By Palmer R. Cook, OD
You want patients to get the best performance from the lenses they purchase and manufacturers go to great lengths to design lenses able to give that “best performance.” Lens manufacturers take center-of-rotation location, vertex, entrance pupil size and many other factors into account when designing lenses, while offering a smorgasbord of design choices to help you do the best job of patient-lens matching.
Great technology is just one part of the story. Patients can only appreciate technology’s complete benefits if that technology is properly applied. A good case in point involves the positioning of MRP levels. MRP stands for the Major Reference Point. It is the point in the lens giving the exact prescription called for. For non-prism prescriptions, the MRP and the optical center (OC) of the lens are the same. Lateral positioning of the MRP is universally (well, almost universally) done by including PDs or even half-PDs when placing orders for prescription eyewear. Specifying the vertical position of the MRPs is another matter. A recent informal survey of several large labs indicated over 90 percent of non-PAL orders did not include an MRP height specification. Failure to specify MRP heights sometimes leads to situations in which angry patients return to complain. More often, not specifying MRP heights causes a needlessly prolonged and uncomfortable period of adaptation and the full benefit of the lens technology is never completely available to the patient. Needless to say these are patients who often search for another provider for their next eyecare and eyewear experience.
ACHIEVING THE BEST PERFORMANCE
The absolute best performance of an ophthalmic lens is reached when the line of sight is perpendicular (or normal) to the lens surfaces and passes through the OC. This is true regardless of the base curve, dioptric power of the lens or the material used in fabricating the lens. When the line-of-sight is away from the OC, lens performance deteriorates. Factors such as pupil size, prescription, lens material and patient sensitivity affect how far the line-of-sight can deviate from the OC before aberrations become significant.
Patients who must have prism unavoidably experience aberrations that would not be at issue if their lines-of-sight passed only through the OCs of their lenses. As most eyewear designers realize, patients with prism are also more likely to be troubled by chromatic aberration. Because of this they favor lens materials with higher Abbe values for these patients.
Labs typically place single-vision MRPs at the mounting line if no MRP height is specified. The mounting line passes through the geometric centers of both eyewires. It is located halfway from the highest point of the lenses to the lowest point of the lenses. If this MRP location is too low or too high, patients must carry their chins higher or lower for the best lens performance.
You can think of a well-designed lens as having a peripheral area of moderate performance, an intermediate area of good performance and a central area of best performance. This is especially true for aspheric lenses and because of this you should never give your lab a “false” PD to achieve needed prism when ordering aspherics. The sizes of these moderate, good and best areas are relative, not absolute. They depend upon the Rx, pupil size, location of the center of rotation of the eye, vertex distance, pantoscopic tilt, the patient’s tolerance to blur and the way in which the lens is being used.
Differences in the powers of the vertical meridians can cause the areas of best and good performance to become oval and quite limited in the vertical. MRP placement is critical in such cases (Fig. 1).
IDEAL MRP LOCATION
It is to the patient’s advantage to place the MRP of each lens so that the line-of-sight passes through it most of the time and especially when visual needs are most critical. For this reason alone it makes sense to maximize lens performance by specifying both the lateral positioning (the PD) and the vertical positioning (the MRP height) of every lens you order. Clifford Brooks, OD suggests a quick and easy way to accurately determine MRP height in the 2007 edition of System For Ophthalmic Dispensing. Dr. Brooks’ technique is to properly adjust the frame including the correct pantoscopic tilt and any needed bridge adjustment. The patient then raises his chin until the plane of the front is perpendicular to the floor. The vertical measurement from the lowest point of the eyewire to the center of the pupil, with your eye at the same height as the patient’s, will give the height at which the lab should position the MRP. Dr. Brooks cautions that, especially with aspherics, the total drop via this method should be 5mm or less.
The OC of the lens in Fig. 2 is on the Mounting Line. Although the absolute best lens performance occurs when the line-of-sight passes through the OC, the area out to the first circle represents an area within which the away- from- optical- center decrease in performance is not significant.
The area between the two circles represents an area of decreased, but still good, performance. The area between the outer circle and the edge of the lens is an area of further decreased performance or moderate performance.
If the MRP is placed at the mounting line (Fig. 3) the patient must raise his chin to have the full width of the best performance area available when he looks from side to side. He must also raise his chin excessively for his lines-of-sight to pass through the MRP when looking straight ahead.
If the OC is raised to the base of the pupil (Fig. 4), the patient can more easily stay within the area of best performance. To move the MRP so far from the geometric center of the eyewire may cause excessive lens thickness at the top and center of plus lenses and excessive edge thickness at the bottom of minus lenses.
In Fig. 3 the frame could be adjusted higher on the patient’s face to bring the OC up, but the cosmetic result might not be very good. It is important to always adjust the front so that it is positioned correctly before measuring the MRP height or taking other measurements. Correct positioning of the front includes the establishing the correct vertical position, the correct wrap, the correct vertex distances, the correct pantoscopic angle and the correct tilt (usually meaning the top of the eyewire more or less parallels the eyebrows).
SOLVING HYPER EYE PROBLEMS
Special techniques can be used to optimize lens performance for hyper eye patients. In Fig. 5 Mary’s right eye is 3mm higher than her left. Mary has no vertical muscle imbalance. Her refractive error is -4.00DS OU and she reports a lengthy adaptation period whenever she is fitted with new glasses. MRP heights were not previously measured when eyewear was ordered for her so the lab always placed the OCs at the mounting line. This causes her right eye to be above the OC inducing 1.2sBD while the left eye is looking through the left OC. Usually unwanted vertical prism of .5s or more is not tolerated well by patients. Often patients will “self adjust” new eyewear to eliminate unneeded vertical prism. The eyewear is dispensed so that it follows Mary’s level brow line. She gradually “worries” the eyewear in a clockwise direction to eliminate the prism. She interprets the 10 days needed to get the glasses re-oriented as a “period of adaptation” and often complains, “new glasses are always hard to get used to.” Cylinder would further complicate this situation.
A CAUTION FOR THE DOCTORS
Doctors measure for vertical muscle problems in a number of different ways. The best ways insure that the measurement does not in part (or wholly) neutralize prism induced by lenses in the trial frame or refractor. If you avoid induced prism during testing, you should alert the optician to place the MRPs at unequal heights for hyper eye patients. In Mary’s case (Fig. 5), if the right eye is 3mm above OC of her -4.00 correction when her left eye is at OC of the left lens, the amount of vertical prism will be incorrect by 1.2sBD OD. If the lab simply places her MRPs at the mounting line, the 1.2sBD prescribed will shift the OC 3mm higher in the right lens and she will do fine. But if the eyewear designer is thorough and measures the right MRP higher by 3mm, the prism will be incorrect by 1.2sBD OD. A common understanding about testing and MRP placement between doctor and optician is especially important for hyper eye (and anisometropic) patients.
Eyewear dispensers should look carefully at how the previous eyewear is being worn when working with patients with a hyper eye. Mary presented with her previous Rx rotated so that both eyes are equidistant from the OCs. This is a strong indication that vertical prism may not be needed. Care should always be taken by the examining doctor to assure vertical phoria measurements are free of any prism effects contributed by test lenses.
If the prescribing doctor incorrectly measures a vertical muscle problem due to induced prism, when Mary becomes presbyopic problems will arise. Since labs typically place the MRP 4 or 5mm above the seg line she may suddenly find that she cannot adapt to new eyewear because tilting the eyewear to eliminate the unneeded prism will misalign the bifocals. If Mary is fitted with a progressive the same problems will exist.
BIFOCALS AND TRIFOCALS FOR HYPER EYE PATIENTS
Bifocal and trifocals represent a special challenge for your lab if you do not specify an MRP height. Many labs place the MRPs a “standard” 4 or 5mm above the seg lines if the distance MRP heights are not specified. For many patients this works, but for patients with one eye higher than the other (hyper eye), or for those who have powers in the vertical meridian that are not about the same in both eyes, significant problems can arise.
Placement of the MRPs too far above or below the pupil centers or dealing with a hyper eye is only part of the story. Anisometropia is the condition in which the refractive errors differ between the two eyes. This is especially troubling if the difference in power falls in the vertical meridians. Patients are not very tolerant of vertical prism. If a patient’s prescription is: OD -2.00DS and OS -4.00, and if his lines-of-sight are 5mm above the OCs, then the right eye will be experiencing 1sBU and the left eye will be experiencing 2sBU. This imbalance of 1scan cause headaches, asthenopia, diplopia and general dissatisfaction with the eyewear.
With progressives, aniso-metropia related problems can be minimized by using relatively short corridors and selecting PAL designs with the shortest drop from the prescription neutralization point to the prism neutralization point.
BIFOCALS & TRIFOCALS FOR ANISOMETROPIA
If you allow your lab to decide where to place the distance MRPs they will probably be placed 4 or 5mm above the seg line. This can lead to problems with adaptation, comfort and patient satisfaction especially if there is a significant power difference between the vertical meridians. Usually patients with such power differences are more comfortable if the OCs are placed as close as practical to the seg lines. The seg lines in these cases should be placed as high as the patient can comfortably and safely tolerate.
ONE OR A PAIR OF REPLACEMENT LENSES
When replacing a pair of lenses, the placement of the MRPs in the previous lenses should be considered. Raising or lowering the MRP position can usually be done as long as the relative heights are kept equal. If a single lens is to be replaced it is absolutely necessary to specify where you want the MRPs placed.
THE BOTTOM LINE
Today’s lens technology is good, but it can only perform at its best if it is ordered correctly. Labs try to do their best on every order they receive, but incorrect or missing measurements may yield results that are only acceptable or patient tolerable at best. If patients are to receive the best level of performance from the lens technology you must: 1. Specify an MRP level for every lens you order. 2. Look at your patient’s old eyewear and how it is being worn. This can give you important clues about designing the new eyewear. 3. If there is a difference in power between the vertical meridians be especially cautious. Your anisometropic patients will always experience unwanted vertical prism when looking above or below the MRP levels. Only you can decide how to place the MRPs to minimize this problem.
For purposes of clarity, use the term MRP routinely and always specify the vertical MRP position along with the PD. This will help all your patients get better lens performance, easier adaptation and more satisfaction from their eyewear.
Palmer R. Cook, OD, is director of professional education for Diversified Ophthalmics in Cincinna