Excimer laser beams (193 nm) of uniform fluence were studied to find out why they produce corneal ablations deeper at the edge than the center. Ablation depth profiles were taken of porcine corneas, including five dehydrated samples. Hydrated corneas and polymethyl methacrylate were ablated with and without central masks. Ablation plumes were photographed. Hydrated porcine corneas showed patterns of central underablation. As the incident beam increased, the crater exhibited increasingly shallower central ablation while maintaining nearly constant depth at the edges. Dehydrated corneas did not vary significantly. Masks did not alter the depth or shape of craters near ablation edges, but depth adjacent to the images of the masks was more than twice that with no mask. Depth adjacent to the mask image was nearly the same as at the edge of the zone. The rate of change in depth with position was nearly equal in both areas. Maximum plume density was centered over the entire ablation with and without the mask. Redeposition of plume particles is not the major cause of central underablation. Propagating transverse energy from the absorption of photons by peptide bonds increases pressure on excited components within the irradiated area, increasing recombination, which raises the ablation threshold.
A study was conducted to investigate why 193 nm excimer laser beams of
uniform fluence produce corneal ablations that are deeper at the edge than at the center.
Enucleated porcine eyes were ablated and measured with an optical profilometer. A
dehydrated cornea was also ablated. Enucleated porcine eyes and PMMA were ablated
with and without narrow central masks, and ablation plumes were photographed.
Characteristic patterns of central underablation were present in the porcine corneas.
Ablation craters ranging in diameter from 2.0 to 6.5 mm exhibited increasingly shallower
central ablation and nearly constant depth at the edges. There was no significant depth
variation in the dehydrated cornea. Masks did not change the depth or shape of craters
near the edges of the zone; but depth adjacent to the images of the masks was more than
twice that with no mask. The depth adjacent to the mask image was nearly the same as at
the edge of the zone, and the rate of change in depth with position was nearly equal in
both areas. The area of maximum plume density centered over the entire ablation with or
without the mask. Redeposition of plume particles is not the major cause of central
underablation; rather, propagating transverse energy from the absorption of photons by
the peptide bonds increases pressure on the excited components within the irradiated
area, increasing recombination, which in turn raises the ablation threshold.
This paper presents an analysis of the effects of axial and transverse displacement on the optical quality and accuracy of lenses created during excimer laser photoablation. Tolerance levels for axial positioning of the cornea prior to and during surgery are presented. The axial tolerance levels are dependent upon a number of parameters which include the intended dioptric correction and laser system cone angle. A collimation lens is introduced as a means of desensitizing the laser system to axial displacement. Transverse displacement tolerances during laser treatment are shown to depend upon the treatment diameter, dioptric correction and acceptable distortion level in the lens ablated into the anterior corneal stroma. A video and computer analysis of transverse motion during seven randomly selected excimer laser refractive surgeries is presented. Although transverse displacement exceeded the tolerance levels presented, it did not appear to affect the quality of correction in the eight patients analyzed.
Using 193 nrn Excimer Laser light to reshape the cornea has been shown to be an effective way to correct myopia in man"2 One way to achieve the controlled shape is to create a large diameter Excimer beam, and pass it through a computer controlled ins diaphragm. As the diaphragm closes, progressively less laser pulses reach the outer portions of the treatment area, while all the pulses reach the center region, effectively flattening the cornea. The resultant correction is spherical, and does not correct astigmatism. This paper discusses the use of a motor controlled slit to correct astigmatism in a similar manner to myopia. The mechanism described combines an iris diaphragm and adjustable slit for correcting myopia, astigmatism, or a combination of the two. The slit mechanism also rotates so the slit axis can be aligned with the patient's astigmatic axis. All motions are motor driven under computer control, with encoder feedback to ensure correct positioning. Initial test medium was PMMA, which has ablation characteristics similar to human stromal tissue. Mechanical profile scans and lensometer tests of the PMMA test blocks were performed to verify the correct ablation pattern. Animal eyes were ablated and tested for astigmatic induction.
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