5 Reasons Progressive Lenses Fail
Progressive, or multifocal, lenses have developed an undeserved reputation for being difficult to use. People have reported that they feel unstable, can’t read or even can’t see out of them at all.
The vast majority of adaptation issues with progressives lenses aren’t from the lenses at all, rather they are caused by the fitting of the lenses to the patient. Here are five of the most common errors that cause non adaptation to progressive lenses.
1. PD (Pupillary Distance)
There may have been a time when using a ruler for inter pupillary measurements sufficed, those days are long gone. Modern lens designs require the most accurate measurements available, this means monocular PD’s measured with sub millimetre accuracy.
Using binocular measurements fails to account for the fact that our eyes are rarely symmetrical and any uncompensated asymmetry will narrow the field of vision. In extreme cases this can result in complete failure of the patient to fuse their image at near or have their field of vision offset to the point where clear vision is only achievable by significantly turning the head to the side.
I have worked with practitioners who claimed that taking the correct fitting heights for their progressive lens patients was unnecessary. Their argument was that the patient should be able to move their head to find the correct zone in the lens. What was the result? The wearer had to either drop their chin to drive or raise their chin up high to read. The instances where the fitting just so happened to match what the patient needed were so rare as to be remarkable and the failure rate was so high that the lens companies would send out their representatives to figure out what was going wrong. Of course they didn’t admit to the ‘no heights’ policy to the rep which was proof enough
that they were not doing the right thing.
Progressive lenses are designed to be fit at a specific point relative to the eye, this is considered carefully by the lens designers and changing any parameters can have quite significant effects.
Progressive lenses are made to several standards which tend to be based around the level of customisation. Basic lens designs make a set of assumptions about how the frame fits the face and these are not always correct. For example, if the lens design assumes a vertex distance of 14mm but the frame is worn at 10mm then there will be a fundamental power difference between the prescribed lenses and the final result. In short, the resultant prescription may not be correct!
Another common source of problems are the tilt and wrap of the frame. Unless the design specifically calls for these parameters it is unlikely that they will be compensated in the final lens. As we all remember from our basic optics, tilting or wrapping a frame can lead to changes in the amount and orientation of cylinder, rendering once good lenses completely unwearable.
On the subject of parameters, how many different measurements are taken for progressive lenses? Is it just heights and PD?
What about the ‘top of the line’ or ‘premium’ lens?
What extra measurements are being taken to allow the premium lens designs to perform at their best? Yes, basic progressive lenses necessarily make assumptions on the designs for tilt, vertex, wrap etc, that is how the cost is kept down.
However, the whole point of the premium lenses is that they are more customised for the patient. Unless the extra measurements are being taken then assumptions are being made at the lab and this erodes the benefit to the patient. On that note, the range of tools available for taking a full suite of measurements is larger than ever before. It ranges from simple kits to full video centration systems so that practitioners can ensure they are taking the most accurate measurements possible.
While the best lens designs have many excellent features, their excellence is predicated on a foundation of accurate, detailed data.
What design did your patient wear before?
Considering the previous wear patterns has a large impact on the success of a lens, if a specific design has been worn in the past we need to know how to consider the way the style and feel of the lens will relate to the new design, prescription and fitting.
Patients will occasionally elect to get the ‘best lens available’ only to have it fail, two of the big causes of this are failure to take adequate measurements and failure to relate the new design to previous wear. Talking to the lens companies directly will help a practitioner get a feel for how a given lens performs and relates to it’s predecessors.
I know that some people will say ‘but that is a design consideration’. Yes, it is, but it is so often overlooked that it deserves it’s own point.
The Minkwitz condition dictates the relationship between the add power and the corridor length. It is possible to completely change the way a patient views the world by simply changing the corridor length. Similarly, when we consider the previous wear patterns we can carefully manipulate the corridor length to manipulate the design of the new lens, effectively making the change to a new progressive lens more comfortable.
What can we do for you?
Grant Hannaford and the team at Hannaford Eyewear are dedicated to providing the best in optical education for their patients and the ophthalmic industry as a whole. Why not get in touch with them today to see if they can help you, or if you’re in the industry we can help your practitioners grow their knowledge of optics and dispensing beyond the graduate level.