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Phillip Gooding, Michael Wiltberger, Alexander Vankov, Georg Schuele; Tool for Evaluation of Target Tissue Instability Induced by Head and Eye Movement with Fs Laser Docking Schemes. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4183.
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© ARVO (1962-2015); The Authors (2016-present)
Stability of the target tissue during femtosecond laser cataract surgery is important for creating high quality cuts. Applying physical constraints directly to the patient to assure absolutely no movement is impossible. This ex-vivo study provides insight in to the relationship between patient head and eye movement to target tissue stability with the intent of creating a useful tool for evaluating stability of patient interface designs and performance of active target tracking schemes.
A test fixture was constructed to simulate the patient docking geometry of the CATALYS ® System. A linear motor actuator under software control was used to generate motion of the test globes with respect to the patient interface. Controlled forces were applied to test globes by variation of globe motion amplitude and frequency. Target tissue movement was measured by recording fiducial mark motion through a video microscope and performing image analysis (Figure 1). Reasonable parameters for patient induced force magnitude and frequency were obtained from literature and generalized patient data from over 850 CATALYS ® System procedures. The globe displacement and frequency forcing function was categorized in to two regimes: low frequency (0.25 Hz) full cycle to emulate gross head movement due to patient breathing, and high frequency (1 to 100Hz) half-sine pulses to emulate saccadic movement. Fresh cadaver globes (2.5 ±0.9 days) of typical appropriate age for cataract surgery (79 ± 8.1 years) were secured to a spherical holding cup with adhesive.
Under low frequency cycling, cadaver eyes (N=8) exhibited a non-linear response with a maximum effective spring rate of 140 microns per kgf yielding a displacement of 22 microns at 150 grams applied force (Figure 2). Under high frequency half-cycle pulse, cadaver eyes (N=4) exhibited a highly non-linear response with effective spring rate ranging from 144 microns per kgf at low frequency to 68 microns per kgf at high frequency. This yields displacements ranging from 22 microns to 10 microns across the 1 to 100 Hz frequency band.
This tool proved useful for observation of target tissue stability under a variety of simulated patient induced movements enabling a method for relative performance evaluation of future patient interface designs and target tissue tracking architectures.
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