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

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.