Article

PRESCRIBING FOR ASTIGMATISM

ASPHERIC LENSES, ASTIGMATISM, AND ABERRATIONS

The design of aspheric contact lenses can often be abstract and difficult to understand. The following are nine clinical pearls to help eyecare practitioners understand how contact lenses can induce or reduce visual aberrations.

Pearls of Wisdom

  1. The cornea itself is aspheric. The term aspheric refers to a surface that does not have a uniform radius of curvature as a perfect sphere does. Normal corneas have a prolate shape that has a steeper central curvature that flattens out toward the limbus. Patients who have undergone refractive surgery often have corneas with an oblate shape that have a flatter central curvature that steepens out toward the limbus.
  2. Higher-order aberrations (HOAs) are less visually significant than are lower-order aberrations (LOAs). LOAs consist of zeroth, first-, and second-order aberrations, which are prism, refractive error, and astigmatism, respectively. HOAs (third through tenth) include coma, spherical aberration, and trefoil, among others. Typically, the higher the order, the less visually significant the aberration is. This means that practitioners should consider HOAs as a possible cause of visual blur only if they have already corrected the LOAs.
  3. HOAs become more visually significant with increasing pupil size. A common patient complaint is ghosting or halos around sources of light at night. This is because the larger pupil size increases the magnitude of HOAs, which are mostly spherical aberrations.
  4. Clinically, aberrometry measurements can be useful when trying to determine the cause of a patient’s vision complaints. Aberrometers measure HOAs by calculating the root-mean-square (RMS) in microns. An RMS value of approximately < 0.300μm is considered normal.
  5. Aspheric lenses aim to improve visual quality by decreasing HOAs, which are mostly spherical aberrations. Lenses, especially those with high amounts of power, can induce HOAs. One goal of aspheric lenses is to reduce these induced aberrations.
  6. Many multifocal contact lenses achieve add power with high degrees of eccentricity. Eccentricity is a measurement of the deviation from a sphere. Multifocal optics are achieved in lenses that have high eccentricity and rapidly steepen or flatten toward the edge of the lens.
  7. Aspheric soft lenses generally have the asphericity on the front surface. Aspheric optics are more beneficial on the front surface of the lens due to the greater difference between the indices of refraction between air and the contact lens versus the lens and the tear film.
  8. Corneal GP lenses can be front-aspheric, back-aspheric, or both (bi-aspheric). Front-surface asphericity in GP lenses can be utilized to decrease HOAs or to create multifocal optics. Back-surface asphericity may be utilized to better contour a patient’s cornea. A bi-aspheric GP lens has both front- and back-aspheric surfaces.
  9. Aspheric optics do not correct residual astigmatism. In the past, there were claims that aspheric optics could correct small degrees of residual astigmatism. This is untrue; aspheric optics can affect the magnitude of HOAs, but they do not correct or decrease the amount of residual astigmatism, which is an LOA.

In Summary

With dozens of soft and GP lens options available, the term “aspheric” may be used rather loosely. Communicating with laboratories may be helpful to gain insight on the different aspheric lens designs. It is important to understand the potential benefits of these designs to provide the best possible care for contact lens-wearing patients. CLS