optics of refractive procedures amy c. nau, od, faao clinical optics 3 rd edition, elkingtpon, frank...

$: Optics of Refractive Procedures Amy C. Nau, OD, FAAO Clinical Optics 3 rd edition, Elkingtpon, Frank and Greaney$
Optics of Refractive Optics of Refractive ProceduresProcedures

Amy C. Nau, OD, FAAOAmy C. Nau, OD, FAAO

Clinical Optics 3rd edition, Elkingtpon, Frank and Greaney

• Ways to alter the refractive state of Ways to alter the refractive state of the eyethe eye– Change the refractive power of any Change the refractive power of any

mediamedia– Change the depth of the ACChange the depth of the AC– Change in the axial length of the eyeChange in the axial length of the eye

OutlineOutline

• SurfaceSurface– CorneaCornea– Excimer laserExcimer laser

• PRKPRK• LasikLasik• LasekLasek• PTKPTK

– IncisionalIncisional• AKAK• PK (DLK etc)PK (DLK etc)• RKRK

– OtherOther• CKCK• IntacsIntacs• orthokeratologyorthokeratology

• InvasiveInvasive– LensLens

• Clear lens extractionClear lens extraction• Single visionSingle vision• MultifocalMultifocal• AccommodatingAccommodating• PhakicPhakic

– Silicone OilSilicone Oil

Corneal approach v Lens Corneal approach v Lens approachapproach

• Majority of power occurs at the air/tear Majority of power occurs at the air/tear interface interface – FFairair=50, F=50, Fwaterwater 5D 5D

• Corneal surgery functionally “limited” +4 to -Corneal surgery functionally “limited” +4 to -10D10D

• Second most powerful is the lensSecond most powerful is the lens– Accommodation eliminated Accommodation eliminated

• Unless multifocal or accommodatingUnless multifocal or accommodating

– Does not eliminate corneal astigmatismDoes not eliminate corneal astigmatism• Problem : Problem : predicting residual RX from lent/corneal cyl pt.predicting residual RX from lent/corneal cyl pt.

Predicting residual Predicting residual refractionrefraction

Rx: -5.00+2.50x 180K: 44.00/45.00@090

Question: when you implant a distance only IOL, what will be the predicted residual refractive error?

What will be the patients expected visual acuity?

What are some ways that you can take care of this for the pt?How would you counsel them during the pre-operative visit?

mailto:44.00/45.00@090

Basic Considerations for good Basic Considerations for good refractive outcomes-refractive outcomes-• Consequences of creating anisometropiaConsequences of creating anisometropia

is the fellow eye being operated upon? If so, when?is the fellow eye being operated upon? If so, when?• IOL for MV- please check the dominant eye!!IOL for MV- please check the dominant eye!!• Unlikely to tolerate 2.50 D of anisometropiaUnlikely to tolerate 2.50 D of anisometropia

– Some pts cannot wear contact lenses…Some pts cannot wear contact lenses…• Think about stereo/ occupational considerations. Think about stereo/ occupational considerations. • Don’t have to make them emmetropesDon’t have to make them emmetropes• 50% of pts unable to adapt to MV- 50% of pts unable to adapt to MV- • Undercorrect myopes if unsure, they are used to Undercorrect myopes if unsure, they are used to

taking off their glasses for near- cannot stand to taking off their glasses for near- cannot stand to be hyperopic….be hyperopic….

Basic Considerations for good Basic Considerations for good outcomesoutcomes• Make sure refraction and topography Make sure refraction and topography

are are stable and accuratestable and accurate – Check old glasses/ cl Rx if unsureCheck old glasses/ cl Rx if unsure– Peds pts- notoriously difficultPeds pts- notoriously difficult

• RGP’s may take 15+ weeks to baselineRGP’s may take 15+ weeks to baseline

• Soft cls if EW may take 5-10 weeks. Soft cls if EW may take 5-10 weeks.

• Many persons will complain loudly or go Many persons will complain loudly or go elsewhere – let them go… elsewhere – let them go…

OutlineOutline




– OtherOther• CKCK• IntacsIntacs




Corneal Correction- general Corneal Correction- general principlesprinciples

– Myopia: corrected by reducing the Myopia: corrected by reducing the refractive power of the cornea (making it refractive power of the cornea (making it flatter)flatter)

– Hyperopic and presbyopic correction Hyperopic and presbyopic correction must increase the refractive power of the must increase the refractive power of the cornea (making it “steeper” )cornea (making it “steeper” )

• F=n’-n/rF=n’-n/r

Hyperopic correctionHyperopic correction

• Ablate tissue peripherally, making Ablate tissue peripherally, making the central cornea “steeper”the central cornea “steeper”– Less predictableLess predictable– More aberrationsMore aberrations– Custom preferred over traditionalCustom preferred over traditional– Try CK…Try CK…

Excimer Laser-Excimer Laser- PRK, Lasik, PRK, Lasik, Lasek, PTKLasek, PTK

• Excimer laser= “excited dimer”Excimer laser= “excited dimer”• Two atoms form a pseudo molecule in the Two atoms form a pseudo molecule in the

excited state by adding electric energy but excited state by adding electric energy but dissociate into the ground state. dissociate into the ground state.

• Argon (inert)-Fluorine (reactive) dimer emits Argon (inert)-Fluorine (reactive) dimer emits 193nm UV radiation. 193nm UV radiation.

• The cornea absorbs UV, so intraocular The cornea absorbs UV, so intraocular penetration is greatly reduced. penetration is greatly reduced.

• Each photon has 6.4eV, sufficient to break Each photon has 6.4eV, sufficient to break intramolecular bondsintramolecular bonds

Excimer LaserExcimer Laser

• Delivery of high energy to small area Delivery of high energy to small area causes tissue removalcauses tissue removal– (ablation/disintigration)(ablation/disintigration)

• The temperature is very high, but the The temperature is very high, but the amount of heat produced is small, amount of heat produced is small, minimizing collateral damage. Absorbed in minimizing collateral damage. Absorbed in first nanometer of tissue. first nanometer of tissue.

• USED for PRK, Lasik, Lasek,PTK USED for PRK, Lasik, Lasek,PTK (phototherapeutic keratectomy)(phototherapeutic keratectomy)

ExcimerExcimer

• Broad beam- traditional lasikBroad beam- traditional lasik• Scanning Slit- Nidek EC-5000. allows more Scanning Slit- Nidek EC-5000. allows more

tissue removed from center than periphery tissue removed from center than periphery (or vice versa)(or vice versa)

• Flying SpotFlying Spot– Laser system including a laser bean delivery Laser system including a laser bean delivery

system and eye tracker responsive to movement system and eye tracker responsive to movement of the eye. Shots are fired in a sequence and of the eye. Shots are fired in a sequence and pattern such that no laser shots are fired in a pattern such that no laser shots are fired in a consecutive location and no consecutive shots consecutive location and no consecutive shots overlap. The pattern is a response to the overlap. The pattern is a response to the movement of the eye.movement of the eye.

– http://www.patentstorm.us/patents/7220255.htmlhttp://www.patentstorm.us/patents/7220255.html

PRKPRK

• Each laser pulse ablates surface Each laser pulse ablates surface tissue to a dept of .4-.5 micronstissue to a dept of .4-.5 microns

• Diameter of the laser is 1-7mm Diameter of the laser is 1-7mm (controlled by apeture)(controlled by apeture)– Haloes if apeture is 3.5-4mmHaloes if apeture is 3.5-4mm

•Alphagan, pilo, artificial pupil contact lensAlphagan, pilo, artificial pupil contact lens

– 6-7mm is better, but need to ablate 6-7mm is better, but need to ablate more corneamore cornea

PRKPRK

• To correct myopia, successive To correct myopia, successive concentric applications of increasing concentric applications of increasing diameter are made (traditional broad diameter are made (traditional broad beam)beam)– More tissue ablated centrally than More tissue ablated centrally than

peripherallyperipherally

“terraced” profile will result in an increase in higher order aberrations

PRKPRK

• For higher myopia, make more rings For higher myopia, make more rings and try to smooth out the junctionsand try to smooth out the junctions– Still, less predictable outcome, more Still, less predictable outcome, more

likely to regress and higher risk of likely to regress and higher risk of scarringscarring

LasikLasik

• Same profiles as PRKSame profiles as PRK– Less scarring, regression, earlier Less scarring, regression, earlier

stabilization, better predictabilitystabilization, better predictability

Formula for figuring out if sufficient Formula for figuring out if sufficient thickness exists- must do for the thickness exists- must do for the wavescan datawavescan data

Don’t use pachy- orbscan or pentacam is Don’t use pachy- orbscan or pentacam is betterbetter

(pentacam probably best)(pentacam probably best)

• Larger refractive errors have greater Larger refractive errors have greater chance of scarring. chance of scarring.

• Lasik is like a scl, will correct for both Lasik is like a scl, will correct for both internal and lenticular astigmatism. internal and lenticular astigmatism.

• What power to consider for MV? – What power to consider for MV? – 2.50? 1.50 (probably better). Leaves 2.50? 1.50 (probably better). Leaves margin for hyperopic outcome. Which margin for hyperopic outcome. Which eye to do first? eye to do first?

Custom LasikCustom Lasik

Visx- CustomVue OverviewVisx- CustomVue Overview

• WaveScan WaveFront system and Star S4 EximerWaveScan WaveFront system and Star S4 Eximer• AcquireAcquire- fourrier analysis /psf - fourrier analysis /psf • DesignDesign- using Wavescan software- using Wavescan software• AlignAlign- Iris Registration ensures delivery of tx to - Iris Registration ensures delivery of tx to

proper corneal site, well centered in spite of proper corneal site, well centered in spite of hippushippus– 3-d eye tracking x,y,z (limit 1.5mm)3-d eye tracking x,y,z (limit 1.5mm)– Virtual Reticle (grid overlay)Virtual Reticle (grid overlay)

• DeliverDeliver- Variable Spot scanning with Variable - Variable Spot scanning with Variable repetition repetition ratehttpratehttp://www.visx.com/professionals/visx_technology/deliver.://www.visx.com/professionals/visx_technology/deliver.phpphp

What to do with What to do with astigmatism? astigmatism?

• Have to reduce the surface curvature Have to reduce the surface curvature more in the steep meridian more in the steep meridian compared to the flat one. compared to the flat one. – Slit beamSlit beam– Elliptical ablation zoneElliptical ablation zone– Scanning beamScanning beam– Ablatable maskAblatable mask

http://www.emedicine.com/oph/topic655.htm#target3

Myopic astigmatic corrections are achieved by applying the laserenergy in an elliptical pattern along the central part of the flat meridian, hereby flattening the steep axis

Hyperopic astigmatic correction is achieved by applying the laser energy preferentially in the periphery, steepening the flat axis

http://www.emedicine.com/cgi-bin/foxweb.exe/makezoom@/em/makezoom?picture=%5Cwebsites%5Cemedicine%5Coph%5Cimages%5CLarge%5C476523untitled7.jpg&template=izoom2


Slit beamSlit beam

• Widens for successive applicationsWidens for successive applications

• Uniform deep ablation is produced in Uniform deep ablation is produced in the long axis of the slitthe long axis of the slit

• The surface curvature is reduced The surface curvature is reduced only in the meridian where the slit only in the meridian where the slit widenswidens

Ablatable maskAblatable mask

• A plate of PMMA placed in the path of A plate of PMMA placed in the path of the laser beam to shield the corneathe laser beam to shield the cornea

• Thinner areas of the mask are ablated Thinner areas of the mask are ablated first and thus allow deeper ablation of first and thus allow deeper ablation of the corresponding area of the corneathe corresponding area of the cornea

• Also used to treat hyperopiaAlso used to treat hyperopia

Post op irregular Post op irregular astigmatismastigmatism• Usually because corneal thickness is Usually because corneal thickness is

insufficientinsufficient

• Less than 400 microns pre-op is troubleLess than 400 microns pre-op is trouble

• 250 microns in bed may be insufficient250 microns in bed may be insufficient

• Problems seem to occur with thicker Problems seem to occur with thicker than expected flaps- intraoperative than expected flaps- intraoperative pachymetry, post op can do Visante OCTpachymetry, post op can do Visante OCT

OutlineOutline








RKRK

• Irreversibly flattens the central Irreversibly flattens the central corneal curvature to reduce Rxcorneal curvature to reduce Rx

80-90% depth incisions are make in the mid peripheral and peripheral corneaWeakens the cornea , IOP can cause wound gapeThe mid and peripheral cornea begins to bulgeAdult cornea does not stretch and so the conformationalchange causes a flattening of the central cornea.

RKRK

•Greater effect: longer or deeper incisions, more incisions, or a smaller central zone.

•Diameter of center 3-5mm to avoid glare•The more central the incisions, the less

the effect. •Usually use only 4 or 8 incisions•Best for less than -5D•6 months to stabilize

RKRK

RKRK

• Consecutive hyperopiaConsecutive hyperopia– Hyperopic shift continues as cornea Hyperopic shift continues as cornea

loses integrity. loses integrity. – May be difficult or impossible to fit with May be difficult or impossible to fit with

a lensa lens– May need transplant or running sutures May need transplant or running sutures

to maintain integrityto maintain integrity

Astigmatic (arcuate) Astigmatic (arcuate) KeratotomyKeratotomy

• Surgical correction of astigmatismSurgical correction of astigmatism– CosmeticCosmetic– Post IOLPost IOL– Post PK, trauma, etc.Post PK, trauma, etc.

– Results somewhat unpredictable. Results somewhat unpredictable. – Hope for CL that fits or spectaclesHope for CL that fits or spectacles

AKAK

• Place 1-2 deep curved incisions Place 1-2 deep curved incisions perpendicular to the steep meridianperpendicular to the steep meridian

• Flatten the steep meridia is matched by a Flatten the steep meridia is matched by a steepening 90 degrees away (coupling steepening 90 degrees away (coupling effect)effect)– Coupling ratio flattening/steepeningCoupling ratio flattening/steepening– SE is about the same ratio about 1SE is about the same ratio about 1

• Effect is immediateEffect is immediate• Subsequent healing affects outcomeSubsequent healing affects outcome

Wilkins, 2005- nomograms innacurate for PKWilkins, 2005- nomograms innacurate for PK

Astigmatic keratotomyAstigmatic keratotomy

• all sutures should be removed because all sutures should be removed because they can be the cause of the astigmatism. they can be the cause of the astigmatism.

• Keratoplasty wound should be inspected Keratoplasty wound should be inspected for focal abnormalities. Wound dehiscence for focal abnormalities. Wound dehiscence and graft override cause flattening of the and graft override cause flattening of the central cornea in that meridian and may central cornea in that meridian and may be best corrected by opening and be best corrected by opening and resuturing the wound, despite the lengthy resuturing the wound, despite the lengthy recovery. recovery.

Astigmatic keratotomyAstigmatic keratotomy

Arcuate incisions flatten the steeper Arcuate incisions flatten the steeper meridian the same amount as they meridian the same amount as they steepen the flatter meridian; steepen the flatter meridian; therefore, the net effect is no change therefore, the net effect is no change in the spherical equivalent. Arcuate in the spherical equivalent. Arcuate incisions greater than 90° are not incisions greater than 90° are not recommended because of the risk of recommended because of the risk of late wound dehiscence. late wound dehiscence.



Controlling astigmatism in Controlling astigmatism in cataract surgerycataract surgery

• Place incision in steepest meridian of Place incision in steepest meridian of the cornea to reduce cylthe cornea to reduce cyl– May be able to use spherical IOL and get May be able to use spherical IOL and get

good outcomegood outcome

– Ex: -7.00+3.00x090Ex: -7.00+3.00x090– K K 43.00/[email protected]/44.00@180 1D residual cyl 1D residual cyl– Place incision at 180Place incision at 180

mailto:43.00/44.00@180

Relaxing incisionsRelaxing incisions

• An incision causes the cornea to An incision causes the cornea to bulge at that site (weakens the bulge at that site (weakens the structure)structure)

• This reduces the surface curvature of This reduces the surface curvature of the central cornea in the meridian in the central cornea in the meridian in which the incision is made and which the incision is made and induces increased curvature in induces increased curvature in meridian 90 degrees awaymeridian 90 degrees away

Reducing PK cyl/ scarring Reducing PK cyl/ scarring cylcyl• Incising over 60 degrees over the steep Incising over 60 degrees over the steep

meridianmeridian• If incisions within the graft (not at If incisions within the graft (not at

junction) effect is more predictable and junction) effect is more predictable and preserves wound healingpreserves wound healing

• Arcuate is preferred (uniform thickness)Arcuate is preferred (uniform thickness)• Longer and more axial have greater Longer and more axial have greater

effecteffect

Wedge resectionsWedge resections

• >10D cyl>10D cyl• Remove deep arcuate wedge measuring 60-90 Remove deep arcuate wedge measuring 60-90

degrees from the graft/host junction in the FLAT degrees from the graft/host junction in the FLAT meridianmeridian

– Opposite of relaxing incisionOpposite of relaxing incision– Wound sutured to shorten the cornea and steepen the Wound sutured to shorten the cornea and steepen the

curvature in that meridiancurvature in that meridian– The overall effect of wedge resection is to steepen the flat The overall effect of wedge resection is to steepen the flat

meridian approximately twice as much as it flattens the meridian approximately twice as much as it flattens the steeper meridian. The net effect is an increase in myopia or a steeper meridian. The net effect is an increase in myopia or a decrease in hyperopia. The surgical technique involves decrease in hyperopia. The surgical technique involves removing a wedge of tissue along the flat meridian of the removing a wedge of tissue along the flat meridian of the cornea. cornea.

Compression suturesCompression sutures

• Tight suture in the flat meridian will increases the Tight suture in the flat meridian will increases the curvature of the cornea adjacent to it and displace curvature of the cornea adjacent to it and displace the apex away from itthe apex away from it– Causes the apex of the cornea to bulge awayCauses the apex of the cornea to bulge away– Note the topography will change if suture dissolves or Note the topography will change if suture dissolves or

breaksbreaks– Compression sutures used simultaneously with astigmatic Compression sutures used simultaneously with astigmatic

keratectomy can markedly increase the effect of the keratectomy can markedly increase the effect of the incisions. Compression sutures are placed 90° from the incisions. Compression sutures are placed 90° from the incisions. Suture depth should be approximately 80% of incisions. Suture depth should be approximately 80% of the corneal thickness. The sutures are tied with a slipknot, the corneal thickness. The sutures are tied with a slipknot, and tension is adjusted under intraoperative keratometric and tension is adjusted under intraoperative keratometric or keratoscopic control until an overcorrection of 25-50% or keratoscopic control until an overcorrection of 25-50% is achieved. is achieved.

OutlineOutline








Conductive Conductive KeratoplastyKeratoplasty

• Increases the asphericity of the corneaIncreases the asphericity of the cornea• Initially conceived as an alternative to Initially conceived as an alternative to

hyperopic excimer procedures. hyperopic excimer procedures. • MV trial with progressive CL useful.MV trial with progressive CL useful.• Some glareSome glare• Leave room to moveLeave room to move• Set up expectationsSet up expectations• Nobody really sure how it worksNobody really sure how it works

Post operative Post operative

• Still have to use reading or driving Still have to use reading or driving glassesglasses

• Glare/ monocular diplopia- usually Glare/ monocular diplopia- usually resolvesresolves

• Somewhat difficult to fit with lenses- Somewhat difficult to fit with lenses-

• Steep bc with aspheric surfacesSteep bc with aspheric surfaces

• Useful post CE or LasikUseful post CE or Lasik

IntacsIntacs

Up to -3D myopia (normals)Sufficient thickness (minimum 450 microns)

Removable/ adjustable

Think about Intacs then Lasik for High myopes with thinner corneas

Intacs for EctasiaIntacs for Ectasia

• Where to put the Where to put the segmentsegment

• One versus two One versus two segmentssegments

• May not produce May not produce great VA, but allow great VA, but allow for CL use or for CL use or prevent PKprevent PK

• May be CL May be CL intolerantintolerant

PKPK

PKPK

• High amounts of irregular cylinderHigh amounts of irregular cylinder– 19% have 5D or more19% have 5D or more

• May need subsequent AKMay need subsequent AK

• Up to 50% still need RGP designUp to 50% still need RGP design

• ExpectationsExpectations

• Lamellar techniques promisingLamellar techniques promising

• Relaxing incisions and/or compression Relaxing incisions and/or compression sutures, wedge resection, LK, excimer sutures, wedge resection, LK, excimer proceduresprocedures

Suture Removal (lacs)Suture Removal (lacs)

• Basis is the same as for PKBasis is the same as for PK

OutlineOutline








The lensThe lens

• Change the powerChange the power

• Change the positionChange the position

• Increasingly, to manage presbyopiaIncreasingly, to manage presbyopia

IOL CalculationsIOL Calculations

• Various formulasVarious formulas– Regression (SRK, SRKII)Regression (SRK, SRKII)– Theoretical (Hoffer, Haigis, Holladay)Theoretical (Hoffer, Haigis, Holladay)

• based on reduced eye models based on reduced eye models – One refractive surface for the cornea and thin One refractive surface for the cornea and thin

lens for IOLlens for IOL

IOL calculationsIOL calculations

• SRK SRK

• IOL Power – A (IOL specific) -0.9 x IOL Power – A (IOL specific) -0.9 x K(mean) – 2.5 x ALK(mean) – 2.5 x AL

• SRK II- A constant varies w/ axial SRK II- A constant varies w/ axial lengthlength

Theoritical basisTheoritical basis

• Power-[n/(AL-d)]-(n/[n/(K+TR)]-d])Power-[n/(AL-d)]-(n/[n/(K+TR)]-d])– d is effective lens positiond is effective lens position– TR = target Rx at corneal planeTR = target Rx at corneal plane

What’s the difference between What’s the difference between various theoretical formulas?various theoretical formulas?

• Estimated position of the IOL and the Estimated position of the IOL and the effective refractive index to effective refractive index to transform corneal radius to powertransform corneal radius to power

• In the US (holladay, SRK/T etc) use In the US (holladay, SRK/T etc) use the AL and K for prediction of IOL the AL and K for prediction of IOL positionposition

Post Refractive SX Post Refractive SX calculationscalculations

• K’s and topography tend to K’s and topography tend to overestimate corneal refractive overestimate corneal refractive power after RSpower after RS

• This will cause underestimation of This will cause underestimation of the IOL power leading to a hyperopic the IOL power leading to a hyperopic outcomeoutcome

Solutions post Refractive SXSolutions post Refractive SX

• Clinical History MethodClinical History Method

• Subtract SE change induced by the Subtract SE change induced by the sx form the K (diopters) measured sx form the K (diopters) measured before the refractive surgery then before the refractive surgery then use formulause formula

• Make sure it the Rx after stabilization Make sure it the Rx after stabilization and not a phakic myopic shift. and not a phakic myopic shift.

Solutions post RS for IOL’sSolutions post RS for IOL’s

• Hard contact lens methodHard contact lens method• Determines difference between MR both with and Determines difference between MR both with and

w/o a regp of known bc and subtracts this w/o a regp of known bc and subtracts this difference from the BCdifference from the BC– Refract and calculate SE at spectacle planeRefract and calculate SE at spectacle plane– Place pl rgp of some known bc and perform spherical Place pl rgp of some known bc and perform spherical

refractionrefraction– IF SE the same, cornea has same power as bc of the lensIF SE the same, cornea has same power as bc of the lens– Hyperopic shift= power is greater than bcHyperopic shift= power is greater than bc– Myopic shift= power is smaller than bcMyopic shift= power is smaller than bc– Calculation of postop mean K: bc of cl + change in Rx Calculation of postop mean K: bc of cl + change in Rx

Solutions for post RS IOL’sSolutions for post RS IOL’s

• Look at difference between anterior Look at difference between anterior and posterior lens surfaceand posterior lens surface

• Pre sx, they should be the same. Pre sx, they should be the same.

• Post RX, the posterior should mimic Post RX, the posterior should mimic what the pre-op K’s arewhat the pre-op K’s are– Newer topography devices have this Newer topography devices have this

capabilitycapability

Solutions for post RS iol’sSolutions for post RS iol’s

• Calculate with std formula using pre-Calculate with std formula using pre-op biometry dataop biometry data

• Enter initial ametropia as target Enter initial ametropia as target refractionrefraction

Solutions for post RS IOL’sSolutions for post RS IOL’s

• Empirical correctionEmpirical correction

• We know that the true corneal power can We know that the true corneal power can be predicted from the measured power be predicted from the measured power – The effect of overestimation of corneal power The effect of overestimation of corneal power

is linear with respect to the amount of is linear with respect to the amount of correction d/t refractive surgery.correction d/t refractive surgery.

- Zeiss instrument : 24% has to be subtracted - Zeiss instrument : 24% has to be subtracted from measured K’s to get true K’sfrom measured K’s to get true K’s

Calculation of toric IOLsCalculation of toric IOLs

• Ok to use conventional formulas if all Ok to use conventional formulas if all axes of all toric surfaces are aligned.axes of all toric surfaces are aligned.Just calculate each meridian separatelyJust calculate each meridian separately

If use formula that bases lens position is If use formula that bases lens position is based on K, you will get two different based on K, you will get two different predicted positionspredicted positions

Cylinder of toric lens must be aligned with Cylinder of toric lens must be aligned with axis corneal astigmatism axis corneal astigmatism

Toric IOL’sToric IOL’s

• Spherical front and toric back Spherical front and toric back surfacessurfaces– May cause meridional magnification May cause meridional magnification

issuesissues– Bitoric IOL normalizes meridional Bitoric IOL normalizes meridional

differencesdifferences– Ray tracing schemesRay tracing schemes

Post Intacs IOL CalculationsPost Intacs IOL Calculations

• Who knows?Who knows?

Pediatric IOL CalculationsPediatric IOL Calculations

• None work wellNone work well

• Issues with emmetropizationIssues with emmetropization

• PCOPCO

• Current formulas based on adult eyesCurrent formulas based on adult eyes

Monovision IOL CalculationsMonovision IOL Calculations

• Try to aim for -1.50 and set up Try to aim for -1.50 and set up expectationsexpectations

• Same for any surgical monovisionSame for any surgical monovision

• Can try MF IOL plus modified MVCan try MF IOL plus modified MV

ReStor IOLReStor IOL

• Diffraction based system, sort of like Diffraction based system, sort of like simultaneous bifocal soft contact simultaneous bifocal soft contact lenseslenses

• Again, expectationsAgain, expectations

Array IOLArray IOL

AccommodatingAccommodating IOLIOL

Work on the focus shift principle

Accommodative effort by CM exertsPressure on vitreous

Increases pressure in vitreous whichCauses anterior displacement of IOL

Brings focus in front of retina, functionalpseudoaccommodation

1mm forward shift = 1.6D accom

• PseudoaccommodationPseudoaccommodation– Increased depth of focusIncreased depth of focus– Spherical aberrationSpherical aberration

CrystalensCrystalens

• SiliconeSilicone

• Plate haptics ensure posterior Plate haptics ensure posterior position in capsular bagposition in capsular bag

• Hinged haptic facilitates maximal Hinged haptic facilitates maximal forward movementforward movement

Phakic IOLsPhakic IOLs

Phakic IOLPhakic IOL

Silicone Oil- effectsSilicone Oil- effects

• Silicone Oil has higher n than lens Silicone Oil has higher n than lens and changes the posterior lens from and changes the posterior lens from a converging to a diverging interface. a converging to a diverging interface. – Induces hyperopic shift from 5-7DInduces hyperopic shift from 5-7D– In an aphakic eye, the curved anterior In an aphakic eye, the curved anterior

surface and higher refractive index of surface and higher refractive index of silicon oil compared to the lens will silicon oil compared to the lens will cause a myopic shift, causing more cause a myopic shift, causing more limited hyperopic Rx (+6 or so)limited hyperopic Rx (+6 or so)

Scleral BucklesScleral Buckles

• Induce myopiaInduce myopia

• Induce CylInduce Cyl

GasGas

• Phakic eye will increase the refractive Phakic eye will increase the refractive power of the posterior lens surface power of the posterior lens surface and cause a large myopic shiftand cause a large myopic shift– May be able to do indirect w/o a lens!May be able to do indirect w/o a lens!

– Aphakic eye will makes posterior cornea Aphakic eye will makes posterior cornea highly diverging and almost neutralizes highly diverging and almost neutralizes the power of the corneathe power of the cornea

optics of refractive procedures amy c. nau, od, faao clinical optics 3 rd edition, elkingtpon, frank...

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