Current paradigms in refractive surgery

Abstract

Refractive surgeries have evolved from the archaic incisional corneal procedures to the use of sophisticated femtosecond lasers and new-generation phakic intraocular lenses (pIOL) for surgical correction of refractive errors. The armamentarium of modern-day refractive surgery includes corneal-based procedures such as photorefractive keratectomy, laser-assisted in situ keratomileusis and keratorefractive lenticule extraction, as well as lensbased pIOL implantation. The current procedures are associated with a high index of efficacy and predictability, with enhanced safety and a significant reduction in sight-threatening complications. Patient counselling and case selection is imperative to achieve optimal visual outcomes and patient satisfaction. This review article provides a comprehensive overview of current refractive surgery procedures, with an emphasis on decision-making. Evolving frontiers in refractive surgeries like customised corneal ablation and presbyopia correcting pIOL are also discussed.

Introduction

The field of refractive surgery has evolved from the now obsolete incisional corneal surgeries such as radial keratotomy, to the use of sophisticated femtosecond lasers and new-generation foldable phakic intraocular lenses (pIOL). Modern-day refractive surgical procedures include surface ablation, laser-assisted in situ keratomileusis (LASIK) and keratorefractive lenticule extraction (KLEx) amongst the corneal-based procedures; and pIOL amongst the lens-based procedures. In addition to improved efficacy and predictability of the modern-day refractive procedures, there has been a concomitant increase in the safety of these surgeries with a decrease in the associated complications.

We herein review the refractive surgical procedures, with their advantages, complications and refractive outcomes. There is an emphasis on decision-making, enabling the surgeon to choose the procedure best suited for the patient.

Decision-making in refractive surgery

The major challenge in any refractive surgery is to select the right patient and the right procedure to deliver the best refractive outcomes with optimal patient satisfaction. An extensive preoperative workup is very crucial to achieve the desired visual outcomes.¹ The preoperative workup should include clinical history, detailed ocular examination including dry eye workup, corneal topography and corneal biomechanical strength assessment. In addition, adequate preoperative counselling is essential to address patient expectations and explain the potential pros and cons of the procedure.

Clinical history

Stability of refractive error should be documented before any refractive procedure, defined as less than 0.5 D change in sphere or cylinder over the past one year. Table 1 mentions the US FDA-approved age criteria and Table 2 mentions the approved range of refractive correction for different refractive surgeries. The indication for undergoing refractive surgery should be elicited in history. Flap-based procedures may be avoided in patients engaged in active contact sports. In addition, the professional requirements must be taken into account if the patient is undergoing a refractive procedure for professional purposes that require medical fitness.

Table 1.

Minimum age criteria for respective refractive surgery procedure.

S.No.	Type of Refractive Surgery	Minimum Age Criteria
1.	PRK/LASIK	18 years or more
2.	SMILE	22 years or more
3.	Phakic IOL	21 years or more
4.	Presbyopic LASIK	40 years or more

Table 2.

US FDA-approved ranges of refractive error correction for refractive surgeries.

Type of refractive procedure	US FDA-approved ranges of refractive error correction for refractive surgeries
	Myopia	Hyperopia	Astigmatism
PRK	0 to −13.0 DS	0.5 to +6.0 DS	0 to 4 DC
LASIK	0 to −14.0 DS	0 to +6.0 DS	0 to 6.0 DC
SMILE	−1.0 to −10.0 DS	Not approved	−0.75 to −3.0 DC
Visian ICL	−3.0 to −20.0 D SE	Not approved	+1 to +4 DC
EVO	−3.0 to −18.0 D
EVO+	−3.0 to −14.0 D
Artisan phakic IOL	−5 to −20 D	Not approved	up to +2.5 DC

History of contact lens usage is particularly important. Rigid gas permeable lens and soft contact lens use should be discontinued for 4 weeks and 2 weeks respectively before refractive planning to ensure reliable corneal topography and eliminate corneal warpage.

History of ocular herpes simplex is a relative contraindication for refractive surgery in view of high risk of recurrence of the disease after the surgery. Uncontrolled connective tissue disorders, diabetes mellitus, pregnancy, lactation and keloids are relative contraindications for refractive surgery.^2,3 One should also enquire about family history of corneal ectasia. Concurrent use of medications like isotretinoin, amiodarone and hormone replacement therapy can significantly impact the outcomes of refractive surgery.

Ocular examination

The presence of deep-set eyes, prominent brows, small palpebral fissure and blepharospasm should be documented as docking in such eyes can be challenging with higher chances of suction loss.

Distance and near visual acuity should be assessed for each eye separately with cycloplegic and manifest refraction. A detailed slit lamp evaluation is of paramount importance to rule out any ocular co-morbidities. Large number of patients undergoing refractive surgery often have pre-existing dry eye disease (10–55%) with majority of patients having meibomian gland dysfunction (72%).⁴ Slit lamp examination should include ocular surface staining to detect signs of dry eye disease combined with Schirmer test and tear film break-up time to assess ocular surface status.

Investigations

Normal corneal topography is a prerequisite for undergoing routine refractive surgery. Eyes with characteristics of keratoconus or other corneal ectasias should not undergo corneal refractive surgery. Scheimpflug technology-based tomographers are commonly used for pre-surgical screening. Combining topography with biomechanical assessment indices like corneal biomechanical index (CBI) and tomographic biomechanical index (TBI) using Corvis ST has been proven superior in detecting preclinical ectasia.⁵ Numerous preoperative scoring systems like Randleman score can also be used to predict the risk of ectasia.⁶

Corneal thickness is a limiting factor, especially for cornea-based refractive procedures as an adequate residual stromal bed thickness (RSBT) of 250–300 microns must be ensured to maintain the tensile strength of the cornea and reduce the risk of postoperative ectasia. Approximately, 12–14 microns of stromal tissue is ablated for each diopter of correction. As per the Munnerlyn formula, the depth of ablation is directly proportional to the magnitude of myopic correction (in D) and square of the optical zone (in mm).

Ablation Depth = [Degree of Myopia (D) ∗ Optical Zone Diameter² (in mm)]/3

One should also take the percentage of tissue altered (PTA) into consideration during planning, as PTA value of >40% is a strong predictor of postoperative ectasia.

Pupil size should be assessed, as patients with mesopic size larger than optical zone or phakic intraocular lens (IOL) optic may experience symptoms like glare and haloes, especially at night. Ocular biometry, including measurement of white-to-white distance and aqueous depth is essential when planning for pIOL implantation.

Algorithm for deciding suitable refractive surgery

After a detailed refractive workup, guiding the patient to reach the right decision is the next step. The refractive surgeries can be divided into cornea-based and lens-based procedures [Fig. 1].

• •Patients with high refractive error, RSBT <250 microns and PTA >40% are not suitable for cornea-based procedure. These patients should be evaluated for lens-based refractive surgeries based on age and crystalline lens status.
• •Patients with mild to moderate refractive errors with RSBT and PTA within permissible range with no evidence of corneal ectasia are best suited for cornea-based procedures. Patients actively engaged in contact sports may be counselled for surface ablation or KLEx. LASIK is the most commonly performed refractive procedure and is suitable for majority of patients. KLEx is the current preferred modality of treatment in patients with lower magnitude of astigmatism as it preserves corneal biomechanical strength and is associated with lower risk of postoperative dry eye disease.
• •Patients with mild to moderate refractive error with mild corneal ectasia can undergo Photorefractive Keratectomy (PRK) Xtra or LASIK Xtra which combines collagen crosslinking with PRK/LASIK.⁷

Fig. 1.

Algorithm for decision making in refractive surgery.

Most individuals achieve good visual outcomes after a single refractive procedure, but 10–20% patients might require retreatment or enhancement or second type of procedure.

Surface ablation

Surface ablation or PRK involves the use of excimer laser for stromal ablation to correct refractive error. The overlying epithelium may be removed mechanically, alcohol-assisted removal or laser-assisted removal as in transepithelial PRK.

Indications

PRK may be the preferred procedure for patients with contact sports to avoid flap-related complications. It is preferred in thinner corneas as it involves ablation from the bowman membrane. PRK is also preferred in patients where there is a higher probability of retreatment because it preserves higher RSBT as compared to LASIK. It is of utility in patients with anterior corneal pathologies like epithelial basement membrane dystrophy, recurrent corneal erosions and post-adenoviral subepithelial infiltrates. Patients with cornea either too flat (<40 D) or too steep (>48D) are at high risk of flap-related complications; PRK may be preferred in these cases.

Outcomes

Current surface ablation procedures are safe and efficacious with faster recovery time, with 86–94% patients achieving uncorrected visual acuity (UCVA) of 20/20 or better; and 83–96% patients having spherical equivalent within +-0.5D of target at 18 months.⁸ For hyperopia correction, the number of patients with UCVA of 20/20 or better at 1 year varies from 46 to 70% with 75–80% achieving spherical equivalent within +-0.5D of target.^9,10 No significant difference in terms of refractive outcomes has been noted between PRK and Transepithelial Photorefractive Keratectomy (t-PRK), as well as PRK and LASIK in the late postoperative period.^11,12

Complications

The complications unique to surface ablation include postoperative pain, corneal haze and delayed epithelial healing.

In addition, complications related to excimer laser ablation may occur, such as decentered ablation, glare and haloes, under-correction, overcorrection and regression, dry eyes, infections and inflammation.

• •Postoperative pain: It is related to de-epithelisation and injury to corneal sensory nerves. The use of intraoperative chilled balanced salt solution after the excimer laser or use of t-PRK technique can help in reducing the pain. Treatment options include the application of bandage contact lens after surgery and prescription of topical NSAIDs (0.1% diclofenac and 0.4–0.5% ketorolac).
• •Corneal Haze: Postoperative haze is associated with healing response of cornea, and keratocytes play a major role in wound healing post-PRK. Predisposing factors include high refractive error corrections with deeper stromal ablation, smaller optic zone, ocular surface disorders and exposure to ultraviolet rays.^13,14 Early-onset haze is usually transient and clinically insignificant; late-onset haze may adversely impact visual outcomes. The severity of corneal haze can be assessed using grading systems proposed by Fantes et al or Hanna et al.^15,16 Intraoperative use of mitomycin C (MMC) has reduced the incidence of postoperative corneal haze.

• Treatment options include topical corticosteroids, mechanical debridement with MMC 0.02% application for 2 min, or transepithelial phototherapeutic keratectomy (PTK) with MMC.¹⁷ Repeat PTK with or without amniotic membrane grafting or superficial anterior lamellar keratoplasty can be done for recurrent corneal haze.¹⁸

• •Delayed Epithelial Healing: Management of delayed epithelial healing involves application of bandage contact lens (BCL) and topical lubricants. A careful preoperative assessment is crucial for its prevention.

Flap-based ablation

LASIK is the most commonly performed refractive surgery which involves creation of flap followed by excimer laser ablation of stromal bed and replacement of flap. The methods of flap creation have evolved from free hand dissection to automated microkeratome to femtosecond lasers. Femtosecond lasers have also been constantly evolving from high energy, low-frequency platforms like IntraLase to low-energy, high-frequency platforms like WaveLight, VisuMax and Zeimer which enable the creation of more planar, smoother and uniform flaps as compared to meniscus-shaped flaps generated by microkeratome.

Types of excimer laser ablation

Conventional ablation used for myopia correction changes the prolate-shaped cornea into oblate-shaped cornea which results in introduction of higher-order aberrations (HOAs) which can degrade the visual quality. Wavefront-optimised (WO) treatment prevents introduction of new HOAs due to laser treatment, wavefront-guided (WG) treatment also treats the pre-existing ocular HOAs and topography guided (TG) treatment involves treatment of only corneal HOAs. Other platforms include Custom Q and optical ray-tracing-based ablation.^19,20

Indications

WOLASIK is suitable for majority of patients with HOAs less than 0.4 microns and regular cornea without any scarring. It does not treat the pre-existing HOAs but overall generates a prolate-shaped cornea by delivering peripheral laser pulses.

WGLASIK is indicated for patients with HOAs more than 0.4 microns and associated glare and haloes. Repeatable, reliable and accurate wavefront data captured by aberrometer is a prerequisite for WG treatment.

TGLASIK considers only corneal HOAs. The indications for TG LASIK include normal cornea with regular astigmatism (US FDA-approved), high irregular astigmatism, post-LASIK ectasia, decentered ablation, patients with large angle kappa, post-keratoplasty and post-RK astigmatism. It also requires good-quality topography data which is entered into the laser platform.

Outcomes

• •Outcomes of LASIK for Myopia: UCVA of 20/20 or better has been achieved in 84.1–93.9% patients at 6 months after WO-LASIK. The manifest spherical equivalent (MSE) was within 0.5D of target in 75.9–94.6% eyes.²¹ On comparing WO and WG LASIK, no significant difference was observed between either group with regard to visual acuity for myopia up to 7D and myopic astigmatism.²²
• •Outcomes of LASIK for Hyperopia: Reinstein et al. reported outcomes of 1383 eyes treated for hyperopic LASIK, wherein 75% eyes achieved UCVA of 20/20 or better and 17% eyes lost one line of corrected distance visual acuity (CDVA).²³ Higher rates of regression have also been reported with hyperopic LASIK with majority of eyes achieving stable refraction after 3–6 months of surgery.

Complications

These can be categorised as intraoperative complications, which are mainly related to the creation of the flap, and postoperative complications.

Intraoperative complications:

• •Flap-related complications: Partial or incomplete flap is the most common complication associated with microkeratome LASIK, observed in deep-set eyes, excessive squeezing, chemosis or narrow palpebral fissure. Free cap may be observed in flatter corneas and buttonholing of flap is seen in steeper corneas.
• •Intraoperative suction loss: Suction loss can occur either during flap bed creation or during side cut formation. The FS LASIK can be restarted with redocking in the former case, while reduction of side cut diameter by 0.5 mm followed by reactivation of side cut can be done in the latter situation.
• •Vertical gas breakthrough (VGB): FS-LASIK involves laser-induced optical breakdown (LIOB) which results in separation of corneal layer to create flap. VGB occurs due to the escape of cavitation gas bubbles into subepithelial space which leads to focal area of uncut flap. The procedure should be abandoned if VGB is present centrally, as flap lifting in such cases would be associated with flap tears.
• •Opaque bubble layer (OBL): OBL formation occurs due to inability of gas to escape in the stroma which results in temporary infiltration of stromal bed. The incidence of OBL ranges from 5 to 56%.¹⁴

Postoperative complications

• •Post-LASIK infectious keratitis: Post-LASIK infectious keratitis can be early-onset (by staphylococcus species, pseudomonas) and late-onset (by atypical mycobacteria, nocardia and fungi). Management involves the identification of causative organism and administration of appropriate antibiotic therapy. Flap lifting combined with irrigation with antibiotics can also be done. Flap amputation is often required in more fulminant infections
• •Flap striae: Fine flap striae or microstriae do not adversely impact the visual acuity, and are amenable to treatment with intensive lubricants and frequent topical corticosteroids. Full-thickness striae which degrade the visual quality (macrostriae) warrant treatment with repositioning of flap after re-lifting it.
• •Diffuse lamellar keratitis (DLK): DLK is a sterile, inflammatory condition observed in the immediate postoperative period characterised by presence of white granular infiltrates deposited at the interface with minimal conjunctival congestion. Intensive corticosteroid therapy is required for management.²⁴
• •Post-LASIK dry eye: Dry eye disease (DED) is the most common complication after LASIK owing to iatrogenic damage to the sub-basal nerve plexus and loss of conjunctival goblet cells. It is usually self-limiting; the symptoms peak during the immediate postoperative period which is responsible for high patient dissatisfaction. Topical preservative-free lubricants should be prescribed to all patients undergoing surgery for at least 6 months to combat DED. Severe chronic dry eyes might require additional topical cyclosporine therapy and steroids as well.
• •Post-LASIK corneal ectasia: It is the most dreadful complication leading to decrease in UCVA with progressive increase in myopia or astigmatism. Abnormal preoperative topography is the most significant risk factor. There is a questionable role of corneal collagen crosslinking (CXL) in patients with post-LASIK ectasia. Severe or refractory cases may require DALK or PK.
• •Other complications: Night vision disturbances like glare and haloes, regression, overcorrection, under correction, epithelial ingrowth, transient light sensitivity syndrome and pressure-induced intralamellar stromal keratitis.²⁵

Keratorefractive lenticule extraction (KLEx)

KLEx uses femtosecond laser to create a refractive lenticule which may be removed via a small side cut incision to correct myopia and myopic astigmatism. Small incision lenticule extraction (SMILE, Zeiss Visumax) is the first commercially available platform for KLEx with FDA approval for correction of myopia up to −10 D and myopic astigmatism up to 3 D. Many advanced laser platforms have received CE mark approval recently (Table 3).

Table 3.

Commercially available keratorefractive lenticule extraction (KLEx) platforms.

Parameters	SMILE	CLEAR	SILK	SmartSight
Femtosecond Laser Platform	ZEISS VISUMAX 500	Ziemer Z8	Elita	SCHWIND ATOS
Approval	CE/FDA	CE	CE	CE
Possible refractive cuts	Flap/Lenticule	Flap/Lenticule/Arcuate	Lenticule	Flap/Lenticule
Refractive correction range	−0.5 to −10 DS 0 to -5DC	−0.5 to −10 DS 0 to -5DC	−0.5 to −12 DS 0 to -6DC	−0.5 to −12 DS 0 to -6DC
FS Energy (Nanojoules)	110–150	<100	40–50	75–135
Patient Interface	Machine fixed	Handheld	Machine fixed	Machine fixed
Automated Pupil Detection	Absent	Present	Present	Present
Cyclotorsion Compensation	Absent	Present	Present	Present

Indications

KLEx preserves corneal biomechanical strength and is associated with less corneal nerve damage and is preferred for patients with high myopia and preoperative mild dry eye disease respectively. Also, it avoids flap formation, which makes it a better option for patients with contact sports. SMILE is not FDA-approved for hyperopia correction.

Outcomes

SMILE offers excellent reliability, predictability and safety profile. 90–100% eyes were able to achieve UCVA of 20/40 or better at postoperative day 1 with 60–100% eyes obtaining UCVA of 20/20 or better at 6 months after surgery.²⁶ Long-term published data showed that UCVA of 20/20 or better was noted in 90% eyes at 3 years and 92% eyes at 4 years.²⁷ Reinstein et al. demonstrated higher corneal biomechanical strength in patients who underwent SMILE as compared to LASIK.²⁸ Li et al. showed less severe damage to sub-basal nerve plexus in SMILE as compared to LASIK at 3-month post-operative period.²⁹

Complications

SMILE surgery is associated with a steep learning curve. The two main intraoperative challenges are suction loss and lenticule misdissection with cap-lenticular adhesion. In addition, cap and side-cut tears, black spots, opaque bubble layer, dry eye disease, under and overcorrection, regression, epithelial defects etc. may be observed.

• •Suction loss: Higher rates of suction loss have been associated with surgeon inexperience. Other predisposing factors include tight speculum, deep-set eyes, prominent nasal bridge, anxious patients and loose conjunctival tissue. Suction loss can occur at different stages. Fig. 2 depicts the appropriate management of suction loss based on the stage at which it occurs.
• •Difficult lenticule extraction: The failure to identify correct dissection plane is the most common reason for the complication and can be directly attributed to surgeon's experience. We observed decrease in incidence of this complication from 16% in the initial 50 cases to 2% in the next 50 cases.³⁰ Other predisposing factors include thin lenticule, dense OBL, micro adhesions and poor intralamellar dissection due to black spots or inadequate FS energy. The intraoperative use of “meniscus sign” described by Titiyal et al. has been invaluable in the identification of correct dissection plane.³¹ Customised strippers can be used to extract the lenticule. Failure to extract the lenticule can lead to retention of partial or complete lenticule. Retained lenticule can lead to irregular astigmatism, and may warrant retreatment using surface or flap-based ablation or circle pattern of VisuMax laser.

Fig. 2.

Management of Intraoperative Suction Loss during SMILE.

Phakic intraocular lenses

Lens-based refractive surgeries either involve the removal of crystalline lens at the cost of losing the natural power of accommodation (Refractive lens exchange) or implantation of IOL over the crystalline lens (Phakic IOL). The outcomes of refractive lens exchange heavily rely on the optical performance of multifocal or extended depth of focus IOL. However, the pIOL technology offers superior visual outcomes without sacrificing crystalline lens and accommodation. pIOL technology has tremendously evolved from the implantation of anterior chamber IOLs which were associated with corneal decompensation, glaucoma and cataract, to iris-fixated lenses and posterior chamber pIOLs currently. Current generation posterior chamber pIOLs are extremely safe to implant and offer great patient satisfaction.

Only two pIOL designs are FDA-approved; iris-fixated rigid PMMA lens called Verisyse/Artisan and posterior chamber phakic IOL-implantable collamer lens (ICL). Currently, V4C model of EVO/EVO + ICL which consists of a central aquaport measuring 360 microns in diameter is widely preferred in cases requiring pIOL implantation.

Indications

pIOLs can be used for correction of myopia, hyperopia (not FDA-approved) and associated astigmatism. The endothelial counts should be >1900–3825 cells/mm³ for ICL and >2000–3550 cells/mm³ for Artisan. The anterior chamber depth should be at least 3 mm or more for ICL and 3.2 mm or more for Artisan. Iridocorneal angle should be open with angle aperture of at least 30° (Shaffer grade 3 and 4 or Scheie grade 0 and 1).

Outcomes

Good refractive outcomes were reported with the use of Artisan lens with 84% eyes achieving UCVA of 20/40 or better and 51.9% eyes achieving UCVA of 20/25 or better when myopic correction ranging from −4.5D to −22D was performed.³² A meta-analysis of cataract development after ICL implantation noted 223 out of 1210 eyes developed cataract with anterior subcapsular cataract being the most common morphology.³³ However, the cataract morphology appears to remain stable over years and rarely cause reduction in visual acuity. EC loss of around 5% and 7% has been reported after ICL implantation at 1 year and 7 years respectively.³⁴

Complications

• •Intraoperative ICL complications: Chipping or breakage of IOL can occur due to excessive pulling force while loading the lens or damage to trailing haptic with plunger. The lens can be injected in inverted orientation, and should immediately be explanted followed by implantation in the correct orientation.
• •Complications specific to iris-fixated pIOL: The list includes uveitis glaucoma hyphema (UGH) syndrome, pupil ovalisation, chronic iritis, pigment dispersion, progressive endothelial cell loss, disenclavation or dislocation of pIOL, cataract and high surgically induced astigmatism due to large incision size.
• •Raised Intraocular Pressure: The reported incidence of high IOP post-ICL implantation is noted as 0.8–26%.³⁵ High IOP can be observed in the immediate postoperative period which can be attributed to steroid response, retained viscoelastic device or less commonly pupillary block. Transient rise in IOP can be managed with antiglaucoma drugs or rapid tapering of steroids. However, sustained high IOP can occur due to pupillary block which can be treated by creating additional functioning PI, or due to excessive vault of ICL which may necessitate explant.

Summary

Modern refractive surgery practice provides multiple options to patients for refractive error correction. It is very crucial for a refractive surgeon to accurately guide his patient to choose the surgery best suited according to his requirements. Corneal excimer laser-based and lenticule extraction procedures have been approved for a wide range of refractive error correction. pIOL may be preferred for high magnitude of refractive errors not amenable to corneal-based procedures. Recent advancements in the field include the introduction of ray tracing-based LASIK for customised laser ablation and presbyopic pIOLs to provide extended depth of focus without compromising distance visual acuity.³⁶

Acknowledgements (if any)

Nil.

Source of support

Nil.

Disclosure of competing interest

The authors have none to declare.