By Palmer R. Cook, OD
A lens prescription usually consists of little more than a power formula and the PD. The challenge is to convert that information into eyewear that performs well, fits comfortably and has an attractive appearance when worn. Doing this successfully takes knowledge of mechanical and physiological optics, and occasionally some skill as an amateur psychologist. The task is further complicated by the wide array of lens designs, lens and frame materials, and the many claims and counter-claims of the industry’s manufacturers.
Today, digital technology allows us to produce aspheric curvatures, and adds that are not easily detectable by the casual observation of friends, family, coworkers and anyone else. Lined lenses clearly advertise to all that the wearer is moving beyond the vitality of youth. As a result, PALs continue to grow in popularity and performance.
Every patient wants to see clearly and comfortably, and they want appearance enhancing eyewear with easy adaptation. This is challenging, because you are working with our most sensitive sensory system, and because PALs require head positioning so the lines-of-sight must simultaneously pass through the lens within specific zones for clear, comfortable vision.
The upper red dots represent the DRPs, and the lower green dots represent the NRPs. The black crosses are the Fitting Crosses. The frame pd is 70, the sum of the A measurement (50 mm) and the DBL (20 mm). The monocular frame pd is 35 (i.e., 70 ÷ 2). The decentration OD is 2 mm (the distance between the red solid line and the red broken line) and it is found by subtracting the patient’s right monocular pd from the frame monocular pd (35 – 33 = 2). The left decentration is found similarly and it is 4 mm (larger than the guideline of from 1 to 3 mm decentration). If the frame were a 48o20 the monocular frame pd would be 34 OU so the patient’s narrow left monocular pd would be just within the guideline. This would reduce the horizontal distance from the left fitting cross from 29 mm to 27 mm which would be insignificant for a –1.00 Rx, but if it were a –7.00, the change in edge thickness could be substantial.
A metal Distometer (vertexometer) is again available and can be ordered from BERNELL, A Division of Vision Training Products, Inc. (www.bernell.com). Although it is not autoclavable, chemical sterilizing solutions are available. The instrument represents the most practical method of approximating the fitting vertex now available. The manufacturer recommends adding .5 to 1 mm to account for lid thickness, although there are estimates for as much as 3 mm for lid thickness for younger patients, and a bit more should probably be added for older patients with blepharochalasis or floppy lid syndrome so some clinical judgement should be used.
For purposes of layout and accuracy, wrap frames starting at about 10 degrees need a pd increase, as well as some BI prism for a really good outcome. Using a plastic tipped caliper allows you to more accurately measure the pd when verifying the finished eyewear, which should be the pd used during the examination.
Measuring the patient’s head tilt using gravity as your bench mark is quite easy. The patient covers his left eye, and the optician stands about 36 to 48 inches or so to the patient’s right. The patient is instructed to view his right eye in a mirror that is mounted flat on a vertical wall. An inclinometer or smartphone with a leveling app can be held at about 16 inches from the optician’s eye and its left edge is brought into alignment so that it seems tangent to the patient’s glabella (the small bump between the eyebrows and the pogonion, the most forward point of the patient’s chin). The patient should be asked to hold his head at a comfortable chin-up, chin-down tilt. In this case, the inclinometer indicates his head is in a seven-degree retro, or back-leaning tilt. Adult patients should then be asked if this is how they hold their head when they drive. Many patients use a chin-up while driving head position, and that may be the best tilt to use when measuring for the fitting crosses and the pantoscopic tilt.
The pantoscopic tilt is not the angle that the temple makes with the front of the frame. The pantoscopic tilt is the angle the level line-of-sight makes with the lens plane. Here the patient is wearing his aligned frame and covering his left eye and viewing his right eye in the vertical mirror. The inclinometer or smartphone is then aligned with the front. In this case, the pantoscopic angle is tilted back at the bottom and forward at the top which is termed a panto angle. Immediately after taking the reading, the angle of the G-P line can be re-checked. These measurements are quick and easy, and can be done accurately with only a little practice. The vertical positioning of the right DRP can then be marked (the patient should be cautioned to not change his head position). In order to not block his view of his own right eye, a horizontal mark can be made lateral to the center of his pupil. The optician’s eye level should be at the patient’s eye level. The patient should then remove his glasses and a second mark can be made at the location of his monocular right pd and the same height as the first mark. This would be his DRP level and if he is re-checked with his head at the same tilt, the mark should be directly in his line-of-sight when he attempts to view his pupil. If the optician’s eye was too low or too high, the mark will be respectively too low or too high. The mark should be moved so that it is correctly positioned, and the process should be repeated from his left side for the left eye. Once both DRPs are marked alternate occlusion from one side to the other should reveal that each mark intersects his line-of-sight with his head in the same correct tilt. The Fitting Cross locations will be about 5 to 6 mm lower in each eye depending on the lens design.
The parallel blue arrows represent the lines-of-sight when distant objects are viewed. The solid green arrow shows separation of the parallel blue arrows and represents the distance pd, which is constant regardless of where the spectacle plane is located. The red arrows represent the lines-of-sight when the eyes converge to view a near object. Their separation at the spectacle plane (orange and heavy red arrow) represents the near pd, which varies according to the vertex distance.
The vertex for reading is significantly greater compared to the straight-ahead vertex because of the downward angle needed for the lines-of-sight to intersect the NRPs. The green broken line shows an average distance vertex, and the red broken line represent a possible near vertex distance when the pantoscopic tilt is minimal. If the pantoscopic tilt were increased, the near vertex would be shortened as shown by the orange broken line, and the lines-of-sight are further separated at that distance
Avoid excessive wrap with higher powers. Digital technology and good bench work by your lab can do wonders in making lenses work well in wrap frames, but ask your lab for sphere and cylinder power guidelines when your patient is interested in a strongly wrapped frame.
Use frame sizes that limit the lens decentration to between 1 to 3 mm whenever possible. Too much decentration makes lenses thicker and heavier. Patients with higher powers or prism Rxs are more troubled with chromatic aberration when the decentration is excessive (Fig. 1). Allowing patients to select frames requiring high decentration or with undue wrap may lead them to fault you for problems that arise although the selected frame was the culprit.
Never fit a lens at its advertised “minimum fitting height.” Otherwise, you may cut off the lower half of the reading zone. To avoid this, use a centration chart to be sure the Near and Distance Reference points are well within the usable area of the lens area. To avoid awkward situations, do a quick centration chart check before your patient falls in love with the frame under consideration.
LENS MATERIALS AND AR
For those doing critical seeing out-of-doors, who wear large lenses, have excessive decentration or who have prism prescriptions, divide the power in the strongest meridian of the lenses by the Abbe value of the material. For values below .15, the material is likely a good choice regarding blur related to chromatic aberration. The higher this number climbs, the more likely it becomes that your patient will experience difficulties particularly with distance vision. If the Rx is OD –6.25DS, OS –6.00 –1.00 x 090, the strongest power is in the 180 meridian of the left eye (i.e., –7.00). If you use Trivex (Abbe 45) –7.00 ÷ 45 = .156, poly (Abbe 30), –7.00 ÷ 30 = .233, or 1.60 (Abbe 42, MR-8) –7.00 ÷ 41 = .171, you can make an educated guess as to which material will work best. In this case Trivex or Mitsui’s MR-8, 1.60 material could be good choices.
Using a top-quality anti-reflective lens improves the performance of every lens material, every lens design and every prescription, so AR lenses as opposed to standard lenses should always be considered.
It should be standard practice to only take measurements after the frame is correctly aligned. Wrap is easily measured using a wrap guide available from any lab. Be sure your lab compensates for both induced prism and PD modification for frames with wraps of about 10 degrees or higher. Ophthalmic lenses, other than those with plano front curves, induce unwanted BO prism which should be neutralized with added BI prism if the lens is wrapped (rotated around a vertical axis).
Your lab should be able to tell you the vertex distance assumed for every PAL design they offer. It would be ideal if every doctor refracted at the intended vertex of the PAL that they prescribe. About 80 percent of lens prescriptions are of a low enough power that vertex distance is not highly critical. The point at which vertex becomes critical varies according to patient tolerance, pupil size, illumination and many other factors. A good vertex-critical benchmark would be for powers in the strongest meridian that are ≥ four diopters.
The vertex question is not limited to the power changes when lenses are placed closer to the eyes (minus powers become greater, plus powers are decreased), or furher from the eyes (minus lenses are weakened, and plus powers become greater). Vertex distance also plays a role in the reduction of peripheral aberrations. For lower power prescriptions, vertex distance is often not highly critical, but for mid-range and higher powers, it is best to fit adjustable pad frames which allow you to vary the vertex.
Vertex distance can be estimated from the side using a PD rule, but because of parallax, a better technique is to use a Distometer (vertexometer Fig. 2).
With some ingenuity you can modify a slit lamp so that the microscope’s travel is measured as you move from a focus on the apex of the patient’s cornea to the back surface of the lens for very strong lens prescriptions.
The final measurement to assure a great outcome is pantoscopic tilt. For measuring the pantoscopic tilt and the Fitting Cross height, the patient’s chin-up, chin-down preferred head position should be measured with his lines-of-sight level as though a point on the distant horizon was being viewed. Gravity is the constant that permits the pantoscopic tilt to be measured (Fig. 3a and 3b). Your lab cannot determine the pantoscopic tilt because the patient is needed when taking the measurement.
Manufacturers of PALs can provide centration charts for each of their designs. These measuring tools are invaluable for getting the best outcomes when ordering PALs. Every dispensary should have a notebook with laminated centration charts for every design they dispense, and every optician should be trained in their use. Each chart must be a manufacturer’s original because reproduction with a copier or emailing can cause changes in their dimensions.
Centration charts are manufacturer and design specific, and you must use the one matching the lens design you are using. Placement of the frame on the chart is critical. Both the DRP and the NRP should be well within the usable lens areas when the frame is oriented on the chart, and your marked Fitting Crosses should also be positioned correctly.
Reasonable benchmarks for DRP and NRP placement could be ≥ 5 mm from the DRP up to the highest usable point of the lens and ≥ 4 mm downward from the NRP to the lowest usable point of the lens. Although frames cannot be tilted or rotated on the centration chart, if you have an “almost fit” the frame can be moved upward or downward a bit to get the DRP or NRP into acceptable locations. For this you must mark the “new” location of the Fitting Cross and evaluate that position on the patient. If the new Fitting Cross location is higher, the patient will need to lower his chin to center it on his pupil, and if it is lower he will need to raise his chin to center it on the pupil. The clinician must then determine if the new head tilt is acceptable. Alternatively, when fitting a frame with a marginally acceptable B measurement, use the centration chart to locate an optimal Fitting Cross height accommodating acceptable DRP and NRP locations, mark the Fitting Cross heights and then evaluate the result on the patient.
INDIVIDUALIZED LENSES AND DEFAULT VALUES
Digital equipment allows the production of curvatures designed to give the effect of the test lenses used during testing even though the “as worn” positioning includes rotation around both vertical and horizontal axes. Some PALs are offered with parameters that you can “customize.” This is a valuable feature for stronger lens powers. Such lenses are known as individualized or “as worn” designs.
In fact, digital lens designs are based on average or “common” values for the fitting vertex distance, wrap, pantoscopic tilt and center of rotation values. For non-individualized designs, the tacit assumption is that the testing was at the refractor’s designed vertex distance, which is supplied to the lab with the fitting vertex. Since lensmeters can’t read lenses in the as-worn orientations, your lab will supply lensometer findings that represent a match for the needed Rx. For stronger prescriptions, individualized lenses represent a huge step forward in providing better performing eyewear. It would be helpful if manufacturers revealed the fitting value assumptions that were made for their non-individualized designs.
Manufacturers do provide “default” findings for individualized designs. These values should not be used unless the patient’s vertex distance, pantoscopic tilt and wrap match or are close to the default values for the lens design. Using default values is not a good practice.
What about that term “close to?” If the default fitting value is 13.5 mm, and you measured it as 12 mm, you are better off including your measurement when ordering. After all, who is more likely to be correct, you, or someone who is just trying to find an average? If the average adult male has a PD of 66.5, would you fit all adult males at that PD? The bottom line is that for superior results, use individualized or as-worn lens designs and take the needed measurements instead of using default values. If your measurements match the default values, use them.
Distance PDs are constant regardless of the vertex distance, but near PDs vary with the near vertex distance and the viewing distance, becoming greater when the vertex distance is shortened and narrower when the vertex distance is lengthened (Fig. 4). Near PDs for PALs are pre-set by the manufacturers, so the one tool you have to get a match between the separation of the lines-of-sight for near work and the separation of the NRPs is to vary the near fitting vertex distance. This is most easily done by changing the pantoscopic tilt. The width of the reading area of the near zones of PALs is maximized when the near PDs match the actual separation of the NRPs (assuming the NRP locations are appropriate for the add power). For every millimeter of error in this match, the usable width of the reading area decreases by 2 mm.
PALS AND PRISM
When ordering PALs, always estimate the vertical prism powers that the patient will encounter for distance vision. Theoretically, that is the prism found at the DRPs. For example: If the Rx is OD plano DS, OS -3.00 OS, to fit a PAL with a drop from the DRP to the PRP of 8 mm, the patient will have an imbalance of 2.4∆. If the drop from the PRP to the DRP is 14 mm, an imbalance at near of 4.2∆ will occur. That implies the patient will need about 4∆ of slab-off. However, if the patient has been wearing single vision spectacle lenses or a slab-off, the examining doctor should be consulted about the use and strength of slab-off.
Another prism problem with PALs is the displacement of the lines-of-sight when prism moves the apparent location of the object of regard. Fitting Cross (FC) locations are critical for these Rxs.
If 4∆ BI is prescribed OU, both FCs will need to be moved 1 mm outward. If 2∆ or vertical prism is Rxs the FCs must be moved .5 mm in the direction of the prism’s apex OU. For large amounts (≥ 6∆) of prism and a more complete explanation of calculating the FC movements that are needed when prism is prescribed, please refer to System for Ophthalmic Dispensing by Brooks and Borish (3rd Ed. Published by Butterworth Heinemann).
Yoked prism effects should also be given consideration with PAL lenses of all types. Yoked prisms have the same amount and same direction OU, deviating the eyes in such a way that the intersection of each line-of-sight with the lenses will be deviated by about .3mm toward the apex of the prism. Patients tolerate this effect rather well up to about 3 prism diopters depending on patient sensitivity and other factors such as the Abbe value of the lens material. There are some anecdotal reports that with strong encouragement toward enduring a prolonged adaptation period, larger powers of yoked prisms may be acceptable. By checking the yoked prism effect of previous successfully worn glasses, you may be able to anticipate whether the patient is likely to respond well to a new prescription or lens design. If the prescription is +3.00 OU in the vertical meridian, and you select a PAL design that has a drop from the DRP to the PRP of 10 mm, the patient will experience 3∆ BD OU at the DRP. By selecting a lens design with an 8 mm drop from the DRP to the PRP, the yoked prism effect will drop to 2.4∆ BD OU. Although yoked prism effects tend to be greater at near because of an increased drop from the PRP to the NRP, they may be less troublesome. This is because the effects of chromatic aberration are reduced at near, and because patients are less likely to be moving around in their environment when doing near tasks.
Both lens thickness and lens weight can be reduced through prism thinning. Laboratories typically make the prism thinning decision, so it is not necessary to request it. The thinning is achieved by adding the same amount of BD prism to each lens for plus lenses and low minus lenses. This allows a thinner center thickness and provides sufficient edge thickness at the bottom of the lens. High minus lenses can also be thinned at the bottom when the FC is placed high by adding BU prism in equal amounts. Although whether to use prism thinning, and how much, is best left to your lab, top-notch dispensers always record the amount of prism thinning when verifying the finished Rx.
This is done by simply recording the prism powers that are read at the PRPs during verification. The amount will be equal if no vertical prism was prescribed. When vertical prism is prescribed you may find that the yoked effect which otherwise might be 1∆ BD OD and 1∆ BU OS has become .5∆ BU OD and 2.5∆ BU OS, which maintains the effect of 2∆ more BU OS or was required by the Rx. If one lens needs to be replaced, the prism power on the patient’s note of verification should be ordered. Also, the total amount of yoked prism may become excessive due to prism thinning at the DRP leading to complaints about adaptation and lens performance.
THOUGHTS ON DISPENSING PALS
When dispensing compensated prescriptions, also dispense the lensometer values that should be read if the Rx is correct. Consumers who are experiencing adaptation difficulties with their new eyewear may visit another dispensary asking to have the prescription checked, only to be told that the new lenses fail to match their written prescription. A few may then come storming back, and you will be stuck making defensive explanations. Others may delay returning to you as they gradually discover that the eyewear is working satisfactorily. In either case, your reputation may be tarnished, and the patient may decline having you fill subsequent prescriptions.
After the new eyewear is aligned (and with clean lenses), ask the patient to view a distant target while changing his head positions to locate the Sweet Spot that gives the best vision. Repeat with a near target. Let the patient know that the Sweet Spots gradually enlarge as the glasses are worn. Tips to speed up adaptation include: 1. Wear the new eyewear as directed and do not switch back and forth between the old lens prescription and the new. 2. Spend time moving about with the new eyewear and try walking out-of-doors, especially where the ground is uneven. 3. Use brighter than normal lighting for near work for the first three days. This is especially important for PAL wearers because it will actually widen the corridors and reading areas of the lenses by increasing the depth-of-field for both eyes, and it makes binocular fusion easier, and 4. The patient should be instructed to contact you if adaptation is not noticeably progressing after three days of wearing the eyewear as directed.
Use individualized technology for Rxs when the strongest meridian in either lens is ≥ four diopters or when frame wraps of about 12 degrees or greater are encountered, and respect your lab’s guidelines on acceptable wrap powers. When you dispense PALs that approach or exceed the above guidelines, call the patient five to 10 days after dispensing, and inquire whether their vision is improving and whether their eyes are still adapting. For such calls, avoid asking about problems because they will make efforts to gratify your curiosity. Instead focus on improved clarity, less strain and extended ability to address prolonged visual tasks. If your patient is having significant problems with the new eyewear he will let you know, and you can address the issue.■
Contributing editor Palmer R. Cook, OD, is an optometric educator and optical dispensing expert.