Keratometry is a fundamental, non-invasive diagnostic test used during a comprehensive eye examination. It provides a precise measurement of the curvature of the eye’s clear, dome-shaped outer layer, known as the cornea. The cornea is responsible for two-thirds of the eye’s total focusing power, making its shape an important parameter in determining visual acuity. This measurement is foundational for addressing refractive errors and for planning specific eye procedures.
The Critical Applications of Corneal Measurement
The data obtained from keratometry is used for several important purposes in vision correction and surgical planning. One of its primary uses is the identification and quantification of astigmatism, a common condition where the cornea is shaped more like a football than a perfect sphere. By measuring the curvature along two principal meridians, the test determines the precise extent and axis of this irregular curvature. This information is then used to prescribe the correct cylindrical power and axis in glasses or contact lenses to neutralize the refractive error.
Corneal curvature measurements are necessary for ensuring the proper fit of contact lenses, particularly for more rigid designs. For rigid gas permeable (RGP) lenses, the lens must rest correctly on the corneal surface to allow for adequate tear film exchange and prevent damage. The keratometry readings define the base curve of the lens, which is the inner curve that mates with the front surface of the eye. A poor fit, whether too steep or too flat, can lead to discomfort, blurred vision, and potential long-term corneal health issues.
A third application is in the pre-surgical planning for cataract removal and the implantation of an intraocular lens (IOL). During this procedure, the natural lens is replaced with an artificial one, and the correct power of this replacement lens must be calculated precisely. Keratometry readings provide the necessary corneal power data, often called “K readings,” which are factored into biometric formulas to select the IOL power. Inaccuracies in the corneal measurement can result in a significant refractive error after surgery, affecting the desired visual outcome.
The Keratometry Procedure and Technology
The measurement is performed using an instrument called a keratometer, sometimes referred to as an ophthalmometer. This device works on the principle that the anterior surface of the cornea acts like a convex mirror, reflecting light from a projected object. The keratometer projects illuminated targets, known as mires, onto the patient’s cornea. The operator focuses on the reflection of these mires to measure their size and shape.
The patient is seated comfortably with their chin on a rest and their forehead against a support, looking straight into the instrument. The operator aligns the instrument so the mires are centered on the cornea. Since a steeper cornea produces a smaller reflected image, the instrument uses the size of the reflection to mathematically calculate the corneal radius of curvature. The process is quick and non-contact.
Traditional manual keratometers require the operator to align and superimpose the reflected mire images using internal prisms and adjustments. Modern automated versions are now widely used. Automated keratometers use an electronic sensor to instantly analyze the reflected light pattern and calculate the curvature. Regardless of the type, the measurement is typically taken over a small, central area of the cornea, usually within a 2 to 3-millimeter diameter, which is the most optically active zone.
Interpreting Keratometry Readings
Keratometry results are presented in two related units: diopters and millimeters. Diopters (D) represent the refractive power of the cornea, indicating how strongly it bends light. Millimeters (mm) express the radius of curvature, the physical measurement of the corneal shape. These two values are interchangeable through a standard optical formula that assumes a fixed refractive index for the cornea.
A typical measurement for an average cornea falls within the range of approximately 42 to 44 diopters. A reading lower than this range indicates a flat cornea, while a higher reading suggests a steeper cornea. The keratometer provides two primary values, known as the K readings, representing the power of the steepest and flattest meridians.
The difference between these two meridian values directly quantifies the amount of corneal astigmatism present. For example, a reading of 43.00 D at 180 degrees and 44.50 D at 90 degrees indicates 1.50 diopters of astigmatism. This specific notation also reveals the axis, or orientation, of the astigmatism, which is necessary for correcting the vision. The precision of these readings allows eye care professionals to accurately assess the overall shape of the central corneal surface.