Clinical Skills Lecture 3: Keratometry

Keratormetry

measures the curvature of the from surface of the eye

cornea

tera film

Keratormetry is part of the _____ refraction

objective

keratometry is performed before

retinoscopy

Cornea

major refractive surface of the eye

the refractive index change of air to the tear film

keratometry uses the ____ nature of the cornea to measure its curvature

reflective

Keratometry is the objective method for determining

curvature of cornea

amount and orientation of corneal astigmatism

quality of cornea and tear film

monitoring of corneal conditions

Corneal conditions we can monitor using keratometry

keratoconus or other degenerative conditions

progressive myopia

Clinical uses of keratometry

Objective methods

nature of anisometropia

stability of cornea

contact lenses

refractive estimates

calculations of IOL power

Anisometropia

significant unequal refractive error between two eyes

Nature of Anisometropia

are the unequal errors due to

Axial: axial lengths are different

Refractive: the curvatures are different

when should the stability of cornea be monitored

post contact lens wear

orthokeratology -- fit lenses to change corneal shape while sleeping so lenses don't need to be worn during the day

following cataract or refractive surgery

Keratometry is useful for contact lenses because

determine base curve of lens

detect abnormalities on the lens

Post CL wear

Verify base curve of lens

We use keratometry for refractive estimates when

- unresponsive patient

-children

- ambylopes

- high ametropes (high refractive error)

- pts with poor retinoscopy reflexes

We use keratometry for cataract surgery

to calculation the IOL power appropriate for patient

dependent on patient

Keratometry measures the radius of curvature in

2 principle meridians

uses mires that

reflect of the cornea

Measure their size

It measures only the

center of the of the anterior surface (3.1mm)

4 paracenteral points of a 3.1mm annulus ring

What formula can we use to calculate the dioptric value of the cornea in the 2 meridians

D = n - 1/r

index of refraction = n = 1.3375

7.40 mm radius = 45 D

Keratometry assumes the cornea is

a spherical mirror

(using reflective convex surface of cornea)

The image is produces _____mm bending the cornea

4mm

smaller, erect and virtual

Radius is calculated by measuring the size of the image of an object of known size and proportion

assume the distance between the object and image is equal to the distance between the

object and the focal point of the "mirror"

Formaula to find radius

2bh'/h

b and h are constants in instrument

h' image size

Scheiner's disc

allows for focusing

when focused - single

when unfocussed - double

Doubling prism

measure the image size --> radius of curvature

Types of Keratometers

- one position (B&L)

- Two position (Haag Streit, Schlotz)

one position (B&L) keratometer

H & V doubling done simultaneously, can measure both primary meridians at the same time

(We have!)

Two position (Haag Streit, Schiotz) keratometer

1 doubling system, you must rotate to measure the second primary meridian

allows for measurement of irregular astigmatism (primary meridian's are not 90 degrees apart)

* dioptric range of the B & L keratometer *

36 - 52 D

Mean corneal power is....

43D

To increase the upper range of the keratometer

place a +1.25 trial lens in front of the aperture

this extends range to 61D

Multiply power by 1.1659 to get D value

To increase the lower range of the keratometer

Place a -1.00 D trial lens in front

extends range to 30 D

multiply power by 0.8576 to get correct D value

USE CONVERSION TABLE

Clinical Procedure

1. Clean!

2. Focus eye piece (start full ccw)

3. Unlock intrument

4. Ask Pt to place chin in chin rest w forehead against the bar

5. Align instrument to the eye

6. Look into ocular for mires

7. Center mires in view

8. Focus the bottom right mire

9. lock instrument

10. Locate the principle meridians

11. measure radius and power horizontal meridian

12. measure radius and power of vertical meridian

If you can not see the plus sign

adjust the occluder position, it is blocking the mire from reflecting off the cornea

If you can not see the minus sign

the patients head is positioned incorrect

the patient make have a droopy eyelid

If in between axis

always round up or always round down

Power is recorded in

Truncation of 1/8th diopter

0.12

0.25

0.37

0.50

0.62

0.75

0.87

Recording

Power at 1° meridian (deg)/ Power at 2° meridian (deg); mires

Minus cylinder power x axis of flatter meridian, AM lower power; mires

What if the horizontal and vertical meridians are the same?

the cornea is spherical

43.62 @ 173 / 43.62 @ 083

Sph x _______ AM 43.62 mires

Sources of error

- improper calibration

- poor pt position

- poor pt fixation

- didn't focus eyepiece

- not placing the reticule on the bottom right circle

- not rotating the instrument to determine the true axis

- corneal distortions, excessive tearing, abnormal lid position

- poor focus

Regular astigmatism

2 principal meridians are perpendicular to each other

mires not distorted

3 types: WTR, ATG, Oblique

irregular astigmatism

2 principal meridians are NOT perpendicular to each other

mires are usually distorted

With-the-rule (WTR) astigmatism

more power in the vertical

horizontal meridian is flatter

horizontal meridian is the axis meridian

180 +- 30

Against the rule (ATR) astigmatism

More power in the horizontal

vertical is flatter

vertical meridian is the axis meridian

90 +- 30

Oblique astigmatism

principal meridians between 31 to 59 and 121 to 149

Which type of astigmatism is more common?

WTR

As we age, astigmatism moves more towards

ATR

Corneal Astigmatism is measured by

Keratometry

Refractive astigmatism is measured by

retinoscopy and/or subjective refraction

Refraction astigmatism can be ESTIMATED by

Javal's rule

Why do the corneal and refractive astigmatisms not match?

Lenticular astigmatism

effectivity change

posterior corneal curvature

corneal refractive index variations

aberrations of these

lenticular astigmatism

astigmatism found in the crystalline lens

Lenticular astigmatism is always ____ The Rule

against

Effectivity change

when move from corneal plan to spectacle plane there is an increase in the amount of power needed

the higher the corneal cylinder the _____ the adjustment

higher

We do not measure ____ corneal curvature with keratometry

posterior

Corneal refractive index vartion

n=1.3375 matches tears better than cornea

Javal's rule

Total astigmatism = (Effectivity constant 1.25)(Corneal Astigmatism) + (Lenticular astigmatism)

At = (P)(Ac) + Al

Al = -0.50 x 090

Javal's rule round to the

closest 1/8th diopter

Javal's Rule ATR

normal

Javal's Rule WTR

must flip signs and axis

Jamal's rule sphere

-0.50 x 090

WTR corneal cylinder -->

less WTR spectacle cylinder

ATR corneal cylinder -->

more ATR spectacle cylinder

JAval's rule does not apply to

oblique axis

must use corn cylinder to estimate the spectacle correction

if calculations have a + cylinder, what do you do?

flip the sign, flip the axis 90 degrees

must convert to minus cylinder

Modified Javal's rule

At = Ac +(-0.50 x 090)

Ways to estimate refractive error

spherical equivalent

sphero-cylindrical equivalent

spherical equivalent

1/2 cylinder + sphere

monocular uncorrected visual acuity*

keep in 0.25D steps

Three ways to measure spherical equivalent

Egger's Table myopic +

Hyperopia

Punctum remotum

Egger's Table myopic

20/25 - 20/200

Egger's Table hyperopic

VA 20/20 or better

measure with +2.50D

Punctum remotem

worse than 20/200

100/cm

Spherical equivalent

keeps the blur at a minimum by keeping the blur cicle close to the retina

Circle of least confusion

Circle of least confusion

located at dioptric mean of two focal lines

Spherical equivalent uses

- JCC (which is better 1 or 2)

- Fitting pt w mild astigmatism into spherical CL's

- Pts with high astigmatism (gradually work up to correct cylinder)

- selecting trial lens for VF

- refraction, does the change make sense

How to estimate SC Rx

subtract half the cylinder from original unaided acuity SE

SEE NOTES for problems

Rounding for SC Rx Sphere

closest 0.25D FIRST

if in 1/8th D round in the minus direction

Rounding for SC Rx Cylinder

Closest 0.25D FIRST

If in 1/8th D, round in the plus direction