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6. Why do I get astigmatism after my Ortho-K mould?

6. Orthokeratology TroubleshootingCharl Laas

You also need to consider induced cylinder when the Forge lens' alignment curve did not seal 360 degrees. In this scenario, the one meridian achieved a myopic reduction, but in the other meridian the myopic reduction was unsuccessful or under corrected. The meridional correction will result in an induced cylinder due to improper moulding in the one meridian. Using the axial power map will highlight this scenario where increased astigmatism is seen in the treatment zone.

This meridional under correction can sometimes be used to our advantage, where we do wrong to do right. Consider the case above with corneal cylinder but no spectacle cylinder. Usually, you will fit a Forge Toric lens on a toric cornea, but this will induce residual cylinder as the refraction had no cylinder, indicating the presence of internal astigmatism. To explain this better, internal astigmatism corrected for corneal astigmatism resulting in a spherical refractive prescription. If you mould the cornea with the Forge Toric lens you will correct all the corneal astigmatism but the lenticular astigmatism will now manifest.

By changing the design to a Forge RS design you will achieve under correction in the steep meridian and therefore not fully correct for the corneal astigmatism which now remains to neutralise the internal lenticular astigmatism. The post fit map will not be a beautiful textbook bull's eye pattern, BUT the patient will have improved vision compared to using a Forge Toic lens.

However, 'cheating' with a Forge RS lens to leave corneal astigmatism uncorrected can lead to unpredictable outcomes and is generally not recommended.