Progressive addition lenses can be notoriously difficult to successfully dispense, but there are several techniques that can be used to ensure patients are left satisfied.
How come some people love their progressive addition lenses (PALs) and others just can’t quite get used to them?
The answer is often in the prescription.
It’s important to know that lenses are not prescribed to magnify objects, but purely so that the object we are looking at is focused onto the retina. Magnification (or minification in the case of minus lenses), is a direct result of prescription power and actually an unwanted by-product of lens power.
When a PAL wearer looks down to read, they only have a small area in which to view their reading matter and so for plus lens wearers (hyperopes), the reading material they are looking at is actually magnified; they see less of the page in their limited reading zone and the higher the add power, the worse this becomes.
Often we find that some presbyopes do not appreciate their new freeform design lenses and ask why “these new and more expensive lenses are not as good as my old lenses?”
What happens, is that the wearer’s prescription is governing where and how the wearer needs to look to read. There is also prismatic effect at near point, which complicates what they can see and, more to the point, what they cannot see.
Plus lenses will create some base out prism on convergence. The image is displaced towards the apex of the prism, further complicating the reading image by necessitating slightly more convergence.
Minus prescription wearers (myopes) on the other hand, experience the opposite and generally will see a wider area through the reading zone because their prescription actually minifies the image, and they can see a lot more through their reading zone. The prismatic effect of the minus prescription will be base in (or less base out, considering the add power), which also works to benefit these short-sighted PAL wearers by lessening the amount of convergence required for reading.
Understanding this should help to realise that all wearers’ prescriptions are different and so too will be their level of acceptance of their new progressive addition lenses.
Remember, for two presbyopes with the same add (say +2.00), one a hyperope with a +2.00 distance Rx and one a myope with a -2.00 distance Rx, the power at near point will be very different – +4.00D and Plano respectively. So it is common for low add myopes to not like their PALs and prefer to remove their glasses to read.
But that’s not all, when the patient undergoes a refraction, the prescription is determined by the optometrist who uses either a phoroptor head or a trial frame. Both of which use optically precise, 38mm flat form, glass bi-convex or bi-concave lenses positioned perpendicularly in front of the patient’s eyes – the perfect script in the circumstances and the perfect starting point for lens recommendation.
You see, there is another important difference to be considered here, the difference of how each individual lens is positioned on each diverse wearer.
The final position of the lenses in the spectacles will be different to the lenses used to determine the prescription in the first place. To experience the effects of this, take a powered spherical stock lens and focus it on a lensmeter, then tilt the lens and watch the focus change.
Most frames will have some pantoscopic angle
What happens with this small experiment is you have created astigmatic error on a spherical lens by tilting it, which replicates what happens when the spectacle frame is positioned on the wearers face at a different angle to the phoropter head or trial frame. Of course, there will be varying degrees of astigmatic error depending on power and how the frame sits on each individual face.
However, lens laboratories can optimise freeform lenses accordingly – to do this they use default measurements if you do not provide them. But by providing the actual position of wear measurements, your lab can personalise the lenses for your patients with lenses that place the true power in the as worn position of the frame and provide a better visual experience for the wearer, and better adaptation rates.
Taking accurate and considered position of wear measurements are crucial for patient wearing success. These facial measurements include monocular PDs for both distance and near, pupil heights for both right and left eyes, pantoscopic tilt, Back Vertex Distance (BVD) and facial wrap.
Most frames will have some pantoscopic angle; the angle of the plane of the spectacle frame front with the frame temple. When the frame is positioned on the wearer, this becomes the measurable pantoscopic tilt.
To understand this difference, just imagine your frame sitting on me and my frame sitting on you, the tilt will be different on us both.
Position of wear measurements will ensure that every wearer benefits from the best vision possible on every prescription, and will directly contribute to the success of your practice and its reputation.
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At Golden Eye Optometry, we view good vision care as front line protection at every age. A routine eye exam can detect more than poor vision. It can shed early light on glaucoma, macular degeneration, cataracts and diabetes.