Chapter 1: The Science of Refractive Surgery
Refractive surgery aims to reduce dependence on contact lenses or spectacles for use in routine daily activities. A wide variety of surgical techniques are available, and all require an appropriate preoperative evaluation to determine the best technique and ensure the optimal outcome for each patient individually.
Refractive surgical procedures can be categorized broadly as corneal or intraocular (Table 1-1). Keratorefractive (corneal) procedures include incisional, laser ablation, lamellar implantation, corneal collagen shrinkage, and corneal crosslinking techniques. Intraocular refractive procedures include phakic intraocular lens (PIOL) implantation and cataract surgery or refractive lens exchange (RLE) with implantation of a monofocal, toric, multifocal, accommodative, or extended depth of focus intraocular lens. Each technique has advantages and disadvantages and should be specifically matched to the patient.
This chapter reviews the fundamental corneal properties relevant to refractive surgery (focusing on keratorefractive procedures), corneal imaging for refractive surgery, and the effects of keratorefractive surgery on the cornea. It includes review of the optical principles discussed in BCSC Section 3, Clinical Optics; refractive errors (both lower- and higher-order aberrations); corneal biomechanics; corneal topography and tomography; wavefront analysis; laser biophysics and laser–tissue interactions; corneal biomechanical changes after surgery; and corneal wound healing.
The air–tear-film interface provides the majority of the optical power of the eye. Although a normal tear film has minimal deleterious effect, an abnormal tear film can have a dramatic impact on vision. For example, either excess tear film (eg, epiphora) or altered tear film (eg, dry eye or blepharitis) can decrease visual quality.
The optical power of the eye derives primarily from the anterior corneal curvature, which produces about two-thirds of the eye’s refractive power, approximately +48.00 diopters (D). The overall corneal power is less (approximately +42.00 D) as a result of the negative power (approximately –6.00 D) of the posterior corneal surface. Standard keratometers and Placido-based (ie, based on an analysis of corneal reflections of a concentric ring image) corneal topography instruments measure the anterior corneal radius of curvature and estimate total corneal power from these front-surface measurements. These instruments extrapolate the central corneal power (K) by measuring the rate of change in curvature from the paracentral 3–4-mm zone; this factor takes on crucial importance in the determination of IOL power after keratorefractive surgery (see Chapter 11). The normal cornea flattens from the center to the periphery by up to 4.00 D (this progressive flattening toward the peripheral cornea is referred to as a prolate shape) and is flatter nasally than temporally.
Table 1-1 Overview of Refractive Procedures
The majority of keratorefractive surgical procedures change the refractive state of the eye by altering corneal curvature. The tolerances involved in altering corneal dimensions are relatively small. For instance, changing the refractive status of the eye by 2.00 D may require altering the cornea’s thickness by less than 30 μm. Thus, achieving predictable results is sometimes problematic because minimal changes in the shape of the cornea may produce significant changes in postoperative refraction.
Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.