Introduction
Refractive surgery is one of the most rapidly evolving fields in ophthalmology. The introduction of the excimer laser, for example, has replaced corneal incisions for routine cases in a very short time. Additionally, over the past decade the femtosecond laser for laser-assisted in situ keratomileusis (LASIK) has added to the ease, safety, and efficacy of refractive procedures. Today, the femtosecond laser also is used for:
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Incisional refractive surgery.
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Corneal tunnels or pockets for the insertion of ring segments or disc-shaped implants into the stroma.
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Lamellar or penetrating corneal transplantations.
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Removal of corneal stromal lenticule to change the refraction of the eye small incision lenticule extraction (SMILE).
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(Refractive) lens exchange surgery.
Many new approaches to correct presbyopia have been introduced in the last couple of years, including laser ablation profiles and intracorneal and intraocular implants. The greatest paradigm shift may have been the Nobel prize–winning method for adjusting the refractive power of an intraocular lens (IOL) after implantation into the eye by targeted radiation with UV light. Corneal cross-linking for the treatment of keratoconus is now a Food and Drug Administration (FDA)–approved treatment that also may have some potential as a refractive procedure.
In this chapter, we discuss excimer laser and ablation profiles; the classification of the different refractive surgery procedures, their utilization, advantages, and limitations; and briefly describe new procedures.
Excimer Laser and Ablation Profiles
Excimer laser corneal surgery was introduced as a precise tool for linear keratectomies by Trokel et al . in 1983 but was later used for corneal reprofiling or photorefractive keratectomy (PRK) in 1988. The ultraviolet laser (193 nm excimer or 213 nm solid state) allows the anterior corneal surface to be reshaped precisely to change its radius of curvature ( Fig. 3.1.1 ).
Numerous technological developments—such as flying-spot lasers, eye trackers, and use of femtosecond lasers for flap preparation—have improved clinical outcomes. The advent of wavefront measurement technology also enabled the quantification of ocular aberrations.
Laser Ablation Profiles
The excimer laser can be used to flatten or steepen differentially the corneal meridians and hence to treat compound myopic and compound hyperopic astigmatism. Mixed astigmatism can be treated by flattening the refractively more powerful meridian or by steepening the weaker one.
There are currently several ablation profiles available for laser vision correction. Challenging cases include high and mixed astigmatism, higher degrees of defocus, large pupil size, higher amounts of higher-order aberration (HOA) or specifically spherical aberrations, thin corneas, pre-existing corneal opacification, or night driving problems. Assessment and treatment of an eye may be complicated by previous corneal or lenticular surgery leading to false measurements or an unpredicted response to the ablation. Moreover, a refractive procedure may have therapeutic aspects in recurrent erosion syndrome.
Existing laser ablation profiles include :
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Munnerlyn’s formula.
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Wavefront-guided ablation.
- •
Wavefront-optimized or aspherical or Q factor–adjusted laser profile.
- •
Topography-guided ablation.
- •
Presbyopia correcting profiles.
In addition, combinations of these existing variants have been introduced recently, and even more are announced for the future (for example, ray-tracing optimized ablations). Ablation profiles in corneal laser surgery can be divided into those based on the total optical system and those based on the cornea.
Munnerlyn’s Formula
The classic ablation profile for the correction of myopia and myopic astigmatism is based on Munnerlyn’s formula, which removes a convex–concave lenticule of corneal tissue with spherocylindrical surfaces to remodel the corneal curvature. The laser profile is based only on subjective and objective measurements of refraction. It does not take spherical aberrations into account, which may lead to an increase of spherical aberration, resulting in an oblate cornea. Munnerlyn’s formula does not compensate for the loss of fluence in the periphery of the ablation zone, which occurs because the energy of a laser pulse is spread out over a larger area (i.e., an oval rather than a circle), nor the increased reflectance of the laser beam due to an oblique angle of incidence when the laser is not targeting the corneal apex. Moreover, the reduction in tissue removal is greater than the reduction in fluence. These three laser-related factors together with different biomechanical and wound-healing responses in the periphery are now compensated for by algorithms proprietary to the laser manufacturer.
Wavefront-Guided Ablation
The principles of wavefront deformation measurements are discussed in greater detail in Chapter 3.6 . In a perfect optical system, all the refracted rays are focused on a single plane (wavefront). Optical aberrations induce deformations on this plane and can be quantified. They represent the optical performance of the entire visual system, not only the anterior surface of the cornea, as in corneal topography. The lower-order optical aberrations (sphere and astigmatism) can be corrected with spherocylindrical glasses. The HOA (including spherical aberration and coma) correspond to what is clinically known as irregular astigmatism ( Fig. 3.1.2 ).
In theory, the higher the amount of HOA, the greater the benefit of performing a wavefront-guided ablation. However, wavefront-sensing may be negatively affected by the use of mydriatic eyedrops. Furthermore, new HOA are induced by the laser treatment itself even if the radial loss of efficacy is compensated by less than perfect alignment of the ablation pattern on the cornea and less than perfect eye-tracking, including cyclotorsional movements before and during ablation. In addition, aberrations are induced when a LASIK flap is created.
Lenticular aberrations increase with age, whereas corneal aberrations remain fairly stable throughout our lives in the absence of anterior corneal disease or dry eye.
Topography-Guided Ablation
Based on topography measurements by different topographers, including the Orbscan IIz ( Fig. 3.1.3 ), the Pentacam ( Fig. 3.1.4 ), and others, the elevation profile of the anterior corneal surface is calculated. The desired corneal surface is determined with the goal of correcting the refractive error and HOA induced at the cornea. The difference between the preexisting surface and the desired surface is used to calculate the ablation profile. Because of the calculated wavefront component of the cornea in this laser profile, the term corneal wavefront ablation is sometimes imprecisely used.
Topography-guided ablations have their greatest theoretical superiority in cases in which the problem is clearly located in the anterior cornea, like consequences of earlier surgery. Examples include decentered laser ablations, corneal grafts, and corneal scars. The reason is that a corneal topographer has a much higher resolution than wavefront sensors. A corneal topographer may evaluate the whole cornea, whereas ocular aberration measurements are only possible over the entrance pupil. Finally, a topographer directly evaluates the surface on which there are imperfections. Topography-guided ablations that were combined with collagen cross-linking were introduced in the treatment of keratoconus or forme fruste keratoconus.
Wavefront-Optimized /Aspherical /Q-Factor-Adjusted Laser Profiles
The term wavefront-optimized refers to laser treatment software that has been designed with certain corrections preprogrammed, although a true and customized wavefront plan is not employed. Spherical aberrations are the most disturbing optical imperfections after the lower-order terms sphere and cylinder . Because standard spherocylindrical excimer ablations induce positive spherical aberrations, wavefront-optimized laser profiles have been developed to preserve the eye’s pre-existing optical aberrations without introducing new aberrations.
The aim of preserving the natural cornea shape of each patient is propagated by other companies as aspherical or Q factor–adjusted ablation. This treatment is designed to improve the eye’s optical quality by optimizing the asphericity of the cornea. The Q-factor is a measure of the asphericity of the cornea. In the normal population, the mean Q-factor is −0.25, which indicates a slightly prolate shape. After determining the ideal asphericity of the cornea in a model, some researchers have suggested the optimum Q-factor should be around −0.4 or −0.5.
Presbyopia Correction
Presbyopia correction can be attempted by creating multifocal corneas or hyperprolate corneas when aiming for a postoperative Q-factor close to −1.0. Attempts to address presbyopia by corneal ablation date back to the last millennium.
Laser Ablation Profiles
The excimer laser can be used to flatten or steepen differentially the corneal meridians and hence to treat compound myopic and compound hyperopic astigmatism. Mixed astigmatism can be treated by flattening the refractively more powerful meridian or by steepening the weaker one.
There are currently several ablation profiles available for laser vision correction. Challenging cases include high and mixed astigmatism, higher degrees of defocus, large pupil size, higher amounts of higher-order aberration (HOA) or specifically spherical aberrations, thin corneas, pre-existing corneal opacification, or night driving problems. Assessment and treatment of an eye may be complicated by previous corneal or lenticular surgery leading to false measurements or an unpredicted response to the ablation. Moreover, a refractive procedure may have therapeutic aspects in recurrent erosion syndrome.
Existing laser ablation profiles include :
- •
Munnerlyn’s formula.
- •
Wavefront-guided ablation.
- •
Wavefront-optimized or aspherical or Q factor–adjusted laser profile.
- •
Topography-guided ablation.
- •
Presbyopia correcting profiles.
In addition, combinations of these existing variants have been introduced recently, and even more are announced for the future (for example, ray-tracing optimized ablations). Ablation profiles in corneal laser surgery can be divided into those based on the total optical system and those based on the cornea.
Munnerlyn’s Formula
The classic ablation profile for the correction of myopia and myopic astigmatism is based on Munnerlyn’s formula, which removes a convex–concave lenticule of corneal tissue with spherocylindrical surfaces to remodel the corneal curvature. The laser profile is based only on subjective and objective measurements of refraction. It does not take spherical aberrations into account, which may lead to an increase of spherical aberration, resulting in an oblate cornea. Munnerlyn’s formula does not compensate for the loss of fluence in the periphery of the ablation zone, which occurs because the energy of a laser pulse is spread out over a larger area (i.e., an oval rather than a circle), nor the increased reflectance of the laser beam due to an oblique angle of incidence when the laser is not targeting the corneal apex. Moreover, the reduction in tissue removal is greater than the reduction in fluence. These three laser-related factors together with different biomechanical and wound-healing responses in the periphery are now compensated for by algorithms proprietary to the laser manufacturer.
Wavefront-Guided Ablation
The principles of wavefront deformation measurements are discussed in greater detail in Chapter 3.6 . In a perfect optical system, all the refracted rays are focused on a single plane (wavefront). Optical aberrations induce deformations on this plane and can be quantified. They represent the optical performance of the entire visual system, not only the anterior surface of the cornea, as in corneal topography. The lower-order optical aberrations (sphere and astigmatism) can be corrected with spherocylindrical glasses. The HOA (including spherical aberration and coma) correspond to what is clinically known as irregular astigmatism ( Fig. 3.1.2 ).
In theory, the higher the amount of HOA, the greater the benefit of performing a wavefront-guided ablation. However, wavefront-sensing may be negatively affected by the use of mydriatic eyedrops. Furthermore, new HOA are induced by the laser treatment itself even if the radial loss of efficacy is compensated by less than perfect alignment of the ablation pattern on the cornea and less than perfect eye-tracking, including cyclotorsional movements before and during ablation. In addition, aberrations are induced when a LASIK flap is created.
Lenticular aberrations increase with age, whereas corneal aberrations remain fairly stable throughout our lives in the absence of anterior corneal disease or dry eye.
Topography-Guided Ablation
Based on topography measurements by different topographers, including the Orbscan IIz ( Fig. 3.1.3 ), the Pentacam ( Fig. 3.1.4 ), and others, the elevation profile of the anterior corneal surface is calculated. The desired corneal surface is determined with the goal of correcting the refractive error and HOA induced at the cornea. The difference between the preexisting surface and the desired surface is used to calculate the ablation profile. Because of the calculated wavefront component of the cornea in this laser profile, the term corneal wavefront ablation is sometimes imprecisely used.
Topography-guided ablations have their greatest theoretical superiority in cases in which the problem is clearly located in the anterior cornea, like consequences of earlier surgery. Examples include decentered laser ablations, corneal grafts, and corneal scars. The reason is that a corneal topographer has a much higher resolution than wavefront sensors. A corneal topographer may evaluate the whole cornea, whereas ocular aberration measurements are only possible over the entrance pupil. Finally, a topographer directly evaluates the surface on which there are imperfections. Topography-guided ablations that were combined with collagen cross-linking were introduced in the treatment of keratoconus or forme fruste keratoconus.
Wavefront-Optimized /Aspherical /Q-Factor-Adjusted Laser Profiles
The term wavefront-optimized refers to laser treatment software that has been designed with certain corrections preprogrammed, although a true and customized wavefront plan is not employed. Spherical aberrations are the most disturbing optical imperfections after the lower-order terms sphere and cylinder . Because standard spherocylindrical excimer ablations induce positive spherical aberrations, wavefront-optimized laser profiles have been developed to preserve the eye’s pre-existing optical aberrations without introducing new aberrations.
The aim of preserving the natural cornea shape of each patient is propagated by other companies as aspherical or Q factor–adjusted ablation. This treatment is designed to improve the eye’s optical quality by optimizing the asphericity of the cornea. The Q-factor is a measure of the asphericity of the cornea. In the normal population, the mean Q-factor is −0.25, which indicates a slightly prolate shape. After determining the ideal asphericity of the cornea in a model, some researchers have suggested the optimum Q-factor should be around −0.4 or −0.5.
Concepts in Development
An online pachymetry-guided ablation could aid in removing the anterior portion of the cornea without perforating Descemet’s layer before a lamellar graft is transplanted.
In routine laser vision correction, the combination of existing principles will bring even better outcomes than we are used to today. Such a combination of a wavefront-guided ablation with an aspherical profile according to the cornea’s preoperative asphericity has delivered excellent results in our hands. Optical ray-tracing algorithms, on the other hand, may allow the highest degree of customization. The idea behind ray tracing is that no single measurement of an eye can provide all the data required to achieve utmost individualization of the ablation. Therefore the information of several types of measurements such as ocular wavefront, corneal topography including the topography of the cornea’s back surface, corneal thickness, anterior chamber depth, lens thickness, and axial length may be considered. The systematic induction of HOA by means of wavefront-guided treatments may be overcome by such a method.
Classification of Refractive Procedures
The refractive power of an optical system, such as the eye, can be modified by changing the curvature of the refractive surfaces, the index of refraction of the different media, or the relative location of the different elements of the system.
Several classifications of keratorefractive surgery have been proposed based on the mechanisms of action of the surgery or on the type of surgery. A simplified classification in which the site of action of the surgery on the cornea—either over the optical zone or peripheral to it—is matched against the four different mechanisms of action of corneal surgery: addition, subtraction, relaxation, and coagulation–compression. The procedures that act on the optical zone are further subdivided into superficial or intrastromal ( Table 3.1.1 ). In addition to keratorefractive surgery, the use of intraocular implants is the second class of procedures to modify the ocular refraction.
| Optical Zone | Addition | Subtraction | Relaxation | Coagulation–Compression |
|---|---|---|---|---|
| Superficial |
| PRK, LASEK, epiLASIK, epi-Bowman keratectomy | Corneal molding | |
| Intrastromal |
|
| Lamellar keratotomy | |
| Peripheral cornea | Intracorneal ring segments | Wedge resection |
|
|
Cornea
Approximately two-thirds of refraction occurs at the air–tear–corneal interface, which generally parallels the anterior surface of the cornea. The cornea is readily accessible, and its curvature can be modified as an extraocular procedure. Most keratorefractive procedures to date modify the radius of curvature of the anterior surface of the cornea.
Central Cornea
Most procedures used to modify the corneal optical zone, or central cornea, change the relationship between its anterior and posterior surfaces; the thickness of the cornea is also modified. The central cornea may be modified either on the surface or intrastromally.
Corneal Surface: Addition
Epikeratophakia.
Epikeratophakia (also known as epikeratoplasty and onlay lamellar keratoplasty) was introduced by Werblin et al. It involves removal of the epithelium from the central cornea and preparation of a peripheral annular keratotomy. A lyophilized donor lenticule (consisting of Bowman’s layer and anterior stroma) is reconstituted and sewn into the annular keratotomy site.
Use of epikeratoplasty for the general treatment of myopia and hyperopia has been abandoned largely because of the potential loss of best-corrected visual acuity due to complications like irregular astigmatism, delayed visual recovery, and prolonged epithelial defects.
Synthetic materials and improved means of attaching the lenticule to the cornea may allow epikeratoplasty to become a more useful refractive technique in the future.
Corneal Surface: Subtraction
The surface ablation procedures PRK, laser subepithelial keratomileusis (LASEK), and epiLASIK have excellent results in terms of safety, efficacy, and stability with low-to-moderate myopic astigmatic corrections. In prospective trials, no clinically significant superiority of any of these three methods could be established regarding epithelial closure time, pain perception, haze formation, safety, and efficiency. Epi-Bowman keratectomy (EBK) is the latest variant of these surface ablation techniques.
Photorefractive Keratectomy.
In PRK, the epithelium is removed by mechanical scraping, utilizing ethanol or with an excimer laser ablation, before the stroma is ablated to correct the ametropia. The stroma is eventually covered by the epithelium, which heals from the periphery toward the center in about 4 days. After PRK, the corneal epithelium undergoes a hyperplastic phase in which the refractive status of the eye may be modified. The deposition of new collagen and glycosaminoglycans by activated stromal keratocytes after PRK is a common phenomenon after deep ablations and in younger individuals ( Fig. 3.1.5 ) and manifests as corneal haze or subepithelial scarring. The activation of the keratocytes seems to stem from interaction of epithelial cells and raw corneal stroma as the epithelium migrates to cover the defect or from activation of keratocytes by soluble tear factors that percolate through the initial epithelial defect after PRK. The haze may be associated with regression of the refractive effect or focal topographical abnormalities; it peaks in humans 3–6 months after the operation and disappears after 1 year for most patients. Many surgeons now use mitomycin-C prophylactically during the initial treatment to prevent haze formation or therapeutically to remove haze.
Laser Subepithelial Keratomileusis.
LASEK involves cleaving the epithelial sheet at the basement membrane or at the junction of the epithelium to Bowman’s membrane with dilute alcohol, applying the laser as in conventional PRK, and repositioning the epithelium afterward. The first LASEK procedure was performed by Azar. The term LASEK was coined by Massimo Camellin, who also popularized this method of surface ablation.
EpiLASIK.
EpiLASIK is an abandoned surface ablation procedure designed to create an epithelial flap with an epikeratome that is equipped with a blunt separator instead of a sharp blade, as in microkeratomes used during LASIK.
Epi-Bowman Keratectomy (EBK).
Epi-Bowman keratectomy was recently introduced and does not employ a metallic blade but a soft instrument to manually remove the epithelium before stromal ablation.
Corneal Stroma: Subtraction
Keratomileusis.
The term keratomileusis refers to the technique of “carving” (Greek smileusis ) the cornea. Dr. José I. Barraquer first reported clinical results with the technique in 1964.
Classic keratomileusis involves the excision of a lamellar button of parallel faces from the cornea with a microkeratome, freezing and reshaping the lamellar button, and replacing it in position with sutures. The procedure was modified by Krumeich and Swinger, who reshaped the disc with a second microkeratome pass without having to freeze it, in a procedure known as BKS (Barraquer–Krumeich–Swinger) keratomileusis. Ruiz and Rowsey made further modifications by applying the second microkeratome pass to the stromal bed instead of the resected disc, in a procedure called in situ keratomileusis. Even though the refractive cut with the microkeratome gave a disc of parallel surfaces with no optical power, a dioptric effect was achieved because of the remodeling of corneal tissue, as described by Barraquer in the law of thickness. The development of a mechanized microkeratome, or automatic corneal shaper, provided a more consistent thickness and diameter of the corneal disc and improved the predictability of the procedure. This procedure is known as automated lamellar keratoplasty (ALK). The fact that the corneal cap does not have to be modified led to the use of a hinged flap instead of a free cap. This, in turn, led to sutureless repositioning of the flap, which simplified the procedure further.
Laser-Assisted in situ Keratomileusis.
LASIK refractive correction is the most commonly performed refractive surgery in the world today. The early model was first performed in rabbits by Pallikaris et al. in a modification of Ruiz’s keratomileusis in situ ( Fig. 3.1.6 ). Buratto and Ferrari first performed this procedure in humans after inadvertently obtaining a thin resection with the microkeratome while performing a modification of Barraquer’s classic keratomileusis using the excimer laser instead of the cryolathe to modify the corneal cap.
In PRK, LASEK, and epiLASIK the laser is applied directly to Bowman’s layer, whereas in LASIK it is applied to the midstroma after a flap has been lifted from the cornea. The flap is then replaced. LASIK causes a minimal degree of epithelial hyperplasia (much less than PRK) that causes regression of the effect. No visually significant haze follows uncomplicated LASIK, but when the flap is too thin, haze may occur, suggesting that a critical amount of unablated flap keratocytes is needed to inhibit haze formation after routine LASIK.
Femto-LASIK.
Traditionally, the corneal flap cut during LASIK was created with a microkeratome blade. In contrast, Femto-LASIK uses the femtosecond laser, which is coupled to the patient’s eye with an interface fixated by suction. The femtosecond laser beam separates the corneal tissue by causing numerous microexplosions at a preprogrammed depth and position. The remaining tissue bridges between these cavitation bubbles are then bluntly dissected using spatula-like instruments. As no actual cut is performed with the femtosecond laser, in the rare event of a suction loss during flap preparation, a second attempt can be done immediately. This is not possible after a suction loss of a mechanical microkeratome, which necessitates changing to a surface ablation or waiting for approximately 3 months. This feature is a clear advantage to mechanical microkeratomes, but other flap-related complications like buttonholed flaps, flap striae, flap dislocation, and keratectasia may still happen. Transient light sensitivity—a new complication seen with initial femtosecond flap makers that occurred in some patients and resolved spontaneously after a couple of weeks—seems to be overcome with state-of-the-art femtosecond lasers by reducing the amount of energy delivered into the cornea.
Intrastromal Laser Ablation.
Intrastromal, solid-state, picosecond lasers are being developed that are more compact and portable than excimer lasers. Intrastromal ablation is made to flatten the central cornea, the epithelium and Bowman’s layer are spared, and thus fewer keratocyte fibroblastic responses are seen.
Intrastromal Lenticule Extraction.
A new procedure, small incision lenticule extraction (SMILE), takes place entirely within the cornea and is performed exclusively with a femtosecond laser system, that is, no excimer laser is needed. The SMILE procedure consists of these steps ( Fig. 3.1.7 ):
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The femtosecond laser is used to outline a small lens-shaped segment of tissue (lenticule) within the center of the cornea and a small incision in the midperiphery of the cornea.
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The lenticule is removed through this self-sealing incision and discarded.
The removal of the lenticule reduces the curvature of the cornea, thereby reducing myopia. Without a corneal flap, SMILE causes less postsurgical dry eye and may pose less risk for ectasia than LASIK. Also, without a corneal flap, no risk exists of flap displacement from trauma to the eye after surgery. SMILE has recently been approved by the FDA for the correction of myopia and myopic astigmatism and may soon become a popular alternative to LASIK for vision correction. However, currently it is not possible to perform SMILE for hyperopia.
Corneal Stroma: Addition
Keratophakia.
Keratophakia is the technique by which a corneal lens is inserted to change the shape of the cornea and modify its refractive power. Traditionally, a lamellar keratectomy was performed with a microkeratome on the recipient’s cornea. A fresh or preserved donor cornea also underwent a lamellar keratectomy. A stromal lens was created from the donor cornea and placed intrastromally in the recipient. In the future, lenticules obtained during a SMILE procedure may be reshaped according to the refractive needs of the recipient cornea and implanted in an intrastromal interface created by a femtosecond laser.
Intracorneal Inlays.
Intracorneal inlays may prove beneficial in the treatment of various refractive errors. Barraquer, working in Bogotá, Colombia, performed experiments with corneal implants as early as 1949. Early inlays were composed of flint glass and Plexiglas for the correction of aphakia and high myopia. Claes Dohlman first described the use of a permeable lenticule in 1967 in Boston. Hydrogel inlays were developed so as not to impede metabolic gradients across the stroma, including nutrient flow to the anterior cornea.
Today, corneal inlays mainly are designed to treat presbyopia. The mechanisms behind the current generation of inlays can be divided into three categories:
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Small-aperture corneal inlays that increase the depth of focus.
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Space-occupying inlays that create a hyperprolate and thus multifocal cornea.
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Refractive annular addition lenticules that work as bifocal optical inlays to create separate distance and near focal points.
Corneal Stroma: Relaxation
Lamellar Keratotomy (Hyperopic Automated Lamellar Keratoplasty).
In deep lamellar keratotomy (hyperopic ALK), a microtome performs a deep keratectomy to elevate a corneal flap that is replaced without additional surgery. The stromal bed then develops ectasia under the flap. Hyperopic ALK works best for low levels of hyperopia, but the predictability is low, and the risk of progressive ectasia ended the use of this procedure.
Peripheral Cornea
Several keratorefractive procedures are used to change the shape of the central cornea through their action on the peripheral cornea. This is achieved without changing the thickness or the relationship between the anterior and posterior surfaces over the corneal optical zone.
Peripheral Cornea: Addition
Intracorneal Rings.
Krumeich introduced the concept of titanium rings to alter the corneal curvature in keratoconic eyes or in combination with corneal transplant surgery ( Fig. 3.1.8 ). In keratoconic eyes, first, a dedicated trephination system (GTS) is used to create a circular groove in which the ring is placed and secured with a double running antitorque suture. The suture may be removed after completion of wound healing. The rings may be inserted in the interface of corneal transplants. The idea was to modify corneal curvature by altering the shape of the implanted ring with special instruments. However, this concept yielded no sufficiently predictable effect, and extrusion of the rings has been observed.
Intracorneal Ring Segments.
Intracorneal ring segments are placed in the peripheral cornea and take advantage of the fact that the arc of the cornea remains constant at all times, so when the anterior surface is lifted focally over the ring, a compensatory flattening of the central cornea occurs ( Fig. 3.1.9 ). An advantage of intracorneal segments over other refractive surgical techniques is removability as opposed to reversibility, as at least the tunnel preparation is permanent. The main drawbacks are the limited range of correction and poor predictability compared with excimer laser ablative procedures. As a result, intracorneal ring segments today are almost solely used for high cylindrical corrections in keratoconic corneas and may be combined with corneal cross-linking.
Peripheral Cornea: Subtraction
Wedge Resection.
Troutman developed the use of wedge resections and resuturing in the flat meridian, often with relaxing incisions in the steep meridian. Although the procedure effectively decreases astigmatism, clinical results are highly unpredictable and it is now reserved for the correction of postkeratoplasty astigmatism of high degree. The use of the femtosecond laser to facilitate wedge resection surgery has been shown to be effective for postkeratoplasty astigmatism.
Peripheral Cornea: Relaxation
Radial Keratotomy.
Radial keratotomy (RK) for myopia involves deep, radial corneal stroma incisions that weaken the paracentral and peripheral cornea and flatten the central cornea ( Fig. 3.1.10 ). Sato et al. in Japan used anterior and posterior corneal radial incisions to treat keratoconus, astigmatism, and myopia. The procedure was abandoned because of the long-term complication of bullous keratopathy secondary to endothelial cell loss. Anterior RK was performed by several ophthalmologists in the former Soviet Union in the early 1970s and was later popularized by Fyodorov and Durnev. RK has been performed in the United States since 1978.
The stability of refraction after RK is lower than with many other refractive surgical procedures. Therefore RK has been replaced by excimer laser procedures.
Hexagonal Keratotomy.
Proposed by Gaster and Yamashita in 1983, hexagonal keratotomy, first performed in humans by Mendez in 1985, consists of making circumferential, hexagonal, peripheral cuts around a clear optical zone. It “uncouples” the central cornea from the periphery, which allows the cornea to bulge or steepen, thereby decreasing hyperopia. The procedure has been largely abandoned because of the complications of poor healing and irregular astigmatism.
Astigmatic Keratotomy.
The first modern cataract extraction through a corneal incision, performed by David in France in 1747, introduced ophthalmologists to surgically induced astigmatism. Several investigators in the latter part of the nineteenth century, including Snellen, Schiotz, and Bates, attempted to correct corneal astigmatism with transverse relaxing corneal incisions. The first systematic study of the correction of astigmatism was performed by Lans in 1898.
Astigmatic keratotomy (AK) involves making transverse cuts in an arcuate or straight fashion perpendicular to the steep meridian of astigmatism to produce localized ectasia of the peripheral cornea and central flattening of the incised meridian, thereby decreasing the astigmatism. Although important in its time, AK is no longer used except during a corneal graft.
For cataract surgery, limbal-relaxing incisions have gained popularity because they are more comfortable for the patient than are arcuate or transverse midperipheral incisions, although their effect is smaller as they are farther away from the corneal center. However, today limbal-relaxing incisions are inferior to the implantation of toric IOLs during cataract surgery.
Peripheral Cornea: Coagulation–Compression
Thermokeratoplasty.
Radial intrastromal thermokeratoplasty shrinks the peripheral and paracentral stromal collagen to produce a peripheral flattening and a central steepening of the cornea to treat hyperopia. Unable to produce satisfactory results with relaxing incisions, Lans used cautery to selectively steepen a corneal meridian in rabbits. It was not until 1914 that Wray performed the procedure in humans in a case of hyperopic astigmatism. The procedure was later modified to correct hyperopia and popularized by Fyodorov. Although an initial reduction in hyperopia was observed, the lack of predictability and significant regression are persistent problems.
The solid-state infrared laser holmium:yttrium–aluminum–garnet (Ho:YAG) laser has been used in a peripheral intrastromal radial pattern (laser thermokeratoplasty) to treat hyperopia of 2.50 diopters (D) and less. The long-term refractive stability of Ho:YAG laser thermokeratoplasty is poor. A handheld radiofrequency probe to shrink the peripheral collagen also has been employed.
Microwave-Induced Thermokeratoplasty: Keraflex Procedure.
During an investigational Keraflex procedure, a microwave generator delivers a single low-energy microwave pulse lasting less than 1 second. Energy is applied to the cornea using a dielectrically shielded microwave emitter that contacts the epithelial surface. Through capacitive coupling, the single pulse raises the temperature of the selected region of corneal stroma to approximately 65°C, shrinking the collagen and forming a toroidal lesion in the upper 150 µm of the stroma. The lesion created is intended to flatten the central cornea to achieve myopic correction without compromising the biomechanical integrity of the cornea.
Circular Keratorrhaphy.
A suture placed in a circular fashion on the peripheral cornea to constrict the cornea and steepen the central cornea was first attempted by Krasnov in Russia in 1985 to treat hyperopia and aphakia. The principal problems are the development of irregular astigmatism by differential tension and loss of the effect as the suture elongates and “cheese-wires” through the tissue.
Peripheral Cornea: Oppression
Orthokeratology.
Orthokeratology is used as a nonsurgical option for the correction of myopia. An orthokeratology lens is flatter, looser, and larger than a conventional lens. Theoretically, the lens mechanically alters the central corneal contour over time. Because of safety concerns, orthokeratology did not gain widespread acceptance.
Reverse-geometry lenses are fitted with a base curve flatter than the central corneal curvature to apply pressure to a central corneal zone that flattens during wear to reduce the myopic refractive error. The current approach to orthokeratology using reverse-geometry lens designs results in rapid reductions in myopic refractive error.
Intraocular Lenses and Refractive Lensectomy
Refractive Lens Exchange
Extraction of the clear lens to correct high myopia was performed by Fukala in Germany in 1890. The procedure was later abandoned because of an unacceptably high rate of complications. With more sophisticated operative techniques, recently there has been renewed interest in managing high refractive errors by clear lens extraction. Because of retinal problems in high myopes, the procedure seems safer in high hyperopes For young patients, one major drawback is the loss of accommodation. To date, a variety of methods to restore near vision are available after lens removal, including correction with glasses and contact lenses, monovision, scleral expansion techniques, and various IOLs. However, none of them is perfect.
Toric Intraocular Lenses
Intraocular lenses with a toric or bitoric surface with corrections of up to 5 D to correct even high amounts of astigmatism are available. They have demonstrated reasonable rotational stability.
Multifocal Intraocular Lenses
Pseudoaccommodative bifocal or trifocal lenses with a refractive and/or diffractive design have been in routine refractive surgical use for the last couple of years. For acceptable correction of presbyopia, the achieved refractive error must be negligible. However, this is often not the case after lens exchange alone even with toric multifocal IOLs, and the use of modern biometry devices, as well as sophisticated lens power calculation formulae. Then an enhancement with an excimer laser may be an option. However, even if emmetropia is achieved, compromises in night vision, glare, and halos remain inherent drawbacks of this approach.
Potentially Accommodative Intraocular Lenses
An alternative approach to a true accommodative IOL is to use the “focus shift” principle produced by an increase in effective lens power with forward movement of the optic. The haptics of the accommodative IOL fixate in the capsular bag and allow the optic to move in reaction to the contraction of the ciliary muscle so that the patient can focus on nearby objects ( Fig. 3.1.11 ). However, based on simple calculations, the forward shift possible in the capsular bag on its own is not sufficient to allow for the restoration of a reasonable amount of accommodation of 2–3 D for a single optic lens.
Another variant is to combine two optics that move relative to each other within the capsular bag into a single IOL (Synchrony dual-optic lens, Abbot Medical Optics, AMO, Santa Ana, CA). Using this intraocular telescope effect, small excursions may allow for sufficient accommodative response. The FluidVision IOL (PowerVision, Belmont, CA) uses liquid channels to harness the accommodative forces from the ciliary body expressed through the capsule similar to the crystalline lens. The NuLens (NuLens, Ltd., Herzliya Pituah, Israel) is a two-piece IOL that is placed outside the capsular bag. With the Tetraflex lens (Lenstec, St Petersburg, FL) the rationale is that accommodation results from an increase in HOA caused by deformation of the IOL through ciliary muscle contraction and/or increased vitreous pressure analogous to the natural lens. However, no long-term data are available yet for any of these potentially accommodating IOLs.
Light-Adjustable Intraocular Lenses
The Light Adjustable Lens (LAL, Calhoun Vision, Pasadena, CA) utilizes a Nobel prize–winning technology that allows it to change its refractive power after implantation in the eye. The LAL has properties similar to standard monofocal IOLs, but it differs with the special macromers incorporated in the makeup of the lens. These macromers are sensitive to light of a certain wavelength. When irradiated by such light, the macromers are photopolymerized.
After LAL implantation using a standard cataract surgery technique, allowing 2–3 weeks for corneal incisions to heal and refraction to stabilize, the lens in the eye will be irradiated for approximately 2 minutes with a digital light delivery device specially designed to deliver the exact dose and profile of light onto the lens. This light exposure is limited to certain portions of the lens and lets the macromers form an interpenetrating network via photopolymerization. Over the next 1–2 days, unreacted macromers from the nonexposed areas physically migrate to the irradiated areas, thus re-establishing a chemical equilibrium. This physical diffusion causes the irradiated parts to swell and change their curvature, which results in a change of refractive power. Myopia, hyperopia, and astigmatism may be corrected by customized irradiation patterns. Even multifocal treatments or the induction of positive or negative asphericity are thus possible. Once the targeted power adjustment is achieved, the entire lens is irradiated to polymerize the remaining unreacted macromers.
Phakic Intraocular Lenses
In the 1950s, the use of phakic IOLs was attempted first by Strampelli and Barraquer but abandoned at that time because of multiple complications. Improvements in IOLs have renewed interest in the procedure. The iris-claw lens originally devised by Worst for the correction of aphakia was later modified by Fechner et al. to correct high myopia in phakic patients. It is enclaved in the midperipheral, less mobile iris and presently requires a 6.0-mm incision for its insertion. The angle-supported phakic IOL was introduced by Baikoff and Joly for the correction of myopia and has gone through several modifications ( Fig. 3.1.12 ). Long-term follow-up has reported progressive pupil ovalization with an older model.
The posterior chamber phakic IOL was introduced by Fyodorov et al. in 1990. Several new models have been developed since. They must accommodate to the space between the posterior iris and the crystalline lens. Sizing is crucial. If the IOL vaults too much, pigment dispersion and even papillary block glaucoma can result. If it lies against the anterior surface of the crystalline lens, cataract can result.
Long-term follow-up is needed for all types of phakic IOLs regarding endothelial cell loss ( Fig. 3.1.13 ), glaucoma, iris abnormalities, and cataract formation.
Add-on Intraocular Lenses in Pseudophakic Eyes
In contrast to obsolete “piggyback” procedures in which two IOLs were implanted in the capsular bag, the “add-on” concept involves placement of an additional IOL in the sulcus ciliaris after routine implantation of another lens into the capsular bag. The first “add-on” lens (HumanOptics, Germany) was first implanted in 2000. An add-on IOL can be performed immediately following cataract surgery in a single session or years later. Its indications include correction of residual ametropia after cataract surgery (spherical and/or cylindrical) and the treatment of pseudophakic presbyopia. This method avoids the risk and hazards of IOL explantation from the capsular bag. The exchangeability of the additional IOL may be advantageous in cases with expected change of refraction as in keratoplasty patients, in pediatric patients after cataract surgery, and for refractive power compensation in vitrectomized eyes filled with silicone oil.
New or Alternative Approaches
Photorefractive Intrastromal Cross-Linking (PiXL)
Corneal collagen cross-linking, recently FDA approved to halt progressive ectatic disorders, uses UV light and a photosensitizer (riboflavin) to strengthen chemical bonds in the cornea. A mild flattening of the corneal curvature and a tendency toward centration of the apex are observed. Clinical studies in low myopic eyes have shown promising early results.
LASIK Extra
The most common form of ectasia is naturally occurring keratoconus. However, corneal ectasia is also feared as a rare but potentially devastating complication after laser vision correction such as LASIK. Corneal cross-linking has been reported to be beneficial for this condition. Recently it was proposed to prophylactically apply corneal cross-linking immediately following LASIK.
Prophylactic
In the future, it may be possible to prevent the development of corneal astigmatism or corneal ametropia by cross-linking, even when performed on nonectatic corneas with different parameters than used today for ectatic diseases.
IntraCor
IntraCor was a minimally invasive femtosecond laser procedure for the treatment of presbyopia. The laser formed a series of concentric rings within the stroma, which caused a central steepening of the cornea to treat the presbyopia ( Fig. 3.1.14 ).
Ciliary Muscle–Zonular Complex
Attempts have been made to treat presbyopia based on an alternative theory of its pathogenesis: relaxation of the equatorial zonules. These zonules have been made taut by either scleral expansion or infrared laser application. However, this theory of accommodation is not supported by independent studies. The studies undertaken all support the classic accommodative mechanism described by Helmholtz.
Axial Length
Presently, procedures that modify the axial length of the eye—by either resection of the sclera or reinforcement of the posterior pole in cases of high myopia—have a role in the management of staphyloma but not in the management of refractive error.
Refractive Indexes
Although not intended to be a refractive procedure, the use of compounds with a different index of refraction during retinal surgery must be considered. In an aphakic eye, a convex bubble of silicone oil (with a higher index of refraction) will act as a positive IOL, rendering the eye more myopic while the oil stays in place. A gas bubble with a lower index of refraction will act as a diverging IOL, rendering the eye hyperopic while the gas stays in place.
Cornea
Approximately two-thirds of refraction occurs at the air–tear–corneal interface, which generally parallels the anterior surface of the cornea. The cornea is readily accessible, and its curvature can be modified as an extraocular procedure. Most keratorefractive procedures to date modify the radius of curvature of the anterior surface of the cornea.
Central Cornea
Most procedures used to modify the corneal optical zone, or central cornea, change the relationship between its anterior and posterior surfaces; the thickness of the cornea is also modified. The central cornea may be modified either on the surface or intrastromally.
Corneal Surface: Addition
Epikeratophakia.
Epikeratophakia (also known as epikeratoplasty and onlay lamellar keratoplasty) was introduced by Werblin et al. It involves removal of the epithelium from the central cornea and preparation of a peripheral annular keratotomy. A lyophilized donor lenticule (consisting of Bowman’s layer and anterior stroma) is reconstituted and sewn into the annular keratotomy site.
Use of epikeratoplasty for the general treatment of myopia and hyperopia has been abandoned largely because of the potential loss of best-corrected visual acuity due to complications like irregular astigmatism, delayed visual recovery, and prolonged epithelial defects.
Synthetic materials and improved means of attaching the lenticule to the cornea may allow epikeratoplasty to become a more useful refractive technique in the future.
Corneal Surface: Subtraction
The surface ablation procedures PRK, laser subepithelial keratomileusis (LASEK), and epiLASIK have excellent results in terms of safety, efficacy, and stability with low-to-moderate myopic astigmatic corrections. In prospective trials, no clinically significant superiority of any of these three methods could be established regarding epithelial closure time, pain perception, haze formation, safety, and efficiency. Epi-Bowman keratectomy (EBK) is the latest variant of these surface ablation techniques.
Photorefractive Keratectomy.
In PRK, the epithelium is removed by mechanical scraping, utilizing ethanol or with an excimer laser ablation, before the stroma is ablated to correct the ametropia. The stroma is eventually covered by the epithelium, which heals from the periphery toward the center in about 4 days. After PRK, the corneal epithelium undergoes a hyperplastic phase in which the refractive status of the eye may be modified. The deposition of new collagen and glycosaminoglycans by activated stromal keratocytes after PRK is a common phenomenon after deep ablations and in younger individuals ( Fig. 3.1.5 ) and manifests as corneal haze or subepithelial scarring. The activation of the keratocytes seems to stem from interaction of epithelial cells and raw corneal stroma as the epithelium migrates to cover the defect or from activation of keratocytes by soluble tear factors that percolate through the initial epithelial defect after PRK. The haze may be associated with regression of the refractive effect or focal topographical abnormalities; it peaks in humans 3–6 months after the operation and disappears after 1 year for most patients. Many surgeons now use mitomycin-C prophylactically during the initial treatment to prevent haze formation or therapeutically to remove haze.
Laser Subepithelial Keratomileusis.
LASEK involves cleaving the epithelial sheet at the basement membrane or at the junction of the epithelium to Bowman’s membrane with dilute alcohol, applying the laser as in conventional PRK, and repositioning the epithelium afterward. The first LASEK procedure was performed by Azar. The term LASEK was coined by Massimo Camellin, who also popularized this method of surface ablation.
EpiLASIK.
EpiLASIK is an abandoned surface ablation procedure designed to create an epithelial flap with an epikeratome that is equipped with a blunt separator instead of a sharp blade, as in microkeratomes used during LASIK.
Epi-Bowman Keratectomy (EBK).
Epi-Bowman keratectomy was recently introduced and does not employ a metallic blade but a soft instrument to manually remove the epithelium before stromal ablation.
Corneal Stroma: Subtraction
Keratomileusis.
The term keratomileusis refers to the technique of “carving” (Greek smileusis ) the cornea. Dr. José I. Barraquer first reported clinical results with the technique in 1964.
Classic keratomileusis involves the excision of a lamellar button of parallel faces from the cornea with a microkeratome, freezing and reshaping the lamellar button, and replacing it in position with sutures. The procedure was modified by Krumeich and Swinger, who reshaped the disc with a second microkeratome pass without having to freeze it, in a procedure known as BKS (Barraquer–Krumeich–Swinger) keratomileusis. Ruiz and Rowsey made further modifications by applying the second microkeratome pass to the stromal bed instead of the resected disc, in a procedure called in situ keratomileusis. Even though the refractive cut with the microkeratome gave a disc of parallel surfaces with no optical power, a dioptric effect was achieved because of the remodeling of corneal tissue, as described by Barraquer in the law of thickness. The development of a mechanized microkeratome, or automatic corneal shaper, provided a more consistent thickness and diameter of the corneal disc and improved the predictability of the procedure. This procedure is known as automated lamellar keratoplasty (ALK). The fact that the corneal cap does not have to be modified led to the use of a hinged flap instead of a free cap. This, in turn, led to sutureless repositioning of the flap, which simplified the procedure further.
Laser-Assisted in situ Keratomileusis.
LASIK refractive correction is the most commonly performed refractive surgery in the world today. The early model was first performed in rabbits by Pallikaris et al. in a modification of Ruiz’s keratomileusis in situ ( Fig. 3.1.6 ). Buratto and Ferrari first performed this procedure in humans after inadvertently obtaining a thin resection with the microkeratome while performing a modification of Barraquer’s classic keratomileusis using the excimer laser instead of the cryolathe to modify the corneal cap.
In PRK, LASEK, and epiLASIK the laser is applied directly to Bowman’s layer, whereas in LASIK it is applied to the midstroma after a flap has been lifted from the cornea. The flap is then replaced. LASIK causes a minimal degree of epithelial hyperplasia (much less than PRK) that causes regression of the effect. No visually significant haze follows uncomplicated LASIK, but when the flap is too thin, haze may occur, suggesting that a critical amount of unablated flap keratocytes is needed to inhibit haze formation after routine LASIK.
Femto-LASIK.
Traditionally, the corneal flap cut during LASIK was created with a microkeratome blade. In contrast, Femto-LASIK uses the femtosecond laser, which is coupled to the patient’s eye with an interface fixated by suction. The femtosecond laser beam separates the corneal tissue by causing numerous microexplosions at a preprogrammed depth and position. The remaining tissue bridges between these cavitation bubbles are then bluntly dissected using spatula-like instruments. As no actual cut is performed with the femtosecond laser, in the rare event of a suction loss during flap preparation, a second attempt can be done immediately. This is not possible after a suction loss of a mechanical microkeratome, which necessitates changing to a surface ablation or waiting for approximately 3 months. This feature is a clear advantage to mechanical microkeratomes, but other flap-related complications like buttonholed flaps, flap striae, flap dislocation, and keratectasia may still happen. Transient light sensitivity—a new complication seen with initial femtosecond flap makers that occurred in some patients and resolved spontaneously after a couple of weeks—seems to be overcome with state-of-the-art femtosecond lasers by reducing the amount of energy delivered into the cornea.
Intrastromal Laser Ablation.
Intrastromal, solid-state, picosecond lasers are being developed that are more compact and portable than excimer lasers. Intrastromal ablation is made to flatten the central cornea, the epithelium and Bowman’s layer are spared, and thus fewer keratocyte fibroblastic responses are seen.
Intrastromal Lenticule Extraction.
A new procedure, small incision lenticule extraction (SMILE), takes place entirely within the cornea and is performed exclusively with a femtosecond laser system, that is, no excimer laser is needed. The SMILE procedure consists of these steps ( Fig. 3.1.7 ):
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The femtosecond laser is used to outline a small lens-shaped segment of tissue (lenticule) within the center of the cornea and a small incision in the midperiphery of the cornea.
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The lenticule is removed through this self-sealing incision and discarded.
