Laser Procedures for Myopia

In PRK, the excimer laser is applied to the anterior surface of the cornea for reshaping ( Fig. 1.2 ). The laser may be used to remove the corneal epithelium. Alternatively, the epithelium may be removed by scraping with a surgical blade or by using dilute ethanol and a cellulose sponge. For myopia of 1 D to 7 D, PRK has been shown to result in a high rate of preservation of best corrected visual acuity (BCVA) and minimal complications. In most series, 90% of patients achieve 20/40 or better uncorrected acuity and are within 1 D of emmetropia. In this moderate myopia group, the initial overcorrections generally regress toward emmetropia over several months, with stabilization after 6 to 12 months. Highly myopic patients often regress 6 to 12 months after surface PRK, presumably because of stromal regeneration and/or epithelial hyperplasia, which cause resteepening of the ablated zone. Dense subepithelial haze occurs rarely but is greater in PRK treatments exceeding 6 D and may reduce the BCVA. Mitomycin C has been applied during PRK treatments in order to decrease the incidence of haze formation. Artola et al. found that induced corneal aberrations after PRK for myopia created a multifocality that enhanced near acuity, which may delay the onset of presbyopic symptoms. However, this multifocality also reduced the quality of the retinal image for distance at low contrast.

Schematic illustration of myopic photorefractive keratectomy. The shaded area refers to the location of tissue subtraction. More stromal tissue is removed in the central as compared to the paracentral region.

LASEK and epi-LASIK are modifications of the PRK procedure in which the corneal epithelium is preserved, displaced prior to surface ablation, then replaced after laser application. Advantages over PRK include decreased postoperative discomfort, reduced postoperative scarring, and faster visual recovery. Prior to laser application, the epithelium is treated with 15% to 20% ethanol. This treatment weakens hemidesmosomal attachments between the corneal epithelium and the underlying Bowman membrane. The epithelial sheet can then be easily displaced and protected by moving it outside of the ablation zone. Following stromal ablation, the epithelial sheet is returned to its original location, covering the ablated area. Pallikaris et al. have described epi-LASIK, using an automated blade to remove the corneal epithelium mechanically, without the application of alcohol. They suggest that this technique should provide improved comfort and decreased haze formation compared to PRK, and histologic studies show better preservation of the corneal epithelial sheet when compared to LASEK.

LASIK is a two-stage procedure that combines lamellar surgery with laser application. It has become the most widely performed refractive procedure in the United States. Its main advantages over surface ablation procedures include faster visual recovery, less postoperative discomfort, and decreased incidence of postoperative corneal scarring or haze in patients with higher refractive errors. During LASIK, an anterior corneal flap is created and then is lifted, the excimer laser is applied to the stromal bed, and the flap is returned to its original position ( Fig. 1.3 ). The corneal flap can be created with either a microkeratome or an intrastromal laser. Microkeratomes are affixed to the globe via a suction device and the blade is passed via a manual or automated mechanism. The femtosecond (FS) laser is a solid-state laser with a 1053-nm wavelength that can be used to photodisrupt the corneal stroma with a preset depth and pattern. When used for LASIK, the laser creates the corneal flap prior to excimer laser application.

Schematic illustration of myopic and hyperopic laser in situ keratomileusis. A superficial corneal flap is raised. The shaded area refers to the location of tissue subtraction under the flap. After treatment, the flap is repositioned.

Customized corneal ablations use Q-based or “wavefront” aberrometers to detect and treat both spherocylindrical error and higher-order aberrations (HOAs) that can affect visual acuity. At the time of publication, these devices are approved in the United States for the treatment of myopic and astigmatic refractive errors. These custom lasers offer the possibility of improved vision compared to traditional excimer lasers because they address additional factors that may be contributing to blur in an individual’s optical system. A study of 132 eyes undergoing LASIK using the NIDEK Advanced Vision Excimer Laser (NIDEK) showed that fewer HOAs were induced when compared to non-custom LASIK, and 93% achieved uncorrected vision of at least 20/20. Preoperative sphere and cylinder ranged to −8.25 D and −3 D, respectively.

SMILE is a refractive procedure in which an FS laser is used to create a corneal stromal lenticule, which is extracted whole through a 2- to 3-mm incision ( Fig. 1.4 ). Outcomes have been noted to be similar to those of LASIK: in a meta-analysis by Zhang et al. comparing SMILE and FS-assisted LASIK (FS-LASIK) in 1101 eyes, no significant difference was found in refractive outcomes. SMILE was found to result in higher postoperative corneal sensitivity but fewer dry-eye symptoms than FS-LASIK. The biomechanical stability after SMILE surgery is expected to be greater than that after LASIK and may be comparable to PRK and LASEK. Fig. 1.5 compares RK, PRK, LASIK, and SMILE corneal biomechanics. Long-term follow-up has demonstrated a reduction in HOAs and minimal refractive regression, though some potential advantages, such as improved biomechanical stability and postoperative inflammation, have yet to be established.

Small-incision lenticule extraction (SMILE).

Simulated displacements in corneal shape on the surface resulting from the four refractive surgical procedures at a normal intraocular pressure of 15 mm Hg. The dark-red areas involve maximum displacements (>0.5 mm) outwards (body expansion), and the dark-blue areas involve zero displacement near the constrained boundary of the models. The “preoperative surface” is displacement of the normal cornea. (A) Radial keratectomy: maximum displacements located at middle incisions; (B) photorefractive keratectomy: maximum displacement at central cornea; and (C) LASIK and (D) SMILE: maximum displacements located around the central cornea (unit: mm).

(From Shih P-J, Wang I-J, Cai W-F, Yen J-Y. Biomechanical simulation of stress concentration and intraocular pressure in corneas subjected to myopic refractive surgical procedures. Sci Rep . 2017;7(1):13906. doi:10.1038/s41598-017-14293-0.)

Laser Procedures for Myopic Astigmatism

Compound myopic astigmatism can be treated with negative or positive cylinder ablation. Negative cylinder ablation flattens the central cornea in both the flat and the steep meridians. Positive cylinder ablation may allow a larger optical zone with no change in the central depth of ablation. One study examined 74 eyes with compound myopic astigmatism treated with the Meditec MEL 10 G-Scan (Zeiss) excimer laser. Patients were followed for 1 year and had myopia from −4.50 D to −9.88 D and astigmatism up to 4.00 D. At 1 year, mean postoperative spherical equivalent was −0.49 and mean cylinder refraction was 0.59.

Incisional Procedures for Myopia

RK achieves the best results in patients with low and moderate degrees of myopia (up to 5 D). In patients with higher amounts of myopia (6–10 D), the response to surgery is much more variable and undercorrection is more common. The age of the patient partially determines the upper limit of attainable correction. Older patients achieve a greater correction by approximately 0.75 D to 1.00 D per 10 years of age exceeding 35 years. Other patient variables may affect outcomes but are difficult to quantitate. For example, reports show that a premenopausal female with a flat cornea, low intraocular pressure, and a small corneal diameter may achieve less correction than would be generally predicted for a particular RK technique.

RK has been studied thoroughly, most notably by the National Eye Institute (NEI)–funded, multicenter Prospective Evaluation of Radial Keratotomy (PERK) study, a collaborative effort of 9 clinical centers. Predictability of results remains problematic. Early studies of predictability showed that about 70% of eyes have a residual refractive error within ±1 D of the predicted result and 90% within ±2 D. Later studies, with a staged approach, report 80% to 90% of eyes within 1 D of emmetropia. Stability of refraction after radial keratotomy is also inadequate. The 10-year PERK results revealed long-term instability of refractive errors; 43% of eyes changed refractive power in the hyperopic direction by 1 D or more (hyperopic shift) between 6 months and 10 years.

RK has essentially been replaced by newer excimer laser keratorefractive procedures. In 2003, one survey showed that 4% of cataract and refractive surgeons performed RK, down from 46% in 1996.

Incisional Procedures for Myopic Astigmatism

Naturally occurring astigmatism is very common and up to 95% of eyes may have some clinically detectable astigmatism in their refractive error. Between 3% and 15% of the general population has astigmatism greater than 2 D. Although there is some variability, approximately 10% of the population can be expected to have naturally occurring astigmatism greater than 1 D, where the quality of UCVA might be considered unsatisfactory. Surgically induced astigmatism can occur following cataract surgery. The incidence of astigmatism following extracapsular cataract extraction greater than 2 D is approximately 25% to 30%. With clear corneal incision phacoemulsification procedures, the incidence of astigmatism is much less. Beltrame et al. showed 0.66 D to 0.68 D of surgically induced astigmatism 3 months after phacoemulsification through a 3.5-mm clear cornea incision.

Astigmatic keratotomy (AK) involves performing transverse (also called tangential, or T) cuts in an arcuate or straight fashion perpendicular to the steep meridian of astigmatism ( Fig. 1.7A ). AK offers the patient a very good chance of significant improvement by correcting astigmatic errors. In general, patients with greater than 1.5 D of astigmatism may be candidates for AK. Deeper and longer incisions closer to the center of the cornea produce greater effect, but cuts beyond 75 degrees are not recommended. Effects of cuts increase dramatically with age. This procedure is now performed with the femtosecond laser and, rarely, with a diamond blade.

Correction of myopic astigmatism. (A) Astigmatic keratotomy. (B) Limbal relaxing incision. (C) Ruiz procedure.

Relaxing incisions in the steep meridian were developed by Troutman ( Fig. 1.7B ). These decrease astigmatism in the steep meridian, but the results can be unpredictable. This procedure may be combined with wedge resection or suturing in the flat meridian. These techniques have been used to correct postkeratoplasty astigmatism and surgically induced astigmatism at the time of cataract surgery. A study of 52 eyes showed a mean astigmatic change of −0.8 D in patients who had clear cornea cataract surgery with placement of limbal relaxing incisions (LRIs). The control group of 47 eyes had a mean astigmatic change of +0.50 D.

The Ruiz procedure, now rarely used, employs trapezoidal cuts, four transverse cuts inside two radial incisions ( Fig. 1.7C ). Although important in its time, stacking multiple rows of astigmatic incisions is no longer felt to be prudent because of poor predictability. A pair of tangential or arcuate incisions achieves significant correction. Additional incisions have minimal added benefit.

Corneal Implants for Myopia

Synthetic materials can be embedded between corneal stromal lamellae to correct myopia. Intracorneal rings can be threaded into a peripheral midstromal tunnel or placed in a peripheral lamellar microkeratome bed to effect flattening of the central cornea. Their advantage lies in the avoidance of manipulation of the central cornea and visual axis ( Fig. 1.10 ). Studies have also examined synthetic intracorneal lens implants that are placed in a centrally dissected corneal stromal pocket for the correction of aphakia and myopia ( Fig. 1.11 ). These lenses have high indices of refraction and are made of materials such as polysulfone.

Corneal intrastromal ring segments. (A) The ring is placed in the stroma (top) resulting in central flattening (middle) ; the central cornea is not manipulated (bottom) . (B) Photograph of intrastromal segments (arrows) .

Schematic illustration of an intracorneal lens inlay. The synthetic lens is placed in the corneal stroma after creation of a lamellar flap (illustrated here) or within a lamellar pocket (not shown).

Laser Procedures

Excimer laser techniques—such as PRK, LASEK (or epi-LASEK), and LASIK—can be used to treat hyperopia. An ablation pattern allows for maximum ablation in the midperiphery for an overall steepening of the optical zone. At present, custom corneal ablations are not approved for hyperopic corrections in the United States.

Incisional Procedures for Hyperopia

Hexagonal keratotomy, devised by Mendez in 1985, is an incisional treatment for hyperopia consisting of circumferential connecting hexagonal peripheral cuts around a clear 4.5-mm to 6.0-mm optical zone. This procedure allows the central cornea to steepen, thereby decreasing hyperopia ( Fig. 1.12 ). A second procedure using nonintersecting hexagonal incisions was described by Casebeer and Phillips in 1992. A study in 1994 of 15 eyes reported complications that included glare, photophobia, polyopia, fluctuation in vision, overcorrection, irregular astigmatism, corneal edema, corneal perforation, bacterial keratitis, and endophthalmitis. These authors concluded that hexagonal keratotomy was unpredictable, unsafe, and had high rates of complications.

Conductive keratoplasty (CK). Spot algorithm used to predict the effect of CK. A greater effect is obtained with neutral-pressure CK.

Nonlaser Lamellar Procedures for Hyperopia

ALK, keratophakia, and epikeratophakia have been used to treat hyperopia. In hyperopic ALK (also known as keratomileusis), a deep lamellar keratectomy is performed with a microkeratome, elevating a corneal flap. The stromal bed subsequently develops ectasia under the flap, which is replaced without additional surgery. Alternatively, the stromal side of the resected disc is remodeled into a convex hyperopic lenticule that, when placed in the original stromal bed, results in steepening of the central cornea. Hyperopic ALK has poor predictability and the risk of progressive ectasia limits its usefulness. Homoplastic ALK has been performed to hyperopia from 4 D to 10 D. In this procedure, the microkeratome removes a small disc (80–100 mm in thickness, 5–7 mm in diameter) that is discarded and replaced by a 350- to 400-µm thick donor lenticule (generated using the microkeratome). The safety and efficacy of hyperopic and homoplastic ALK have not been fully established.

Keratophakia is a technique developed by Barraquer for treating high hyperopia or aphakia. A lamellar keratectomy is first performed on the patient’s cornea using a microkeratome. Donor corneal tissue is then shaped into a lens after removal of the epithelium, Bowman layer, and anterior stroma. This donor lens is placed intrastromally within the recipient and the anterior lamellar cap is sutured in place. This process creates a steeper anterior cornea and increases refractive power. Synthetic intracorneal lenses have also been developed for implantation in the lamellar bed but are investigational. Hyperopic epikeratophakia uses a prepared donor lenticule without microkeratome removal of tissue. Although theoretically safer than keratomileusis, it lacks predictability and may induce irregular astigmatism.

Thermal Procedures for Hyperopia

Thermal energy can be used to shrink collagen of the corneal stroma and increase central corneal power. When applied to the paracentral or peripheral cornea, these techniques result in increased central corneal curvature and peripheral corneal flattening. Three methods are described: radial intrastromal thermokeratoplasty, laser thermokeratoplasty, and conductive keratoplasty.

Radial intrastromal thermokeratoplasty shrinks the peripheral and paracentral stromal collagen, producing a peripheral flattening and a central steepening of the cornea to treat hyperopia. Radial thermokeratoplasty (hyperopic thermokeratoplasty [HTK]) for the correction of hyperopia was developed in the then Soviet Union in 1981 by Fyodorov. A retractable cautery probe tip produces a series of preset-depth (≈ 95%) stromal burns in a radial pattern similar to that used in RK. Although an initial reduction in hyperopia was observed, lack of predictability and significant regression are problems. However, there may be less induced astigmatism with radial thermokeratoplasty than with hyperopic ALK or hexagonal keratotomy.

Solid-state infrared lasers, like the holmium:yttrium aluminum garnet (Ho:YAG) laser, have been used in a peripheral intrastromal radial pattern (laser thermokeratoplasty [LTK]) to treat hyperopia of 4 D and less. LTK works by causing thermal shrinkage of stromal collagen in the paracentral cornea, with a resultant steepening of the central corneal curvature, thereby reducing hyperopia. Recent work on human eyes has demonstrated appropriate topographic changes with at least short-term stability. This laser energy can be delivered by a handheld probe or slit beam system and appears most useful for limited amounts of hyperopia and hyperopic astigmatism. However, the long-term effects and refractive stability of Ho:YAG LTK are unknown.

Conductive keratoplasty (CK) is a technique that has been recently approved by the US Food and Drug Administration (FDA) for the treatment of hyperopia and presbyopia. CK uses a special probe to deliver radiofrequency wave energy to the deep stroma of the midperipheral cornea, causing focal shrinkage of collagen fibers, steepening the central cornea and flattening the periphery (see Fig. 1.12 ). Applications are made in concentric 6-, 7-, or 8-mm circles; the amount of effect depends on the number of spots placed. At the present time, CK has been approved for the treatment of hyperopia (0.75–3.25 D, with no more than 0.75 D of astigmatism) and presbyopia in emmetropes and hyperopes (by induction of myopia, −1.00 D to −2.00 D).

Monovision

Monovision improves near vision by giving one eye a slightly myopic correction, usually −1 D to −2 D. The other eye is corrected fully for distance. Myopia remaining in the dominant eye is called uncrossed monovision , and myopia remaining in the nondominant eye is called crossed monovision . Monovision treatments can be applied to myopes, hyperopes, and emmetropes. For patients with myopia, the “near” eye is not treated for the full amount of myopic refractive error; rather, it is left with a residual myopic correction. In hyperopes, myopia must be created by “overcorrecting” the near eye. Keratorefractive options to achieve monovision have expanded in the past decade and include PRK, LASIK, and conductive keratoplasty. One challenge to creating monovision with laser and conductive procedures is irreversibility.

Following monovision treatment, patients must adapt to its effect. Monovision patients have been found to perform relatively worse with low levels of illumination, near-threshold levels of stimuli, and tasks requiring good depth perception. However, among patients who underwent PRK and LASIK monovision correction, between 88% and 96% were satisfied with their visual outcome.

Conductive Keratoplasty

While conductive keratoplasty was approved in the United States for the treatment of presbyopia in emmetropes, the advantages that it offers being a nonincisional, nonablative approach are limited by a high rate of refractive regression. In a retrospective consecutive single-surgeon study, Ayoubi et al. compared FS-LASIK and conductive keratoplasty for monovision treatment of the nondominant eye in presbyopic emmetropic patients. FS-LASIK monovision provided stable correction with less induced astigmatism and HOA; the retreatment rate was 3% after FS-LASIK compared to 50% after CK ( P <.0001). Stahl et al. evaluated long-term follow-up for unilateral CK performed in the nondominant eyes of near-plano presbyopic patients. The postoperative refraction for these eyes eventually stabilized, with no statistically significant change in mean manifest spherical equivalent or keratometry between 1 and 3 years.

Multifocal Corneal Ablation and PresbyLASIK

Multifocal corneal ablation is still an experimental process in which the excimer laser is used to produce different optical zones within the cornea that can serve distance or near vision (see Fig. 1.12 ). PresbyLASIK, a multifocal corneal ablation procedure based on traditional LASIK, creates a multifocal surface able to correct any visual defect for distance while reducing the near spectacle dependency. This multifocal cornea produces simultaneous images on the retina, and the patient processes the appropriate image when performing distance or near tasks. For example, when looking at a distance target, the image produced by the optical zone(s) for distance will be in focus while light passing through the near optical zone(s) will create blur. Side effects include postoperative glare, halos, ghost images, and monocular diplopia. Treatment may be limited by pupil size and the degree of refractive error.

Pseudo-accommodative corneas may take on two possible patterns: peripheral presbyLASIK creates a peripheral concentric near zone, while central presbyLASIK creates a central near zone ( Fig. 1.13 ). A recent study of presbyLASIK in myopes and hyperopes found that presbyLASIK induced significant changes in spherical aberration. In myopes, this yields the advantage of an increased depth of focus relative to LASIK; in hyperopes, the spherical aberration is more consistent, independent of refractive change. Alió et al. demonstrated predictability, stability, safety, and good visual outcomes with central presbyLASIK in presbyopic patients with hyperopia. PresbyLASIK has also been combined with micro–monovision to allow for better intermediate vision stereoacuity than monovision alone.

Differences between ablation patterns. In peripheral presbyLASIK, the center of the cornea is treated for distance vision and the periphery for near. In central presbyLASIK, the center of the cornea is treated for near vision and the periphery for distance vision.

(Modified from Vargas-Fragoso V, Alió JL. Corneal compensation of presbyopia: PresbyLASIK: an updated review. Eye Vis . 2017;4:11.)

Corneal Inlays

Corneal inlays are lenticules that are inserted into an FS-created corneal stromal pocket for the treatment of presbyopia. There are currently 3 types of corneal inlays available: the KAMRA (AcuFocus) inlay uses a pinhole effect; PresbyLens (ReVision Optics) is based on corneal shape changes; and Flexivue Microlens (Presbia) has a central plano zone surrounded by peripheral ring segments of different refractive indices. These inlays differ from monovision by preserving distance vision in the implanted eye. The KAMRA inlay was the first FDA-approved implant in this class; long-term studies demonstrate good uncorrected near and intermediate vision, without an unacceptable decrease in distance vision. However, the KAMRA inlay restricts entering light with the small aperture; in a small percentage of patients, this causes glare, halos, and reduced contrast and night vision. Compared to ablative procedures, inlays carry the benefit of reversibility. Complications are uncommon; the most common complication of a decentered inlay may be corrected with recentration.

Hybrid

Hybrid techniques combine the benefits of these approaches and intend to suppress their drawbacks. Laser-blended vision provides moderate multifocality in both eyes combined with a small degree of monovision in the near eye). In Supracor and PresbyMAX, reduced multifocality in the distance eye is combined with full multifocality and monovision in the near eye. Supracor is an aberration-optimized algorithm that creates a 3.0-mm hyperpositive area of +2.00 D for near vision, with either symmetric or asymmetric surrounding distance correction. Presbymax creates a biaspheric multifocal corneal surface with a central hyperpositive area of +0.75 D to +2.50 D for near vision correction, surrounded by an area of distance correction. Intracor uses FS laser to create several concentric intrastromal rings at different depths to steepen the central cornea of the nondominant eye and is used in low hyperopic, emmetropic, and low myopic eyes. Since Intracor requires no ablation, it protects the integrity of the cornea with a stable gain in uncorrected near visual acuity (UNVA).

A systematic review of presbyopic correction of the cornea by Mosquero and Alió concluded that PresbyMAX provided excellent UNVA and distance corrected near visual acuity, with high predictability and a 1% reversal rate. KAMRA provided similarly excellent uncorrected distance visual acuity with a 1% retreatment rate but a 6% reversal rate. In contrast, presbyLASIK, laser-blended vision and Supracor all had high subsequent retreatment rates. Nearly all forms of presbyLASIK yield a loss of at least two lines of distance visual acuity, generally caused by dry eye or the induction of HOAs. Intracor was found to have a high (9%) loss of two or more lines of corrected distance visual acuity. Subsequent reports on corneal ectasia and concerns regarding retreatment and reversibility have raised safety concerns.