Keratometry Walter Huang, OD Yuanpei University Department of Optometry

Keratometry Definition “Kerato” = cornea “Kerato” = cornea “metry” = measurement of “metry” = measurement of

Keratometer

Anatomy

Schematic

Keratometer It is used to determine the curvature (i.e., steepness or flatness), refracting power, and toricity (i.e., astigmatism) of the anterior central cornea in each of the two principal meridians The keratometer is used to measure the front surface corneal radii It is used to assess the integrity of the cornea and/or tear film

“K” Reading Uses Fitting and evaluating contact lenses Evaluating the corneal topography for any distortions or irregularities Patients with irregular corneas are often not correctable to 20/20 vision Patients with irregular corneas are often not correctable to 20/20 vision Determining whether the cornea or axial length is the cause of refractive error Evaluating the corneal healing process after surgery or injury affecting the cornea

How Does It Work? The cornea is both a convex refracting surface and a convex mirror An object of known size is reflected from a known distance to the corneal surface The size of the reflected image is measured with a telescope The refracting power of the cornea is calculated based on an assumed index of refraction (1.3375)

Calibration Should be done regularly to ensure the accuracy of “K” readings Mount a 5/8 inch steel ball bearing at the position close to that normally of the patient’s eye The steel ball has a known radius of curvature, which upon proper calibration of the keratometer, can be correctly read

Preparation Focus the eyepiece of the keratometer for the examiner’s eye Turn on the power Turn on the power Set the adjustable eyepiece as far counter- clockwise as possible Set the adjustable eyepiece as far counter- clockwise as possible Place a white sheet of paper in front of the instrument’s objective lens to retroilluminate the reticle (i.e., cross hairs) Place a white sheet of paper in front of the instrument’s objective lens to retroilluminate the reticle (i.e., cross hairs) Turn the eyepiece clockwise until the reticle is first seen in sharp focus Turn the eyepiece clockwise until the reticle is first seen in sharp focus

Preparation Adjust the instrument for the patient Adjust the height of the patient’s chair and the instrument to a comfortable position for both the patient and the examiner Adjust the height of the patient’s chair and the instrument to a comfortable position for both the patient and the examiner Unlock the instrument controls Unlock the instrument controls Instruct the patient to place his chin in the chin rest and his forehead against the forehead rest and adjust for the patient Instruct the patient to place his chin in the chin rest and his forehead against the forehead rest and adjust for the patient

Preparation Align the instrument for the patient Raise or lower the chin rest until the patient’s outer canthus is aligned with the hash mark on the upright support of the instrument Raise or lower the chin rest until the patient’s outer canthus is aligned with the hash mark on the upright support of the instrument From outside the instrument, roughly align the barrel with the patient’s right eye by raising or lowering the instrument and by moving it to the left or right until a reflection of the mire is seen on the patient’s cornea From outside the instrument, roughly align the barrel with the patient’s right eye by raising or lowering the instrument and by moving it to the left or right until a reflection of the mire is seen on the patient’s cornea

Procedure Instruct the patient Keep eyes open wide and blink normally Keep eyes open wide and blink normally Try not to move the head or speak Try not to move the head or speak Look at the reflection of own eye in the keratometer barrel Look at the reflection of own eye in the keratometer barrel

Procedure Look into the keratometer and refine the alignment of the image of the mires (three circles) on the patient’s cornea Focus the mires and adjust the instrument so that the reticle is centered in the lower right hand circle Lock the instrument in place

Procedure Focus the keratometer Out of focus Out of focus

Procedure Focus the keratometer In focus with the reticle centered In focus with the reticle centered

Procedure Adjust the horizontal and the vertical power wheels until the mires are in close apposition To locate the two principal meridians of the patient’s cornea, rotate the telescope until the two horizontal plus signs of the mires are perfectly continuous with one another

Procedure Locate the principal meridians Off the principal meridian in one direction Off the principal meridian in one direction

Procedure Locate the principal meridians On the principal meridian On the principal meridian

Procedure Adjust the horizontal power wheel until the plus signs of the mires overlap into one image The primary meridian is closest to 180 degrees The primary meridian is closest to 180 degrees

Procedure Adjust the vertical power wheel until the minus signs of the mires overlap into one image The secondary meridian is 90 degrees from the primary meridian The secondary meridian is 90 degrees from the primary meridian

Procedure Determine the refractive power Neither horizontal meridian nor vertical meridian is measured Neither horizontal meridian nor vertical meridian is measured

Procedure Determine the refractive power Horizontal meridian is measured (+ sign) Horizontal meridian is measured (+ sign)

Procedure Determine the refractive power Vertical meridian (- sign) is measured Vertical meridian (- sign) is measured Endpoint is reached Endpoint is reached

Procedure Throughout the procedure, adjust the focus and recenter the reticle as needed

Recording Record for each eye separately Record to two significant digits to the nearest eighth diopter using one of three methods Remember that it is the power meridian, NOT the axis, being recorded in keratometry

Recording Method 1 Horizontal Power/Horizontal Meridian Horizontal Power/Horizontal Meridian Vertical Power/Vertical Meridian Vertical Power/Vertical Meridian Example 1 OD 41.25/ /110 OD 41.25/ /110 OS 44.12/ /100

Recording Method 2 Horizontal Power at Horizontal Meridian / Horizontal Power at Horizontal Meridian / Vertical Power at Vertical Meridian Vertical Power at Vertical Meridian Example 2 OD at 020 / at 110 OS at 010 / at 100

Recording Method 3 Horizontal Power/Vertical Power at Vertical Meridian Horizontal Power/Vertical Power at Vertical Meridian Example 3 OD 41.25/42.87at 110 OD 41.25/42.87at 110 OS 44.12/43.50 at 100

Recording Record the amount of corneal astigmatism in diopters (i.e., the difference in power between the two principal meridians) Record the type of astigmatism WTR – more power in the vertical meridian WTR – more power in the vertical meridian ATR – more power in the horizontal meridian ATR – more power in the horizontal meridian OBL – major meridians within 15 。 of 045 and 135 OBL – major meridians within 15 。 of 045 and 135 Irregular – two principal meridians are not 90 。 apart Irregular – two principal meridians are not 90 。 apart

Recording Record the conditions of the mires Mires clear and regular Mires clear and regular Mires irregular and distorted Mires irregular and distorted

Interpretation The difference in power between the two principal meridians is the amount of corneal astigmatism In Optometry, the astigmatism is corrected by the minus cylinder lens From the K readings, the meridian of least refracting power indicates the position of the minus axis of the correcting cylinder

Interpretation Example 4 OD 42.50D at 180 / 44.50D at 090 OD 42.50D at 180 / 44.50D at 090 Corneal astigmatism = 2.00D Corneal astigmatism = 2.00D Correcting cylinder = -2.00DC x 180 Correcting cylinder = -2.00DC x 180 WTR astigmatism!! WTR astigmatism!!

Interpretation Example 5 OS 42.75D at 180 / 42.00D at 090 OS 42.75D at 180 / 42.00D at 090 Corneal astigmatism = 0.75D Corneal astigmatism = 0.75D Correcting cylinder = -0.75DC x 090 Correcting cylinder = -0.75DC x 090 ATR astigmatism!! ATR astigmatism!!

Expected Findings Average K readings are 43.00D to 44.00D The two principal meridians are expected to be 90 。 apart