Purpose: : Different intraocular lens designs may provide more or less stability after surgery. To improve IOL centration and stability outcomes, an instrument to measure both parameters is required. Along this objective, we developed a new instrument to quantify IOL misalignments and stability from the dynamic recording of Purkinje images.
Methods: : The prototype is based on a previous version of a system recording Purkinje images (Tabernero et al., Optics Express 2007), but includes significant improvements and modifications. The system allows video recording at 50 Hz and the capture during saccadic eye movements (9º amplitude) induced by two flickering fixation LEDs placed in a central and a peripheral position with respect to the objective-camera axis. Flickering frequency can be switched to 1, 0.5 or 0.33 Hz. In order to investigate directional IOL stability, the peripheral positions of the flickering LEDs included four possible orientations (nasal, temporal, inferior and superior). When an IOL suffers from wobbling, a quick oscillation of Purkinje images PIII and PIV (from anterior and posterior IOL surfaces) can be observed immediately after a change of gaze. Stability is assessed by fitting the relative movement of PIII and/or PIV with respect to PI (reflection from the anterior corneal surface) to the solutions of a classical dumped harmonic oscillator system. The wobbling effect for each IOL is characterized in terms of the “dumping ratio” values resulting from the fitting analysis. Dumping ratios ≥ 1 means that wobbling is not present (the ideal situation in this case) and null values means that the system oscillates without dumping.
Results: : Purkinje images were recorded in five pseudophakic patients (implanted with clinically stable monofocal IOLs) during a series of six saccadic movements (amplitude 9º; frequency 1 Hz). The damping ratio values ranged from 0.07 to 0.30 (average 0.13; standard deviation 0.05), showing a measurable wobbling effect in each case. The IOLs returned to a stationary point in a time that ranged from 1/10 to 1/2 seconds. The analysis revealed that the origin of the wobbling came from tilt oscillations (up to 1 degree). The IOL was also slightly decentered (0.1 mm) as a consequence of the saccadic movements, but in this case it returned to its original position with nearly no oscillations.
Conclusions: : A new instrument to measure IOL misalignments and stability has been developed and successfully tested in patients. It may be useful for a wide range of future studies involving IOL stability.