Non‐invasive phakometric measurement of corneal and crystalline lens alignment in human eyes

Abstract

We describe a non-invasive phakometric method for determining corneal axis rotation relative to the visual axis (beta) together with crystalline lens axis tilt (alpha) and decentration (d) relative to the corneal axis. This does not require corneal contact A-scan ultrasonography for the measurement of intraocular surface separations. Theoretical inherent errors of the method, evaluated by ray tracing through schematic eyes incorporating the full range of human ocular component variations, were found to be larger than the measurement errors (beta < 0.67 degrees , alpha < 0.72 degrees and d < 0.08 mm) observed in nine human eyes with known ocular component dimensions. Intersubject variations (mean +/- S.D.: beta = 6.2 +/- 3.4 degrees temporal, alpha = 0.2 +/- 1.8 degrees temporal and d = 0.1 +/- 0.1 mm temporal) and repeatability (1.96 x S.D. of difference between repeat readings: beta +/- 2.0 degrees , alpha +/- 1.8 degrees and d +/- 0.2 mm) were studied by measuring the left eyes of 45 subjects (aged 18-42 years, 29 females and 16 males, 15 Caucasians, 29 Indian Asians, one African, refractive error range -7.25 to +1.25 D mean spherical equivalent) on two occasions.

Effects of Reference Axes Used During Measurements of Ocular and Corneal Higher-Order Aberrations in Patients Following LASIK

“…We believe that the main reason for the different dominance between ocular and corneal HOAs may be that the optical axis of the cornea is not coaxial with that of the lens, as has been suggested previously. 17 In that case, spherical aberrations of the cornea or lens can be asymmetric aberrations in the other optical system. Our results showed that there was an effect due to the axes of measurements in a wavefront analysis, at least in normal subjects and in patients following conventional LASIK.…”

Assessment and Statistics of Surgically Induced Astigmatism

“…A 14°tilt around the vertical axis of the crystalline lens would theoretically give this amount of RA 50 (p.209) . Actually, the tilt of the crystalline lens is much smaller 72,73 , which points to the posterior corneal surface as the source of the RA. Both anterior and posterior corneal surfaces have the same principal meridians 74 .…”

Advances in anterior segment imaging

“…Phakometry is a noninvasive technique for measurement of the crystalline lens radii of curvature, usually involving photography of the Purkinje-Sanson images [37]. It has now been over a century since Tscherning devised the ophthalmophakometer for the purpose of measuring and determining the spherical radii of ocular surfaces, the depths or thickness of ocular structures and their degree of alignment to an optical axis [38].…”