The goal of refractive surgery is to reduce dependence on contact lenses or glasses for use in routine daily activities. A wide variety of surgical techniques and technologies are available to accomplish this goal. To ensure optimal refractive outcomes, surgeons must conduct careful preoperative evaluations using appropriate diagnostic tools to determine patient candidacy.

Refractive surgical procedures can be categorized broadly as corneal, scleral, or intraoc­ ular (­Tables 1-1, 1-2). Keratorefractive (corneal) procedures include incisional, ­laser ablation, refractive lenticule extraction (ReLEx, FLEx, SMILE), corneal inlays and onlays, corneal collagen shrinkage, and corneal crosslinking techniques. Scleral procedures involve ­either scleral implants or ­laser excision of the anterior ciliary sclera. Intraocular refractive proce­ dures 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, focusing on keratorefractive procedures, provides an overview of rel­ evant fundamental corneal properties, imaging for refractive surgery, and the effects of such surgery on the cornea. It includes review of the optical princi­ples discussed in BCSC

­Table 1-1  ​Overview of Keratorefractive Procedures

Section 3, Clinical Optics; refractive errors (both lower-­ and higher-­order aberrations); corneal biomechanics; corneal topography and tomography; wavefront analy­sis; ­laser bio­ physics and ­laser–­tissue interactions; and corneal wound healing.

The majority of the optical power of the eye derives from the combined effect of the air–­ tear interface and the corneal curvature. The normal tear film has minimal deleterious ef­ fect on vision. However, an abnormal tear film—­such as excessive tear film (eg, epiphora) or altered tear film (eg, dry eye or blepharitis)—­can dramatically decrease the quality of vision. The anterior corneal curvature contributes approximately two-­thirds of the eye’s refractive power, about +48.00 diopters (D). However, the overall corneal power is ap­ proximately +42.00 D as a result of the negative power (approximately –6.00 D) of the posterior corneal surface.

­Table 1-2  ​Overview of Scleral and Intraocular Refractive Procedures

IOL = intraocular lens; PMMA = polymethyl methacrylate.

Standard keratometers and Placido-­based corneal topography instruments mea­sure the anterior corneal radius of curvature and estimate total corneal power from the 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 9). The normal cornea flattens from the center to the periphery by up to 4.00 D and is flatter nasally than temporally; this shape is described as prolate (Fig 1-1).

The majority of keratorefractive surgical procedures change the refractive state of the eye by altering corneal curvature. The tolerances involved in this process are relatively small. For example, achieving a refractive change of 2.00 D may require altering the cornea's thickness by less than 30 μ m. Thus, attaining predictable results is sometimes problematic because minuscule changes in the shape of the cornea may produce large changes in refraction.