The contact lens prescription is a document containing several specific measurements to ensure the lens fits the eye correctly and provides the necessary vision correction. These parameters typically include the sphere or power (PWR/SPH), which corrects for nearsightedness or farsightedness, the base curve (BC), which matches the curvature of the eye, and the diameter (DIA) for proper fit. When an eye requires correction for astigmatism, two additional measurements appear: Cylinder (CYL) and Axis (AX). The Axis measurement is a number that tells manufacturers and eye care professionals exactly where to position the corrective power within the lens.
Understanding the Axis Measurement
The Axis (AX) is a degree measurement that specifies the precise angle of the cylinder power required for vision correction. This number does not indicate the strength of the prescription, but rather the orientation of the corrective lens component on the eye. The value is listed as a number between 0 and 180 degrees, which corresponds to an imaginary protractor superimposed over the front surface of the eye.
A measurement of 90 degrees represents the vertical meridian of the eye, while 180 degrees indicates the horizontal meridian. This angular coordinate serves as a map, telling the lens exactly where the vision problem exists so the corrective power can be placed in the correct direction. The Axis value must be accurate because even a slight rotation of the lens on the eye can lead to blurred or fluctuating vision.
The Role of Astigmatism
The Axis measurement addresses astigmatism, a common type of refractive error. Astigmatism occurs when the cornea, the clear front dome of the eye, or sometimes the internal lens, is not perfectly spherical. Instead of having a uniform, round shape like a basketball, the surface is shaped more like the side of an American football.
In a perfectly round eye, light rays entering the eye are bent evenly and focus at a single point on the retina, resulting in clear vision. However, the football-like shape of an astigmatic eye means it has two different curves—a steeper curve and a flatter curve. This uneven curvature causes light entering the eye to focus at multiple points instead of one, leading to blurred or distorted vision at all distances. The severity of the astigmatism is indicated by the Cylinder (CYL) value on the prescription. The Axis value then pinpoints the specific angular location of this uneven curvature, which is essential because the corrective lens needs to align precisely with the eye’s unique orientation of astigmatism.
How Toric Lenses Use the Axis
Contact lenses designed to correct astigmatism are called toric lenses, and the Axis value is central to their design and function. The Axis dictates the specific orientation of the corrective cylinder power that is ground into the lens material. Unlike a standard spherical lens that has the same power across all meridians, a toric lens has different powers in two perpendicular meridians to neutralize the eye’s uneven curvature. For a toric lens to work, the cylinder power must remain constantly aligned with the prescribed Axis on the eye. Since contact lenses can rotate with blinks or eye movements, manufacturers incorporate various stabilization features to maintain this alignment.
Stabilization Methods
One common technique is prism ballasting, where the lower edge of the lens is made slightly thicker to use the pressure from the eyelid to force the lens to settle in the correct position. Another method is dynamic stabilization, which uses thin zones on the top and bottom of the lens to interact with the eyelids during blinking, helping the lens quickly return to its intended orientation. Some newer designs, like those using an accelerated stabilization method, feature four thicker stability zones to harness the blink for realignment. These solutions ensure the lens does not rotate away from the prescribed Axis.