2020–2021 BCSC Basic and Clinical Science Course™
13 Refractive Surgery
Chapter 1: The Science of Refractive Surgery
The cornea consists of collagen fibrils arranged in approximately 200 parallel lamellae that extend from limbus to limbus. The fibrils are oriented at angles to the fibrils in adjacent lamellae. This network of collagen is responsible for the mechanical strength of the cornea. The fibrils are more closely packed in the anterior two-thirds of the cornea and in the axial, or prepupillary, cornea than they are in the peripheral cornea (see BCSC Section 8, External Disease and Cornea).
Structural differences between the anterior and posterior stroma affect the biomechanical behavior of the cornea. These include differences in glycosaminoglycans as well as more lamellar interweaving in the anterior corneal stroma; thus, the anterior cornea swells far less than the posterior cornea does. Stress within the tissue is partly related to intraocular pressure (IOP) but not in a linear manner under physiologic conditions (normal IOP range).When the cornea is in a dehydrated state, stress is distributed mainly to the posterior layers or uniformly over the entire cornea. When the cornea is edematous, the anterior lamellae take up most of the strain. Most keratorefractive procedures alter corneal biomechanical properties either directly (eg, radial keratotomy) or indirectly (eg, excimer laser stromal ablation by means of tissue removal). The lack of uniformity of biomechanical load throughout the cornea explains the variation in corneal biomechanical response to different keratorefractive procedures. LASIK has a greater overall effect on corneal biomechanics than does photorefractive keratectomy (PRK) and small-incision lenticule extraction (SMILE), not only because a lamellar flap is created but also because the laser ablation occurs in the deeper, weaker corneal stroma, modifying a greater amount of corneal tissue.
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.