would be the 060 meridian, and the 10 o’clock to four o’clock line is the 150 meridian. It is a good idea to indicate meridians with three digits, that way you can be sure that a mistake such as the 0 being left off an axis 110 is not taken as axis 011.
Minimum Lens Thickness: There is a limit to how thin a lens can be surfaced. This minimum thickness is determined by practicality and by established standards such as the ANSI Z80-1999. In plus lenses, lens blanks are sometimes overly thick to allow a greater diameter. For small eyesizes it is often necessary to surface the lens thinner, rather than simply edging down a large blank, to achieve a first-rate result.
MRP: The MRP (major reference point) is that point in the lens that gives the prescription. Bifocals have two MRPs. One is the distance MRP and the other is the near MRP.
PD: This is the distance from the center of one pupil to the other. It varies depending on whether the patient is looking at distance or converging to look at a near object.
Prentice’s Rule: The amount of prism at any point in a lens is equal to the distance in centimeters from the optical center of the lens times the power of the lens in diopters. The base of the prism will be toward the optical center for plus powered lenses and away from the optical center for minus powers. (e.g. 10 millimeters (1cm) from the center of a +2.00 lens the amount of prism is 1 x 2.00 or 2 prism diopters—with the base toward the optical center of the lens.) For cylinder lenses, the power used in the calculation is the power in the meridian that passes through the point being considered.
Refracting distance: This is the distance from the strongest test lens in the refracting instrument to the front of the eye. Many refractors have devices to help estimate this distance.
Spectacle Lens Vertex Distance: This is the distance from the lens to the eye. Except for steeply curved lenses in large eyewires, simply measuring the distance from the cornea to the plane of the spectacle front is adequate to determine a spectacle lens vertex.


Understanding The ED
The ED of a frame is the “effective diameter” and represents the largest diameter circle that can be drawn with its center at the geometric center of the eyewire and its edge just touching the edge of the eyewire at one point. Frame manufacturers provide the ED so that you know if the blank you are working with is large enough to fill out the eyewire. Their diagrams show the meridian in which the ED falls.

l If the meridian of strongest power falls at or near the ED meridian, the possibility of an undesirable weight or thickness problem is increased.
l A longer ED generally requires thicker, and therefore heavier, lenses.

Prism
Prism requires the line of sight to change direction in order to view a given object. When a lens formula specifies prism, it will specify the direction of the base (thickest part) of the prism. The amount of prism is usually, but not always, the same in both eyes. Usually, but not always, horizontal prism is either base in (BI) in both eyes or base out (BO) in both eyes. Usually, but not always, vertical prism is downward in one eye and upward in the other.
l When the prism part of a lens formula varies from what is “usually” done, a call to the prescribing doctor may help you avoid making an error in ordering.
l When lateral prism (i.e. BI or BO) is specified, try to avoid decentration as much as possible by careful frame selection, unless the optical center of the lens is shifted toward the geometric center of the eyewire by the addition of the prism component.

•When vertical prism—i.e. base up, (BU) or base down (BD)— is specified, avoid, as much as is practical, using a frame with a deep (large B measurement) lens shape.

Prism Imbalance
Patients look down, away from the centers of their lenses, to read. With bifocals, they must look down to read. Looking away from the optical center of a lens induces a prism effect. The base of the induced prism is toward the thickest part of the lens and the amount of the prism is equal to the power of the lens times the distance from the optical center in centimeters. Looking down one centimeter (10mm) in a +3.00D lens induces three prism diopters (3s) of base up power (Prentice’s Rule).

Always comparing the right and left lens powers is good technique. This comparison is especially important for the powers in the up-and-down (90°) meridians. For every cylinder lens formula, the power in the 90° meridian can be easily calculated by combining the sphere power with the power of the cylinder in the 90° meridian (see sidebar at right).

•Patients do not easily tolerate vertical prism differences—often called vertical prism imbalance—between the two eyes. Individual tolerance varies, but a difference between the right and left lens of one prism diopter (1s) or more of vertical prism can cause asthenopia, adaptation problems, reading difficulties and even diplopia. Patients with single-vision lenses and vertical prism imbalance usually learn to drop their heads to read through the lenses at a point closer to the optical centers. Careful placement of the MRPs helps these patients avoid induced prism problems.

•For patients with vertical prism imbalance who must look down to read, slab-off design or reading lenses should be considered. It is especially important to select a design with a short distance from the prism neutralization point to the reading level when ordering PAL lenses for these patients. The distance from the prism neutralization point to the distance neutralization point also requires careful consideration because this shows the amount of induced prism that must be tolerated for distance vision tasks when there is a vertical imbalance.

The Outcome Is What Really Counts
Don’t risk wasting your time, resources and reputation only to come up with a disappointing result. Know the implications of the little factors implicit in the lens formula and act on them. You’ll have fewer regrinds, better results and happier patients every time.

Palmer R. Cook, OD is director of education for Diversified Ophthalmics in Cincinnatti.

 

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