Binocular lens tilt and decentration measurements in healthy subjects with phakic eyes

Tilt and decentration of the natural crystalline lens affect optical quality of the foveal image. However, little is known about the distributions of these variables in healthy subjects with phakic eyes and about their correlations in both eyes. A simple, portable, easy-to-use, and partially automated device was developed to study lens tilt and decentration in both eyes of 11 healthy subjects with phakic eyes.

The first, third, and fourth Purkinje images (P1, P3, P4) were visualized using a single infrared (IR) light-emitting diode (LED), a planar lens (F = 85 mm; f/number of 1.4), and an infrared sensitive analog video camera. Software was developed to mark pupil edges and positions of P1, P4, and P3 with the cursor of the computer mouse, for three different gaze positions, and an automated regression analysis determined the gaze position that superimposed the third and fourth Purkinje images, the gaze direction for which the lens was oriented perpendicularly to the axis of the IR LED. In this position, lens decentration was determined as the linear distance of the superimposed P3/P4 positions from the pupil center. Contrary to previous approaches, a short initial fixation of a green LED with known angular position calibrated the device as a gaze tracker, and no further positional information was necessary on fixation targets.

Horizontal and vertical kappa, horizontal and vertical lens tilt, and vertical lens decentration were highly correlated in both eyes of the subjects, whereas horizontal decentration of the lens was not. There was a large variability of kappa (average horizontal kappa -1.63 degrees +/- 1.77 degrees [left eyes] and +2.07 degrees +/- 2.68 degrees [right eyes]; average vertical kappa +2.52 degrees +/- 1.30 degrees [left eyes] and +2.77 degrees +/- 1.65 degrees [right eyes]). Standard deviation from three repeated measurements ranged from 0.28 degrees to 0.51 degrees for kappa, 0.36 degrees to 0.91 degrees for horizontal lens tilt, and 0.36 degrees to 0.48 degrees for vertical lens tilt. Decentration was measured with standard deviations ranging from 0.02 mm to 0.05 mm. All lenses were found tilted to the temporal side with respect to the fixation axis (on average by 4.6 degrees ). They were also decentered downward with respect to the pupil center by approximately 0.3 mm.

Lens tilts and positions could be conveniently measured with the described portable device, a video camera with a large lens. That the lenses were tilted to the temporal side in both eyes, even if corrected for kappa, was unexpected. That they were displaced downward with respect to the pupil center could be related to gravity.