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M.E. Arnoldussen; Comparison of Axial Displacement Tolerances of VISX Excimer Laser Systems . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2679.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The original STAR was designed to minimize the effect of axial displacements away from the intended treatment plane caused by such factors as initial miss-focus on the corneal apex or patient movement during surgery. Successive generations of the excimer laser system have incorporated adjustments for smoother ablations at the cost of a narrower focusing tolerance. This study compares the respective axial sensitivities of these systems based on spherical treatments centered on-axis. This historical perspective demonstrates how the axial displacement tolerance has successively improved after the advent of smoothing ablations. Methods: A platform stage with a micrometer adjustment for varying treatment depths was attached to the housing around the final turning mirror of different VISX excimer laser systems (STAR, S2, standard S3, VSS S3). Each system was calibrated to meet normal operating specifications, with the zero reference height standardized to the corneal treatment plane. A –4 D lens was cut for multiple axial displacements in millimeter increments moving both closer to and farther away from the laser (N > 15 per system). The power of each lens was read with a standard lensometer. Additionally, lenses made by the S3 systems (the newest in the series at the time of the study) were scanned along the X-axis using a profilometer (Tencor Model P-10). Astigmatism was not measured in this experiment. Results: The lens power readings were fit to linear curves as a function of axial displacement (r^2 > 0.96 for all cases). The diopters per millimeter found were: 0.06 for STAR, 0.069 for STAR S2, 0.067 for STAR S3 using standard concentric ablations, and 0.06 for STAR S3 using Variable Spot Scanning. Scans of the S3 ablations show that the effective power is not only a function of the ablation area covered but also of the angle of the beam incident to the ablation plane. Conclusions: The less sensitive a laser system is to axial displacements, the more consistent the treatment results should be. Although other factors have affected the overall design through the years, the axial displacement tolerance of the STAR S3 system with scanning ablations shows that the sensitivity has returned to the level originally seen in STAR.
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