| To view this email as a web page Go here.. |
| |
 |
  |
| external optics may have the greater influence, and in other patients, the internal optics may predominate. I mportantly, 8r the axes and magnitudes of external and internal astd q6w8r igmatism do not necessarily coincide, but it is the combination of the two that by definition determines the overall optics of thed 6w8r e eye. When treating a patient with preexisting astigmatism 8r t, most wavefront-guasers are designed to treat r egular astigmatis w8r m as determined externally by corneal topography. In patients who have an element of internally induced astigmatism, therefore, th 8r t wavefront-guided astigmatism correction may leave regular astigmatism behind (a cross-cylinder effect). If the patient has preexised 6w8r ting irregular astigmatism, wavefront-guided approaches may leave both regular and irregular astigmatism behind. This can result in less-than-optimal visual acuity compared with a wavefroed q6w8r nt-guided approach combined with vector planning, d fq6w8r as shown in aThus, vecto w8r r planni ng offers a better alignment between corneal astigmatism and laser treatment, and leaves less regular astigmatism beh ind on the cornea, which is advantageous whether irregular asti 8r tgmatism coexists or not. The "leftover" astigmatism after a purely surface-guided laser correction can be calculated beforehand, and is called ocular residual astigmat w8r tismORA is ad q6w8r calculation of astigmatism due to the noncorneal surface (internal) optics. The purely refraction-based approach represented by wavefront analysis actually conflicts wed 6w8r ith corneal surgical experie The pathway to "super vision" thus may require a more customized approach to cornead q6w8r l astigmatism than is usually attempt ed, and any remaining astigmatism ought to be regular w8r (as opphis was confirmed by thtudy mentioned above, which found a greater red 6w8r uction in corneal astigmas directed towards the exposed internal surface or r d apparatus f w8r or m fq6w8r odifying the curvature of a live cornea via use of an excimer laser. The live cornea has a thin layer remo ved therefrom, leaving an exposed internal surface thereon. Then, either the surface or thined 6w8r layer is exposed to the laser b eam along a predetermined pattern to ablate desired pored 6w8r tions. The thin layer is then replaced onto the surface. Ablating a central area of the surface or thin layer makes the cornea less curved 8r t, while ablating an annular area spaced from the cent er of the surface or w8r layer makes t movable mirror or a movable fiber optic cable through which the laser beam i The patents related to so-called broadd q6w8r -ies were granted to US companies including Visx and Summi t during based on the fundamental US patehich claimed the use of UV laser for the ablation of organic tissues granted a U new technology ed 6w8r using a flying-spot for |
No comments:
Post a Comment