Application of femtosecond lasers to cataract surgery has added unprecedented precision and reproducibility but ocular safety limits for the procedure are not well-quantified. We present an analysis of safety during laser cataract surgery considering scanned patterns, reduced blood perfusion, and light scattering on residual bubbles formed during laser cutting. Experimental results for continuous-wave 1030 nm irradiation of the retina in rabbits are used to calibrate damage threshold temperatures and perfusion rate for our computational model of ocular heating. Using conservative estimates for each safety factor, we compute the limits of the laser settings for cataract surgery that optimize procedure speed within the limits of retinal safety.
Femtosecond lasers have added unprecedented precision and reproducibility to cataract surgery. However, retinal safety limits for the near-infrared lasers employed in surgery are not well quantified. We determined retinal injury thresholds for scanning patterns while considering the effects of reduced blood perfusion from rising intraocular pressure and retinal protection from light scattering on bubbles and tissue fragments produced by laser cutting. We measured retinal damage thresholds of a stationary, 1030-nm, continuous-wave laser with 2.6-mm retinal spot size for 10- and 100-s exposures in rabbits to be 1.35 W (1.26 to 1.42) and 0.78 W (0.73 to 0.83), respectively, and 1.08 W (0.96 to 1.11) and 0.36 W (0.33 to 0.41) when retinal perfusion is blocked. These thresholds were input into a computational model of ocular heating to calculate damage threshold temperatures. By requiring the tissue temperature to remain below the damage threshold temperatures determined in stationary beam experiments, one can calculate conservative damage thresholds for cataract surgery patterns. Light scattering on microbubbles and tissue fragments decreased the transmitted power by 88% within a 12 deg angle, adding a significant margin for retinal safety. These results can be used for assessment of the maximum permissible exposure during laser cataract surgery under various assumptions of blood perfusion, treatment duration, and scanning patterns.
Pan-retinal photocoagulation in patients with diabetic retinopathy typically involves application of more than
1000 laser spots; often resulting in physician fatigue and patient discomfort. We present a semi-automated patterned
scanning laser photocoagulator that rapidly applies predetermined patterns of lesions; thus, greatly improving the
comfort, efficiency and precision of the treatment.
Patterns selected from a graphical user interface are displayed on the retina with an aiming beam, and
treatment can be initiated and interrupted by depressing a foot pedal. To deliver a significant number of burns during
the eye's fixation time, each pulse should be considerably shorter than conventional 100ms pulse duration. We
measured coagulation thresholds and studied clinical and histological outcomes of the application of laser pulses in the
range of 1-200ms in pigmented rabbits.
Laser power required for producing ophthalmoscopically visible lesions with a laser spot of 132&mgr;m decreased
from 360 to 37mW with pulse durations increasing from 1 to 100ms. In the range of 10-100ms clinically and
histologically equivalent light burns could be produced. The safe therapeutic range of coagulation (ratio of the laser
power required to produce a rupture to that for a light burn) decreased with decreasing pulse duration: from 3.8 at 100ms, to 3.0 at 20ms, to 2.5 at 10ms, and to 1.1 at 1ms. Histology demonstrated increased confinement of the thermal damage with shorter pulses, with coagulation zone limited to the photoreceptor layer at pulses shorter than 10ms. Durations of 10-20ms appear to be a good compromise between the speed and safety of retinal coagulation. Rapid application of multiple lesions greatly improves the speed, precision, and reduces pain in retinal photocoagulation.
Cryogen spray cooling (CSC) is used to minimize the risk of epidermal damage during pulsed laser treatment of port wine stain (PWS) birthmarks. Unfortunately, the current approach to CSC does not provide the necessary epidermal protection for all patients, particularly those with darker skin types. Therefore, alternative approaches need to be sought to improve PWS laser therapy.
On a previous numerical study we showed that using multiple-intermittent CSC spurts and laser pulses could permit, under certain conditions, the use of higher laser doses while providing sufficient epidermal protection. In this study we show some results of ongoing experimental to study the feasibility of implementing clinically such an approach.
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