Article

SCLERAL LENSES: THE BENEFITS OF TORIC LANDING ZONES

Incorporating toric landing zones for asymmetric scleras can resolve many scleral lens fitting complications.

By definition, a scleral lens rests exclusively on the scleral conjunctiva while vaulting the cornea and limbus.1 The two principal parameters for an optimal scleral lens fit are the vault height and the landing zone width, which determine the lens total diameter.

Vault height and landing zone width are associated with each other. The vault height depends on the sagittal height of the cornea, and it is greater in conditions including advanced keratoconus, keratoglobus, and prominent/protruding grafts. The higher the vault, the more likely that the weight of the landing zone may apply excessive pressure on the conjunctiva. Therefore, the landing zone design in scleral lens fitting is crucial because it is the only zone that rests on the sclera.

The landing zone design depends on the scleral shape. A good fitting relationship and alignment are important to avoid fitting problems with scleral lenses.

SCLERAL ASYMMETRY

It is well known that the sclera is asymmetrical. In 1977, after describing scleral lens fitting problems including trapped air bubbles and localized conjunctival vessel compression/blanching, Bier and Lowther recommended the use of spherical oval fitting or toroidal shells in the presence of high scleral toricity.2 The advent of Scheimpflug imaging, profilometry, and optical coherence tomography (OCT) confirmed this scleral asymmetry and has resulted in a better understanding of scleral shape and scleral lens fitting anomalies.

The sclera is not spherical, but rather non-rotationally symmetrical. One study reported a significant difference between opposing corneoscleral junction (CSJ) angles, showing that the difference was greater in the horizontal meridian compared to the vertical meridian, as the differential between superior and inferior CSJ angles was small.3 The more marked CSJ angle was nasal, followed respectively by temporal, inferior, and superior.3 Another study reported similar results, showing different CSJ angles in the four quadrants; the most acute angle was nasal, followed by inferior, superior, and temporal, which had the flattest and smoothest CSJ.4 A more marked CSJ angle indicates a flatter scleral curvature. In fact, in additional research, scleral curvature was reported to be steepest in the temporal quadrant (12.32mm) compared to the nasal quadrant (13.33mm).5 These outcomes can explain the temporal lens decentration observed clinically, as the lens tends to be displaced in the area with lower elevation (steeper curvature).

Furthermore, the sclera, similarly to the cornea, may present steep and flat meridians6 even though corneal and scleral toricity are not associated.7 Scleral toricity (height differential between the two principal meridians) at the 15.0mm chord may range from approximately 50µm to more than 300µm.7 Another study suggested that scleral toricity was 100µm on average.8

However, it was established that scleral asymmetry was less in the limbal area (10.0mm to 15.0mm) and increases further into the periphery (beyond 15.0mm).9 Likewise, a more recent study demonstrated that scleral asymmetry starts at the more symmetrical limbus and increases in asymmetry toward the extraocular muscles.10 The OCT measurement at a 15mm chord presented a low toricity. Therefore, it was suggested that scleral lens diameters of 14.5mm or less can be rotationally symmetric, and scleral lens diameters greater than 14.5mm may benefit from toric landing zones or quadrant-specific designs.10

Therefore, it is crucial that there is an optimal fitting relationship between the haptic zone and the sclera. Scleral lenses, especially larger lenses, have to land in both principal meridians. This can be achieved with toric landing zones, which provide several advantages when dealing with lens decentration, lens distortion,11 air bubble formation, localized conjunctival vessel blanching,12,13 lens impingement,14,15 conjunctival prolapse, and debris influx into the lens reservoir.11 Toric landing zones can allow improved comfort, increased wearing time, overall satisfaction, better visual quality, and enhanced optical correction.16-18

LENS DECENTRATION

Clinically, practitioners may find inferior-temporal decentration with scleral lenses, inducing a fluorescein thinning pattern in the superior-nasal quadrant.19 Lens inferior decentration may result from excessive vault height, large vault diameter, and lens mass. The higher the lens sagittal height, the greater the lid pressure on the lens.20 Lenses that have a large vault and diameter, which are not properly aligned on the sclera, may also cause lens decentration because the “watermelon seed” effect of the upper lid may be greater. When squeezing a watermelon seed between the thumb and index finger, the fingers tend to push out the rounded portion of the seed. The higher the rounded part of the seed, the greater the leakage from the fingers. Thus, reducing vault height and diameter, if possible, may reduce lens inferior decentration. Finally, because of gravitational forces, lens mass may induce inferior decentration. Reducing the lens diameter or making a lens thinner may decrease the effects of gravity and lid pressure.

Temporal lens decentration is induced by scleral asymmetry, which increases toward the extraocular muscles. The lens touches the sclera at the point of greatest elevation, or flattest curvature (nasally), then is pushed in the opposite direction with lower elevation, or steeper curvature (temporally). Toric landing zones or a quadrant-specific design may be indicated to improve temporal centration.

Another important factor influencing lens decentration is scleral toricity. DeNaeyer affirmed that putting spherical haptics on a toric sclera would exhibit a poor-fitting, with-the-rule appearance; the lens will touch the horizontal meridian and will lift off in the vertical meridian.21 Landing on both meridians can be achieved only with toric landing zones, which are necessary to ensure lens stabilization and centration.

LENS FLEXURE

Ultra-high-Dk rigid materials allow oxygen supply to the cornea, thus promoting anterior ocular surface health during scleral lens wear. However, high-Dk materials may result in lens flexure on eye, causing lens warpage and residual astigmatism. Corneal topography or keratometry may be performed over the lens to detect any warpage and deterioration of the optical quality of the lens.

Flexure is usually managed by decreasing lens diameter or increasing the central lens thickness. Decreasing lens diameter is not always possible. Increasing the center thickness of the lens may resolve flexure, but it will also reduce the oxygen supply to the cornea; plus, the increased lens mass may result in inferior decentration.

Choosing a toric-back-surface design will improve scleral alignment in all meridians and may improve lens flexure and enhance visual quality (Figure 1). This option allows any scleral lens diameter to be utilized and helps prevent lens decentration and reduced oxygen delivery to the cornea.

Figure 1. Topography over scleral lenses to detect the presence of lens flexure/torsion. (A) Topography showing lens torsion. (B) Topography over toric landing zone managing lens torsion.

In some cases, lens flexure is not completely resolved with toric landing zones, and small amounts of astigmatism may still be manifest. But, a toric landing zone provides greater lens stability, subsequently allowing a front-surface toric to correct any residual astigmatism.

CONJUNCTIVAL PROLAPSE

Conjunctival prolapse, or “hooding,” occurs when the conjunctival tissue is drawn beneath the lens at the limbus. It is most frequently observed in elderly patients, those who have undergone multiple ocular surgeries including strabismus or retinal surgery, those who have loose conjunctival tissue, and/or those who have dermatochalasis. Conjunctivochalasis, which is redundant, non-edematous tissue, is more common with age. There are also minor changes and alterations in the conjunctiva that include conjunctival thinning and a loss of tissue transparency with age. The conjunctival vessels become increasingly dilated, prominent, and tortuous.

Although aesthetically displeasing to patients and practitioners alike, conjunctival prolapse is typically benign. It does not usually have long-term consequences; however, there are exceptions. When the conjunctiva adheres to the cornea, it creates a pseudopterygium. If the prolapsed tissue recedes after scleral lens removal, conjunctival prolapse is not a problem. However, it is a concern if the prolapsed tissue remains adherent even with digital manipulation. This manipulation can be performed with a clean finger or a cotton swab. Adherent prolapsed tissue has been observed in patients predisposed to atopy, inflammation, and limbal stem cell deficiency. Another instance in which conjunctival prolapse is a concern is if neovascularization is present in the area of the conjunctival prolapse.

If conjunctival prolapse is present and concerning, methods to alleviate it include reducing excessive central clearance, reducing excessive limbal clearance, and adjusting the peripheral curves to more closely align to the sclera. Toric landing zones may improve scleral lens alignment when conjunctival prolapse is present.11

Keep in mind that sometimes, even with an ideal scleral lens fit, conjunctival prolapse cannot be eliminated. Rare instances of adherent prolapsed tissue or neovascularization can be managed with conjunctival resection or refitting into a different contact lens modality such as soft (if indicated), hybrid, small-diameter GP, or a piggyback lens system.

CONJUNCTIVAL IMPINGEMENT

When the outer edge, or periphery, of the scleral lens pinches into the conjunctiva, conjunctival impingement occurs. In such cases, scleral lenses will initially be comfortable without any patient complaints. With increased wearing time during the day, comfort will gradually decrease. Bulbar hyperemia will develop after lens removal.

Depending on the severity of conjunctival impingement, the patient may or may not be symptomatic. With scleral lens removal, conjunctival staining with sodium fluorescein or lissamine green may occur. If possible, try to avoid/resolve conjunctival staining associated with conjunctival impingement.

Conjunctival impingement occurs due to an excessively steep scleral lens landing zone. Rectify this by flattening the landing zone to evenly distribute the lens weight over the conjunctiva and to reduce localized pressure. In addition, toric lens peripheries may be specified.

If localized conjunctival hyperemia is due to impingement on a conjunctival elevation such as a pinguecula, pterygium, glaucoma drainage device, or filtering bleb, the scleral lens may be designed with a localized area of vault over the elevation or a notch to avoid contact with the elevation.

Scleral lens wear for extended hours during the day could lead to sensitivity and discomfort after scleral lens removal, resulting in scleral lens intolerance and/or reduced wearing time. Improving alignment by flattening the scleral lens periphery or landing zone will eliminate vessel impingement and also escalate tear exchange. Incorporating toric peripheries will help to properly align the lens to the sclera. Utilizing a smaller lens diameter may also be beneficial.

LOCALIZED CONJUNCTIVAL VESSEL BLANCHING

Some scleral lens wearers have conjunctival vessel blanching in a specific area. Based on factors including scleral toricity and conjunctival anatomy, this blanching may be in one or more quadrants. It is beneficial to evaluate the scleral lens on eye (outside of the slit lamp) to determine the area and degree of blanching. Misalignment of the scleral landing zone may result in blanching in a specific area. This excessive impingement and landing zone compression could result in significant conjunctival injection, which may be more prominent in one area. A patient may experience sensitivity and discomfort along with conjunctival hyperemia after extended hours of daily scleral lens wear. This may eventually lead to reduced wearing time and scleral lens intolerance.

Blanching of vessels under the scleral landing zone is evident with scleral lens compression. Conjunctival hyperemia adjacent to the compression may be visualized. After the scleral lens is removed, rebound hyperemia may occur in the area of compression or blanching. To eliminate compression or blanching, flatten the periphery; incorporate toric peripheries if the compression or impingement is in a specific meridian.16,18 Toric peripheries can also increase tear exchange. Quadrant-specific lens designs can further improve alignment.

AIR BUBBLE FORMATION

One of the major issues in fitting scleral lenses is when an air bubble is trapped beneath the lens in the post-lens fluid reservoir (Figure 2). If a bubble is present, it may negatively interfere with vision, cause discomfort, and lead to corneal desiccation. If the bubble is large, it can result in lens disruption.

Figure 2. Inferior bubble in the post-lens fluid reservoir.

If this phenomenon occurs only occasionally, it may be due to an improper lens application technique or to use of an aerated solution to fill the bowl of the lens.

If it happens frequently after lens application, then the cause may be a poor fit. Important factors that can lead to bubble formation include excessive corneal clearance associated with a small landing zone and at least one area in the lens periphery that is considerably lifted away from sclera.

One recommendation is to reduce corneal clearance and to incorporate a larger landing zone and/or a toric landing zone. Likewise, the use of a more viscous preservative-free solution in the bowl of the lens prior to application may reduce the incidence of air bubbles. Be cautious with this option because an excessive amount of more viscous solution may influence visual acuity and oxygen diffusion,22 and it could possibly induce toxic reactions.11

Bubbles may also appear when fitting fenestrated scleral lenses. Fenestrations were thought to facilitate some tear exchange to provide more oxygen delivery to the ocular surface without compromising physiology. Currently, with the availability of high-Dk materials, fenestrations are not required. However, fenestrated lenses may still be used in some cases; the small hole that permits the circulation of fresh oxygenated tears becomes an entrance channel for air bubbles. Choosing a sclera lens without fenestration may prevent this “complication.”

In the presence of a toric or irregular anterior scleral surface, air bubbles ensue because of poor alignment with the sclera. The lens may lift away from the sclera in one or more peripheral areas, resulting in an insufficient seal that facilitates the entry of air bubbles. Toric scleral lenses may avoid the formation of bubbles in such cases.16 Ordering a lens with an asymmetrical back surface to steepen the lens and reduce excessive tear exchange may be beneficial. Choosing a smaller lens may be a good option as well.

MIDDAY FOGGING

The debris inflow into the post-lens fluid reservoir, commonly referred to as “midday fogging” (MDF) (Figure 3), is a multifactorial phenomenon that occurs in 33% of scleral lens wearers who complain of hazy or foggy vision.23 This happens exclusively when fitting scleral lenses and requires patients to remove, clean, and refill the lens with fresh preservative-free saline solution. Causes of MDF may include a predisposition to dry eye and significantly greater central corneal clearance combined with lens edge tightness.23,24

Figure 3. Debris in the reservoir. The images show a turbid liquid layer behind the lens and the presence of small white mucous debris.

If MDF is present, it is pertinent to address and manage meibomian gland dysfunction, blepharitis, and Demodex. Reducing both the central and limbal clearance may help alleviate MDF because it narrows the channel of debris migration.

Another approach that often is effective is to change the filling solution to a preservative-free, non-buffered saline. If the presence of debris persists, a more viscous solution may be utilized in addition to the preservative-free, non-buffered saline.

When using a solution with increased viscosity, the amount may be varied to reduce MDF while maintaining good acuity. Some patients vary the amount depending on their symptoms that day.

The mechanical impact of the lens on the conjunctiva associated with fluid forces may be responsible for MDF.25 Therefore, lowering the mechanical impact on the conjunctiva is a key issue to alleviate debris migration into the reservoir. A toric landing zone may improve this fitting relationship, minimizing tear exchange (Figure 4) and conjunctival stress. A study demonstrated that patients who were switched from spherical back-surface to toric back-surface designs needed fewer breaks to remove, clean, and change the filling solution of the lens compared to those wearing spherical back-surface designs.26

Figure 4. Image of fluorescein entering the fluid reservoir in a toric pattern.

BETTER VISUAL QUALITY AND OPTICAL CORRECTION

In the last few years, scleral lenses have evolved to have a more diverse spectrum of indications; but one primary indication is visual improvement, especially in irregular corneas. The liquid reservoir between the cornea and scleral lens improves both regular and irregular corneal astigmatism and corrects about 90% of higher-order aberrations (HOAs), thus restoring vision.27

However, residual astigmatism can manifest. This is often the result of lens flexure, which can be managed with a toric landing zone, improving visual quality. Also, residual astigmatism can originate in the crystalline lens, which cannot be corrected by the liquid reservoir. When the amount of the residual astigmatism significantly reduces visual acuity, eyeglasses over scleral lenses may be used. Scleral lenses with front-surface toricity may be an option as well. When choosing front-surface toricity, it is necessary to stabilize the lens on the eye for proper axis orientation by using prism ballast or a toric landing zone.

Stabilizing the lens with a toric landing zone is not only useful for avoiding lens flexure, but also as previously mentioned for avoiding lens decentration. Decentered lenses dislocate the lens optics, causing a prismatic effect (Figure 5). Back-toric designs improve centration and stability, allowing superior visual quality.26

Figure 5. Prismatic fluid layer produced by a decentered scleral lens. Vertical decentration may induce a significant prismatic effect, which is a cause of residual astigmatism.

Corneal ectasia, including keratoconus, is one of the main indications for scleral lenses.28 The highest optical consequence of corneal steepening and thinning in keratoconus is the presence of a large degree of HOAs, especially in patients who have large pupils. Studies using advanced technologies have quantified the measurements of HOAs in patients who have keratoconus, reporting that this is approximately five to six times that of normal eyes, deteriorating retinal image quality.29,30 Sclerals with wavefront-guided optics in advanced keratoconus patients demonstrated a substantial reduction of HOAs.31 This optical correction provided a considerable benefit in visual acuity and contrast sensitivity.31 To accomplish HOA reduction, the lens has to be stable and align to the center of the visual axis, especially in patients who have abnormal corneal surfaces.32-33 Incorporating back-surface toricity and quadrant-specific adjustments provides alignment of the haptic to the scleral shape and minimizes the dynamic movement of the lens.

MORE COMFORT, WEARING TIME, AND OVERALL SATISFACTION

End-of-day discomfort is common in some scleral lens wearers. The reasons for this discomfort may include corneal bearing, limbal bearing, toxic reactions to preservatives in solutions, debris in the liquid reservoir, and/or improper alignment on an asymmetric sclera.11

An ill-fitting landing zone may cause awareness on the conjunctiva. When wearing spherical scleral lenses, patients may squeeze their eyes and point to a precise area that is often the quadrant in which the lens does not align properly on the sclera.34 One study indicated that contact lens wearers feel more comfortable with a lens decentering temporally and less comfortable with a lens decentering inferiorly.35 Another study found that the temporal quadrant area of the sclera was the smoothest and flattest; the second highest angle value was the inferior quadrant after the nasal quadrant.4 This may partially clarify the difference in comfort when a lens decenters in a different quadrant.

A good fitting relationship and alignment to more equally distribute pressure on the sclera may relieve discomfort symptoms. In a study of 27 eyes, patients reported increased comfort and wearing time after being refit with a back-toric design from a spherical lens design.17

Another study of 99 eyes that were switched from back-surface spherical to back-surface toric sclerals demonstrated that patients had a preference for back-surface toric scleral designs; these patients reported increased comfort, visual quality, and overall satisfaction.26

CONCLUSION

Fitting scleral lenses may be challenging when challenges occur, especially in cases with asymmetrical scleras and conjunctival anomalies. Understanding scleral shape, the etiology of the adversities, and the mechanisms for fitting scleral lenses clarifies the technique to manage challenges by adjusting and modifying the lens periphery. Finally, it is evident that the landing zone design is crucial for an optimal fitting relationship between the haptic zone and the sclera. Toric landing zones offer many advantages, solving several diverse scleral lens complications. CLS

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