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Etiology, Diagnosis, and Management of Keratoconus: New Thoughts and New Understandings
- Types of Keratoconus
- RGP Fitting Approach
- Surgical Treatments for Corneal Ectasia
- Management of Corneal Problems that can Mimic Keratoconus
The following is our step-by-step approach to fitting aspheric RGP lenses for early keratoconus.
The diagnostic lens fitting begins by performing corneal mapping and viewing the topography as an axial map display. The topography is quantitatively viewed to identify the size, shape, and location of the red zone (steepest area of the cornea) as well as the blue zone (flattest area of the cornea). The dioptric curvature of the corneal apex is also identified for reference. (Figure 46.)
A diagnostic lens is then selected with a base curve radius equal to the dioptric curvature at the corneal apex. The lens is placed on the eye and its position and relationship to the cornea evaluated with fluorescein.
An optimum lens-to-cornea fitting relationship is accomplish when three fitting criteria are present:
- The base curve radius should provide slight clearance across the corneal apex with no fixed, mid-peripheral bubbles.
- The mid-periphery of the lens should “land” (touch) the mid-peripheral cornea at 3 and 9 o’clock to prevent nasal/temporal lens decentration.
The lens should create minimal impingement across the flatter superior cornea. (Figure 47.)
The lower edge of the lens will often lift slightly away from the cornea due to inferior corneal steepening. Additionally, intermittent bubbles or frothing may be noted inferiorly. This is not only acceptable but may be essential to ensure both adequate superior alignment and vertical lens movement with the blink. Any attempt to decrease the inferior edge lift (i.e., steepening the base or peripheral lens design), may result in a tight superior lens-to-cornea fitting relationship. Furthermore, with a steep fitting relationship, the lens edge may cause further mechanical distortion of the cornea that may exacerbate the condition.
Having achieved the desired fit, a sphero-cylinder over-refraction is performed to determine the final contact lens power. A compensating anterior surface aspheric surface is often incorporated to neutralize the radial astigmatism and coma induced by the highly aspheric posterior lens surface.
The final lens is then ordered in a moderate to high Dk rigid gas permeable material. Each lens manufacturing laboratory will incorporate a slightly different aspheric central and/or peripheral lens design, so it is imperative that the diagnostic lens design match the final lens ordered.
Fitting Advanced Keratoconus
Because of the varying peripheral corneal topographies found in advanced keratoconus, no single lens design or fitting philosophy will consistently result in an optimal fit. For this reason, a variety of different fitting approaches must be employed, each based on the status of the central as well as the mid-peripheral corneal topography. The following describes our fitting approach for advanced keratoconus and is based on the nipple, oval, and globus photokeratoscopy and videokeratoscopy classification of corneal topography.
Fitting the Nipple Cone
Because of the rapid change in corneal shape from center to mid-periphery, the nipple-shaped topography creates numerous fitting obstacles. As previously discussed, the nipple cone consists of central ectasia measuring 5 mm or less surrounded by 360 degrees of essentially normal cornea. When fitting a nipple-shaped cornea with a spherical multicurve lens design, it is important to use a diagnostic lens fitting set that incorporates a small overall diameter of 8.1 mm and a small posterior spherical optical zone of 5.5 mm. (Table 1.)
The peripheral lens design should consist of a series of 3 or more spherical peripheral curves that gradually fatten the lens periphery. (Figure 48.) Because of the multiple junctions present on the posterior lens, it is necessary for the laboratory to incorporate multiple blending curves. The resulting lens design is a non-definable aspheric surface.
The nipple-shaped topography is often best managed through the use of designs that incorporate aspheric radii. The aspheric lens fitting technique is identical to that described for the fitting of early keratoconus, but, due to rapid topographical flattening from center to periphery, it is often necessary to increase the amount of posterior lens asphericity. This increase in asphericity flattens the periphery of the lens to provide an improved peripheral lens alignment.
Fitting the Oval Cone
The oval-shaped form of keratoconus is hallmarked by an inferior steepening with varying degrees of normal corneal topography superiorly. When fitting the oval-shaped cone, careful attention must be directed to the status of the superior and horizontal (i.e., along the 180 degree meridian) corneal topography. A superior alignment fitting technique similar to that described in early keratoconus can be considered only if the superior topography and the horizontal topography are relatively normal. In this situation, the normal cornea at 9, 12, and 3 o'clock provides a sufficient base on which to support the superior alignment lens. (Figure 49.)
As the oval cone progresses, the normal corneal topography in the temporal horizontal meridian is lost. The superior alignment fitting philosophy is less likely to provide an adequate fit due to the lack of a horizontal fulcrum (contact point) temporally. This loss of normal cornea temporally is followed by involvement of the superior cornea resulting in the last island of normal cornea being in the superior-nasal quadrant.
A more contoured fitting philosophy can be used with a spherical multicurve lens design that incorporates an overall diameter of 8.6 mm and a posterior optical zone of 6.0 mm. (Table 2.) (Figure 50.) Alternately, the oval-shaped cornea can be fit with an aspheric lens design having adequate peripheral eccentricity to provide the needed peripheral clearance and lens stability. Currently, our aspheric lens of choice for keratoconus is the I-Kone lens design manufactured by Valley Contax in Springfield, Oregon, USA. (http://www.valleycontax.com)
Fitting the Globus Cone
In nipple keratoconus, the ectatic central cornea is surrounded by 360 degrees of normal cornea. In the oval-form, 180 degrees or less of normal cornea remains above the midline. The globus-form is hallmarked by a nearly total involvement and ectasia of the cornea. In this advanced stage, the only normal unaffected portion of the cornea may be located in the superior limbal area.
Because of the size and extensive involvement of the peripheral cornea, contact lens fitting for globus cones requires large lens diameters of 9.1 mm and a 6.5 mm posterior optical zone. (Table 3.) (Figure 51.) Alternately, an aspheric lens design can be used, however it might be necessary to incorporate less peripheral asphericity than that of the diagnostic lens to accommodate the steeper peripheral cornea associated with the globus-shaped topography.
Over-refraction is an integral part of diagnostic fitting. It may be important to over-refract using large diopter steps (e.g., 0.50 D or even 1.00 D) because keratoconus patients often find it difficult to distinguish small changes in power due to the high degree of myopia and irregular astigmatism caused by the disease. It is not uncommon for keratoconus patients wearing rigid contact lenses to manifest moderate to high amounts of residual astigmatism. This can be easily evaluated by placing a plus/minus 0.50 D cross-cylinder at 90, 180, 45, and 135 degrees, and asking the patient if there is any subjective difference in acuity.
Correcting the residual astigmatism is best accomplished with glasses, which often improves visual acuity three to four lines. Front surface toric rigid lens designs have been successfully fitted in keratoconus patients resistant to wearing spectacles over their contact lenses.
All keratoconus contact lenses should be ordered in a moderate to high Dk rigid gas permeable material to avoid epithelial hypoxia and corneal erosion during the long wearing schedule of keratoconus patients. All lenses must be manufactured under the strictest controls possible. Not only must the laboratory be capable of manufacturing one perfect lens for the patient, but they must also be able to duplicate that lens should the patient need a replacement. Every aspect of the lens design (base curve, optical zone, diameter, edge, etc.) plays an integral role in the overall success of the lens. Therefore, the practitioner should carefully evaluate the lens prior to dispensing and communicate any design concerns to the manufacturer.
In certain cases, traditional spherical and aspherical GP lens designs may not provide the desired centration, optics, or comfort required by the patient. In these situations, the patient may benefit from a large diameter (13.5 to 16.0 mm) semi-scleral lens design, (Figure 52.) These lenses can be manufactured in a wide range of parameters (base curves, powers, diameters, flange radii, etc.) in high Dk (100+) materials.
Semi-scleral lenses are best fitted through the use of a diagnostic set. The fitting procedure begins by selecting a diagnostic lens with a base curve radius equal to the steepest keratometric reading. The lens is placed on the eye and evaluated with fluorescein. The ideal fitting relationship is one in which the corneal portion of the lens exhibits apical clearance across the central cornea. (Figure 53.)
The lens should demonstrate a 1.0 mm band of pooling adjacent to the limbus and alignment in the area of the scleral curve. These lenses often incorporate a large limbal fenestration to reduce lens adhesion and facilitate lens removal. Clinical experience has shown that central corneal alignment (or a flatter than alignment fitting relationship) will often result in excessive lens adhesion and subsequent tightening of the lens onto the eye. Therefore, adequate corneal vaulting is essential to prevent this binding phenomenon.
Semi-scleral lenses have proven to be extremely beneficial for patients with highly irregular and/or asymmetric keratoconic corneas. The lens design can dramatically decrease many of the comfort and centration complications associated with more traditional rigid lens designs.
For the past 100 years, rigid corneal contact lenses have offered the best visual correction for keratoconus. However, even with all of their benefits, modern GP lenses may not meet the needs of many patients. The inability to achieve all-day wear, and the discomfort experienced by some patients wearing GP lenses has created a need to investigate the use of soft lenses for keratoconus. Clinically, it would seem unlikely that soft contact lenses could compensate for the refractive error associated with keratoconus, but new soft lens designs and manufacturing techniques have made it possible to correct the complex optics created by keratoconus.
Corneal Sensitivity in Early Keratoconus
Clinical experience has shown that the corneal apex in early keratoconus can go through a period of hypersensitivity. This hypersensitivity may be due, in part, to a dilatation or stretching of the corneal nerve fibers, as seen by biomicroscopy. In addition, the minute ruptures in Bowman’s layer may be a contributing factor to patient hypersensitivity. It has been the authors’ experience that some patients with early keratoconus find it difficult or impossible to tolerate even extremely well-fitted GP lenses. This intolerance may not be psychological, but may be due in large part to hypersensitivity of the corneal apex during the early disease. In later stages of keratoconus, this hypersensitivity decreases and the cornea can become hyposensitive (Axenfeld’s sign).
The use of soft contact lenses in keratoconus encompasses a variety of different modalities each of which have proven successful in providing functional optics and comfort for selected patients. These modalities include piggyback lens designs, soft toric lenses, custom spherical and aspherical lens designs, and hybrid designs.
Piggyback Soft Lenses
The technique of placing a rigid contact lens on top of a soft lens (piggyback) was first described in the mid 1970’s. Early piggyback systems consisted of thick, low Dk, soft lenses in combination with low Dk silicone/acrylate rigid lenses. It was not surprising that this combination frequently resulted in corneal hypoxia and neovascularization, which limited its usefulness. However, with the recent introduction of high Dk silicone hydrogel lenses and stable high Dk GP materials, the dual lens system is now enjoying a rebirth, particularly for keratoconus patients experiencing comfort or lens position issues.
The Traditional Piggyback Lens System
The traditional piggyback system consists of a high Dk silicone hydrogel soft lens over which a high Dk RGP lens is fitted. The fitting procedure begins with the diagnostic fitting of the soft lens to optimize lens movement and position. This is followed by keratometry or videokeratography over the anterior surface of the soft lens to determine the radii of the “new” corneal surface. A GP lens is selected with a base curve radius equal to the flat K and a diameter of approximately 9.0 to 9.5 mm. The base curve is adjusted until an appropriate lens-to-lens fitting relationship is established. The GP lens should accomplish three fitting objectives:
- Apical clearance to prevent the lens from rocking and pivoting over the corneal apex.
- Lens contact or “landing zone” along the horizontal meridian at 3 and 9 o’clock to center the lens along the horizontal meridian.
- Unobstructed lens movement along the vertical meridian. (Figure 54.)
An over-refraction is performed to determine the final power of the RGP lens, which can be manufactured in a spherical or aspherical design in a high Dk material with customary peripheral lens and edge configurations.
Custom Piggyback Lens System
The custom piggyback soft lens differs from the traditional system in that it incorporates a circular, recessed depression in the center of the soft lens. Within the boundaries of the cutout, a high Dk RGP lens is fitted. (Figure 55.) The system provides optimal performance by virtue of the now centered RGP optics and enhanced comfort through the presence of the soft lens.
In the United States, the custom piggyback soft lens is manufactured by X-Cel Laboratories in Duluth, Georgia (http://www.visionslens.com/contact.html). The soft lens is available in a wide range of parameters, including base curve radii from 6.00 to 11.00 mm and lens diameters from 12.5 to 16.5 mm. The recessed cutout can be manufactured in diameters of 7.5 to 11.5 mm. The fitting criteria for the soft lens are identical to that of any lens, with movement and centration as the primary fitting objectives. Diagnostic fitting of the soft lens is enhanced by inserting any rigid lens into the recessed cutout to mimic final lens weight and lid/lens interaction.
Once the appropriate soft lens fit has been determined, the rigid lens can be removed and K readings can be obtained over the central portion of the soft lens. A diagnostic GP lens with a base curve radius equal to flat K is then inserted into the cutout and its fitting relationship evaluated and adjusted.
It is important to select an GP lens with an overall lens diameter 1.0 mm smaller than the cut out diameter to allow for some movement and tear exchange within the soft lens cut-out boundaries. For example, if the cut-out diameter is 9.5 mm, the RGP lens diameter should be 8.5 mm.
Soft Toric Lens Designs
Toric soft contact lenses have been successfully used in the early stages of keratoconus or in form fruste keratoconus. The fitting procedure begins by placing a soft lens with powers equal to the manifest refraction vertexed to the plane of the cornea on the eye. Final lens power is best calculated by performing a sphero-cylinder refraction over a well equilibrated lens. Modern production techniques allow the final lens to be manufactured in a wide range of custom parameters that include, base curve, power, diameter, and thickness.
Custom Spherical and Aspherical Soft Lenses Designs
Most soft lenses designed specifically for keratoconus utilize a tri-curve posterior lens design with increased central thickness to mask much of the regular and irregular astigmatism. (Figure 56.) The base curve is designed to fit the apex of the cone, the intermediate curve parallels the peripheral cornea, and the scleral curve rests approximately 1.0 mm beyond the limbus. These lenses can be custom-designed and manufactured in a broad range of base curves, powers, and diameters. Other parameters that can be controlled include the diameter of the anterior and posterior optical zones, center thickness, and the width and radius of the intermediate curve and scleral curves.
Figure 56. Custom soft lens designed for keratoconus.
Patient Selection for Soft Lenses Designs
The corneal topography of an individual keratoconus patient can play an important role in the successful use of a soft lens. Corneas that seem to do best are the nipple- and globus-types in which there is a relatively concentric 360 degree peripheral topography. (Figure 57.)
Figure 57. Advanced globus-shaped corneal topography successfully fit with custom soft contact lenses.
Patients with large diameter sagging cones may be poor candidates due to corneal asymmetry in which the steepening of the inferior cornea causes the lens to lift inferiorly. It is important to remember that the base curve, as well as the intermediate and scleral curves of the lenses, can be custom-designed to correspond to the topography of the individual cornea. Also, if the patient is being refitted from rigid contact lenses, it is best to fit one eye at a time (the most needy eye first).
Fitting Custom Spherical and Aspherical Soft Lenses Designs
Keratometric readings are taken (as best as possible) to estimate the steepness of the central cornea, as well as the amount and location of corneal astigmatism. A diagnostic lens is selected with a base curve 1.00 mm flatter than the mean K. For example, if the central keratometric readings are 52.00 at 45 / 62.00 at 135, the mean K equals 57.00 D (5.90 mm). A diagnostic lens can be selected that is 1.00 mm flatter than 5.90 mm so the 6.90 mm diagnostic lens curves with a peripheral radius of 8.60 mm and is placed on the cornea to be evaluated with the slit lamp. (Table 4.) (Figure 58.)
The lens-to-cornea fitting relationship is evaluated with traditional soft lens fitting techniques, the lens should exhibit 1.0 to 1.5 mm of limbal clearance, and the lens edges should rest 360 degrees onto the sclera and the lens should exhibit 0.25 mm of movement with a blink.
The thicker soft lens design will be more rigid than a traditional soft lens, therefore a true tear lens will form between the posterior surface of the lens and the cornea. It is the presence of this lacrimal lens that aids in the correction of the regular and irregular astigmatism. (Figure 59.) It is important to note, however, that the tear lens may change due to settling of the lens during the first week of lens wear.
Following evaluation of the trial lens, a spherical and/or sphero-cylinder over-refraction is performed. The power is only an estimate because changes in the tear lens should be anticipated during the first week of lens wear. Due to lens rigidity, it is not uncommon for patients to need only small cylindrical over-corrections or, in some cases, none at all.
Hybrid Lens Designs
In 1977, Precision Cosmet in Minneapolis, Minnesota, USA began work on a hybrid combination GP and soft lens design. Their work culminated in introduction of the Saturn lens in 1985. This lens incorporated a 6.5 mm rigid styrene-based material with a Dk of 14. This was surrounded by a 13.5 mm diameter, 25% water content skirt. The Saturn lens was replaced by the Sola-Barns Hind, Softperm lens in 1989. The new lens incorporated a larger 8.0 mm styrene center in a bi-curve lens design and a 14.3 mm diameter, 25% water content skirt. (Figure 60.)
Figure 60. The SoftPerm II lens design for keratoconus.
The Softperm hybrid design enjoyed only limited success due to physiologic problems secondary to minimal oxygen permeability, frequent loss of adhesion between the GP lens and the soft lens skirt, and limitations in lens design and parameter availability. (Figure 61.)
In September 2001, a California based research group called Quarter Lambda Technologies began development of a new high Dk hybrid lens called SynergEyes (http://www.synergeyes.com). The lens incorporates an 8.2 mm high Dk rigid center, (Paragon HDS 100, Dk 100) and a 28% water content non-ionic soft lens skirt. The overall diameter of the lens is 14.5 mm. (Figure 62.)
We have successfully used this lens for many keratoconus patients, especially those with irregular astigmatism or those who had comfort and centration issues with traditional GP lens designs. The SynergEyes lens is available in two designs for keratoconus: the SynergEyes A is the standard aspherical design ideal for patients with early keratoconus and the SynergEyes KC has been designed specifically for advanced keratoconus patients.
The fitting procedure begins by selecting a diagnostic lens with a base curve radius equal to steep K. High molecular weight fluorescein is placed into the bowl of the lens and the lens is placed on the eye and allowed to equilibrate. The RGP portion of the lens should exhibit central apical clearance and mid-peripheral lens bearing. The soft lens skirt should exhibit 0.25 mm of blink-induced movement. (Figure 63.)
Since the 1950’s, penetrating keratoplasty has been the primary surgical procedure for treatment of keratoconus. (Figure 64.)
The decision to pursue keratoplasty is one that must be made by the individual surgeon and the patient. The factors that can influence the decision include:
- The patient can no longer tolerate or be fitted adequately with contact lenses.
- The contact lenses fail to provide adequate visual acuity due to corneal scarring.
- Sharp bilateral visual acuity is required for recreational or professional requirements.
It is important to recognize that certain patients are perfectly content with 20/50 to 20/60 (6/15 to 6/18) visual acuity. Over the years, these individuals have developed blur interpretation skills that allow them to function surprisingly well in their daily endeavors. The major limiting factor for these individuals is difficulty driving at night.
For most surgeons, keratoplasty is not considered until contact lens options have been exhausted. Surgery involves risks that include:
- graft rejection,
- high post-operative regular and/or irregular astigmatism, and
- steep or flat grafts.
Patients should be informed that keratoplasty for keratoconus has a success rate of approximately 92% for a clear transplant on the first surgery and that occasionally a second or third transplant might be needed secondary to post-operative graft rejection, infection, or wound healing complications. Patients should also be advised that following keratoplasty, approximately 85% of them will find it necessary to wear a contact lens to correct the surgically-induced regular and irregular astigmatism and anisometropia.
Eye bank data from 2004 shows that in the United States 30,668 penetrating keratoplasties were performed with 4,744 of the transplants performed to treat corneal ectasias. (Table 5.)
|Diagnosis||Number of Transplants||Percentage of Transplants|
|Pseudophakia Corneal Edema||5,569||18.4%|
|Endothelial Corneal Dystrophies||4,771||15.8%|
|Regraft Non Rejection||2,411||8.0%|
|Regraft Secondary to Rejection||1,504||5.0%|
|Noninfectious Ulcerative Keratitis||1,257||4.2%|
|Aphakic Corneal Edema||916||3.0%|
|Stromal Corneal Dystrophies||449||1.5%|
|Post Viral Keratitis||407||1.3%|
|Post Microbial Keratitis||295||1.0%|
|Post Syphilitic Keratitis||82||0.3%|
Intacs inserts are 150 degree PMMA ring segments that are placed in the peripheral stroma of the cornea, typically to correct myopia. (http://www.getintacs.com) However, the technique has also been recently the United States Food and Drug Administration approved for the treatment of keratoconus. (Figure 65.)
The inserts are designed to be placed at a depth of approximately two-thirds the corneal thickness and are surgically inserted through a small radial incision into a trough created within the corneal stroma. The inserts shorten the corneal arc length and have a net effect of flattening the central cornea. The amount of flattening is determined by the insert's thickness. Rings are available in thicknesses of 0.250, 0.275, 0.300, 0.325 and 0.350 mm and are oriented horizontally in the cornea at 12 and 6 o'clock. Intacs are indicated for contact lens intolerant patients with early keratoconus who have minimal central stromal scarring. (Figure 66.)
In theory, the inserts are designed to flatten the central cornea and provide reinforcement for the peripheral cornea. The procedure may also decrease the cornea surface irregularity and improve the patient's best-corrected visual acuity.
Laser Surgery for Keratoconus
Clinicians typically rule out LASIK surgery for patients with keratoconus because of a greater risk for scarring and excessive thinning leading to possible post-LASIK ectasia. However, some surgeons have reported success in performing a smoothing procedure for patients over the age of 40 years with stable keratoconus. Candidates for the procedure are most often individuals with mild keratoconus who are contact lens intolerant and reluctant to pursue corneal transplant surgery.
Pellucid Marginal Degeneration
Pellucid Marginal Degeneration (PMD) is a bilateral corneal disorder hallmarked by a thinning of the inferior peripheral cornea. The corneal thinning begins approximately 1.0 to 2.0 mm above the inferior limbus and is separated by a region of uninvolved, normal cornea between the thinned zone and the limbus. (Figure 67.) Ruptures in Descemet’s membrane or acute hydrops may be seen in the area of inferior thinning. (Figure 68.)
PMD commonly manifests between the ages of 20 and 40 years with no apparent hereditary transmission and equal gender distribution. Subjective symptoms are visual secondary to a dramatic increase in against-the-rule corneal astigmatism.
In this condition, the inferior cornea is free of vascularization or lipid infiltration, which differentiates PMD from other peripheral thinning disorders such as Terrien’s Marginal Degeneration and Mooren’s ulceration. (Figure 69.)
PMD can manifest many of the same features as keratoconus however in PMD the central cornea retains a normal thickness and also manifests unique topographic features. Corneal mapping in PMD demonstrates inferior mid-peripheral zones of corneal steepening at 4 and 8 o’clock. This produces the “butterfly wing-like” or “kissing pigeons” pattern, diagnostic of PMD. (Figure 70.)
Management of PMD
The management of PMD involves a wide range of modalities including spectacles, contact lenses, and surgery. Spectacle correction is often satisfactory in the early stages of PMD due to the minimal degree of induced irregular astigmatism, however in the more advanced cases, contact lenses are the suggested mode of treatment. Contact lens management of PMD can be difficult due to the high degree of against-the-rule and asymmetrical astigmatism.
Management of the PMD has recently been enhanced by the wide range of available lens materials and designs that include:
- spheric and aspheric GP lenses,
- bi-toric GP lenses,
- semi-scleral and scleral GP lenses,
- toric soft lenses,
- hybrid lenses, and
- piggyback lenses. (Figure 71.)
Surgical intervention for PMD often involves penetrating keratoplasty. Alternative treatment includes a kidney-shaped penetrating keratoplasty or an inferior lamellar patch graft to more directly manage the localized inferior corneal thinning. (Figure 72.)
Perhaps no other group of patients has benefited more from the science of contact lenses than those with keratoconus. However, all too often the fitting process can be a frustrating experience for both patient and practitioner. To better manage the condition, practitioners must first be aware that no one lens design or fitting technique will provide adequate results in all cases. Additionally, it is important to avoid designing the lenses based on information provided by central keratometry readings. Instead, central and peripheral information obtained from corneal mapping and detailed fluorescein evaluation of diagnostic lenses should be used.
For many practitioners, the use of steeper, apical clearance fitting philosophies will be a major departure from their traditional approach to managing keratoconus patients.
The successful management of the keratoconus patient with contact lenses requires a complete rethinking of corneal topography; it must be recognized that the disease affects the cornea far beyond the range of the keratometer. Therefore, regardless of the fitting technique used, it is important for the contact lens practitioner to envision a three-dimensional picture of the corneal shape during the fitting process so that steep peripheral fitting relationships (which will eventually lead to superior corneal compression, increased lens awareness, and decreased wearing time) can be avoided. It is anticipated that as new and more accurate computer algorithms are developed for the quantitative assessment of corneal topography, our ability to successfully fit even advanced stages of keratoconus will be greatly enhanced.
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