I am often told by new patients who have high amounts of astigmatism that they were told by other eyecare practitioners that they could not wear contact lenses. It is true that these cases present some challenges.

However, practitioners do have great choices for these patients that can work well. Here, I want to briefly revisit the optics of soft toric and bitoric GP lenses.

How Rotation Induces Astigmatism

All toric contact lenses have a potential problem: Lens rotation and axis misalignment will induce residual astigmatism (RA) and will decrease visual acuity. The general rule is that the RA is one-sixth of the original cylinder for every 5º of misalignment. For example, misalignment of 2.00DC by 15º would yield 1.00DC of RA (one-half of 2.00DC) and would likely reduce visual acuity. Note that the more cylinder with which you start, the higher the RA will be for the same amount of misalignment.

Soft Torics

For soft torics, all of the cylinder correction is in the lens itself (i.e., there is no lacrimal lens). When the lens rotates, the full cylinder amount is misaligning. In higher astigmatic powers, lens misalignment can be a significant problem. Let’s say that the lens power is +2.00 –4.00 x 180. With 10º of rotation, the RA will be one-third of this value or about 1.33DC. With modern soft toric designs, 10º of rotation is probably on the high side of between-blink rotation, but even 5º would produce nearly 0.75DC of RA.

Bitoric and Back-Surface-Toric GPs

These GPs are used with higher amounts of corneal astigmatism to improve the fitting relationship (Figures 1 and 2). Suppose that a patient’s refraction is +2.00 –4.00 x 180 and Ks are 43.00/46.50 @ 90 (3.50D of corneal cylinder). Fitting this patient with a bitoric GP will greatly improve the lens fit compared to a spherical GP. But how does lens rotation affect RA for bitoric lenses?

A bitoric lens for this eye will have about 4.00DC (depending on the base curve used). It is counterintuitive, but because of the lacrimal lens, the amount of cylinder error that is rotating isn’t the same as the total amount of cylinder in the lens. It is just the mismatch between the refractive and the corneal cylinder. In this example, it is only 0.50DC.

So, if this bitoric GP misaligns by 10º, there is only 0.16DC (one-third of 0.50DC) RA induced…much less than for the same 10º in a soft toric! Even if this lens rotates 30º, our example patient would have less RA (all of the 0.50DC) than when wearing a soft toric that rotates 5º. This is a significant benefit of the bitoric GP lens compared to a soft toric lens.

Offer the Best Choice

Soft torics can be excellent choices for many patients, even those who have high amounts of astigmatism. But for patients who are bothered by variable acuity from lens rotation or for those who have high visual demands, a bitoric GP lens can often provide crisper, more stable vision. Be sure to offer this option to your high astigmats. CLS