Sphere, cylinder and axis are the clinical refraction components that describe what many refer to as lower-order aberrations. Other optical aberrations in the visual pathway of the eye, such as coma and spherical aberration, which used to be referred to as corneal irregularity or irregular astigmatism, are now collectively called higher-order aberrations.
To conceptualize a wavefront of the aberrations of the eye, consider a light source emanating from the macula and going back out of the eye. Light rays are refracted as they pass through the lens and cornea. A wavefront is constructed by forming a surface using the ends of the exiting light rays. The X-Y dimensions of the wavefront would be the same as the entrance pupil. The Z dimension is representative of the aberration of the eye, also called the optical path deviation. An emmetropic eye without aberrations would have parallel rays exiting the eye and hence a flat wavefront. If the eye is myopic, exiting light rays converge so the wavefront would be concave. If the eye is hyperopic, the light rays diverge, making the wavefront convex. A saddle-shaped wavefront would represent astigmatism. A more complex wavefront would represent higher-order aberrations (Figure 1).
A series of polynomials are used to mathematically describe the shape of the wavefront. Several Zernike polynomials represent aberrations common in clinical practice, in particular defocus (sphere) and astigmatism. Higher-order aberrations are taking on clinical relevance as more is known about how they affect vision. To determine the total amount of higher-order aberrations, one computes the root mean square of the wavefront deviation after mathematically removing the sphere and cylinder components.
Most wavefront-sensing devices used in conjunction with an excimer laser employ a Hartmann-Shack sensor. This device divides light exiting the eye into a grid pattern for analysis by a lenslet array. Other techniques use ray tracing analysis (spatially resolved refractometry), a retinal imaging concept (Tscherning method), or dynamic skiascopy (time-based aberrometry using infrared slit scan of the optical system).
Wavefront technology applied to the eye is a powerful tool for refractive surgery. It represents a paradigm shift in the way optical aberrations can be measured, described, and treated. In addition to considerable utility as a diagnostic tool, wavefront technology has the potential to improve outcomes of refractive surgery and may result in vision better than before surgery. The technology has been used in astronomy for a number of years to improve the image quality of telescopes. It does this by correcting aberrations in reflecting mirrors as well as aberrations induced by the atmosphere. An example is the Hubble space-based telescope for which wavefront analysis was used to correct aberrations caused by an imperfect mirror, resulting in a significant improvement in image quality. Wavefront-guided (WFG) LASIK, also called custom LASIK, is a variation of the surgery where, rather than applying a sphero-cylindrical correction to the cornea, the laser is instructed to ablate a sophisticated spatially variant pattern based on measurements from an aberrometer. The goal of achieving a more optically perfect ablation still depends on appropriate patient selection, high-quality wavefront data, successful surgery, and accurately predicting and managing the changes that occur during healing.