Compensation of corneal horizontal/vertical astigmatism, lateral coma, and spherical aberration by internal optics of the eye

Abstract

Both the anterior surface of the cornea and the internal optics (the posterior cornea, crystalline lens) contribute to the aberration of a wavefront passing through the eye. Artal, Guirao, Berrio, and Williams (2001) reported that the wavefront aberrations produced by the internal optics offset, or compensate for, the aberrations produced by the cornea to reduce ocular wavefront aberrations. We have investigated the wavefront aberrations of the cornea, internal optics, and complete eye on both the population and individual level to determine which aberrations are compensated and probable paths leading to that compensation. The corneal and ocular aberrations of 30 young subjects at relaxed accommodation were measured with the Topcon Wavefront Analyzer, which simultaneously measures refraction, corneal topography (videokeratoscope), and wavefront aberrations (Hartmann-Shack sensor). We found strong evidence for compensation of horizontal/vertical (H/V) astigmatism (Zernike term Z5) lateral coma (Z8) and spherical aberration (Z12). H/V astigmatism compensation is scaled for each individual, suggesting that it is actively determined by a fine-tuning process. Spherical aberration shows no individual compensation, suggesting that is a passive result of genetically determined physiology. Lateral coma shows individually scaled compensation, some of which may be attributable to eccentricity of the fovea.

Association between Offset of the Pupil Center from the Corneal Vertex and Wavefront Aberration

“…This is in agreement with several recent studies on compensation of horizontal coma between the anterior cornea and internal optics. [10][11] This is also consistent with the theoretical account on relationship between spherical aberration and coma aberration in a previous study. 12 But, surprising to us was the non-significant correlations between the y-axis pupil center shift and the vertical coma.…”

Determinants of visual quality after endothelial keratoplasty

“…The internal optics of the eye (posterior corneal surface and crystalline lens) directly oppose and reduce anterior corneal aberrations in normal eyes, 9,33,62,118,139 with compensation by the posterior corneal surface leading to an overall reduction in whole eye HOAs. Posterior corneal changes after EK affect its parallelism with the anterior corneal surface.…”

A review of astigmatism and its possible genesis

“…Kelly, Mihashi and Howland 15 used an instrument that allowed simultaneous capture of corneal and total eye aberrations on a population of young subjects. They found that some corneal aberrations are compensated by the internal optics of the eye, including horizontal/vertical astigmatism, lateral coma and spherical aberration.…”

“…12,15,16 The numerous studies into Javal's rule tend to indicate that this compensation is a passive process (that is, the majority of the population has approximately 0.5 D of internal astigmatism, opposite in sign to the corneal astigmatism). Some authors 15 have suggested the possibility of an active 'feedback driven' process operating to reduce the total astigmatism of the eye (particularly horizontal/vertical astigmatism).…”