Corneal allografts are the most commonly transplanted tissue worldwide; according to the Eye Bank Association of America (EBAA), member eye banks in the United States provided 38,413 corneas for penetrating keratoplasty (PKP) in 2016.¹ Given the corneal histological characteristics of being immune-privileged, it’s not surprising that corneal keratoplasty studies often report good long-term prognosis, with high graft survival rates.^2,3 This procedure is performed mainly for corneal diseases of degenerative, dystrophic, infectious, or inflammatory origin as well as for corneal damages sustained from ocular surface disease.⁴ Specifically, keratoconus, pseudophakic bullous keratopathy, Fuchs’ dystrophy, corneal scarring, and aphakic bullous keratopathy remain the primary indications for PKP in the United States.⁵

Surgical options range from full-thickness PKP or femtosecond laser-assisted keratoplasty to more selective partial forms such as deep anterior lamellar keratoplasty (DALK), which target the anterior structures of the cornea. DALK has the advantage of providing similar refractive changes as a full-thickness keratoplasty, but it may also reduce potential immunologic rejection given that the host Descemet’s membrane and endothelium are preserved. Comparatively, corneal transplants that target the pathological endothelium or Descemet’s layer may revive the host tissue functions without altering the anterior corneal profile. As the corneal refractive profile is predominantly governed by its anterior tissue layers, the postoperative visual recovery course is typically shorter and more predictable. Such procedures are classified as Descemet’s membrane endothelial keratoplasty (DMEK) or Descemet’s membrane automated endothelial keratoplasty (DMAEK).⁶

As a result of high regular and irregular astigmatism following PKP (Figure 1), it is estimated that approximately 47% of patients require a contact lens for improvement of visual acuity.⁷ Traditionally, corneal GP lenses have been the gold standard for visual rehabilitation due to their ability to neutralize high degrees of regular and irregular corneal astigmatism. Nonetheless, their clinical success can be limited by poor patient tolerance or by difficulties in stabilizing a proper fitting position over a severely irregular corneal contour. Fortunately, practitioners now have a wide variety of fitting options to offer patients.

TIMING IS EVERYTHING

Complete healing of the donor-host corneal complex can take up to 24 months; however, the average time frame to begin considering contact lens fitting varies from six to 12 months after surgery. Although optical rehabilitation with contact lenses can start as early as three months after transplantation, the clinical status of the grafts and individualized postoperative protocols can also impact timing for contact lens rehabilitation.⁸

Suture tension and the subsequent suture removal algorithm, for example, can influence the final graft configuration and affect both the type and amount of corneal astigmatism present, which most certainly will impact clinical timing for contact lens fitting. Alternatively, pending individual patient scenarios and pre-operative expectations, surgeons may elect to perform additional limbal relaxing incisions or laser ablative procedures after suture removals to reduce corneal astigmatism. Foreseeably, such adjunctive surgical treatment strategies will also impact overall corneal shape and subsequent lens fitting.^6,9

In addition, postoperative topical medication dosing schedules, the exact type of allograft technique utilized, and/or the baseline corneal diagnosis necessitating keratoplasty can also affect the postoperative optical rehabilitation with contact lenses. Therefore, multidisciplinary communication with corneal surgeons and patients can be essential in determining the exact timing for when the contact lens rehabilitation process should be initiated.

LET THE SHAPE BE YOUR GUIDE

Beyond improving postoperative visual functions, one of the most critical fitting objectives is to minimize trauma to the donor tissue and the host bed. As mechanical and physiological stress can lead to increased risks of infection or graft rejection, it is helpful to select a contact lens design that can be conducive to long-term corneal health.

In general, an ideal lens design is one that closely matches the corneal profile and/or provides central clearance across the graft-host border while maintaining peripheral alignment with the host tissue. Hence, a careful topographical analysis of the graft shape should be performed to help select a lens that may exert the least amount of mechanical stress on the ocular surface. Five classic postoperative corneal configurations are typically described: prolate, oblate, mixed, asymmetrical, and steep-to-flat.¹⁰

A prolate-type graft will exhibit regular astigmatism with a steeper central region that flattens toward the periphery, whereas an oblate graft is flatter centrally with a steep periphery. Regular astigmatism can also be observed throughout the entire topographic area in a mixed-shape graft. Irregular astigmatism is a dominant characteristic of both asymmetrical and steep-to-flat configurations. Similar to an asymmetrical bow tie shape, an asymmetrical graft has two steep hemi-meridians that are not displayed at 180º from each other on the topography map (Figure 2). Lastly, steep-to-flat will demonstrate a topographical pattern in which the steep hemisphere is adjacent to the flat hemisphere (Figure 3). Regardless of the type of topographic map, each type can present its own challenges with respect to contact lens fitting.^10,11

If the ametropia is not extreme and the astigmatic influences surrounding the visual axis are predominantly regular, clinicians may opt to simply prescribe spectacles as a relatively easy and inexpensive option. However, it is rare that a patient can experience a desirable visual outcome with spectacles alone. Anisometropia and aniseikonia can manifest after keratoplasty and often cause sufficient disparity between fellow eyes; use of a contact lens can help equalize retinal image size.¹⁰

SOFT LAUNCH VERSUS HARD LAUNCH

Soft contact lenses can be of use when higher-order aberrations (HOAs) are less of a concern. Custom soft lens designs offer some advantages in that they have an extended range of available fitting curves, diameters, and lens powers. Nonetheless, custom soft lenses are typically made of hydrogel material with an increased center thickness, both of which raise the concern for potential hypoxia and associated neovascularization, which can increase likelihood of rejection. Thus, use of thicker hydrogel lens designs is discouraged. Newer designs have largely mitigated this concern through the availability of higher-Dk silicone hydrogel materials and reduced center thickness (minimum of 0.2mm). This newer technology can be useful in patients who have not been successful with corneal GP lenses.

HOAs in eyes following keratoplasty can, on average, demonstrate magnitudes up to 5.5 fold that of a normal cornea control group. Trefoil, spherical aberration, and coma have been reported to highly contribute to this negative visual outcome.¹² Due to their superior optics in neutralizing the unwanted visual interferences of HOAs, corneal GP lenses remain the gold standard for visual rehabilitation post-keratoplasty. It may be feasible to use a standard spherical-back-surface GP design if the corneal toricity is centrally localized, 5.00D or less in magnitude, assumes a prolate profile, and/or when peripheral eccentricity approximates that of a normal cornea (i.e., 0.5 to 0.7).¹⁰ In cases of mixed-shape grafts, topography reveals corneal astigmatism distributed in an orthogonal pattern extending from center to periphery, which may indicate that satisfactory fitting requirements can be met with standard back-toric GP designs.⁹ However, due to the various graft shapes that can be presented in the postoperative period, practitioners commonly utilize proprietary GP lens options developed to overcome many postoperative fitting challenges.

Proprietary lens designs offer benefits in that a plethora of designs is available to help manage a wide variety of graft types, and they can be an economical choice. Intralimbal GP designs with larger lens diameters may be advantageous for fitting grafts, as it is desirable to select a lens with a posterior optical zone that is larger than the actual graft diameter, which can further help to minimize potential mechanical interactions between a GP lens and the graft tissue.^8,10

Custom keratoconus GP designs can help troubleshoot fitting challenges in grafts that have high eccentricity values, which are often associated with moderate-to-severe central steepening. These designs can help to achieve better apical clearance, lens positioning, and improved tear exchange by reducing excessively tight-fitting edges. A tight fit may impact the health of the cornea due to mechanical stress or tight lens syndrome, which, in worst cases, can cause patients to suffer from iritis or even to have an increased risk of graft rejection related to chronic hypoxia. Also, the long-term impact of a poor-fitting lens may lead to scarring and reduced visual acuity.^8,10

Conversely, reverse geometry designs can better assist in fitting oblate grafts, in which eccentricity can be negative in value. Reverse geometry lenses, with their flatter optical zone and steeper secondary peripheral curve, can closely match the profile of a oblate graft and allow alignment or clearance over a steep graft-host junction. These lenses can also reduce the excessive edge lift that may result when attempting to fit an oblate graft with a conventional spherical GP.

Steep-to-flat topographic patterns, in which the steeper half of the graft is 180º away from the flatter half, are often among the most challenging geometries on which to stabilize a corneal GP lens. This shape can cause a corneal GP to decenter toward the location of the steeper half, potentially reducing its optical performance. Large-diameter designs can be helpful by distributing the weight over a larger surface area, which can result in better centration and stability. Initial trial lenses can be selected based on topography utilizing the dioptric value 3mm from the center or the average keratometry value. It is also possible to utilize the midperipheral curvature(s) via an axial topography map to customize peripheral curve(s) of a given GP lens.^10,13

GP lenses also afford advantages due to their relatively simple handling and care regimen. However, the primary limitations to successful GP wear are a longer adaptation period and a higher rate of lens intolerance. While these drawbacks can be reduced with patient education and an improved lens-to-cornea fitting relationship, they cannot be completely eliminated for some patients.¹⁴

SOFT LANDING ISN’T JUST FOR OUTER SPACE

When acceptable patient comfort or fit cannot be achieved with corneal GPs, a piggyback system using a soft lens (typically low-powered in a high-Dk material) beneath the GP can be an alternative. This modality can aid in GP centration (Figure 4) and can decrease GP bearing that may compromise corneal integrity. Therefore, one of the main fitting goals is to achieve improved clearance across the entire posterior surface of the GP lens. Due to two lenses compounding the barrier to oxygen transmission, chronic corneal hypoxia is a potential concern and will require careful long-term monitoring.

Newer hybrid lenses can simultaneously provide crisp vision through their GP center (130 Dk) and improved patient comfort with the second-generation, higher-Dk silicone hydrogel outer soft skirt (84 Dk). Lens care and handing are also relatively simple for this modality, with hydrogen peroxide being a commonly recommended care system. A simplified care system can improve contact lens care compliance. Nonetheless, dehydration of the soft skirt during wear and low GP-skirt junctional clearance may allow eventual binding to corneal and conjunctival epithelium, which raise possible concerns about hypoxia despite the clinical benefits of the newer lens materials. Thus, close corneal monitoring should be instituted, and total lens wear time may also have to be limited, even with good patient comfort.¹⁵

LOOK BOTH WAYS BEFORE CROSSING

When faced with extreme corneal irregularities, such as those presented in many cases after corneal transplantations, scleral designs may become a viable clinical tool. However, some caveats may exist with current scleral lens technologies and should be kept in mind prior to choosing sclerals for patients who have a corneal graft.

The distance between the front surface of a scleral lens and that of a graft may decrease corneal oxygen bioavailability, which could manifest with undesirable implications to the survival of a graft. The endothelial cell layer, being the most posterior and incapable of regeneration, can be the most vulnerable to hypoxic stress. Thus, it is clinically prudent to ensure the presence of a minimally acceptable endothelial cell count (ECC) prior to considering scleral lenses.

While there is not an absolute agreement on the lowest ECC required for scleral lens fitting, most clinicians currently agree that ECC should minimally be greater than 800 to 1,000 cells/mm². If the ECC is below this minimum standard and yet scleral lenses are considered medically necessary, patients should be properly educated on the possible impact of scleral lenses on the long-term survival of the donor tissue, which includes degradation of vision due to edema and, ultimately, graft failure. Regardless of the ECC, it is beneficial to start by selecting a lens material with high oxygen permeability to maximize delivery of corneal oxygen.^16,17,18

Vessel growth sprouting into the region of a donor graft may also increase the risk of tissue rejection, and caution should be taken in fitting strategies to reduce any potential stimuli related to such an adverse event. As with all contact lens device fittings, it is best to document the location(s) and extent of pre-existing neovascularization in relation to the corneal graft prior to proceeding with contact lens fitting to facilitate future monitoring (Figure 5).

BRIDGING OVER TROUBLED WATER

Scleral lenses, by vaulting the entire cornea, can negate the fitting challenges imposed by the various combinations of surface contours with corneal grafts. Because a scleral lens does not touch the cornea, it can greatly improve patient tolerance while creating a fluid reservoir capable of masking corneal irregularity and rehabilitating visual functions. Additionally, by vaulting over a corneal graft and minimizing its negative fitting influences, the resultant lens stability allows for more effective neutralization of residual astigmatism.

The vast majority of scleral lenses commonly have a prolate design. However, reverse geometry scleral lens designs may be necessary when areas of higher elevation are observed in the graft periphery or at the graft-host junction. Even with full corneal clearance, patient discomfort with scleral lenses can still be reported if the lens haptic is not properly aligned with the sclera or when there are areas in which the lens is excessively steep or flat. When impingement, compression, or edge lift is observed in two or more quadrants (usually 180º away from each other), a toric peripheral curve design on the back haptic surface is indicated. Moreover, if a patient is reporting scleral lens discomfort or difficulty with lens removal despite a relatively optimum fit appearance, staining the eye with sodium fluorescein prior to scleral lens removal may be helpful in targeting areas of discomfort or suboptimal fit. The conjunctival surface will demonstrate an arcuate pattern localized in areas where the haptic contour needs to be modified.

Central clearance of 200 microns and limbal vault of 50 microns have been proposed for scleral lenses after consideration of the oxygen transmissibility limitations of today’s lens materials.¹⁷ However, a clinical consensus on this topic will continue to evolve with future studies and with the development of lens materials that have higher oxygen diffusion constants. However, in general, most clinicians agree that excessive clearance should be avoided to prevent corneal hypoxia.^19,20

Debris entrapment within the lens fluid reservoir can reduce visual performance. It has been postulated that a high vault is the inciting problem; however, this midday fogging phenomenon can still occur even with acceptable lens vault. While reducing central vault and/or adding haptic toricity to aid scleral alignment can help reduce this event, it may be difficult to completely resolve. Conversely, front-surface non-wetting or debris deposition may also be a problem or may present without debris entrapment. Hence, many patients may still have to remove, clean, and reapply their lenses to maintain the best visual performance throughout the wearing period. This need may be reduced with a new polyethylene glycol-based lens coating, but to date, evidence is anecdotal.

Other factors that may present as common barriers to treatment are lens handling skills and patient cost. Discussing pros and cons with patients prior to initiating the fit can be helpful and can reduce practitioner-patient problems following completion of the fitting.²¹

SCLERAL FITTING 2.0

Understandably, the complex geometric configurations of corneal allografts and patients’ underlying disease processes can make it a challenging task to obtain a satisfactory fitting outcome. In these cases, new assistive technologies, such as ocular impression-based lenses and corneo-scleral topographers, are available to enhance the specificities of fitting with improved outcomes.

A hydrophilic, low-viscosity, polyvinyl siloxane impression material is used to capture the exact contours of the anterior globe over and up to 26mm. The resulting impression is sent to the manufacturing laboratory, where the concave surface of the impression is scanned using a 3D scanner. During the manufacturing process, the proprietary software can determine the usable data range in calculating the diameter of the final lens as well as allowing the manufacturer to specify point-by-point sagittal height adjustments independently of each other (Figure 6).²² This impression technology has also demonstrated early success with incorporating aberration-control optics, which may benefit irregular cornea patients including in many post-graft scenarios.²³

Corneo-scleral topographers can also aid in scleral lens fits and complicated fittings (Figure 7). One of the instruments currently available in the United States is a fluorescence-based structured light topographer. It contains two cameras that image a subject’s eye from two different angles to help improve triangulation measurement on the ocular surface. The photo-excitation of the fluorescein compound is required to initiate image capture. This technology allows for simultaneous mapping of corneal and scleral elevation data at both the 16mm and 18mm chord diameter area. A recent study indicates that the instrument’s measurements are highly repeatable with regard to ocular sagittal height and scleral toricity data at a 16mm diameter when fitting scleral lens patients.^24,25

Both of these advanced assistive fitting technologies can be applied clinically to help custom design scleral lenses in highly aberrated corneas that have a severely irregular ocular surface and are unable to be fit with spherical haptics or with standard posterior toric haptics. In addition, they can also create individualized fitting features, such as notches or focalized vaulting areas, to precisely contour over scleral obstacles such as filtering blebs and pingueculae.²⁶

WHAT TO DO WHEN A GOOD DEED GOES UNNOTICED

There are cases in which the clinical lens fit is optimal and patients are satisfied overall with comfort, but the vision may not meet the patients’ expectations. This can occur with both relatively simple corneal transplants or with highly irregular and steep corneas. In these cases, clinicians can now opt for scleral lens designs that have front-surface asphericity, or they can perform a sphero-cylindrical over-refraction and incorporate the result onto the front surface of the lens. In many cases, this can help improve subjective and objective visual expectations.

Corneal hypoxia may be another cause of gradual visual degradation throughout the day, and it is essential for clinicians to differentiate between midday fogging and hypoxic events. Due to hypoxia-driven corneal swelling, patients may report symptoms of starbursts or rainbows around lights, but it is possible that the onset of hypoxic events can precede such visual symptoms. It’s critical to perform a careful biomicroscopic examination after lens removal to identify subtle areas of microcystic edema within the graft and/or on the host tissue bed. Conjunctival prolapse (Figure 8), in which the conjunctiva can override the limbal region either temporarily or can remain attached after lens removal, is of concern. While long-term implications of conjunctival prolapse have yet to be determined, practitioners should note that potential associations with hypoxia and limbal stem cell deficiency have been proposed. If hypoxic response cannot be reduced or eliminated with fitting strategy modifications and/or increased oxygen permeability in the lens material, then scleral wearing time must be limited or discontinued indefinitely.

THE FEAR OF REJECTION

Pending patient compliance and graft response to contact lens wear, post-keratoplasty patients should be monitored annually or even more frequently. In addition to assessing patient vision and comfort, it is also of significance to note the health of the graft and to properly identify clinical signs of graft failure or rejection. Patients may report hyperemia, loss of vision, ocular discomfort, or photophobia. Ultimately, inflammation is the hallmark of immune system-mediated rejection, and endothelial rejection is easily the most common presentation (Figure 9). Biomicroscopy may reveal conjunctival injection, ciliary flush, corneal edema, and endothelial keratic precipitates in scattered form or linearly aligned (known as the Khodadoust line). When noted, and provided that infectious origin has been ruled out, treatment is immediate discontinuation of lens wear and aggressive therapy with topical corticosteroids coupled with close monitoring.²⁷ If the hostile immune reactions against the donor graft cannot be reversed within a reasonable period of time, referral to a corneal specialist is warranted.

CONCLUSION

Contact lens options for post-keratoplasty patients have expanded over the last decade, which have helped improve vision and comfort satisfaction for these patients. With a diverse range of options available, it is vital to customize the lens selection process based on the profile of each individual patient. It’s important to take into consideration the shape of the graft, the health of the donor tissue, and the length of time from surgery prior to initiating the contact lens fit. Other keys to consider are patients’ goals, expectations, and past contact lens history. From there, working with your patients and having access to a wide variety of lens options can allow for an optimal outcome. Once the desirable fitting relationship has been accomplished, good patient compliance with an appropriate follow-up schedule and vigilant corneal monitoring to rule out potential complications are imperative to maintaining the best quality of life for post-keratoplasty patients. CLS

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