3.1.1 Instruments to measure corneal curvature/assess its regularity

Question 1

How does keratometry work:

Answer 1

• Keratometry works on the principle of recording the image size reflected from a known-sized object. Given the object size and distance from image to object, the radius of curvature of the cornea can be calculated.
• In keratometry, the object, which may be two separate mires or two points at distinct distances on a mire, reflects off a 3.2 mm central zone on the cornea

Question 2

Indications for keratometry

Answer 2

• Contact lens assessments
- provide baseline and aftercare examinations values of corneal curvature and any induced changes
- determining side of astigmatic surface
• RGP lens fitting
• NITBUT
- Distorted mires can indicate poor tear quality
• Monitoring corneal pathology

Question 3

Interpreting results

Answer 3

• 0.1 difference = 0.50DC i.e. difference in Ks x5 = corneal astigmatism
• High number = flatter cornea
• Average 7.80mm
• Radius ranges 7.4 – 8.8mm

Example: 8.00/7.40 -2.00/-3.00 x 180
- Sphere power is located at the axis (-2 @ 180)
- The sum of sphere & cyl is located at 90 from axis (-5 at 90)
- 3D of corneal astigmatism

Question 4

Types of astigmatism

Answer 4

With the rule– vertical meridian stays close to 90 & is steepest (steepest = smallest), flattest at 180
- Smallest number is the vertical
- +ve cyl at 90
- -ve cyl 180
- More common in kids

Against the rule– horizontal stays close to 180 & is steepest, flattest at 90
- Smallest number is the horizontal
- +ve cyl at 180
- -ve cyl at 90
- More common in adults

Oblique – principles not at 180 & 90 - 130-160, 120-150

Irregular – principle meridians not separated by 90

Question 5

How to use: keratometer

Answer 5

• Focus & align the instrument along principle meridians
• Corneal radius can then be measured by adjusting mires
• Measuring Ks to an accuracy of 0.1mm
• Two position – body needs rotated before each measurement
• Important to assess clarity of mires/record distortion

Question 6

Bausch and Lomb

Answer 6

• One position– double images produced side by side at 90 deg from each other
• Variable doubling – object size remains constant, which allows for measurement of TBUT
• Fixed mires

Question 7

Javal Schiotz

Answer 7

• Two position - requires rotation about axis to measure each of the principle meridians
• Fixed doubling – the distance between the mires are varied mechanically, when these are lined up, Ks are taken from scale along each meridian in 2 stages. This system only works at certain distance from eye
• mires

Question 8

Zeiss ophthalmometer + with manual keratometry limitations…

Answer 8

• two position / variable doubling / fixed mires
• Limitations: most manuals only estimate central 3mm of cornea – better representation would require corneal topography

Question 9

Automatic

Answer 9

• Two position instruments
• Use servomotors to drive the doubling device until alignment can be assessed optically using light-emitting and detecting diodes
• The machine prints out 3 measures
• Phi 2.4 = central 2.4mm corneal diameter (we use)
• Phi 3.3 = central 3.3mm corneal diameter
• Comparison of the 2 above gives an indication of corneal shape