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aphakia page 1  W F Long 1992 aphakia page 1  W F Long 1992

aphakia page 1 W F Long 1992 - PDF document

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aphakia page 1 W F Long 1992 - PPT Presentation

aphakic eye which has very different optical propertiesfrom the normal or phakic eye correction Thus it is very important to establish the exact vertexdistance at which an aphakic refraction is d ID: 936201

long lenses eye aphakia lenses long aphakia eye page patient refraction image lens calculator acuity spectacles size curves contact

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aphakia, page 1 © W. F. Long, 1992 aphakic eye which has very different optical propertiesfrom the normal or phakic eye. correction. Thus it is very important to establish the exact vertexdistance at which an aphakic refraction is done. This can be done with a distometer, a problem can be avoided by doing an over-refraction. Eschewing thehabitual prescription with a pair of trial clips. When the refraction islike an ordinary refraction. One difference--since the

main source of phakic eyeaphakic eye aphakia, page 2 © W. F. Long, 1992Such high powered corrections produce a number of difficulties. AmongDecreased field of view with ring scotomasome not. There are two main approaches the "foveal philosopy" and the"peripheral philosophy". Both, as we'll see, make use of aspheric curves, scotoma aphakia, page 3 © W. F. Long, 1992trying to give the patient the largest possible dynamic field of view. Aswith spherical curves. Inst

ead aspheric curves are used. In order to cutThis assumes aphakics are head--not eye--turners. It uses front surfaceor Signet Hyperaspheric. These diminish the lens thickness and lower itstaken into account. In equation form, = image size with spectacles = image size with contact lenses aphakia, page 4 © W. F. Long, 1992© W. F. Long, 1992()()The ratio of the image size with spectacles to that withcontact lenses is thenM=(+14.42)/(+12.00)=1.20X,or 20% la

rger with spectacles.Incidentally, sometimes a low vision patient is better corrected withspectacles because the larger image gives better acuity.Problem:If contact lenses. If the patient's Snellen acuity with contact20/70 acuity with contacts. Thus the patient losesFitting a contact lens to an aphakic is not a simple proposition. The highexpensive. Fortunately, most aphakics will be wearing bifocals over their aphakia, page 5 © W. F. Long, 1992implant. These smal

l plastic lenses are routinely inserted in the eye toreplace the patient's own crystalline lens. A patient wearing an implant decade. Initial experiments used "iris clip" lenses which weremechanically attached to the iris and sat in the anterior chamber. Sincecapsule intact). The commonest lenses are made ofPMMA prior to lens fabrication. typical posterior chamber implant aphakia, page 6 © W. F. Long, 1992the A-scan measurement of the axial length of the eye. Thes

e numbers arestatistical fudge factors. The most popular of these equations is the 'SRK'formula used in dedicated calculator and computer programs. The latestformula used in dedicated calculator and computer programs. The latest()()Usually this formula is programmed into a calculator or microcomputer toprevent calculation errors.There has been some experimentation with bifocal IOL's. These are basedcontact lenses. They can be expected to be about as successful as