The posterior cornea: hiding in plain sight.
نویسنده
چکیده
For more than 100 years, eye scientists and clinicians have recognized that there is a difference between the refractive and anterior corneal astigmatism of the human eye. Javal postulated an approximately linear relationship between corneal astigmatism and refractive astigmatism, which became known as Javal’s rule: refractive astigmatism 1⁄4 1.25 (keratometric astigmatism) 0.50 90. For most eyes, this means that, compared with anterior corneal astigmatism, refractive astigmatism in most eyes has less net plus power along the vertical meridian. In traditional refraction terminology, refraction tends to have less withthe-rule (WTR) or more against-the-rule (ATR) astigmatism. Until recently, the source of this difference was unknown. Many, myself included, assumed that the crystalline lens was solely responsible for creating a refractive ATR effect. Other candidates were the posterior cornea and the retina. The role of the posterior cornea was dismissed because of the small difference between the indices of refraction of the cornea and aqueous. For the posterior cornea to produce the same amount of refractive astigmatism as the anterior cornea, the difference in curvature between the steep and flat posterior corneal surfaces would have to be approximately 10 times greater. The retina was dismissed on the basis of theory and confirmed by a clever retinoscopic study by Flüeler and Guyton.
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ورودعنوان ژورنال:
- Ophthalmology
دوره 122 6 شماره
صفحات -
تاریخ انتشار 2015