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.
13. 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.
14. 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.
15. 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.
16. • 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.
17. • 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.
18. • 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.
19. 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°
20. 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)
21. 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.
22. 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.
23. 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
24. • 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.
