Contact Lens Practice Pearls
Fitting Toric Corneas With GPs
By Jason R. Miller, OD, MBA, FAAO
Uncorrected astigmatism results in a loss of “crispness” in visual acuity. This can cause frustration, headaches, and eye fatigue. Spherical equivalent contact lenses and soft lenses that provide “ok” vision are often prescribed due to the ease in prescribing them. This is where there are definite opportunities for patients and the practice in fitting GP lenses.
Many of these patients have been to other practitioners unsuccessfully. They may be difficult to satisfy and frustrated with the process. Many options are available that may turn such patients into the most loyal to your practice. But where do we start?
What GPs to Use When
Even though GPs require longer adaptation times compared to soft contact lenses, they are optically pure and allow a high amount of oxygen permeability and tear exchange that make them excellent options. Spherical GPs are often a great starting lens, but when the amount of astigmatism reaches a higher level, back toric or bitoric GPs can improve the stabilization of the contact lens on the eye.
In addition to these options, there are also reverse geometry GPs as well as mini-scleral and scleral GPs for irregular corneas and corneal disease. In the specialty market, it seems like the bigger the GP, the better the visual acuity can be for those who have a high amount of irregularity in their corneas.
Back to the Toric Cornea
One of the biggest challenges we face is fitting moderate-to-high astigmatic patients. Many more options are available in the soft contact lens market, but stability is often a challenge and may be a reason for poor visual clarity (Figure 1).
Figure 1. A highly astigmatic patient of mine developed this picture to demonstrate his visual instability with his soft toric contact lenses.
From the GP perspective, there may be a challenge when fitting a spherical GP lens on a toric cornea. Stability can be improved by increasing the diameter of the GP lens or by steepening its base curve radius. This will be successful only up to a certain level of corneal toricity. In addition, the issue of lens flexure may complicate the fitting of a spherical GP lens on a toric cornea.
Eventually, you may need to consider creating a toric back surface to stabilize a GP lens on a highly toric cornea. This will also minimize the effects of lens flexure. It is important to remember that at times the corneal toricity will not correspond with the refractive astigmatism; in those cases, a bitoric GP lens (toric surface on both the back and the front of the lens) will be the ideal choice.
Don't Forget About Hybrid Contact Lenses
Hybrid lenses can deliver the optics of a GP lens without the lens edge interacting with the palpebral conjunctiva, which leads to most of the initial comfort issues with GP lenses. Hybrid contact lenses are designed with a GP center and soft skirt surrounding the GP. Just as a spherical GP lens will correct corneal astigmatism, so will hybrid contact lenses. In addition, just as a spherical GP will be susceptible to lens flexure on an astigmatic cornea, so will a hybrid lens. Fortunately, newer designs exist that are less likely to flex because of increased lens thickness, and this design works best when there is minimal residual lenticular astigmatism.
Think Outside the Soft Lens
When your next patient is looking for optimal visual acuity and is motivated to wear contact lenses, consider discussing and fitting GPs with that patient. CLS
|Dr. Miller is in a partnership private practice in Powell, Ohio, and is an adjunct faculty member for The Ohio State University College of Optometry. He has received honoraria for writing, speaking, acting in an advisory capacity, or research from Alcon, Allergan, CooperVision, and Visioneering Technologies. You can reach him at firstname.lastname@example.org.|