Irregular astigmatism causes
Examples of causes of irregular astigmatism include forme frustra keratoconus, post-myopic LASIK, post-hyperopic LASIK, and penetrating keratoplasty, Dr. Kohnen said.
"Patients with irregular astigmatism desire the best vision possible and spectacle independence," he added.
To achieve that, Dr. Kohnen pointed out that physicians should keep in mind the differences between corneal and lens astigmatism, internal and external astigmatism, anterior and posterior astigmatism, pre-operative and postoperative astigmatism, astigmatism in the IOL plane, and astigmatism in the spectacle plain.
Until recently, researchers have dismissed the differences between astigmatism of the anterior and posterior corneal surfaces because of the small differences between the indices of refraction between the two, and the limited options for measuring the differences, Dr. Kohnen said.
Today, new measurement capabilities are leading to studies on these differences. In three large studies, the mean magnitude of corneal posterior astigmatism was about -0.3 D, he added.
"When you look at the anterior surface, you have to distinguish between anterior with the rule, anterior against the rule, and anterior oblique," Dr.Kohnen said. "But this does not go in alignment with posterior astigmatism."