Year : 2022 | Volume
: 34 | Issue : 1 | Page : 12--18
Refractive Corneal surgeries: A Review
Jyothi Vengalil Menon
Jyothi The Lasik Vision Centre, Bengaluru, Karnataka, India
Dr. Jyothi Vengalil Menon
Villa 141, Ferns Habitat, Doddenakundi, Bengaluru - 560 037, Karnataka
Refractive corneal surgeries are procedures for correction of refractive errors. Modern keratorefractive procedures include laser in situ keratomileusis, surface ablation, relatively newer refractive lenticule extraction, and corneal inlays. This article aims to present an overview of currently performed refractive corneal procedures. Basic aspects of patient selection, principles, advantages, limitations, complications, and clinical outcomes are discussed.
|How to cite this article:|
Menon JV. Refractive Corneal surgeries: A Review.Kerala J Ophthalmol 2022;34:12-18
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Menon JV. Refractive Corneal surgeries: A Review. Kerala J Ophthalmol [serial online] 2022 [cited 2022 Jun 30 ];34:12-18
Available from: http://www.kjophthal.com/text.asp?2022/34/1/12/343664
Introduction and Methods
Refractive corneal surgeries modify the contour of the cornea to correct the refractive error. This can be done by using excimer laser ablation with or without flap, by femto laser, or by inserting corneal inlays. The field of refractive surgery has seen tremendous advances in the last two decades in terms of refinement in surgical techniques, advances in instrumentation, availability of newer diagnostic tools, and better understanding of corneal biomechanics. This article aims to give an outline of the cornea-based options available in the armamentarium of refractive surgeon. Several studies have shown that modern keratorefractive procedures are efficient, safe, predictable, and stable for correction of refractive errors., Complications, though infrequent, can be further minimized by diligent patient selection and strict adherence to safety protocols.
Studies used in this article were obtained from PubMed search from January 2000 to January 2022. The filters applied were “abstract available,” “meta-analysis,” “randomized control trials,” “review,” and “systematic review.” The keywords used for search were “Refractive corneal surgery,” “Keratorefractive surgery,” “LASIK,” “PRK,” “Surface ablation,” “Refractive Lenticule Extraction,” “Presbyopia treatment,” and “Radial Keratotomy.”
To be a candidate for refractive corneal surgery, patients must be above 18 years of age with stable refraction and normal corneal topography. However, there are specific indications (albeit controversial) for performing refractive surgery in patients below 18 years. Studies have shown that refractive surgery improves visual acuity and stereopsis in children with anisometropic amblyopia and strabismus., The question remains as to the age at which to perform surgery and eligibility criteria to qualify for pediatric refractive surgery.
The contraindications for performing refractive surgeries as per the FDA guidelines are discussed below.,
They include unstable refractive error, corneal ectatic disorders, herpetic keratitis, Avellino corneal dystrophy, cataract, and uncontrolled glaucoma. Patients with ocular allergies and dry eye are also relative contraindications.
Pregnancy and lactation are absolute contraindications for refractive surgery. Patients on isotretinoin for acne must stop medication for at least 6 months prior to keratorefractive procedure. Uncontrolled diabetes mellitus or diabetes mellitus with ocular or systemic complications is a contraindication.
Autoimmune disorders such as rheumatoid arthritis, Sjogren's syndrome, and SLE are relative contraindications. There are several studies indicating that patients with well-controlled autoimmune disorders have undergone refractive corneal surgeries with good visual outcomes., Primary Sjogren's disease is an exception to this. More studies are needed to address the role of laser refractive surgeries in patients with auto immune disease.
Preoperative evaluation for corneal refractive procedures
A comprehensive eye examination must be performed. Contact lens wearers must discontinue lenses for a minimum period of 2 weeks for soft lenses and 6 weeks for RGP lenses and longer in the presence of corneal warpage.
Special attention must be paid to deep set eyes and small palpebral fissure which can interfere with placement of speculum and microkeratome resulting in flap-related complications.
Accurate refraction must be performed both after cycloplegia and subjective correction. UCVA and BCVA must be recorded. Patients in presbyopia age group must be informed about near vision requirement after laser procedure and options for the same must be discussed.
Blepharitis and meibomanitis must be treated prior to surgery to avoid diffuse lamellar keratitis and infective keratitis.
Tear film evaluation is done. Several studies have documented that laser refractive surgeries can adversely affect tear film. Laser in situ keratomileusis (LASIK) is the worst offender in causing iatrogenic dry eye.,
Corneal scars must be carefully looked for since they can cause artifacts on corneal imaging and interfere with creation of femto flaps.
Detailed posterior segment evaluation is performed and predisposing retinal lesions must be identified and treated.
Mesopic pupil size must be recorded, and the ablation zone must be bigger than mesopic pupil to avoid glare and haloes., Various pupillometers such as Colvard pupillometer are commercially available and give accurate pupil size in standard illumination conditions.
Keratometry readings must be noted. It is important not to steepen the cornea more than 50D or flatten the cornea less than 35D as it can induce ocular aberrations. During microkeratome LASIK, cornea steeper than 46D is at risk for flap buttonholes and cornea flatter than 42D is at risk for free flap.
Tonometry must be recorded as patients with glaucoma pose a special challenge to refractive surgeons. Suction during flap creation can increase IOP. Reduction in corneal thickness can give fallacious IOP readings post refractive surgery. Steroids used postoperatively can increase IOP in steroid responders. All these factors must be considered prior to performing refractive surgery in a patient with glaucoma.,,
Corneal pachymetry is measured using ultrasonic pachymetry or Orbscan or Pentacam. It is mandatory to leave a minimum residual stromal bed of 250 microns to maintain corneal stability. Current practice is to maintain residual stromal bed of 300 microns or 50% of original pachymetry whichever is higher. Percentage of tissue altered (PTA) is a recent metric used in screening patients considering flap thickness, ablation depth, and central corneal thickness. PTA higher or equal to 40% should be considered as a higher risk for post laser ectasia.,
Abnormal corneal morphology is by far the commonest cause of post-LASIK ectasia. Placido-based corneal topography is an excellent diagnostic tool to map anterior corneal surface and rule out conditions such as keratoconus and pellucid marginal degeneration. Orbscan is a hybrid system that uses Placido-based system and scanning slit system while Pentacam uses Scheimpflug imaging to scan the cornea. Both Orbscan and Pentacam give anterior and posterior corneal maps along with point-to-point pachymetry. These devices help to evaluate posterior elevation changes which are often the earliest changes in keratoconus. Belin Ambrosio display in Pentacam is a useful software to detect ectatic corneal conditions early.
Ocular response analyzer and Corvis ST are two systems that give an idea of corneal biomechanics indicated by corneal hysteresis (CH) and corneal resistance factor (CRF). A weaker cornea is more prone to ectasia following refractive procedure.
Aberrometers (Wavefront analyzers) can measure higher order aberrations of the optical system of the eye which can be incorporated into the laser machine for correction.
Anterior segment OCT can give pachymetry and flap thickness which is useful while planning enhancements.
Laser in situ keratomileusis
LASIK procedure consists of two steps: creation of corneal flap and excimer laser ablation.
Corneal flap can be created using a microkeratome or femto laser.
Microkeratomes are manual or automated, geared or gearless, preassembled or assembled on the eye. Newer automated, gearless microkeratomes have increased the ease and safety of flap creation. Flap thickness can be 140, 160, 180, or 200 microns depending on the depth plate of microkeratome. Thinner flaps are safer due to increased residual stromal bed.
Flaps can also be created in a bladeless manner using water jet microkeratome.
In femto LASIK, femto laser creates the flap and refractive ablation is by excimer laser. Femto laser can make customized flaps of varying shape, thickness, and hinge position. Studies have indicated that femto flaps are more planar and predictable as compared to microkeratome flaps. However, the final visual outcomes after microkeratome LASIK and femto laser-assisted LASIK procedure appear to be similar.
Excimer laser which corrects the refractive error has evolved from conventional broad-beam lasers to slit beam and finally modern flying spot lasers with spot size ranging from 0.5 to 0.9 mm. Smaller spot size produces a smoother stromal bed with a better ablation profile and translates into better outcome.
Excimer lasers have built in eye trackers that compensates for eye movements in X- and Y-axis during ablation. Modern eye trackers come with dynamic eye tracking, higher frequencies, and cyclotorsion control to actively track the pupil and ensure optimum centration during ablation with improved visual outcomes.
Types of laser in situ keratomileusis procedures
Depending upon patient's preexisting refractive parameters, LASIK procedure can be conventional, wavefront optimized, wavefront guided, or topography guided.
Conventional laser in situ keratomileusis surgery
Excimer laser does refractive ablation to correct refractive errors. During this process, prolate shape of cornea becomes oblate. This can induce higher order aberrations impacting the quality of vision in conventional LASIK procedure.
Wavefront-optimized laser in situ keratomileusis
In order to reduce the induced higher order aberrations, the laser spots are delivered in a predetermined and optimized manner on the cornea to reduce induced aberrations. This is known as wavefront-optimized LASIK and is a refinement over conventional LASIK.
Wavefront-guided laser in situ keratomileusis
The preexisting HOA of eye can be measured using aberrometers. Wavefront-guided LASIK (WFG) is customized to correct the patient's preexisting higher order aberrations. Studies have indicated that WFG LASIK appears to be better than conventional LASIK for treating eyes with higher order aberrations more than 0.30, in re-treatments, and in patients with large scotopic pupil size.,
Topography-guided laser in situ keratomileusis
In patients with astigmatism, irregular cornea, decentered ablations, and preexisting corneal pathology, most of the aberrations arise from the cornea. In such eyes, it is difficult to capture reliable wavefront maps. The data obtained from corneal topography are used to perform topography-guided LASIK to smoothen the irregular corneal surface and improve the quality of vision.,
LASIK can be used to correct myopia up to 12D hyperopia up to 6D and astigmatism up to 6D, provided patient's keratometry and pachymetry values permit.
This procedure is thin-flap LASIK using 90-micron flap. It combines safety of photorefractive keratectomy (PRK) with minimum pain and speedy recovery of LASIK. Thinner flaps increase residual stromal bed, but they are difficult to handle and are prone for flap striae.
Post laser in situ keratomileusis enhancements
Post LASIK enhancements can be treated by flap recut or lifting the flap and doing relaser, provided adequate RSB is present. Flap lift is preferred to flap cut. PRK is also an option. Some studies have shown that relifting the flap may result in more accurate refractive outcome as compared to PRK, but epithelial ingrowth is a risk.,,,
There has been a resurgence of surface ablation due to better pain control and use of mitomycin C. Surface ablation has less impact on corneal biomechanics as compared to LASIK with and no flap-related complications. The epithelium is removed, and laser is applied on the bed consisting of Bowman's layer and anterior corneal stroma. 0.02% mitomycin C is applied on the corneal stroma. Epithelium may or may not be replaced. A bandage contact lens is placed to promote epithelial healing and to reduce pain. The following methods are used to separate epithelium
Mechanical scraping of epithelium is done in conventional PRK.
Laser-assisted subepithelial keratomileusis (LASEK)
Epithelium is removed using 20% alcohol and replaced after refractive ablation.
Epi-laser in situ keratomileusis
Microkeratome is used with a blunt blade to remove the epithelium that can be replaced.
With the use of bandage contact lens, studies have not conclusively demonstrated significant advantage with epithelium on or epithelium off methods.
Excimer laser is used to remove the epithelium followed by refractive ablation. This can be done in two ways. In “two-step trans-PRK,” epithelium is removed first by laser ablation followed by refractive ablation. In “one-step trans-PRK,” the epithelium removal and refractive ablation occurs sequentially as a single step significantly reducing the time taken.
Surface ablation can be conventional, wavefront optimized, wavefront guided, or topography guided. Studies have shown that surface ablation with WFG treatment has better visual outcomes as compared to LASIK with WFG treatment.
Advantages and limitations
Surface ablation is preferred in patients with deep set eyes, small palpebral fissures, flat or steep cornea, thin cornea, and in patients actively involved in contact sports. However, in patients actively involved in outdoor activities, there is an increased risk of haze due to ultraviolet light exposure. Problems faced in surface ablations are delayed visual recovery, postoperative pain, and early or late haze formation, especially with higher ablation depth. Pain can be managed by using bandage contact lens and oral NSAID. Application of 0.02% MMC after laser ablation on the stromal bed reduces haze allowing treatment of higher powers. Studies have shown that mitomycin at a concentration on 0.02% is safe for use on stromal bed after excimer laser ablation. While there is consensus on dose of mitomycin at 0.02%, there is no consensus on time of application which can vary from 12 s to 2 min. Doses less than 0.02% are less effective in preventing haze.,
Refractive lenticule extraction
Refractive lenticule extraction is an all-femtosecond laser-based refractive technique. This procedure uses femto laser for refractive lenticule extraction. Two methods used are femtosecond lenticule extraction (FLEx) and small incision lenticule extraction (SMILE). FLEx requires the creation of a flap to remove the lenticule. SMILE is a flapless and minimally invasive procedure that extracts the lenticule from a small cornea incision. Currently, SMILE is used to correct myopia unto 10D and myopic astigmatism up to 6D. Studies have shown that SMILE has similar efficiency, predictability, and safety as compared to femtosecond LASIK. SMILE is minimally invasive with minimum pain and postoperative discomfort.,,, Studies have indicated that SMILE may have a lesser impact on corneal biomechanics, and tear film as compared to LASIK. Infrequent complications of SMILE include suction loss, lenticule tears, incision tears, epithelial ingrowth, diffuse lamellar keratitis, and residual refractive error. Surface ablation or CIRCLE procedure can be used for retreatment. In CIRCLE procedure, the lenticule is converted into a flap and laser is performed to correct the residual power.,,
Exciting developments are taking place in presbyopia correction, and various methods are currently available.
The dominant eye is corrected for distance and non-dominant eye is corrected for near in presbyopia patients, when correcting for their refractive errors. The near vision correction by +1.5D is well tolerated.
PresbyLASIK is another option. PresbyLASIK is performed mono ocularly on the nondominant eye. In this procedure, in-built algorithms are used to make cornea multifocal to correct near vision. Mono-ocular biaspheric PresbyLASIK is reported to give good results in presbyopes.
Corneal inlay is a new treatment modality for presbyopia. Corneal inlays are implanted in the anterior corneal stroma of the nondominant eye beneath the flap or beneath laser-created pockets. They improve the near vision by increasing the depth of focus. Synthetic and allogenic inlays have been described. Flexivue microlens, Kamra, and Raindrop inlays are some of the corneal inlays available.,
This is a laser-independent procedure used for correction of presbyopia and hyperopia. Radio frequency of 350 Hz is delivered into the peripheral corneal stroma to cause steepening of central cornea. Studies have shown conductive keratoplasty to be safe and effective for low-to-moderate hyperopia, hyperopic astigmatism, and presbyopia.,
Radial keratotomy (RK) was used to correct myopia by using corneal incisions to alter the shape of the cornea. Centripetal or centrifugal radial incisions are made in the cornea avoiding the central optical zone. The drawbacks of RK are variability in results and fluctuating vision. With the advent of lasers, RK has become outdated. Both LASIK and PRK with mitomycin C are useful in post RK residual errors.
Astigmatic keratotomy and limbal relaxing incisions
Astigmatic keratotomy (AK) is another technique used to correct corneal astigmatism usually prior to cataract surgery by placing corneal incision on the steep axis. Limbal relaxing incisions are made in the peripheral cornea just inside the limbus and are less likely to induce irregular corneal astigmatism. Femtosecond can be utilized for AK.
Intacs are intracorneal ring segments. They are useful in correcting low myopia in patients with thin cornea or forme fruste keratoconus who are otherwise unsuitable for laser refractive procedure. Intacs are inserted creating corneal pockets using a mechanical technique or by creating channels using the femtosecond laser.
Complications of keratorefractive procedures
Cornea-based refractive surgeries have been refined over past two decades to yield safe, predictable, and excellent visual outcomes. Complications are infrequent and can happen during operative, immediate postoperative, and late postoperative period and can range from mild to vision threatening.
In microkeratome LASIK, flap complications such as suction loss with flap tear, free caps, buttonholes, flap striae, and rarely flap melt can occur. Decentered ablation and central islands can degrade the quality of vision.
Postoperative complications include residual refractive error, diffuse lamellar keratitis, and infectious keratitis epithelial ingrowth.
Dry eye can be troublesome and may need punctal plugs.
Femto procedure complications include vertical break through, opaque bubble layer, and transient light sensitivity.
In surface ablations, postoperative pain can make patients anxious. Delayed epithelial healing can occur. Corneal haze formation can degrade visual outcome.
One of the dreaded complications of corneal refractive surgery is corneal ectasia. The cornea progressively becomes ectatic with reduction in best-corrected visual acuity. To a large extent, ectasia can be prevented by careful patient selection.
Risk factors for corneal ectasia
Abnormal corneal topography, high myopia, ablation depth above 75 microns, thin cornea, less residual stromal bed due to increased ablation, or thicker flap can lead to keratectasia.,,
Studies have shown that postoperative corneal ectasia occurs at a lower rate in eyes undergoing PRK than LASIK and least of all in SMILE. However, there are some recent reports of post SMILE ectasia.
There are several studies and reviews on the safety, predictability, stability, and efficacy of various refractive surgeries. Most of these studies indicate excellent visual outcome with a few complications following laser vision correction. LASIK, PRK, and SMILE give safe, predictable, and comparable visual outcomes. Based on the current data, it is difficult to comment on the superiority of one technique over the other in terms of long-term visual outcomes.,,,,,,
Corneal refractive surgeries have come a long way from inception and provide excellent options to spectacle free vision. Various procedures are available to correct refractive errors. A thorough knowledge is essential to achieve best results and minimize complications. The results are encouraging, and the best is yet to come.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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