By Dr. Joel Silbert
A 28-year-old female patient presented for contact lens fitting, with no prior contact lens experience. She is in good general health and takes no medication other than for birth control. She works as a consultant for a financial firm and does do a fair bit of computer work and reading throughout the day. Currently she is wearing glasses on a constant-wear basis and is unhappy about cosmesis.
She has thought about refractive surgery, but at her last eye examination two years earlier, her practitioner told her that she was not a candidate due to her high astigmatism. Additionally, she is not prepared for the costs involved with such a procedure, even if she were a suitable candidate.
Habitual Spectacle Rx
OD +2.00 = -4.00 x 180 (20/20-)
OS +2.50 = -3.75 x 178 (20/20-)
OD 42.00 @ 180 / 46.50 @ 90 (good mire quality)
OS 41.75 @ 180 / 45.75 @ 90 (good mire quality)
OU Cornea & adnexae clear; no vascularization, staining, scarring, no evidence of ectasia (thinning), striae, or pigment ring
Based on the above data, what are her options for contact lens fitting?
Is she a good candidate for soft lenses? If so, what type?
Rigid lenses? Can we fit her with a rigid spherical base curve lens?
What about an aspheric base curve? Or is a toric lens indicated?
If a toric lens, what type of toric? (Front, back, bitoric?)
Given the above history, what additional tests would be helpfulprior to initiating contact lens fitting?
Is it possible that she has keratoconus?
What signs would rule this in or out?
The patient was presented with options of a custom soft contact lens vs. a GP lens. Because of the high cylinder, a soft toric would by nature be expensive, and subject to visual fluctuation, as even a small mislocation off-axis would result in reduced acuity or variability in vision.
Given the close approximation of corneal cyl and refractive cyl, this patient was deemed a good candidate for a GP toric contact lens.
Specifically, a bitoric GP lens would be able to correct the refractive error well, while at the same time retain stability (as it would not likely rotate off-axis). Additionally, as with all GP lenses, longevity of lens materials, high oxygen transmissibility, and economy of wear were additional pluses in favor of GP lenses over soft lenses. Once past the initial adaptive phase, the patient would be expected to do very well with this type of lens.
Given that GP torics were to be used, there are two techniques that can be used to fit the patient:
- Optical cross method (Mandell-Moore Fitting Guide is more useful here). This is an empirical technique that allows the practitioner to “plug in” the refractive data (vertexed to the corneal plane if any power over 4D), the keratometric data, and provides guidelines for designing the flat and steep corneal meridian, based on toricity. The technique “undercorrects” the steep meridian by an amount listed on a table, to facilitate adequate tear exchange. The lens is ordered after each meridian is calculated independently.
- We chose, instead, to use an SPE Bitoric Fitting set. As inspection of the clinical data shows, almost all the refractive cylinder is found to be corneal. Whenever there is close correlation between refractive and corneal cylinder, and when the axes are similar, then a spherical lens would be appropriate optically. Here, we are using lenses whose base curve is toric to fit the highly toric cornea, but which has been designed to be equivalent optically to a spherical contact lens. This is done by the application of a front surface cylinder placed at the flat axis, which is equal but opposite to the amount of cylinder that is “induced” by the lens/tearlens interface. The lab calculates this amount (approximately equal to 40% of the rear surface cylinder in the contact lens — clinicians often use the “rule of half” or 50% to estimate this magnitude) and applies it as a front surface cylinder. The net result is a lens that has been restored to “spherical” equivalency, and it behaves optically like any ordinary spherical contact lens, except that it fits the cornea like a toric lens.
Because the cornea has approximately 4 to 4.5 D of toricity, I undercorrected the cylinder by 1 to 1.5 diopters in the steep (vertical) meridian and chose a flat meridian about “on K.” Thus, we used a 3D SPE fitting set. All of our lenses are SPE design, and all have 3 diopters of rear surface cylinder, and the air powers on the label also show 3 diopters difference.
Once this lens is placed on the eye and allowed to briefly equilibrate (a drop of anesthetic is useful here), then we evaluate the fluorescein pattern as we would any conventional lens to achieve the desired pattern (alignment to slight with the rule pattern). Then, a final over-refraction (spherical) is done to determine the best acuity. We simply add the over-refraction to the package labeled powers, and then order the lens. Size dimensions are typically the same a conventional GP lenses (often 9.2/7.8), and we can elect to use spherical or toric peripheral curves.
The use of SPE bitoric fitting sets is very helpful, as it gives us a truer representation of the fitting characteristics and centration, as well as a more accurate means of determining the final powers and acuity, than relying on empirical data alone.
The following parameters were ordered for our patient, after the diagnostic evaluation. Each lens is a 3 D. SPE bitoric lens. If you are ordering an SPE lens, be sure to notify the lab of this when placing the order. She achieved 20/20 acuity in each eye and has gradually increased her wearing time to all-day wear, with excellent physiology.
|OD||8.04/7.50||9.2 (.4)||10.5 (.3)||+2.00/ -1.00||9.2||7.8||Boston EO blue|
|OS||8.13/7.58||9.3 (.4)||10.5 (.3)||+2.75/ -0.25||9.2||7.8||Boston EO blue|
Note that on the basis of the diagnostic evaluation, the left eye was fitted 0.25D flatter than the flat K. Thus, there is slightly more plus in the power in this meridian. We chose spherical PCs for this case, for simplicity. You may see a slightly oval optic zone when inspecting these lenses with a reticule magnifier as a result. Had we chosen toric PCs, then the OZ would have been circular in shape. The use of toric PCs can add some additional rotational
stability to the lens if it is needed.
This case will be presented without a case history. I will just give the pertinent K and refractive data for one eye (the fellow eye is similar). Suffice it to say that the decision to use a GP toric lens has already been made.
K OD 44.00 @ 180 / 46.50 @ 90
Rx OD -3.00 = – 3.50 x 177 (20/20)
Can this patient be fit with a spherical GP lens? There is 2.50 D. of toricity, so while a spherical base curve lens could be used, there will be a lot of “rocking” around the flat corneal meridian. This will probably lead to excess lens movement, more discomfort and instability, and may lead to low riding lenses that could become stagnant in terms of tear exchange.
Can the patient be fit with an aspheric GP lens? This would be a better possibility, as an aspheric lens would provide more uniform bearing on the toric cornea with better weight and contact distribution, than the more harsh spherical base lens. This would be done with an aspheric with an ellipsoidal e-value (e approximately .65) and not with an aspheric that is used for multifocal purposes.
What kind of vision would be produced with either of the above two options? There is a high possibility that acuity would be compromised, as in both cases there is residual astigmatism of about 1.00D. This is a with-the-rule astigmatism!
What about a front toric to correct for this residual astigmatism? Well, that is a possibility, but this would be making a difficult situation even more complicated. Is there any other way that we could correct the residual astigmatism without using a front toric prism ballasted design?
Answer: Yes! Why not design a simple “back toric” lens? This is a rear surface toric that has a spherical curve on the front (not a bitoric). We know that the use of a rear surface toric will fit this 2.5D toric cornea much better than a spherical or aspheric lens. The lens should center well, and conform better to the actual corneal shape. We will design it to provide adequate tear exchange, so that it won’t become too snug.
More importantly… We know from our optic studies that when a back toric lens is applied to a toric cornea, there will be a “cylindrical error induced” (the “induced cylinder”) that forms along the flat axis and which is approximately 50% of the toricity of the rear surface of the contact lens! Well, in this case, if we use a lens with 2D of rear surface toricity, it will induce somewhere between 0.75D and 1.00D of cylinder along the flat axis (180). And in our patient’s case, the residual astigmatism is just about the same amount!
In other words, we are using the “induced cylindrical error” as a “correcting lens” for the patient’s residual astigmatism! Even though it is a tear lens, it behaves like any other lens, and will correct the residual nicely, leading to excellent visual acuity for our patient! NEAT!