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Fitting PALs in smaller frames requires careful positioning of fitting cross
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With the trend moving toward smaller frames and the fitting rate of progressive addition lenses (PALs) continuing to rise, dispensers may have a dilemma with this lens/frame combination. When a patient selects a PAL for its cosmetic and functional benefits and then chooses an en vogue 45-mm eye size frame, what do you do? Some find that this predicament does not have a satisfactory solution.
Probably, the easiest solution is to direct the patient to another frame. Others may consider two pairs of eyeglasses to avoid the unsightly bifocal line. Are small frames and PALs incompatible?
For years, we have recommended standard fitting heights for PALs. In an attempt to make fitting the lenses easier, we have made the fitting techniques step-by-step and recommended minimum fitting heights. In the 1970s and early 1980s, 22 mm was the minimum fitting height recommended. However, in the late 1980s, some fitting guides recommended 24-mm fitting heights be cause many of the lenses became "softer" by lengthening the progressive channels. Fortunately, large frame styles in the United States accommodated these longer channels. As you may recall, we began seeing 60- and even 62-mm eye sizes.
In the past few years, frame styles have become smaller and smaller, and dispensers have become reluctant to fit PALs in these smaller frames. This is not a new problem, because many dispensers have been reluctant to fit children in PALs due to the small eye sizes.
Fortunately for young children, it was recommended that the lenses be fitted 2 to 4 mm higher than an adult, depending on the age of the child. Although this was done to allow the child to reach the add power easier, this also compensated for shorter fitting heights in the smaller frames. Vision was minimally compromised with the higher fitting height.
The new frame styles are small ovals and rectangles with A measurements less than 50 mm and B measurements less than 35 mm. Many styles have 42 mm and 44 mm eye sizes with B measurements about 32 and as small as 29 mm. Most patients are fitted about 5 mm above the geometric center. If you do the math, you will find that most patients will have a fitting height of about 20 mm (B/2 + 5 mm=fitting height) or even less. This is well below the minimum fitting height traditionally recommended for PALs. As a result, many dispensers have been reluctant to use certain frames when fitting PALs.
How to fit PALs today
Can we fit lower than 22 mm? Certainly! The minimum fitting height was somewhat derived from the idea that most PALs had channel lengths between 14 mm and 16 mm. Because bifocals required an 8-mm segment depth, the channel length was added to the 8 mm to get a minimum fitting height of 22 mm for shorter channels and 24 mm for longer channels. Keep in mind that these guidelines were created to simplify fitting PALs.
So, should we avoid the small frames? Absolutely not. PALs do well in small frames even with B measurements around 29 mm. If you look at recent history, 60% of eyeglasses in Europe and Japan - where small frame styles have been used for more than 10 years - are PALs. If fitting PALs in small frames was a problem, we would have seen the results by now.
At the University of Alabama School of Optometry, we reviewed nine PAL wearers fitted between 20 mm and 22 mm. The patients were asked to rate their satisfaction with their PALs on a scale of 1 (very dissatisfied) to 5 (very satisfied). All the patients rated the lenses 4 (satisfied) and 5 (very satisfied). All patients fitted at 20 mm rated the lenses as very satisfied.
Without question, some of the reading zone is edged off with smaller frames. Some dispensers recommend increasing the prescribed add power to shorten the corridor and give the patient more reading zone depth. This concept goes back to the old prescribing theories where overplussing the add power was thought to help adaptation.
However, by prescribing an add power that is more plus power than what the patient needs places the patient's necessary add power higher in the corridor where the width of the channel narrows. The widest portion of the PAL reading area is then occupied by unnecessary add power. While this may increase the distance from the bottom of the frame to the necessary add power, it may not provide the patient with a more functional reading zone. This may reduce the patient's ocular depression, but will de crease the usable reading zone width for the patient. Increase the add power only when you have determined that the add power is too weak for the patient's needs.
Fitting PALs in smaller frames will require careful positioning of the fitting cross during the initial fitting to maximize the reading zone. If the fitting cross is measured too low, too much of the reading zone will be edged off. Once that happens, adjusting the frame will not help. Larger frames allow for more vertical adjustment because the reading zone is seldom cut off. The only limiting factor with larger frames is the cosmetic appearance of the frame fit.
It is best to fit on center pupil as much as possible. If you feel the fitting height measurement is too low (<18 mm), then bias the fitting height 1 mm to 2 mm higher. This will allow a little more room for vertical adjustment, because less reading zone will be cut off. However, be sure you do not interfere with distance acuity. Remember, it is very important to adjust the frame to the patient before any measurements or compensations are made. This includes proper pantoscopic tilt, face form and vertex distance.
For Your Information:
- Michael H. Cho, OD, FAAO, can be reached at the Knoxville V.A. Outpatient Clinic, 9031 Cross Park Drive, Knoxville, TN 37923; (423) 545-4592; fax: (423) 545-4488; e-mail: cho.michael_h@nashville.va.gov. Dr. Cho has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any company mentioned.