opthalmology individual work

8/3/2019 Opthalmology Individual Work

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CONTACT LENS CORRECTION

REFRACTIVE SURGERY

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When light rays pass from a medium of onedensity to a medium of a differentdensity they are refracted or bent. This principle is used intheeyeto focuslight onthe retina. Before reaching the retina light rays pass successivelythroughthe cornea, aqueous humor, lens and vitreous which are all more

densethanthe air. Emmetropia is thenormal optical condition oftheeye. Theeye is considered

to beemmetropic when incident parallel rays of light from infinity cometo afocus onthe retina with accommodation at rest. Anemmetropic eye will havea clear image ofdistant object without any internal adjustment of its optics.The average power of a normal emmetropic eye is +58 to + 60 D

The optical condition oftheeye in whichthe incident parallel rays of lightdo

not cometo a focus uponthe light sensitive layer ofthe retina, withaccommodation at rest is known as ametropia.

. In axial ametropia, theeyeball length is either too long (myopia) or too short(hyperopia). In refractive ametropia, thetotal power oftheeye is either toogreat (myopia) or too little (hyperopia) for the axial length oftheeyeball.

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Inmyopia, the focus is in front ofthe retina, anddivergentrays from a near object cometo focus onthe retina. In suchaneye, if a point onthe retina is considered as the object,the image ofthat point will lie somewhere in front oftheeyebut closer than infinity, andthat is, by definition, the far pointofthateye. Its plane is the far point plane which is conjugateto the retina. The correcting lens that focuses parallel rays oflight atthat far point plane oftheeye must be a diverginglens.

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MYOPIA

CORRECTION

Eye care professionals

most commonly correct

myopia through the use

of corrective lenses such

as glasses or contactlenses. It may also be

corrected by refractive

surgery, though there

are cases of associated

side effects. The

corrective lenses have anegative optical power

(i.e. are concave) which

compensates for the

excessive positive

diopters of the myopic

eye.

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Inhyperopia, the focus is behindthe retina. The farpoint plane ofthehyperopic eye lies behindthe retina.The correcting lens must be a converging lens of such

power that parallel rays of light will focus atthe samedistance behindtheeye as the far point plane ofeye.Etiology ofhyperopia:

axial there is short length oftheeyeball;

re

fractive

: curvature

the

re

is flat

curvature

ofthe

cornea (occurs in cornea plana); index there isincrease in refractive index ofthe lens cortex; inaphakia; in posteriorly subluxated lenses; in macularedema.etc.

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HYEROPIA CORRECTION


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The curvature of the cornea or lens may vary in different meridians, thereby

producing astigmatism. Irregular astigmatism is usually the result of distortions in the

anterior corneal surface (scarring, ectasia, edema, ulcer). This is correctable with hardcontact lenses but not with glasses. In regular astigmatism the axes of greatest and

least refractive power are at right angles. It can be corrected with glasses.

Anisometropia is a difference in the refractive errors of the two eyes. It is commonly

said to be present if the difference in the refractive corrections is 2,00 D or more, eitherspherical or astigmatic.

Aniseikonia is a difference in the size or shape of the images from the two eyes, as

perceived by the visual cortex. The most common cause is the differential magnification

inherent in the spectacle correction of anisometropia, producing different-sized retinal

images.

Aphakia is a condition of the eye where lens has been removed, i .e. absence of

lens. It is a classical example of acquired high hyperopia.

Optical condition: the eye is hyperopic; there is loss of accommodation; the retinal

image is about 25% larger; astigmatism (against the rule). Signs: the anterior chamber is

deep; iridodonesis (tremulousness of iris); Purkinje 3rd and 4th images are absent; thepupil is jet black.


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Presbyopia is gradual loss of accommodative response resulting from loss ofelasticity of lens capsule and lens substance. It becomes a clinical problem whenthe remaining accommodative amplitude is insufficient for the patientto carryouthis accustomed visual tasks. Symptoms of presbyopia usually begin after age40. Presbyopia is treated by prescribing suitable spherical lenses for near work..

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Spherical lenses has equal curvature in all meridians. Convex (plus) lens is used in correction ofhyperopia, presbyopia, aphakia. Identification.Whenthe lens is moved in front oftheeye, the objects

move inthe oppositedirection. If an object is held closeto the lens, itappears to be magnified.

Concave (minus) lens is used in correction of myopia. Identification.Whenthe lens is moved in front oftheeye, the objects move inthe samedirection. An object seenthroughthe lens appears to bediminished insize.

Cylindrical lens. It is a segment of a cylinder of glass cut parallel to its axis.The axis of a cylindrical lens is parallel to that ofthe cylinder of which it ispart.

Identification. Two marks are seen onthe lens indicating the axis ofthelens.Whenthe lens is moved inthedirection ofthe axis, there is nomovement ofthe objects.Whenthe lens is moved in a direction at rightangles to the axis: convex cylinder the objects move inthe oppositedirection; concave cylinder the objects move inthe samedirection.

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The Role ofOptical Defocus

Optically Imposed Myopia

To compensate, theeye must

become morehyperopic.

Optically Imposed Hyperopia

To compensate, theeye must become

more myopic.

Positive Treatment

Lens

Negative Treatment

Len

s


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Contact lenses arethin, clear disks of plastic

that float onthe surface oftheeye. They

correct vision likeeyeglasses do and are safewhen used with care. Contact lenses are used

to correctthe same conditions thateyeglasses

do: myopia (nearsightedness), hyperopia

(farsightedness), astigmatism(blurred vision

dueto the shape ofthe cornea) and

presbyopia (inability to see close up).

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Dynamic refraction is a term usedto

distinguish active refraction (with

accommodation functioning) from staticrefraction (with accommodation paralyzed by

a cycloplegic). The refractive power oftheeye

is increased when accommodation is actively

exerted.

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Determination ofrefraction is done by thefollowingmethods:

Objective:

1. Retinoscopy(sciascopyorshadowtest)isatechniquefordeterminingthe

refractivepowerofaneyeobjectively. Retinoscopyisdoneinadarkroom. The

examinersitsat1mdistancefromthepatient. Thepatientwearsatrialframeandfixesaspotatthefarendoftheroomsothatthelightraysenteringtheeye

areparallel. A lightisplacedbehindandabovethepatient`shead. The

examinerlooksthroughacentralholeintheplanemirrorinthepatient`seye.

Themirrorismovedslowlyfromonesidetotheother. Thedirectioninwhich

theshadow

moves

is

noted

.The

horizontal

meridian

is

observed

first,

and

then

thevertical.

Inhyperopiceyetheshadowmovesinthesamedirectionasthemirror. In

myopiceye(above-1D)theshadowmovesintheoppositedirection. In

emmetropiceyeandmyopiaoflessthan1Dthereisaveryfaintshadow

movinginthesamedirection. Inastigmatismtheshadowappearstoswirl

around(scissor-shaped).2. Auto-refractometry refractionistestedautomaticallyusingelectronicand

computertechnology.

3. Ophthalmometryorkeratometry methodsdesignedformeasuring

cornealastigmatismandfordeterminingtheaxisofthisastigmatism.

4. Ophthalmoscopy.

Subjective:Postmydriatictest.


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Eyesight correctionhas evolvedthe past few centuries.We started witheyeglasses, whichhave beentested by centuries of use, thendevelopedcontact lenses for a more fashionable look. Somedecades ago, a newform ofeyesight correction was developed. This involved surgery andthechange resulting from the operation is more permanentthan previoussolutions.

Several ways of performing eye surgery weredeveloped, butthe mostpopular of all is Lasik (short for Laser-assisted in situ keratomileusis). Lasikuses lasers to changethe shape ofthe cornea (the front part oftheeyethatdoes most ofthe focusing for our eyes) to improve vision. However,several complaints came up after treatment so it became importanttocreatenew Lasik technologies to improve results and increase safety.

Inthe recenttimes the following procedures have been accepted by therefractive surgeons:

- radial keratotomy (RK); - photorefractive keratectomy (PRK); - Laser-assisted in situ keratomileusis (LASIK);

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Refractive eye surgery is any eye surgery usedto improvethe

refractive state oftheeye anddecrease or eliminate

dependency on glasses or contact lenses. This can includevarious methods of surgical remodeling ofthe cornea or

cataract surgery. The most common methods today use

excimer lasers to reshape curvature ofthe cornea. Successful

refractiveeye surgery can reduce or cure common visiondisorders such as myopia, hyperopia and astigmatism.

Wave front technology:

Wave fronttechnology was developed intheearly 21st

century. It is usedto create a three-dimensional map ofthecornea. This improved Lasik becausethe surgical plans

developed from the maps are customized for each patients

eye.


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Radial Keratotomy or RK and Photorefractive Keratectomy or PRK areother refractive surgeries usedto reshapethe cornea. In RK, a very sharpknife is usedto cut slits inthe cornea changing its shape. PRK was the firstsurgical proceduredevelopedto reshapethe cornea, by sculpting, using alaser. Later, LASIK was developed. The sametype of laser is used for LASIKand PRK. Oftentheexact same laser is used for thetwo types of surgery.The major difference betweenthetwo surgeries is the way thatthestroma, the middle layer ofthe cornea, is exposed before it is vaporizedwiththe laser. In PRK, thetop layer ofthe cornea, calledtheepithelium, isscraped away to exposethe stromal layer underneath. In LASIK, a flap iscut inthe stromal layer andthe flap is folded back.

Another type of refractive surgery is thermokeratoplasty in whichheat isusedto reshapethe cornea. The source oftheheat can be a laser, but it isa different kind of laser than is used for LASIK and PRK. Other refractivedevices include corneal ring segments that are inserted into the stromaand special contact lenses thattemporarily reshapethe cornea(orthokeratology).

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Radial keratotomy (RK) is a refractive surgical procedureto correctmyopia.

In radial keratotomy (RK), incisions are made with a precisioncalibrateddiamond knife. Ithas been foundthat incisions that

penetrate only the superficial corneal stroma are less effectivethanthose reaching deep into the cornea, and consequently incisionsare made quitedeep. One study cites incisions madeto a depthequivalentto thethinnest of four corneal-thickness measurementsmadenear the centre ofthe cornea.Other sources cite surgeriesleaving 20 to 50 micrometers of corneal tissue unincised (roughlyequivalentto 90% of corneal depth based onthickness norms).

Accurate keratotomy is still popular to correct astigmatism. It isalso done with a diamond knife but inthese cases, cuts are madecircumferentially, parallel to theedge ofthe cornea.


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Radial

keratotomy19


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Photorefractive keratectomy (PRK) andLaser-Assisted Sub-Epithelial Keratectomy (or Laser Epithelial Keratomileusis) (LASEK)are laser eye surgery procedures intendedto correct a person'svision, reducing dependency on glasses or contact lenses.

Although PRK and LASEK basically usethe sametechnique, thereare minor differences betweenthem. In PRK, epithelium isremoved andthe outermost layer below theepithelium is treatedwith laser. In LASEK, epithelium is not removed, but an alcoholicsolution is usedto causetheepithelial cells to weaken; the surgeonwill foldtheepithelial layer out ofthe laser treatment field, andfold it back in its original place after cornea has been reshaped bylaser. Iftheepithelial flap is not strong enoughto be laid back in itsoriginal place, it will be removed, andthe LASEK procedurebecomes a PRK procedure

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All-LaserLasik: Also called IntraLasik and produced by the IntraLase Corporation,

this new Lasik technology no longer needs to use a microkeratome(A microkeratome uses a bladeto createthe corneal flap, the first

step ofeye surgery). Instead a laser is usedto create bubbles inthecornea and a flap caneasily and precisely be lifted from the rest oftheeye.

All-Laser Lasik may bethe only option for those who hadnot,initially, been qualified for the surgery. Becausenew Lasiktechnology involves creating flaps with precisethickness, it allowspatients withthin corneas or large pupils to undergo thetreatment. Best of all it minimizes complications like buttonholeflaps (incomplete corneal flaps).

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Lasik Vision Correction (a.k.a. Laser Eye Correction) is theeye surgery performed for correcting the myopia,hyperopia and astigmatism. This is an oldtechniquethathas been used since 1950's but ithas been very popularwiththe latest advancements inthe laser technology.

The Lasik Vision Correction is consisting ofthree phases.The patient's eye is immobilized and a flap is created usinga metal microkeratome. There is also a new techniqueknow as IntraLASIK in which a laser microkeratome is used

instead of a metal one. The flap is lifted and anexcimerlaser is appliedto the patient's cornea. This is the phasewhichthe patient's vision problems are corrected. Theexcimer laser remodels thetissue below the cornea andwhenthe process is finishedthe flap is closed.

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Recovery After theeye laser surgery patients aretoldto keep their eyes away from

direct sunlight, avoid rubbing and keeping their eyes healthy to avoid anypost surgical complications. It's normal to have itchy eye for a few daysbut be surenotto rub your eyes. You will useeyedrops and antibiotics to

avoid infections for a few weeks.When you go back for check-up and yourbandages aretaken off, thedoctor examines your eyes and you will knowif you have clear vision or not.

The Lasik Vision Correction brings in lots of advantages and lots ofquestions within. The laser technology is improving every day andeachyear a new eye laser surgery techniqueemerges. The patients who waitmore can benefit from more advancedtechnology.

There are also lots of complications patients claim to suffer after the LasikVision Correction. The most common amongstthem is; seeing halosaround lights inthenight. This becomes a major problem whiledriving atnight. Also there are many patients who claim to have blurred vision afterthe surgery.

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AdvancedControl Eye tracking (ACE) Technology: This was developed by Techno as Perfect Vision. The

Eyetracker locks onto specific details inthe iris. This

information allows lasers to automatically adjusttoeye movements during the surgery, to accuratelyreshapethe cornea according to the surgical planmade beforethe operation.

Based on a study of 2,000 Lasik cases, the use of ACEincreases the chance of curing astigmatism andreduces the chance of going back to the surgeon foranother operation.

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The latesttechnology ineye surgery is the microwave systems being

developed by Avedro, Inc. Microwaves are used, instead ofneedles

or lase

rs,to corr

ecte

ye

sight.

Be

st

of all,n

otissu

ecu

ttin

g isnecessary withthis technology. However, this device is targetedto

be marketed for those with minimal nearsightedness and is also not

a permanent solutionto eye problems. It will benecessary to come

back to theeye surgeon if vision blurs again. In addition, thedevice

is still indevelopment and may take moretime before it is availableto the public.


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LASER EYE SURGERY VIDEO

(CLICK BELOW TO PLAY)


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HITARTHI MEHTA

GROUP - 42

opthalmology individual work

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