Few studies have examined the effects of toric orthokeratology lenses on moderately astigmatic eyes, but what is available suggests that peripheral toric lenses may have comparable astigmatism-correcting and myopia-controlling effects as spherical orthokeratology lenses on eyes with low levels of astigmatism. Because lens fitting is based on both corneal astigmatism and elevation, researchers sought to examine their relationship in moderate-to-high astigmats and which variable was most effective for determining fit.
The researchers took corneal tomography measurements on the right eyes of 25 adults with moderate-to-high refractive myopic astigmatism. Subjects were fitted with toric orthokeratology lenses based on elevation and wore the lenses for 10 days. Centration, movement and decentration were analyzed.
The researchers found the average corneal astigmatism and peripheral elevation were strongly correlated, with each diopter of astigmatism equating to 25μm of elevation at an 8mm chord. Thirteen lenses decentered, based on a treatment zone determined by slit lamp, and 17 lenses decentered based on a tomography-determined treatment zone. The tomography treatment zone area was negatively correlated with central corneal astigmatism and elevation at an 8mm chord.
The researchers concluded that “from tomography images, central corneal astigmatism was highly correlated with peripheral elevation and may be a more expedient measure for clinical use.” They added that “treatment area decreased as corneal astigmatism and elevation increased.”
Tomiyama E, Logan AK, Richdale K. Corneal elevation, power, and astigmatism to assess toric orthokeratology lenses in moderate-to-high astigmats. Eye Cont Lens. June 19, 2020. [Epub ahead of print].