Keratometry is used to measure the curvature of the cornea by reflecting light off the anterior corneal surface. It involves using a keratometer instrument to project illuminated rings onto the cornea and measuring the reflected image to determine the corneal radius of curvature. Keratometry is useful for assessing corneal astigmatism, estimating refractive error, and aiding in contact lens and IOL power calculations. Modern automated keratometers can measure the cornea in multiple meridians simultaneously.

2. • Keratometry (Ophthalmometry)
• Kerato = Cornea
• Metry = Measurement
• Keratometry
• Measurement of the anterior surface of the
cornea
• usually 2-3 mm,
• Anterior-7.80mm,posterior-6.5 mm
• Refractive power anterior is =48D,posterior is -5D

3. Uses of Keratometry
• Measurement of corneal astigmatism.
• Estimate radius of curvature of cornea
• helps in contact lens fitting.
• Assess integrity of cornea and/or tear film.
• Detection of irregular astigmatism – ex.keratoconus
• Assess refractive error in cases with hazy media (Rough estimate,
comparison of two eyes).
• Establish baseline data – should be done in all patients.
• Patient may later want Contact lens or develop an injured/diseased
cornea.
• IOL Power calculation (Pre-op Cataract Surgery workup).
• Pre & post surgical astigmatism.
• D/D of axial versus curvatural anisometropia.
• Progressive myopia.

4. Principle

5. Doubling Principle
Biprism -2 images Move equally as eye moves
Depending on the position of prism – if
distance↓, doubling ↑

11. optics

12. Caliberation
• 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.

13. Preparation
• Adjust instrument for patient
• Adjust height of patient’s chair & instrument to a
comfortable position for both patient & examiner.
• Instruct patient to place chin on chin rest & forehead
against forehead rest & adjust for the patient
• Raise or lower chin rest until patient’s outer canthus is
aligned with hash mark on upright support of
instrument.
• From outside instrument, roughly align barrel with
patient’s eye by raising or lowering instrument and by
moving it to left or right until a reflection of mire is
seen on patient’s cornea.

14. Procedure
• Instruct patient
• Keep eyes open wide and blink normally.
• Try not to move the head nor speak.
• Look at the reflection of own eye in the
keratometer barrel.

15. • 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.

16. • 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.

17. • oblique Astigmatism 2 + signs will not be
aligned Entire optical instrument is rotated till
the two plus signs are aligned
• A scale associated with it indicates in degrees,
one meridian of oblique astigmatism.
• Corneal radius of power is then measured in
this meridian and in the meridian 90 degrees.

18. astigmatism
• Irregular: principal meridians are not
perpendicular to each other -Produce distorted
mires
• Regular: principal meridians are perpendicular
• With-the-rule: more power in the vertical
meridian (greatest curvature) and horizontal
meridian is flatter
• Against-the-rule: more power in the horizontal
meridian and vertical meridian is flatter
• Oblique: principal meridians lie between 20° &
70° and 110° & 160°

19. Bausch & Lomb Keratometer
• Range – 36.00 to 52.00 D
• Normal values – 44.00 to 45.00 D
• To increase the range – Place +1.25 D lens in
front of aperture to extend range to 61 D
(ADD 9 D)
• Place -1.00 D lens in front of aperture to
extend range to 30D (SUBTRACT 6 D)

20. Automated Keratometers
• Focuses the reflected corneal image on to an
electronic photosensitive device,which intantly
records the size and computes the radius of
curvature.
• No doubling device is needed.
• Measures angle size in many meridians so it
computes angle as well as power in many
meridians.
• Absence of annoying glare of brightly illuminated
mires.
• Do not calculate clarity of cornea.

21. Surgical Keratometer
• Attached to operating microscope.
• Helpful in monitoring the astigmatism during Corneal surgery.
• Accuracy limited
• Difficulty in alligning patients visual axis & Keratometers’s optical
axis.
• Caliberated for a fixed distance from anterior cornea.
• Different microscope objective lenses result in different focal
lengths and therefore different working distance.
• Air in the anterior chamber results in the second target reflection.
• External pressure on the globe results in a change in a corneal
curvature.

22. Limitations of Keratometry
• Measures refractive status of a very small central area
of cornea (3 mm), ignoring the peripheral corneal
zones.
• Accuracy lost when measuring very flat or very steep
cornea.
• Small corneal irregularities would preclude the use of
keratometer due to irregular astigmatism.
• One position instruments assume regular astigmatism.
• Distance to focal point is approximated by distance to
the image.
• Autokeratometers do not evaluate the quality of
cornea

23. • Unable to locate keratometric mires
• instrument and/or patient not aligned properly.
• Transient Mire clarity
• Ask patient to blink & measure quickly/put artificial tears.
• Transient Mire focus-
• Ensure that patient’s forehead is secured against the head rest.
• Unsteady Patient gaze -Close other eye.
• H & V mires cannot be measured concurrently
• Irregular Astigmatism.
• Only 1 minus sign is visible
• Patient’s eyelid drooping
• Only 1 plus sign is visible
• Occluder is in the way.