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Keratometer and Keratometry Readings: All you need to Know while interpreting keratometry readings
Keratometer
A keratometer, which is also called an ophthalmometer, is an instrument which measures the curvature of the cornea. It was originated by the German physiologist Hermann von Helmholtz in 1851, although an earlier model was developed in 1796 by Jesse Ramsden and Everard Home.
Commonly used Keratometers are Javal-Schiotz keratometer, The Bausch and Lomb Keratometer and others such as Automated Keratometer which is available with autorefractor keratometer, handheld keratometer
What is Keratometry?
Keratometry (K) is the procedure to measure the corneal curvature. It helps to determine the power of the cornea. Differences in power across the cornea (opposite meridians) results in astigmatism; therefore, keratometry measures astigmatism.
Uses of Keratometry
- Measurement of corneal astigmatism.
- Estimate radius of curvature of the cornea, which helps in the contact lens fitting.
- Assess the integrity of the cornea and/or tear film.
- Detection of irregular astigmatism – keratoconus/pterygium/corneal scarring.
- Assess refractive error in cases with hazy media (Rough estimate, comparison of two eyes).
- IOL Power calculation (Pre-op Cataract Surgery workup).
- Pre & post-surgical astigmatism.
- D/D of axial versus curvature anisometropia.
- Detect Rigid Gas Permeable lens flexure.
- Progressive myopia
Limitation of Keratometry
- Measures a small region of the cornea (2 points at the 3–4 mm zone)
- Does not provide information about the cornea central or peripheral to these points
- Ignores spherical aberration,
- It is susceptible to focusing on and misalignment errors
- Cannot accurately measure irregular corneas.
Keratometry Readings
K1
- Kf ( k flat )
- Curvature power of the flat meridian of the anterior surface of the cornea
- measured within the 3-mm central zone
- expressed in diopters (D).
- Normal K1 is > 34 D.
In myopic correction
- each –1 D correction reduces flat K by 0.75 D to 0.8 D.
- Final flat K should be > 34 D to avoid spherical aberrations with bad vision quality even with post-treatment zero refraction
K1 should be calculated using the axial curvature method rather than tangential one.
K2
- Ks ( k steep)
- Curvature power of the steep meridian of the anterior surface of the cornea
- measured within the 3-mm central zone
- expressed in diopters (D).
- Normal K2 is < 49 D.
In hyperopic correction
- Each +1 D correction will add 1.2 D to steep K.
- Final steep K should be < 49 D to avoid spherical aberrations with bad vision quality even with post-treatment zero refraction
K2 should be calculated using the axial curvature method rather than tangential one.
KM
- K-average
- Mean curvature power of the anterior surface of the cornea
- Measured within the 3-mm central zone
- Expressed in diopters (D).
- Considered to avoid flap complications.
- Km is should be > 40 D to avoid free-flap
- Km should be < 46 D to avoid button-hole complication.
Kmax
- Maximum curvature power of the whole anterior surface of the cornea
- Expressed in diopters (D)
- Normal Kmax is < 49 D
- The normal difference in Kmax between the two eyes is < 2 D
- The normal (Kmax-K2) difference in the same eye is < 1 D.
- Whenever the difference between kmax and K2 is ≥ 1D, K-max should be used instead of K2 for Hyperopic correction to avoid spherical aberrations with bad vision quality even with post-treatment zero refraction
K-max should calculate using the axial curvature method rather than the tangential one.
Topographic astigmatism (TA)
- The difference between K2 – K1
- Within the 3-mm central zone
- TA should be compared with manifest astigmatism (MA).
References:
1. Articles from Dr Reda Gomah
2.Slideshare.net. 2020. Keratometry. [online] Available at: <https://www.slideshare.net/KaranBhatia25/keratometry-55540991> [Accessed 28 September 2020].
3. Sciencedirect.com. 2020. Keratometer – An Overview | Sciencedirect Topics. [online] Available at: <https://www.sciencedirect.com/topics/nursing-and-health-professions/keratometer> [Accessed 28 September 2020].
