Murat V. Kalayoglu, M.D., Ph.D.
Development of the excimer laser has enabled a variety of refractive surgical procedures to be developed over the past several decades. LASIK remains the excimer procedure of choice for most refractive surgeons aiming to correct myopia. Currently, few alternatives to LASIK are available for patients with moderate to high myopia. However, there are two groups of individuals who may benefit from non-excimer procedures. The first is the mild myopia group – that is, individuals with myopia up to three diopters. A subset of these individuals may benefit from implantation of intracorneal ring segments within the peripheral cornea to correct their refractive error. The second group is composed of individuals with mild hyperopia; appropriate candidates may opt to undergo a thermal corneal treatment – thermokeratoplasty - to correct their refractive error.
Thermokeratoplasty, or the delivery of ‘heat’ to portions of the cornea, enables alteration of the intrastromal collagen in a precise, controlled manner. In effect, the collagen fibrils shrink after therapy. If the treatment is applied to the corneal periphery, then the collagen shrinks to create a “purse-string” effect – that is, it results in a relative elevation of the central cornea in relation to the periphery. The resulting steepening at the visual axis leads to increased refractive power with a subsequent myopic shift. The surgeon can therefore treat a concentric ring of spots at the corneal periphery to “tighten” the purse-string and reduce several diopters of hyperopia. Theoretically, planned, uneven application of spots can also correct astigmatism.
The two alternatives to hyperopic LASIK both rely on the abovementioned concept of controlled delivery of energy in a concentric ring around the corneal periphery, “tightening” the purse-string and inducing a myopic shift at the visual axis. The original procedure, approved for use in the United States in 2000, uses a holmium YAG laser to deliver the energy and is termed laser thermal keratoplasty (LTK). An alternative treatment uses radiofrequency waves to deliver the energy in a similar concentric pattern intrastromally at the peripheral cornea. The latter procedure is named conductive keratoplasty (CK) and also has gained FDA approval to be used for some patients with hyperopia in the United States. LTK produces a gradient thermal effect and a cone-shaped zone of collagen shrinkage. The radiofrequency energy employed by CK is applied by a hair-thin tip to penetrate the cornea, which affects deeper layers to create uniform columns of collagen shrinkage.
LTK is approved for use in individuals 40 years or older and who have +0.75 to +2.50 diopters of hyperopia and +1.00 diopter or less of astigmatism. The treatment involves application of topical anesthesia, placement of a lid speculum, drying of the cornea and delivery of a short burst of YAG laser energy to the cornea. Two concentric rings of laser spots are applied, each requiring 1.4 seconds to create. During the procedure, the patient is asked to look at a fixation light while the surgeon simultaneously delivers the 8 pulses for the first ring of spots, then the next 8 pulses for the second ring of spots just outside (peripheral) to the first treatment zone. The entire procedure therefore takes 2.8 seconds to complete. Furthermore, the treatment is non-contact and painless.
CK is approved for use in individuals over the age of 40 and who have +0.75 to +3.25 diopters of cycloplegic spherical hyperopia as well as less than or equal to +0.75 diopters of refractive astigmatism. The procedure involves application of topical anesthesia, placement of the lid speculum, drying of the cornea and placement of 8 spots in a concentric ring around the peripheral cornea. The spots are applied individually by the ophthalmologist, using a keratoplast tip designed to penetrate 450 um of corneal stroma. Such penetration of corneal stroma to a mid to deep stromal depth could allow for increased stability of results, since the depth at which collagen fibrils shrink may determine the degree of permanency of refractive correction. Ideally, the surgeon places the 8 spots within 1 minute, thereby completing the entire procedure within several minutes. The CK unit is small and mobile, allowing the surgeon to use it within multiple exam rooms.
Both LTK and CK are safe, well-tolerated, and effective thermokeratoplasty procedures for the correction of mild-to-moderate hyperopia. The procedures are approved for minimal astigmatism, and most surgeons obtain computerized topography before the treatment to ensure optimal outcomes. Additional uses of thermokeratoplasty could include treatment of presbyopia by inducing a monovision myopic shift in the non-dominant eye. Regression can occur in treated patients, and since these procedures are relatively new, data from longitudinal studies will be needed to determine the degree of regression after many years. However, since neither LTK nor CK require the keratome, they are attractive alternatives to hyperopic LASIK for many patients.