2020–2021 BCSC Basic and Clinical Science Course™
11 Lens and Cataract
Chapter 7: Preoperative Considerations for Cataract Surgery
Corneal power is another major component of IOL power calculations. A 1.00-diopter (D) error in the calculation of corneal power causes a similar degree of error in the postoperative refraction. It is important to optimize the cornea prior to obtaining measurements; this includes adequately treating any ocular surface disease, as well as minimizing warpage from contact lens wear. Preferences vary, but most surgeons require the eyes to be several weeks free from rigid gas permeable (RGP) lenses, and less time for soft contact lenses. Calculating corneal power in eyes that have undergone refractive surgery can be problematic, because traditional measures assume a particular relationship between anterior and posterior curvature. Laser refractive surgery modifies this relationship by altering the anterior curvature. In addition, accounting for posterior corneal astigmatism is increasingly recognized as an important factor in avoiding postsurgical refractive surprises, especially in patients who will be receiving toric IOLs.
Corneal power may be estimated or measured via several techniques, including manual keratometry, corneal topography, and corneal tomography:
In manual keratometry, a small central portion of the cornea (3.2 mm) is measured, and the radius of curvature is calculated based on the size of a reflected image. This technique, which requires a skilled operator, allows direct visualization of tear film irregularity and can reveal cornea irregularities. It measures only the anterior surface of the cornea and extrapolates the corneal power by assuming a fixed relationship to the posterior surface.
In corneal topography, a map of the corneal contour is created. Various map-creation methods exist. Placido disk–based topography, which measures the anterior surface and can provide additional information about the cornea surface, is particularly helpful in analyzing irregular astigmatism or detecting early keratoconus.
In corneal tomography (ie, Scheimpflug imaging or optical coherence tomography [OCT]), the anterior and posterior curvature and cornea thickness can be measured. Scheimpflug imaging is incorporated into platforms to assist in IOL selection. OCT, which has higher axial resolution, can be useful in the presence of cornea opacities. Tomography may be particularly useful in patients who have previously undergone keratorefractive surgery, desire a toric or presbyopic IOL, or might benefit from astigmatism-correcting corneal incisions.
Excerpted from BCSC 2020-2021 series: Section 11 - Lens and Cataract. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.