Purpose: To measure the dynamic stability of the human crystalline lens by using a custom-made new instrument that tracks the lens oscillations following a controlled saccadic eye movement (lens wobbling effect)
Methods: We developed a modified prototype of a Purkinje-meter that uses a high temporal resolution CMOS camera attached to a telecentric lens and a semicircular ring of IR LEDs placed on the lens aperture‘s rim. Saccadic eye movements of 9‘ amplitude were induced by two flickering visible LEDs (1 Hz) placed in a central and a peripheral position with respect to the objective-camera axis. Four flickering orientations were possible (nasal, temporal, inferior and superior). The wobbling effect is a quick oscillation of Purkinje image PIV (posterior lens surface reflection) with respect to PI (corneal reflection) that follows immediately after a change in the direction of gaze and is captured at high speed (400 frames per second).Stability is assessed by fitting the relative movement of PIV with respect to PI to the solutions of a damped harmonic oscillator. Four parameters are used to characterize the wobbling effect, i) the amplitude (i.e., the overshooting effect with respect to the stable position of PIV), ii) the oscillation frequency, iii) the damping ratio and iv) the stationary time (the time constant of the wobbling effect). Videos were recorded in eight subjects during a series of 15 saccadic movements performed horizontally (center-temporal) and vertically (center-up).
Results: The mean value of the oscillation frequency after a saccadic was 19 Hz (standard deviation 6 Hz) and the mean damping ratio was 0.45 (SD 0.10). The average stationary time was 57 msec (SD 20 msec). Given that the mean post-saccadic suppression time is around 50 msec (Diamond et al, Journal of Neuroscience, 2000) the visual effect of the lens wobbling in normal subjects will not be noticeable. Comparing vertical and horizontal movement, we only observed differences in the amplitude, larger in the case of the horizontal movement (252 microns) than in the vertical upwards movement (194 microns), which perhaps reveals some gravity effects or an asymmetric distribution of tension on the lens zonule.
Conclusions: A new instrument to measure crystalline lens stability has been developed and successfully tested in normal subjects. It may be useful for a wide range of future studies involving lens stability.