ORTHOKERATOLOGY (ortho-k) lens technology has improved a great deal since the early days of reverse-geometry lenses. Correcting a spherical target corrected with ortho-k is usually straightforward, but targeting an eye with astigmatism can be more challenging. Fortunately, we now can tailor the astigmatism correction for a better visual outcome for the patient.

In the early days of reverse-geometry lenses, we only had spherical designs. My master’s thesis looked at astigmatism changes with spherical designs and found that, on average, about half of the corneal cylinder was corrected. This often resulted in patients having residual astigmatism. Practitioners now have toric reverse and alignment curves, which have the ability to correct for more corneal astigmatism and aid in lens centration. It is typically recommended to use toric ortho-k lenses when the corneal elevation difference at an 8 mm chord is more than about 30 µm.

Refractive astigmatism is a mixture of corneal and lenticular cylinder, and it is crucial to consider the amount of corneal astigmatism compared to the total refractive astigmatism. If there is a mismatch, there will be some degree of residual astigmatism; the amount of toricity in the ortho-k lens should take this into account.

Frequently, we find the patient has more corneal cylinder than refractive. The most effective treatment would be to aim for undercorrecting the corneal cylinder. As a case example, a patient had a refractive error of –3.00 –0.50 x 010 and Ks of 42.25/43.50 @ 095. If the lens were to fully eliminate the 1.25 DC corneal cylinder, the patient would have 0.75 DC residual astigmatism after lens wear. The initial topography (Figure 1-left) shows about 32 µm of elevation difference, which would typically call for a toric design. For this case, a spherical reverse curve was used to undercorrect the corneal cylinder. A toric alignment zone, however, was used to help with lens centration.

The posttreatment topography (Figure 1-right) shows the corneal astigmatism only decreased by about 0.33 DC, very close to the baseline refractive astigmatism, and the posttreatment refraction was +0.50 DS, with 20/15 uncorrected vision.

Consider the potential for residual astigmatism in your ortho-k cases and tailor the lens choice or the amount of toricity in the lens to correct the optimal amount of corneal astigmatism. Use of a spherical lens, or customizing the toricity of a reverse or alignment curve, can make this possible. As in the case example above, practitioners could use a spherical reverse curve or at least reduce the amount of reverse curve toricity. The GP lab can advise them on all of the options.