Paraxial geometrical optics Paraxial geometrical optics and relevant clinical issues and relevant clinical issues

in refractive surgeryin refractive surgery

Definition: Paraxial and Marginal RaysDefinition: Paraxial and Marginal Rays

True paraxial rays travel infinitesimally close to the True paraxial rays travel infinitesimally close to the optical axis, and deviate only slightly after being optical axis, and deviate only slightly after being refracted by an imaginary tangential plane. Formal refracted by an imaginary tangential plane. Formal paraxial rays are displaced from the optical axis by paraxial rays are displaced from the optical axis by larger amounts.larger amounts.

Marginal rays: Marginal rays: those ray's originating from the those ray's originating from the periphery, or from the edge of the lens.periphery, or from the edge of the lens.

Marginal Ray

Main RayParaxial Ray

Definitions: Apertures Definitions: Apertures

Aperture:Aperture: any opening in an optical system any opening in an optical system that allows light to pass. that allows light to pass.

Pupils are the beam-limiting apertures of Pupils are the beam-limiting apertures of the human visual system.the human visual system.

Definitions: PupilsDefinitions: Pupils

Entrance pupil:Entrance pupil: image of the physical pupil image of the physical pupil generated by the cornea generated by the cornea

Exit pupil:Exit pupil: image of the physical pupil image of the physical pupil generated by the crystalline lensgenerated by the crystalline lens

Definitions: Optical ZoneDefinitions: Optical Zone Optical Zone:Optical Zone: area of ablation where full optical area of ablation where full optical

correction is applied – the area of useful visioncorrection is applied – the area of useful vision Ablation Zone:Ablation Zone: The overall diameter of the The overall diameter of the

ablated area, which may include a blend zoneablated area, which may include a blend zone

AberrationsAberrations

Most optical aberrations after keratorefractive surgery Most optical aberrations after keratorefractive surgery depend upon the “optical stop” of the ocular systemdepend upon the “optical stop” of the ocular system—the pupillary diameter. —the pupillary diameter.

The center of the line of sight must be positioned The center of the line of sight must be positioned inside the (entrance) pupil. For that reason, when the inside the (entrance) pupil. For that reason, when the pupillary diameter is reducedpupillary diameter is reduced, the center of the line , the center of the line of sight and the geometric center of the pupil must of sight and the geometric center of the pupil must virtually coincide. virtually coincide.

Aberrations with pinhole pupilAberrations with pinhole pupil

At the pinhole level, aberrations become increasingly At the pinhole level, aberrations become increasingly insignificant.insignificant.

Clinically, a pupil of approximately 1.8 mm in diameter Clinically, a pupil of approximately 1.8 mm in diameter mimics a pinhole.mimics a pinhole.

CenteringCentering treatment and treatment and maximizing visual functionmaximizing visual function

Clearly, centering the ablation on the apex of the Clearly, centering the ablation on the apex of the cornea without regard to the pupil is ill conceived. In cornea without regard to the pupil is ill conceived. In order to maximize visual function, the ablation must order to maximize visual function, the ablation must be concentric with the pupil. be concentric with the pupil.

Because the center of the visual axis and the middle of Because the center of the visual axis and the middle of the the miotic pupilmiotic pupil coincide, the central light rays coincide, the central light rays through the central region of the pupil are treated as through the central region of the pupil are treated as though they are entering a pinhole system, that is to though they are entering a pinhole system, that is to say, without deviation. This relationship does not say, without deviation. This relationship does not cause aberrations. The optical zone/pupil cause aberrations. The optical zone/pupil relationship is of supreme importance. relationship is of supreme importance.

Method of locating center of Method of locating center of ablationablation

The most accurate method of centering an ablation is The most accurate method of centering an ablation is to create a miotic pupil (using 1% pilocarpine) and to create a miotic pupil (using 1% pilocarpine) and manually center the ablation in the middle of a small manually center the ablation in the middle of a small pupil. This technique creates an excellent pupil. This technique creates an excellent ablation/pupil relationship. If the pupil is 2 mm wide, ablation/pupil relationship. If the pupil is 2 mm wide, for example, then the margin of error probably for example, then the margin of error probably ranges from 0.1 to 0.2 mm. In addition, the laser ranges from 0.1 to 0.2 mm. In addition, the laser beam is perpendicular to the cornea at all times and beam is perpendicular to the cornea at all times and therefore unaffected by parallax, as in the case of therefore unaffected by parallax, as in the case of eye trackers. eye trackers.

Decentration can erase any Decentration can erase any benefit of customized ablationbenefit of customized ablation

Also, if customized corneal ablations are to succeed, Also, if customized corneal ablations are to succeed, and if changes in treatment location of 10 to 20 µm and if changes in treatment location of 10 to 20 µm are important, then how can a decentration of 300 to are important, then how can a decentration of 300 to 500 µm be acceptable? This degree of decentration 500 µm be acceptable? This degree of decentration would seem to undercut the aims of customized would seem to undercut the aims of customized corneal treatments.corneal treatments.

Clinical examplesClinical examplesCase 1: decentered myopic Case 1: decentered myopic

AblationAblation 26 you WF26 you WF S/P LASIK OU 8/99 S/P LASIK OU 8/99

with enhancements with enhancements OU in 2000OU in 2000

Pre-Op: -6.50, 20/20Pre-Op: -6.50, 20/20 Postop: Postop: – –1.75+0.50 x 145, 1.75+0.50 x 145,

20/25, with 20/25, with glare/haloglare/halo

Case 2: Decentered myopia ablationCase 2: Decentered myopia ablation 19 yo male19 yo male PreOP: -PreOP: -

7.50+2.50 x 657.50+2.50 x 65 (20/20);(20/20); Postop: -Postop: -

0.50+0.75x80 0.50+0.75x80 (20/25);(20/25);

Interestingly, this Interestingly, this patient has no patient has no complaints of complaints of glare, halos or glare, halos or decreased visual decreased visual quality.quality.

Case 3: Hyperopic LASIK Case 3: Hyperopic LASIK resulting in small optical zoneresulting in small optical zone

55 yoWF55 yoWF H-L for monovision H-L for monovision

OSOS Pre-Op: +2.75D with Pre-Op: +2.75D with

LII +0.25+1.25x180LII +0.25+1.25x180 Postop: –2.00+0.75 x Postop: –2.00+0.75 x

09 (reduced BSCVA 09 (reduced BSCVA to 20/40 with night to 20/40 with night vision problem)vision problem)

Case 4: Irregular cornea s/p M-LCase 4: Irregular cornea s/p M-L

Case 5: irregular cornea s/p M-LCase 5: irregular cornea s/p M-L

29 yo WF29 yo WF -3.50 DS, 20/20 PreOp-3.50 DS, 20/20 PreOp Grade 3 DLK S/P LASIK Grade 3 DLK S/P LASIK

with folds centrallywith folds centrally Irregular astigmatismIrregular astigmatism +1.00, 20/30 BVA @ 2 +1.00, 20/30 BVA @ 2

mosmos

Case 5 con’t: post-M-L irregular astigmatism (hx postop DLK)

Case 6: Angle kappaCase 6: Angle kappa35 yo WM, s/p ML for –4.00D, Now is –1.50+0.50x110 (20/25 with blurriness)

Where should the center of treatment be in eyes with large angle kappa?

Take-home message ofTake-home message ofparaxial geometric optics and relevant paraxial geometric optics and relevant

issues in refractive surgeryissues in refractive surgery

Ablation centerAblation center should be the center of myopic should be the center of myopic (less than 1.8mm) pupil;(less than 1.8mm) pupil;

DecentrationDecentration is clinically important and may is clinically important and may abolish any benefit of higher order aberration abolish any benefit of higher order aberration treatment;treatment;

Optical zone sizeOptical zone size and its relationship to pupil can and its relationship to pupil can affect visual function;affect visual function;