keratometry

KeratometryKeratometry 11

KERATOMETRYKERATOMETRY

H.Kangari O.D.


The keratometer is an instrument that is used to measure the front surface (approx 2 - 4 mm radius) curvature of the cornea.


Information that provides areInformation that provides are::

The radii of curvature of the cornea. The directions of the principal meridians

of the eye, determining whether the astigmatism is with-the-rule or against-the-rule -- a key factor in contact lens fitting.

The degree of corneal astigmatism. The presence of any corneal distortion.


KERATOMETERKERATOMETER

Keratometry is essential in the fitting of contact lenses, evaluating and following patients with keratoconus, and in the determination of appropriate intraocular lens implant power for patients undergoing cataract surgery.


Optical PrinciplesOptical Principles

Cornea is a convex refracting surface In order to find the refracting power of the

cornea, we need to reflect an object of a known size at a known distance off the corneal surface. Then determine the size of the reflecting image with measuring telescope and calculate the refractive power of the cornea based on the refractive index of n= 1.3375


The keratometer usually shows the corneal curvature in diopters of power or in millimeters and diopters.

If a millimeter scale is not given, it can be found from tables or by using the surface power formula using the refractive index 1.3375. n - 1 Surface power formula: D = ......... R

D = the dioptric power of the cornea n = the refractive index of the keratometer used (1.3375)

R = the radius of curvature of the cornea in meters


Optics Optics Primary Care Optometry-Grosvenor page 224Primary Care Optometry-Grosvenor page 224


FIGURE 1FIGURE 1


Keratometer Mechanical Keratometer Mechanical Adjustments and PartsAdjustments and Parts

A. Focusing KnobB. Rotating Grip for Locating the AxisC. Chin RestD. Locking KnobE. Elevating KnobEP. EyepieceG. Head Rest Adjusting KnobH. Occluding ShieldHR. Head RestJ. Chin Rest Adjusting KnobK. Leveling SightM. Horizontal Measuring DrumN. Vertical Measuring DrumS. Axis Scale


Bausch & Lomb KeratometerBausch & Lomb Keratometer

One position keratometer: it measures two meridians at the same time

It contains two prisms


PROCEDUREPROCEDURE

1) Eyepiece Adjustment : turn the eyepiece (EP) counterclockwise fully. Place a white sheet of paper in front of the telescope part. Turn the eye piece clockwise until the specified target in the instrument becomes in sharp focus.


2) Adjust the height of the instrument and align the outer canthus of the patient with the leveling sight sign (item K in Figure 2)

3)Occlude the non-tested eye (with H).


4)Ask the patient to look at the center of the instrument. If you are aligned, you can see the circles in the eye. The patient can see the picture of their eye in the instrument.


5)Adjust the focusing knob( item A in Fig 2) such that you get a single image of the central circle.(A,B in Fig 3)

6) Align the side and upper circles with the central circle by rotating the body of the instrument(using item B in Fig 2) .(C in Fig3)


7) Use the horizontal alignment knob(M) to bring the crosses on each other.(E in Fig 3)

8) Use the vertical alignment knob(N) to bring the crosses on each other.

9) Read the values from the scales (M & N) and the angle from (S).


FIGURE 3FIGURE 3


Javal-Schiotz KeratometerJaval-Schiotz Keratometer

Two position Keratometer

Only measures one meridians at one time.


ProcedureProcedure

Follow steps 1-4 of the above instrument 5) Adjust the instrument to see a clear image of

the rectangle and staircase targets


6) Rotate the instrument to align the line in rectangle with the line in the staircase target.

7) Turn the measuring knob until staircase and rectangle just overlap.

8) Read the values from the scales.


RECORDINGRECORDING

K flattest meridian@angle Steepest meridian@angle

Example 42.00@180

45.00@90

-3.00x180 Mires Clear & Regular

Also determine if the mires are clear and regular.

Health & Medicine

keratometry