FITTING SOFT TORICS
Determining Power for Disposable Soft Torics
Use these tips to maximize your success with soft toric lens trial fitting.
By Richard G. Lindsay, BScOptom, MBA, FAAO (DipCL), FVCO, FCLSA
The majority of prescribed soft toric contact lenses are disposable, with a recent survey revealing that less than 1 percent of new soft toric lens fits involved unplanned lens replacement. The majority of disposable lenses have monthly, two-week or daily replacement intervals, and disposable soft toric lenses are now available in these three modalities. One advantage associated with disposable soft toric lenses is that prospective patients can undergo a lens-wearing trial using a disposable soft toric lens with the appropriate back vertex power.
Work with What You Have
Virtually all disposable soft toric designs come in a stock range of lenses encompassing a certain number of cylindrical powers (such as –0.75D, –1.25D and –1.75D), a set choice of spherical powers (for example, from +6.00D to –9.00D) and cylinder axes in 5-degree or 10-degree steps usually the latter most often covering the complete spectrum from 0 degrees to 180 degrees. We're usually limited in our choice of back optic zone radius (BOZR) and total lens diameter (TD) for these lenses also. Therefore, assuming that you've chosen to use a particular type of disposable soft toric lens, your main decision in fitting and prescribing these lenses generally relates to the specification of back vertex power.
In specifying a back vertex power for the disposable soft toric lens that you want to trial on a patient's eye, the limited number of cylindrical powers and axes does place some restrictions on you. However, you still need to decide between certain cylinder powers and axes (in addition to determining the spherical component) when selecting this first (diagnostic) lens. I'll explain what factors you need to consider to arrive at the best prescription.
Figure 1. The effect of lid action on lens rotation for a toric soft lens with the prescription -2.00/-1.25 x 45 being worn on the left eye. The upper lid will initially act on the thicker (135°) meridian, thereby causing the lens to rotate temporally.
Allow for Vertex Distance
While we continually learned that we must convert from spectacle refraction to ocular refraction (at the corneal plane) when determining contact lens back vertex power, we often overlook this important principle in contact lens practice.
The need to take the vertex distance into account (for powers greater than ±4.00D) is especially important when correcting astigmatism with soft toric lenses. For myopes, the degree of astigmatism in the ocular refraction will always be less than the spectacle refraction. For hyperopes, the ocular astigmatism will always be greater than the spectacle astigmatism.
Consider a patient who has a spectacle correction of –5.00 –1.50 x10 worn at a vertex distance of 12mm. The ocular refraction is therefore –4.75 –1.25 x10, so not taking the vertex distance into account when performing a contact lens fitting on this patient would result in an over-correction of the ocular cylinder. Conversely, for a hyperopic patient who has a spectacle correction of +5.50 –1.00 x80 worn at a vertex distance of 13mm, there is potential for under-correction of the ocular astigmatism when prescribing a soft toric contact lens if you don't make appropriate allowance for the vertex distance, given that the ocular refraction in this case is +6.00 –1.25 x80.
Err on Under-correcting the Cylinder Power
As I mentioned previously, virtually all disposable soft toric lenses are available in a stock range of lenses encompassing certain cylindrical powers, usually in 0.50D steps such as –0.75D, –1.25D and –1.75D. Given that we generally work in 0.25D steps when determining a patient's refractive error, ultimately this means that you'll often be faced with the choice of under-correcting or over-correcting a patient's astigmatism when prescribing disposable soft toric lenses.
When this situation arises for example, when you're fitting a patient who has a prescription of –2.00 –1.00 x90 with a disposable soft toric lens that comes only in cylindrical powers of –0.75D and –1.25D then it's best to under-correct the cylinder. The reason for this is twofold. First, lens rotation becomes more significant as the cylinder increases. Secondly, while Bernstein et al (1991) demonstrated that no statistically significant masking of corneal cylinder occurs with standard thickness soft spherical lenses, clinical experience has shown that soft toric lenses with their increased thickness may mask up to 0.50D of corneal cylinder. This is especially the case with disposable silicone hydrogel toric lenses, such as the PureVision Toric, because of the higher modulus of the silicone hydrogel materials.
Allow for Lens Rotation Based on the Lens Thickness Profile
Most disposable soft toric lenses have cylinder axes in 10-degree steps, usually covering the complete spectrum from 0 degrees to 180 degrees. So what should you do when the axis of the cylinder in the ocular refraction is 45 degrees? Or 135 degrees? Or 15 degrees? Obviously, you'll need to make an allowance for some lens rotation by virtue of the fact that you can order the cylinder axes only in 10-degree steps. You'll therefore have to allow for either 5 degrees of nasal rotation or 5 degrees of temporal rotation when ordering the diagnostic disposable soft toric lens. Admittedly, most contact lens laboratories will permit you to order two trial lenses for one eye so that you may experiment with a cylinder axis that's 5 degrees to either side of the cylinder axis in the patient's ocular refraction. Even so, your patients are more likely to be impressed and their chair time reduced if your first diagnostic lens turns out to be the appropriate prescription.
Hanks and Weisbarth (1989) showed that, on average, soft toric lenses will tend to rotate nasally by about 5 degrees to 10 degrees, where nasal rotation is rotation of the inferior aspect of the lens towards the nose. They also showed, however, that significant variability exists among soft toric lens wearers in the actual amount and direction of lens rotation. A major factor causing this variability is the thickness profile of the soft toric lens.
The power of the lens determines the lens thickness profile in particular, the axis and magnitude of the astigmatic correction. Gundel (1989) postulated that the principal factor affecting lens rotation is the initial point of contact between the upper lid and the thicker meridian of the lens. This effect is greatest for toric lenses that have oblique axes, and the implication here is that notable rotational effects will occur as contact from the upper lid will always affect one edge of the thicker meridian before the other. As the upper lid comes down, it will force the lens down at this first point of contact, causing it to rotate in a certain direction.
Consider a patient who has an ocular refraction OS of –2.00 –1.25 x45. Assume that the patient is to wear a soft toric lens with a similar prescription. As the upper lid comes down, it will first act on the lens (and the 135-degree meridian) at around the 10 o'clock position on the cornea. The downward motion on the lens at this point will cause it to rotate temporally (Figure 1). Remember that to allow for temporal rotation on the left eye, you should subtract the amount of rotation from the required cylinder axis. Hence, the recommendation for this example is that the first disposable soft toric lens to be placed on this patient's eye should have a back vertex power of –2.00 –1.25 x40.
Maximize the Spherical Component to Ensure Optimal Lens Stability
Prism ballasting is the most common method of stabilization for disposable soft toric lenses. Most prism-ballasted lenses also incorporate some form of dynamic stabilization in the form of lenticulation, comfort chamfer or selective thinning of the inferior aspect of the lens. The main disadvantage of dynamic stabilization is that the thickness differential at the edge of the lens depends on the spherical power of the lens. In some designs, small spherical components can significantly reduce the achievable thickness differentials, which may compromise lens stability.
One way to overcome the decreased lens stability of soft toric lenses that incorporate low or negligible spherical components is to increase the spherical component of the lens. Contact lens practitioners are generally quite happy to leave 0.50D of cylinder uncorrected when prescribing soft spherical lenses. In doing so, they usually increase the spherical component of the lens in lieu of correcting the cylinder. In other words, they prescribe the spherical equivalent for the back vertex power. For example, a patient who has a spectacle refraction of –1.25 –0.50 x90 will most likely be prescribed a soft spherical lens with a back vertex power of –1.50D.
In fitting soft toric lenses that have small spherical components, the same principle of increasing the spherical component to compensate for an uncorrected cylinder can apply so as to improve lens stability. Consider a patient who has a spectacle refraction of –0.25 –1.25 x180. Rather than prescribing a soft toric lens with the same back vertex power as the spectacle refraction, it may be better to reduce the cylindrical correction by 0.50D and replace this with its spherical equivalent, thereby increasing the spherical component by 0.25D. In this case, you would prescribe a soft toric lens with the power –0.50 –0.75 x180. As I mentioned previously, it's generally best to under-correct the cylinder (to reduce the effect of lens rotation and to make allowance for possible masking of corneal cylinder by the toric lens). In addition, increasing the spherical component in this situation will also result in greater thickness differentials in the periphery of the lens, therefore improving the stability of the soft toric lens on the eye.
Better First Fit Success
In summary, many clinicians initially treated disposable soft toric lenses with skepticism because of concerns about on-eye performance and reproducibility. But as it turns out, these lenses have greatly facilitated the process of soft toric lens fitting by making it possible for contact lens practitioners to perform lens-wearing trials on prospective patients.
When fitting disposable soft toric lenses, the back vertex power will generally be the only parameter that you need to specify. By careful consideration of the factors I've discussed, you can help to ensure that the first lens you choose for a soft toric lens trial provides the optimal correction for the patient.
For references, please visit www.clspectrum.com/references.asp and click on document #128.
Dr. Lindsay, a Diplomate and Fellow of the American Academy of Optometry, is in private practice in Melbourne, Australia and is also a Senior Fellow of the Department of Optometry and Vision Sciences at the University of Melbourne.