FITTING SOFT TORICS
Soft Toric Contact Lens Fitting Pearls
These tried-and-true steps will help improve success with soft toric contact lens fitting.
By Long D. Tran, OD, FAAO
Dr. Tran is an assistant professor at the Southern California College of Optometry and is a consultant for the Teague Training Group. He is also the director of Contact Lens & Refractive Surgery Services at Northpark Optometry in Irvine, CA. You can reach him at firstname.lastname@example.org.
The Teague Training Group was established in 2006 with the goal of providing a hands-on soft toric contact lens fitting experience for second- and third-year optometry school students. Last year, two other optometrists and I conducted contact lens fitting workshops at 13 optometry schools across the United States and Canada. During these workshops, each student intern examines, fits and prescribes toric soft contact lenses for a willing patient. Each student is also mentored by a veteran practitioner.
What I observed at these workshops is that, despite many newly available soft toric lens designs and many different lens fitting philosophies, the strategies that lead to soft toric lens fitting success remain the same. Following are some of my favorite soft toric contact lens fitting pearls.
Know What Lies Ahead
We all know that the eye examination starts the moment you meet your patient. Asking the right questions and looking for the right clues can help you determine whether your patient is a good candidate for contact lens wear. If a patient is new to contact lenses, I recommend performing a detailed case history to find out what the patient desires to achieve through contact lens wear. This will allow you to set the patient's goals at the top of your priority list. By knowing what the patient seeks, you will be better equipped to fulfill his expectations.
In addition to listening to patients, I also recommend observing them. Signs such as excessive blinking and red or injected eyelid margins may indicate eyelid disease and/or dry eyes. Gross observation of lid tonicity, especially in older patients, may also yield valuable information. Soft toric lens stabilization methods such as prism ballast and dual thin zones rely on pressure from the upper and lower eyelids to maintain axis alignment and rotational stability. Even alternative designs of newer soft toric lenses such as Acuvue Advance for Astigmatism (Vistakon) and Air Optix for Astigmatism (formerly O2Optix Toric, CIBA Vision) rely on eyelid pressure and eyelid position for optimal stability and alignment.
I don't discourage my patients who have poor lid tonicity from wearing soft toric lenses. Instead, I discuss with them the potential challenges that we may encounter during the lens fitting process. I find that most patients appreciate this type of information.
If a patient is currently a soft toric contact lens wearer, make certain to find out what attributes he likes most about his current lenses or what improvements over his current lenses he wishes to have.
Get Off to a Good Start
Experienced practitioners are familiar with the term "good data in, good data out." Many times, in their haste, I see student interns selecting diagnostic soft toric lenses by relying on refractive data that was collected by someone else, usually a classmate. This can result in poor initial vision and time consuming lens refitting.
Get off to a good start with an accurate subjective refraction. Find out how much cylinder power a patient needs and err on the lesser side. If the patient's refractive cylinder power is –1.00D, choose a diagnostic lens with –0.75D cylinder power as opposed to one with –1.25D cylinder power.
Patients who have low amounts of cylinder correction are more tolerant to contact lens rotation than are patients who have higher amounts of cylinder correction. This is because a high-cylinder lens will induce more residual cylinder when misaligned than would a low-cylinder lens. Christopher Snyder, OD, MS, FAAO, found that every 10 degrees of uncompensated contact lens rotation yields one-third of the cylinder power in the over-refraction. This serves as a great reference for you to quickly determine how much cylinder has been induced when a toric contact lens has rotated on a patient's eye.
If a patient presents with an outside prescription, verify that it's correct. I find that investing a small amount of time into confirming the prescription can save much time (and headache) down the road. As a reminder, any sphere and cylinder prescription of 4.00D or greater needs to be vertexed back to the corneal plane.
Choosing Axis, Cylinder and Power
If the exact prescription is not available when selecting a diagnostic lens, I recommend trying to first match the diagnostic cylinder axis to the refractive cylinder axis, then the cylinder power and finally the sphere power. Assuming that no on-eye lens rotation occurs, having the correct cylinder axis will eliminate the time consuming need to calculate obliquely crossed cylinders after performing a sphero-cylinder over-refraction (SCOR).
In the event that you need to calculate the resultant prescription of obliquely-crossed cylinders and you don't have access to one of the many online calculators, you can duplicate the combined prescriptions with diagnostic lenses and measure the resultant prescription through a lensometer. For example, if you are fitting a soft toric contact lens with a prescription of –2.00 –1.25 ×45 and you find an SCOR of –1.00 –0.50 ×60, you can insert a –3.00D sphere lens (the combined sphere component), a plano –1.25 ×45 lens (the cylinder component) and a plano –0.50 ×60 lens (the over-refraction) into a trial frame and neutralize the resulting power in a lensometer. You can even account for rotation of the soft toric lens by adjusting the axis position of the trial lens representing the diagnostic contact lens. This method may be crude, but it serves as an efficient way to determine what prescription the patient needs.
Time is of the Essence
In clinical settings, the luxury of waiting the recommended 15 to 20 minutes for a soft toric contact lens to stabilize on the eye is often not possible. To help speed this up, I suggest manually positioning the contact lens' laser marking(s) into the proper alignment while applying the lens. This will reduce the distance that the laser mark has to travel when moving into position.
Many practitioners report that when fitting a patient with soft toric lenses, they will ask a patient to wait before checking for on-eye lens rotation. I commend these practitioners for doing so. Regardless of the lens stabilization method, all toric lenses need time to settle. Increased hydration from the lens packaging can also affect the overall contact lens fit. This is why it's important to not rush to assess lens rotation immediately after lens application, as the fit of the lens can change dramatically over time.
Figure 1. These maps show the difference in oxygen transmissibility between some hydrogel and silicone hydrogel soft toric contact lenses. Courtesy of CIBA Vision.
Fit or Vision?
While some practitioners may argue that assessing the fit, stability and rotation of a toric contact lens before checking a patient's vision will bleach retinal photoreceptor cells and decrease visual performance, I advocate evaluating the fit of the contact lenses before checking the patient's visual acuity. By using a thin, moderate-to-low biomicroscopy light beam to check the lens marking(s), you can quickly assess the fit of the lens without affecting the patient's vision. I like to make sure that there is proper contact lens centration, corneal coverage and rotational stability before progressing onto measuring visual acuity. Should the contact lens have inadequate centration or excessive rotation, I will either adjust the contact lens axis according to LARS (left-add, right-subtract) and select a different diagnostic contact lens or choose a contact lens with new parameters. Vision testing through a poorly fitting contact lens will only yield unusable data and it may even discourage patients, especially those who are new to contact lens wear.
Beyond the Fit
In most cases, we assess visual performance and lens performance while patients are looking straight ahead. However, we shouldn't forget that patients often need to function while not looking straight ahead. Highway lane changes and driving in reverse are two examples of activities that require clear vision in different gazes. In addition to assessing vision and contact lens fit in the straight-ahead position, you should ask patients to assess the quality of their vision while gazing in different directions and to report any blur or visual discomfort. Poor or unacceptable vision in directional gazes, even with excellent vision in the straight-ahead position, may warrant a lens design or parameter change.
Final Power Determination
If the soft toric contact lens proves to have an optimum fit and rotational stability, you can next assess visual acuity. In an ideal situation, a patient should be able to attain excellent visual acuity with some slight adjustment of the sphere component of the toric lens. If the patient is unable to get acceptable visual acuity with a spherical over-refraction, then perform an SCOR.
When performing an SCOR, one of three scenarios can occur. If the SCOR axis is the same as the patient's refractive axis, then modification of the sphere and/or cylinder component is needed. If the SCOR axis is 90 degrees away from the patient's refractive axis, then the cylinder power of the diagnostic lens is too high and should be decreased. If the SCOR axis is oblique to the refractive axis, then you need to calculate the resultant cylinder power and axis.
One method is to use calculators that are available online from Web sites such as www.coopervision.com or www.eyedock.com. Simply enter the toric contact lens power, amount of lens rotation and SCOR power, and the resultant sphere, cylinder and axis components are calculated.
If access to an online calculator is not possible, you can measure the resultant powers using trial lenses and a lensometer as described above.
Some of the challenges related to soft toric lenses that do not occur with soft spherical lenses include complications that arise from thicker lens edge profiles and lower oxygen transmissibility. This has resulted in corneal complications such as hypoxia and corneal vascularization. Eghbali and his colleagues (1996) confirmed that prism-ballasted soft toric lenses can have up to 60 percent lower oxygen transmissibility in the inferior portion of the lens than in the superior portion of the lens.
The availability of silicone hydrogel materials has made soft toric contact lenses a safer option for patients. The increased oxygen transmissibility in these materials as compared to soft torics made in traditional hydrogel materials (Table 1) has reduced the risk of corneal hypoxia and corneal vascularization. Newer lens designs such as Acuvue Advance for Astigmatism and Air Optix for Astigmatism have updated stabilization designs. Both lenses have thinner inferior edge profiles that rest over the vasculature of the inferior corneal limbus (Figure 1).
Within the last few years, soft toric contact lens materials, designs and reproducibility have improved immensely. Their safety profile for compliant patients has improved as well. The availability of new lens designs, high-oxygen-transmissible lens materials, different base curves, and a wide range of parameters make them ideal for almost all astigmatic patients.
Although advancements in soft contact lenses will continue to progress, I believe the key steps to successful toric contact lens fitting will continue to remain the same. CLS
To obtain references for this article, please visit http://www.clspectrum.com/references.asp and click on document #149.