RESIDUAL ASTIGMATISM (RA) can be somewhat mysterious. A properly fitting contact lens for astigmatism can still result in under- or overcorrection of the patient’s cylinder—i.e., RA. Excessive RA will decrease a patient’s acuity and lens success. This month, we will examine how lens rotation affects RA in higher astigmats, in both soft and GP bitoric lenses. This discussion will assume we have a patient with –6.00DC of refractive astigmatism, with corneal cylinder being close to 6.00DC as well.
RA WITH SOFT TORICS
Soft toric lenses must supply all of the cylinder correction within the lens, and there is minimal to no lacrimal lens in play. When the lens rotates on the eye, all of the lens’s cylinder power is a potential source of cross-cylinder optics.
For our example patient, who has –6.00DC of cylinder, it doesn’t take much lens rotation to induce clinically significant RA (Figure 1). The green dots show that it only takes 5° of rotation to have 1.00DC RA with this much cylinder. Soft torics typically rotate this much with blinking, so this patient will likely be bothered by the resulting fluctuation in acuity.
RA WITH GP BITORICS
In a GP lens, a patient who has this much corneal cylinder will need a bitoric lens for best fit. The toric base curve is needed to make the lens align with the highly toric cornea. In general, bitoric lenses generate much less rotation-induced RA than soft torics. This is because the lacrimal lens power compensates for some or all of the cross-cylinder effect of the rotating contact lens.
The math for RA here is tricky, but here’s the basic principle: If the refractive and corneal astigmatisms are an exact match, the bitoric will have spherical power effect (SPE) optics and zero RA with lens rotation (blue dots on the graph). If they don’t match, the lens will have cylindrical power effect (CPE) optics, and lens rotation will induce some RA.
Let’s assume our patient has 6.00DC refractive cylinder but 5.00DC of corneal cylinder. With this mismatch of 1.00DC between them, their bitoric lens will have 1.00DC of CPE optics.
What does that mean? When the lens rotates, it is as though it only has 1.00DC instead of the full 6.00DC. You can see from the graph (red dots) that this means less RA overall. In fact, it takes a full 30° of rotation to reach the 1.00DC RA that we got with only 5° of soft toric rotation. For typical 5° to 10° fluctuations, there is less than 0.50DC of RA.
None of this means you cannot have success with higher astigmats in soft lenses. I routinely fit patients like this example with soft lenses, always keeping in mind the potential for fluctuating vision from rotational instability. For patients who need them, GP bitorics are a great option to provide clear, stable vision. CLS