4. Manual Keratometer
■ Invented in 1880, it was designed to measure the front (anterior) surface
curvature of the cornea
■ Only measures an area 3-4 mm wide…This is the central portion of the
cornea known as the APEX…which is the point of maximum curvature
■ The Keratometer contains prisms and measures 2 “PRINCIPAL
MERIDIANS” at the same time
■ Meridians are lines bisecting the cornea through is apex, perpendicular to
one another (90 °apart)
■ Meridians represent the maximum and minimum corneal power (or
steeper/flatter areas of cornea)
6. Keratometry Readings
Measurements are called “K” Readings” and provide us
with the following information:
Radii of curvature of cornea (determines Base Curve of Contact Lens)
The degree of corneal astigmatism (difference between the 2 readings)
Direction of principal meridians of the eye….Whether WTR, ATR or OBL
(a key factor in choosing contact lenses)
The presence of any corneal distortion
****All this information is utilized to fit contact lenses or monitor changes to
the cornea produced by wearing contact lenses
8. WTR, ATR,
OBL
WTR: Flatter area of the cornea lies
along the horizontal (180°) meridian and
the steepest curvature (most curved
part of the corneal surface) lies in or
close to the vertical meridian.
ATR: Flatter area of the cornea lies
along the vertical (090°) meridian and
the steepest curvature (most curved
part of the corneal surface) lies in or
close to the horizontal meridian.
OBL: Flatter area of the cornea lies
within 24° of 45 or 135°, which means…
between 21˚–69˚ and 111˚–159˚.
10. Direction of Astigmatism
■ WTR Astigmatism: 20°of 180° = 160° to 020°
■ ATR Astigmatism: 20° of 090° = 070° to 110°
■ OBL Astigmatism: everything else
11. “K” Reading Values
Range for “K” Readings:
■ Flat: < 42.50
■ Average: 42.50 – 44.50
■ Steep: > 44.50
***This determines the Base Curve for contact lens
selection
12. “Rule of Thumb Chart”
RULE OF THUMB CHART
(Can extend beyond 40.00) KERATOMETRY READINGS (Can extend beyond 47.00)
40.00 41.00 42.00 43.00 44.00 45.00 46.00 47.00
9.0 8.9 8.8 8.7 (8.6) 8.5 8.4 8.3 8.2
FLAT MEDIUM STEEP
BOZR
TO MAKE THE LENS FIT MORE LOOSELY
CHOOSE A FLATTER (LARGER) BOZR
OR
A SMALLER DIAMETER
TO MAKE THE LENS FIT MORE TIGHTLY
CHOOSE A STEEPER (SMALLER) BOZR
OR
A LARGER DIAMETER
13. Manual Keratometer
■ Range (industry standard) is 36.00 – 52.00 diopters
■ However, sometimes necessary to extend that range...can be done by
placing loose lenses in front of the keratometer:
+1.25D lens increases range by +9.00D
-1.00D lens decreases range by -6.00D
■ Uses refractive index of 1.3375
■ Measures in diopters, but some keratometers show measurements in
mm of radius
14. Manual Keratometer: Basic Components
■ Adjustable eyepiece
■ Adjustable chin rest and forehead rest
■ A knob to raise and lower the instrument to align with the
patient’s eye
■ Two power wheels to measure the corneal power in each of the 2
principal meridians
■ Axis scale to indicate the location of the 2 principal meridians
■ Target …known as “MIRES” which is reflected onto the patient’s
eye
■ Focusing knob
■ Focusing cross or reticle
15. Manual Keratometer Set-Up
■ Always disinfect the machine before use
■ Turn on machine…focus eyepiece by first turning adjustable
eyepiece as far counterclockwise as possible
■ Place a white paper in front of instrument to illuminate the reticle
■ Turn eyepiece clockwise until the reticle is in sharp focus
*****ALWAYS KEEP BOTH EYES OPEN WHEN FOCUSING AND WHEN
TAKING “K” READINGS TO AVOID ACCOMMODATION
Ask patient to place chin in chin rest and forehead against forehead
rest, aligning canthus marker of instrument with patient’s temporal
canthus
Starting with patient’s right eye, look thru eyepiece and start to
align/superimpose mires, making sure the large crosshair or reticle
is in the center of the lower right circle
17. Mires
a) First view seen by the examiner. The
central image is doubled indicating
the instrument is not yet focused
b) Centre cross and superimposed circles
c) Align the horizontal crosses (+)
d) Align the vertical dash (-)
18. Conventions for Keratometer Readings
■ Dioptric values are always recorded using four digits (42 diopters
is recorded as 42.00)
■ The meridian axis always recorded using three digits (90 degrees
is 090)
■ A minimum of three sets of readings must be taken(eg: right
eye/left eye, right eye/left eye, right eye/ left eye,) in order to
avoid error.
19. Conventions for Keratometer Readings
■ From the three sets of readings recorded for each meridian axis one must
be chosen as the Base “K” Reading using either or both of the following
processes:
SUBJECTIVE: Select the set of readings you have the highest degree of
confidence in.
OBJECTIVE: Select the median reading values for each eye. Do not compute
the mathematical average.
■ Readings must be recorded in 1/8 diopter increments (drum scales may
indicate 0.25D increments only, in which case if the dial is between two
markings it is a 0.12 (0.13) increment.)
20.
21. Manual Keratometer
■ 4 Main uses of the Manual Keratometer include:
1) Measuring Corneal Curvature
2) Measuring T.B.U.T.
3) Over-Keratometry (soft lens evaluation)
4) Reading radius of an RGP contact lens (while using a conta-
check)
22. Uses for the Manual Keratometer
1) To obtain corneal curvature reading for:
Determination of contact lens selection
Corneal changes (eg: warpage caused by improper c/l fit)
Confirmation of and comparison to the Spectacle Rx
Diagnosis of pathology (eg: keratoconus)
Determination of amount, location (lenticular vs corneal)
type of astigmatism (WTR, ATR, OBL)
23. Uses for the Manual Keratometer
2) Assessment of tear break up time (TBUT)
T.B.U.T. is indicated by a breakdown of the mire integrity
Lack of clarity & stability of mires indicates excess tearing…
observe frequency of blink, and note any tendency for
patient to stare or flutter.
***Generally, >10 seconds is thought to be normal, 5 to 10 seconds,
marginal, and < 5 seconds is an indication of a dry eye.
24. Uses for the Manual Keratometer
3) Assessment of soft contact lens fitting…evaluation of
mires after the blink….noting:
NO CHANGE-indicates alignment fitting (Clear, Clear, Clear)
BLURRING-indicates flat fitting (Clear, Blurry, Clear)
CLEARING-indicates steep fitting (Blurry, Clear, Blurry)
26. Uses for the Manual Keratometer
4) Determination of rigid lens curvatures:
B.C.O.R.
F.C.O.R.
***(requires the use of an accessory device called a
“Contacheck”)
28. Calibrating the Keratometer
■ Calibration done with a “Lensco Meter” or calibration kit which contain steel
balls of know radius, a magnetic holder and an allen key
■ Magnetic holder attaches to side of headrest
■ Take reading of the steel ball to see if Horizontal or Vertical drum readings are
off (they should match the reading of the steel ball) if off, use allen key to
loosen either H or V drums (or both)
■ Turn drums to the correct “K” reading of steel ball (eg: 42.50D), then tighten
the drums back up with the allen key
■ Recheck calibration
30. QUIZ: IN 2 WEEKS…OCT 2nd
■ Label the parts of the Manual Keratometer on a
diagram.
■ Determine if “K readings” are WTR, ATR or OBL.
■ Uses for the Keratometer
