Technologies and techniques continue to evolve in the correction of myopia, hyperopia, astigmatism, and presbyopia. Patients are increasingly interested in spectacle or contact lens independence. It is important to understand today’s options for vision correction. This chapter provides an overview of this exciting area of clinical advancements and research.
With every surgical innovation, it is important to critically evaluate the outcomes and safety with long-term data. We need to be cautious with any new technology, as many refractive procedures have been abandoned because of lack of efficacy or late complications ( Table 36.1 ).
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Patients typically have high expectations with the available technology. They want their postoperative uncorrected visual acuity (UCVA) to be equal or greater to their preoperative best corrected visual acuity (BCVA). It is important to evaluate the patient and the ocular health to determine if they are good candidates for any refractive procedure. Preoperative findings will guide the surgeon in recommending specific refractive options ( Table 36.2 ). To determine the preferred surgical option, we can differentiate higher-order aberrations of the cornea versus the lens. Advanced wavefront units allow measurement of total higher-order aberrations of the eye, which can be differentiated into those from the cornea versus the lens. In patients with significant higher-order aberrations of the lens, a refractive lens exchange would be the treatment of choice to improve the overall quality of vision.
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Laser vision correction
Laser-assisted in situ keratomileusis and photorefractive keratectomy
More than 35 million laser-assisted in situ keratomileusis (LASIK) and photorefractive keratotomy (PRK) procedures have been performed worldwide with reported improvement in outcomes and safety. Significant advances over the past 30 years in excimer laser technology include improved nomograms, flying spot lasers with smoother ablations, more accurate trackers, larger optical zones, aspheric curves, and customized treatments that reduce not only refractive errors but other optical aberrations of the eye.
PRK provides excellent outcomes similar to LASIK. Although some surgeons prefer PRK over LASIK because of the reduced risk of corneal ectasia, most offer LASIK first because of quicker postoperative healing. Surgeons typically recommend PRK when the cornea is thin, mildly irregular, or has evidence of epithelial basement membrane dystrophy. PRK may also be preferred if the patient has a narrow fissure that complicates flap creation or is at higher risk of flap subluxation because of factors, such as an occupation or sporting activity. PRK improves quality of day and night vision and maintenance of corneal clarity secondary to the use of larger optical zones, flying spot lasers that create a smoother ablation, adjunctive use of mitomycin C to reduce the risk of corneal haze, and custom treatments, such as topography- and wavefront-guided ablations and wavefront-optimized treatments. Custom ablation with PRK offers the same refractive results as small incision lenticule extraction (SMILE) but with fewer induced higher-order aberrations. Patients are relatively comfortable following PRK with application of sterile ice to the surface of the cornea, bandage soft contact lenses, and nonsteroidal drops.
LASIK is among the most frequently performed and successful medical procedures. With proper preoperative screening, visual outcomes are excellent with a low complication rate. In North America, femtosecond lasers for creation of the corneal flap have generally replaced blade microkeratomes. Advances in femtosecond technology have shown predictable flap thickness and the ability to customize the diameter, location, hinge, and edge profile of the LASIK flap. In the rare event of suction loss with a femtosecond laser, the suction ring can be reapplied and the procedure completed. With suction loss using a mechanical microkeratome, the procedure is aborted and the patient must return a few months later for PRK.
In a large LASIK clinical review (97 papers; 67,893 eyes) from 2008 to 2015, 90.8% of eyes achieved a distance UCVA of 20/20 or more and 99.5% achieved 20/40 or more. The spherical equivalent refraction was within ± 0.50 D of target in 90.9% of eyes and within ± 1.00 D of target in 98.6%. These outcomes were superior to earlier reports, which reflect further advances in hardware and software of the lasers, surgical techniques, and improved patient selection. Loss of two or more lines of corrected distance visual acuity (CDVA) was 0.61%, less than one-half the number of eyes that had an increase in CDVA of two lines or more (1.45%). The more advanced treatments (topography- and wavefront-guided or wavefront-optimized) allowed for a UDVA of nearly a full line better than in eyes with conventional treatments. Most treatments in the review were for myopia and myopic astigmatism; hyperopic treatments represented 3% of cases. A 2-line or greater CDVA loss was more common in hyperopic than myopic treatments (2.13% vs. 0.95%); this may be related to more sensitive centration of the hyperopic treatment, which has been shown to be best centered on line of sight versus the center of the pupil. Hyperopic treatments are also associated with a greater risk of regression versus myopic treatments.
The most significant long-term complication of LASIK is corneal ectasia (incidence ~0.03%). The risk has been lowered by improved preoperative detection of forme-fruste keratoconus, keratoconus, and pellucid marginal degeneration with elevation tomography that detects elevation abnormalities on the anterior and posterior corneal surfaces. Other factors accounting for improved LASIK outcomes include avoidance of surgery on thin corneas or those with high myopia, creation of thinner corneal flaps, and leaving a thicker residual bed underneath the flap. The presently preferred treatment of corneal ectasia is with corneal crosslinking and possibly topography-guided PRK or an intracorneal ring to reduce irregular astigmatism. Early ectasia detection and treatment can limit corneal irregularity and provide better visual acuity.
Further research in tracking devices, torsional alignment, the ideal centration of ablations, an understanding of the biomechanical properties of the cornea, and medications or adjunctive procedures to modulate wound healing will enhance our outcomes and patient safety for all laser vision correction procedures.
Small incision lenticule extraction procedure
SMILE is a new method of intrastromal keratomileusis in which a femtosecond laser is used to create two cuts within the cornea and one small superficial cut. A lenticule is produced of a specific shape and thickness, and is pulled out mechanically through a 2 to 3 mm diameter corneal incision. SMILE is an alternative refractive procedure for the correction of myopia and myopic astigmatism. Recent studies have validated the efficacy and safety. Table 36.3 presents a comparison of LASIK, PRK, and SMILE.
SMILE is currently reserved for myopia and myopic astigmatism. Enhancement procedures tend to be with PRK, although some recent evidence supports LASIK to correct residual refractive errors. Current limitations of first-generation SMILE versus LASIK include difficulty in performing low myopic corrections (<3 D) because of a thin and fragile lenticule, lack of effect on hyperopia or hyperopic astigmatism, inability to perform topography- or wavefront-guided treatment, less-smooth cuts with femtosecond laser than excimer, lack of optical centration adjustment when the suction device is placed on the eye, no cyclotorsion compensation, slower return of UCVA, and inferior improvement in best UCVA compared with custom treatments. Because the overlying cap in SMILE is adherent, there is essentially no risk of subluxation, although this is rare after LASIK. Future refinements in the hardware of the laser technology, and software design will improve the outcomes of the SMILE procedure.
A recent metaanalysis compared SMILE (291 eyes) with femtosecond LASIK (FS-LASIK; 277 eyes) for correcting myopia in patients with dry eye. The authors concluded that dry eye occurs transiently after both SMILE and LASIK, and although there are some early postoperative advantages of SMILE, there is no long-term superiority over FS-LASIK in terms of tear breakup, quantity of tears, or subjective symptoms. Previous metaanalyses found a lower risk of postoperative dry eye with SMILE.
Reduction in corneal sensation occurs in both the creation of the LASIK flap and subsequent excimer ablation, as well as the SMILE procedure. Randomized controlled studies suggest that there is no increased risk of dry eye secondary to corneal neuralgia; however, other reports find an association. One metaanalysis, reported that corneal sensitivity in the SMILE group recovered faster than in the FS-LASIK group during the first 3 months postoperatively, but that recovery was similar 6 months after surgery.
Corneal biomechanical properties are critical in laser vision correction to allow stability of the refractive correction and prevent corneal ectasia. Removal of corneal tissue by both SMILE and LASIK reduces corneal tensile strength and is directly correlated with the extent of a myopic ablation. SMILE appears to result in a greater reduction of tensile strength in lower myopic corrections, because more tissue is removed, and a similar reduction in higher corrections. However, finite-element models, mathematical analysis, and cadaver cornea experiments suggest that SMILE may preserve corneal biomechanical properties better than LASIK. Because iatrogenic ectasia is a rare complication, it is recommended that the indications and exclusion criteria for SMILE should follow the same guidelines as LASIK.
Phakic intraocular lenses
Phakic intraocular lens (IOL) insertion is the procedure of choice to treat high degrees of myopia and astigmatism, especially in nonpresbyopic patients with clear crystalline lenses. Unlike with laser vision correction, the procedure is reversible as no tissue is removed, and there is essentially no induced dry eye. The two locations for the phakic IOL include the anterior chamber, such as iris-claw intraocular implantation or posterior chamber. Anterior-chamber iris-supported phakic IOLs have been associated with an increased corneal endothelial cell loss compared with posterior-chamber phakic IOLs.
Implantable contact lenses (ICLs) have shown excellent outcomes and high patient satisfaction rates. The material is a collamer substance that offers ultraviolet (UV) protection. The EVO Visian ICL (Staar Surgical) is the latest innovation in which a microscopic hole has been placed in the optic of the lens to prevent pupillary block glaucoma ( Fig. 36.1 ). This small hole obviates the need for a preoperative yttrium aluminum garnet (YAG) laser iridotomy in which two openings were created. The hole can also decrease the risk of an anterior cortical cataract by enhancing fluid flow in the anterior chamber. Long-term data with non-EVO design has shown a low incidence of cataracts. The EVO Visian ICL is available for the correction of up to 20 D of myopia and 6 D of astigmatism. A non-EVO ICL is available for the correction of hyperopia; however, many hyperopes are unsuitable candidates because of narrow anterior chambers. One special indication for the hyperopic ICL is patients who have had radial keratotomy because these are myopic eyes with deeper anterior chambers.
