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PURPOSE To assess the curvature，power, and toricity of the cornea．Keratometry may also be used to assess the integrity of the corneal/tear surface．
EQUIPMENT
·Keratometer or ophthalmometer
BASIC COMPONENTS OF THE KERATOMETER
Because keratometers differ，the examiner should review the specific instructions provided by the manufacturer for the keratometer or ophthalmometer he is using．The following components are common to all keratometers:
·Adjustable eyepiece for focusing the instrument for the examiner’s eye．
·Adjustable chin rest and forehead rest to sport the patient’s head during testing．
·A knob to raise and lower the instrument to align it with the patient’s eye．
·Two power wheels to measure the corneal power in each of the two principal meridians．
·An axis scale to indicate the location of the two principal meridians．The barrel of the instrument can be rotated to align the keratometer appropriately．
·Target (known as mires) which is reflected onto the patient’s cornea．
·Focus control knob or joystick to focus the mires, on the patient’s cornea.
SET-UP
·Disinfect the chin rest and forehead rest of the keratometer．
·The patient removes his glasses or contact lenses．
·Focus the eyepiece of the keratometer．
a．Turn on the power．
b．Set the adjustable eyepiece as far counterclockwise as possible．
c．Place a white paper in front of the instrument objective to retroilluminate the reticle．
d．Turn the eyepiece clockwise until the reticle is first seen in sharp focus．
·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．This is necessary on some keratometers．
·Instruct the patient to place his chin in the chin rest and his forehead against the headrest．
·Raise or lower the chin rest until the patient’s outer canthus is aligned with the hash mark on the right sport of the instrument or with the pointer on the side of the instrument．
STEP-BY-STEP PROCEDURE
1．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 mires is seen on the patient’s cornea (see Figure 3-4)．
2．Instruct the patient to look at the reflection of his own eye in the keratometer barrel．
3．Look into the keratometer and refine the alignment of the image of the mires (3 circles) on the patient’s cornea (see Figure 3-5A)．
4．Focus the mires and adjust the instrument so that the reticle is centered in the lower right hand circle(see Figure 3-5B)．
5．Lock the instrument in place．This is necessary on some but not all types of keratometers．
6．Adjust the horizontal and the vertical power wheels until the mires are in close apposition．
7．To locate the two principal meridians of the patient’s cornea，rotate the telescope until the two horizontal spurs on the mires are perfectly continuous with one another(see Figure 3-5C)．
8．Adjust the horizontal power wheel until the horizontal mires are coincident (see Figure 3-5D)．
9．Adjust the vertical power wheel until the vertical mires are coincident(see Figure 3-5D)．
Note：If the corneal astigmatism is irregular，the two principal meridians will not be 90° apart．In this case, after the power reading for the horizontal meridian is made, the examiner must readjust the barrel of the instrument to align the vertical components of the mires before adjusting the power wheel．
10．Throughout the procedure，adjust the focus and recenter the reticle as needed．
11．Observe the integrity of the cornea by observing the condition of the mires．
12．Roughly align the telescope with the patient’s left eye as described in step 1．
13．Repeat steps 2 to 11 on the patient’s left eye．
RECORDING
·Record for each eye separately．
·Record the power and the meridian for the horizontal meridian first (the primary meridian)．
·Write a slash mark after the primary meridian and record the power and meridian for the vertical meridian (the secondary meridian)．
·Record the amount of corneal astigmatism in diopters．
·Record the type of astigmatism：
WR-with the rule (more power in the vertical meridian)
AR-against the rule (more power in the horizontal meridian)
OBL-oblique (major meridians within 15°of 45 and 135)
Irregular-the two principal meridians are not 90°apart
· Record the conditions of the mires：mires clear and regular (MCAR) or mires irregular and distorted．
EXAMPLES
·OD 42.50 at 180/43.50 at 90；1.00D WR, MCAR
OS 47.37 at 180/41.37 at 90；6.00 D AR, mires distorted
·OD 41.75 at 180/43.75 at 70；2.00 D irregular astig；mires distorted
OS 43.12 at 135/41.87 at 45；1.25 D OBL；MCAR
If only the secondary meridian is recorded，the position of the primary meridian is assumed to be 90°away．
OD 42.00/43.O0 at 90；1.00 WR MCAR
0S 42.00/42.O0 at 90；sphere MCAR
EXPECTED FINDINGS
·Average K readings are 43.00 D to 44.00 D．
·The two principal meridians are expected to be 90°apart． |
