Optics 1.0: The Unwitting Yoked Prism


man_woman_remove_posters_18549

Progressive Addition Lenses.  Can’t live with ’em; can’t live without ’em.  As we know, some patients swear by their PALs while others swear at them.  There are many reasons for variations in individual response to PALs.  My co-author Dr. D. Leonard Werner and I reviewed many of the issues involved in Rxing PALs in our textbook, Clinical Pearls in Refractive Care.

Clinical Pearls

One of the issues that we reviewed was the incorporation of vertical yoked prism that most PAL manufacturers incorporate to maintain symmetry in thickness throughout the lens, thereby improving the cosmesis.  Typically this is yoked prism bases down, with prism power  = 0.6 x [add power].  For example, if the add is +1.50, there is approximately 1^ yoked BD incorporated.

There is an excellent online CE course in 2020 mag regarding this by Pete Hanlin.  He offers a nice illustration and explanation as follows:

Yoked Prism Thinning

“Prism thinning is applied to progressives to reduce the thickness and weight of a PAL as illustrated in Figure 1. Figure 1a illustrates a PAL with no prism thinning. To create add power (i.e., additional plus power), the curvature of a progressive lens increases at the bottom of the lens. In order to be fit into a frame, the lens must have some thickness over a certain diameter (i.e., the lens must be thick enough to provide a lens diameter large enough to be edged down to the frame shape). For many prescriptions, the end result is a lens in which the top is thicker than the bottom. Typically, progressive powered lenses will have a positive combined near power and will tend to be thicker at the top edge. Example: A lens with a distance power of -1.00D sphere and an add of +2.00 will have a combined near power of +1.00. Without prism thinning, this lens will likely be thicker at the top when edged.

The lens (Fig. 1a) cannot be ground any thinner, because any reduction in thickness will decrease the lens area (diameter) at the bottom of the lens and the lens will not be large enough to support the frame size.

By adding base down prism to the lens (Fig.1b), thickness between the top and bottom is equalized- which is why prism thinning is sometimes referred to as “equi-thinning.” Generally speaking, the amount of prism used to thin the lens is usually equal to about 2/3 of the power of the addition. Therefore, a lens with a +1.50 add would have around 1.00 diopter of base down prism to thin the lens. This general rule also means prism thinning should rarely, if ever—exceed 2.00 diopters of base down prism.

Figure 1c illustrates the combination of the base down prism and the original lens shape. Because the top and bottom of the lens are of equal thickness, the entire lens may be ground thinner without reducing the diameter (size) of the lens. The result is a thinner/lighter lens that is more cosmetically appealing and comfortable (Fig.1d).

Prism thinning is generally considered a cosmetic element of the finished ophthalmic lens—it is rarely (if ever) prescribed. Customarily, the decision to apply prism thinning to a lens is made by the Rx calculation program at the laboratory, which computes the edge thicknesses and determines if the addition of base down prism will reduce the overall thickness of the lens. Nevertheless, prism thinning does affect the optical characteristics of the lens-and may therefore affect the wearer’s visual perception.”

sidewalk_writing_19083

  1. Yoked prism bases down are usually incorporated for PAL Rxs with plus power in the vertical meridian at distance.  The higher the plus power and the greater the add power, the more yoked BD.  Occasionally yoked bases up are incorporated when there is minus lens power in the carrier, and this is routinely determined by the lab.
  2. When you Rx vertical yoked prisms intentionally, be aware that if you Rx PALs you’ll likely be adding to or diminishing the yoked Rx power if your lab routinely incorporates yoked prism thinning.
  3. Yet another reason why PALs are not a great idea for patients with ABI (aside from distortion of the periphery, yoked prism often induces postural effects).
  4. Vertical prism power is determined from the PRP (Prism Reference Point) marking on the PAL and based on the following variables as reviewed by Meister:
  • Distance power (in the vertical meridian)
  • Add power
  • Fitting cross height
  • Fitting cross decentration
  • Frame shape

 

 

4 thoughts on “Optics 1.0: The Unwitting Yoked Prism

  1. Quite helpful, Dr. Press. It’s good to know the profession is prescribing base down OU prism for their esophores. Professor Bill Ludlam used to repeat and repeat that any doctor who prescribes lenses is doing vision therapy–wittingly or unwittingly. That patients seem to be able to learn to move in the world whatever distortions we prescribe is a testament to their adaptability. Or not.

  2. I thank you Dr. Press and of course Charles Prentice who stated “a lens is not a pill”. On the other hand a little prism or a little plus (+.50’s, my favorite) seems to go a long way toward negotiation between the patient’s brain and the lenses to correct and as Dr. Cook stated “do in fact vision therapy”.
    A non-adapt to PAL’s might be that patient whose negotiation with the lenses results in failure. For me, my brain will not negotiate at all as it likes one pair of eyeglasses for reading and one pair of eyeglasses for distance….luckily, I get them for no charge and they fix my esophoria. Thank you.

  3. Point added: most labs will routinely provide the best looking final product because most patients routinely value appearance over function (“how I look in my glasses is more important than how I feel in my glasses”). Many of us however deal with patients with seemingly inexplicable visual discomforts/annoyances/challenges. It is for those patients, if one is Rxing a PAL, that it’s important to know what your lab builds into the PAL by default computer calculated yoked prism thinning. You can usually find this out by contacting the lab. See:
    http://www.expertoptics.com/what-amount-of-prism-thinning-will-be-in-these-len/

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s