Scleral contact lenses can offer patients who have compromised corneas or ocular surface disease the ability to see as if they were free from visual disability. Unlike soft contact lenses, scleral lenses vault over the cornea, rest on a fluid-filled reservoir above the cornea, and land on the sclera. The fluid reservoir masks corneal irregularities, providing improved optics. The fluid between the posterior surface of the lens and the front surface of the cornea is often quantified in micrometers (or microns) (µm) using terms such as tear reservoir height, corneal vault height, or apical clearance.

In addition, individuals who have high levels of ametropia or corneal astigmatism can benefit from the vision stability offered by a well-designed scleral lens. Each curvature of the scleral lens must be carefully determined to ensure comfort, quality vision, and optimal health. When fitting a patient’s scleral lens by using diagnostic lenses, prescribers must keep in mind that the scleral lens relationship with the cornea and conjunctiva will change throughout the day due to lens settling.

Lens settling refers to a process in which the landing zone of a scleral lens sinks into the compressible underlying conjunctiva, which results in a flatter and tighter-fitting lens. After just 60 minutes, the assessment of a seemingly aligned scleral landing zone and an optimal corneal vault height can change to a less-than-ideal fit.

It is important to understand what factors influence the total amount of settling and its subsequent effects on vision and comfort. Knowing how to optimally design a lens to prevent complications will result in less chair time, decreased lens remakes, and a more enjoyable overall experience for patients.

DETERMINING INITIAL TEAR RESERVOIR HEIGHT

The landing curves of a scleral contact lens carry the weight of the lens, which results in the initial conjunctival compression. This is further compounded by the downward pressure from the eyelids.^1,2 Consequently, the soft and compressible conjunctiva and Tenon’s capsule beneath the landing curves will slowly yield to the weight of the scleral lens, resulting in a decreased distance between the cornea and the back surface of the lens; this is a phenomenon referred to as lens settling. The settling process begins immediately after application and can continue for eight or more hours.³

To account for lens settling, recommendations for initial vault height vary between 100µm to 400µm,^3,4 depending on the specific lens design, diameter, and cornea type. With this range, under- or overestimating the extent of settling is likely. To further complicate the process, designing a scleral lens that has an excessive or insufficient initial vault height can result in compromised vision and damage to the cornea and conjunctiva.

There are several methods to observe the amount of tear clearance beneath the lens. One method is to decrease the width of a slit lamp beam to an optic section and compare the known lens thickness to the tear height. A more accurate assessment can be made with anterior segment optical coherence tomography (AS-OCT) (Figure 1).^3,5 OCT also provides the benefit of directly viewing the extent of conjunctival compression beneath the landing zones of the lens (Figure 2). In addition, those who have OCT angiography (OCT-A) can assess for compression of the conjunctival microvasculature.⁵

SHORT-TERM SETTLING

A large percentage of lens settling occurs in the first one-to-two hours of wear. Kauffman et al determined that 70% of total lens settling occurred within two hours.⁶ Similarly, Nau et al noted that tear clearance decreased by nearly 50% after two hours of wear.⁷

Even during brief observation times of one-to-two hours, there are large variances in lens settling among studies. Fortunately, there are certain characteristics of a lens fit that can help forecast the total extent of settling after six or more hours of wear, including initial tear clearance and scleral lens diameter.

CORNEAL VAULT

In general, lenses with lower apical clearance heights tend to settle less compared to those with higher apical clearance heights.^2,3 Nau et al observed lens settling for a one-hour period and found that lenses fit with less initial corneal vault settled less compared to those with higher corneal vaults (Table 1). This pattern of lens settling is also associated with long-term wear.⁷ Esen et al observed lens settling patterns in patients who had keratoconus over an eight-hour period and found that the initial corneal vault height and the total extent of settling are inversely related.³ Interestingly, although the total amount of settling varies based on initial corneal vault heights, the rate of settling may be consistent among various vault heights.²

OVERALL LENS DIAMETER

In addition to vault, the overall diameter of a scleral lens can determine the extent of settling. Lenses with larger diameters tend to settle less over time than do those with smaller diameters. This likely results from larger-diameter lenses having a wider scleral landing zone compared to the narrower landing area of smaller-diameter scleral lenses. The wider landing zones will distribute the weight of the scleral lens over a larger area of the conjunctiva. This increased distribution results in less force on the underlying bulbar conjunctiva and Tenon’s capsule and, therefore, in less tissue compression.^1,3,4,6

A great visual provided by one of my mentors, Julie DeKinder, OD, is to imagine the landing curve of a scleral lens as a snowshoe versus a high-heeled shoe in the snow. The high-heeled shoe would penetrate deep into the snow; however, with a snow shoe, the weight distribution of the larger area of contact would create less of an impact in the snow.

Larger-diameter lenses also land further from the limbal tissue. It is possible that the bulbar conjunctiva is less compressible as it transitions further from the limbus. Unfortunately, various designs of scleral lens radius of curvatures complicate genericity (Table 2).

OTHER THEORIES ON SETTLING

Age is thought to be a contributing factor with regard to the compressibility of the bulbar conjunctiva and Tenon’s capsule and, thus, to the overall extent of settling into the conjunctiva. The older the individual, the more compressible the conjunctival tissue may be and, therefore, the more settling may occur.

Another theory relates to forces of suction from the scleral lens. Caroline et al postulated that scleral lenses could create a suctional force that results in a gradual pull of the cornea toward the lens during wear.¹¹

Other theories of various lens settling patterns include differences in eyelid force throughout the day, lens flexure, scleral lens thickness, and saline dissipation from beneath the lens;² however, the overall viscosity of the filling solution used in scleral lenses before application was found to have no effect on the overall amount of settling.¹⁰

WHY APPROPRIATE CORNEAL VAULT IS IMPORTANT

With scleral lenses, the mixing of the fluid beneath the lens with the tears on the ocular surface is minimal when compared to that with corneal GP lenses; therefore, oxygen supply to the cornea is a potential concern.⁸ The overall oxygen permeability (Dk) of the lens material, the thickness of the lens, and the tear reservoir height are all important variables that determine oxygen permeability to the cornea.

Michaud et al investigated the potential oxygen supply that is theoretically available to the cornea in an open-eye environment after traveling through a scleral lens and tear reservoir.¹² The oxygen transport model considered various lens thicknesses and tear clearance heights, with different material permeability. The results were then compared to the Holden and Mertz criterion and Harvitt-Bonanno criteria to determine whether oxygen supply was sufficient to avoid corneal hypoxia. The data suggested that the post-lens tear film should not exceed 200µm, lenses should not have a thickness greater than 250µm, and the highest-Dk material should be used to avoid potential hypoxia and resultant corneal swelling.¹²

Esen et al tested those theoretical models in the clinical setting. In the study, keratoconus patients who did not have endothelial compromise wore lenses of various apical clearances: 100µm-to-200µm (low), 200µm-to-300µm (medium), and > 300µm of clearance (high).³ According to the theoretical models by Michaud et al, the subjects in the medium and high groups should experience hypoxia-related complications; however, there was not a clinically significant increase in corneal swelling compared to the groups of lesser tear clearances. The data from this study were limited to eight hours, with data collected at two-hour intervals. Thus, as most patients in scleral lenses wear lenses for longer than an eight-hour period and for multiple years, the authors concluded that the best practice for prescribing scleral lenses should include prescribing the highest-Dk material possible as well as minimizing the scleral lens thickness and post-lens tear reservoir.³

Similarly, Tan et al conducted a study on healthy corneas that had tear clearances ranging from 200µm to 400µm. They noted that the scleral lenses did induce corneal edema of, at most, 1.65%, which is clinically insignificant. For reference, after overnight eye closure, physiological corneal swelling is about 3.6%.⁸ In conclusion, there is no statistically significant correlation of corneal edema and tear clearance heights ranging from 200µm to 400µm in patients who have normal corneas.

Excessive tear clearance can result in compromised vision secondary to the increase in lower-order aberrations with higher vaults.¹³ Also resulting in decreased vision is the increased likelihood of debris accumulation in the tear reservoir, typically caused by the excessive negative pressure beneath the lens.^3,6 Excessive vaults can also result in inferior lens decentration and in tight-lens syndrome after extended wear. Tight-lens syndrome can lead to discomfort and hyperemia. Furthermore, prolonged wear of lenses that have an excessive tear reservoir could, in theory, lead to corneal neovascularization, limbal stem cell deficiency, and other hypoxia-induced complications secondary to decreased oxygen transmission through a thick tear reservoir and a not-uncommon tight peripheral alignment. This may especially occur in corneas that have endothelial cell compromise.^3,10,12

If the initial tear clearance is too low, settling may result in bearing on the cornea or limbus. Bearing can lead to complications such as epithelial disruption, corneal erosion, discomfort, redness, and other mechanical-related damage.^3,10

IS IT POSSIBLE TO ACCURATELY DETERMINE A DESIRED TEAR CLEARANCE HEIGHT PRE-SETTLING?

Although there are general rules that apply with regard to the extent of lens settling, such as accounting for lens diameter and initial vault height, various studies (such as those listed in Table 2) show that there is extensive variability among patients—and even in the same patient—fit into a different scleral lens design. To predict the extent of settling for a particular patient, authors have suggested observing diagnostic lenses after a certain period of time. Esen et al suggested that four hours of observation could give eyecare professionals (ECPs) an accurate representation of apical clearance post-settling.³ As this suggestion may not apply for those in clinical settings, it was recommended to add an additional 100µm (or 75µm for larger-diameter lenses) to the desired apical clearance post-settling.³ Specifically, for an 18mm lens, Michaud et al concluded that the total amount of lens settling can be estimated by doubling the amount that settled after 30 minutes of wear.¹²

CAN SETTLING CREATE AN OVER-REFRACTION?

Prescribers must consider how the extent of settling may alter the overall prescription of the lens system. Unlike with corneal GP lenses, it is difficult to empirically estimate the required prescription of a scleral lens. There are several factors that complicate calculating the power of the lens, including the base curve of the lens, the depth of the tear reservoir, the keratometry values, the relationship between the keratometry values and the back surface of the lens, etc.⁹ The largest difference—owing to the complexity of scleral lenses versus GP corneal lenses—is the thickness of the tear reservoir beneath the lens.

In theory, as the lens settles, the overall refractive power of the system should change. According to Munnerlyn’s formula, a 70µm decrease should result in an over-refraction of approximately +0.25D;¹⁰ however, in a study conducted by Bray et al, an average amount of settling of 83µm resulted in no change in over-refraction in subjects who had normal corneas after six-to-eight hours of wear.⁹ The authors concluded that, although the change in light vergence of a decreased tear film over a normal eye did not result in a refractive shift, there is still a need to perform an over-refraction in patients who have irregular corneas.⁹

SUMMARY

In conclusion, when prescribing scleral lenses, settling is an important consideration for maximizing corneal health and vision. Unfortunately, the settling patterns vary significantly among lens designs, diameters, vault height, and patients’ corneal or conjunctival characteristics.

To efficiently prescribe scleral lenses in a clinical setting, specific characteristics of the lens, such as initial vault height and diameter, are useful predictors of settling patterns. Practitioners will benefit from applying these fitting characteristics to diagnostic lenses after 20-to-30 minutes of settling time, potentially resulting in less patient chair time and fewer lens remakes. CLS

REFERENCES

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3. Esen F, Toker E. Influence of Apical Clearance on Mini-Scleral Lens Settling, Clinical Performance, and Corneal Thickness Changes. Eye Contact Lens. 2017 Jul;43:230-235.
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12. Michaud L, van der Worp E, Brazeau D, Warde R, Giasson CJ. Predicting estimates of oxygen transmissibility for scleral lenses. Cont Lens Anterior Eye. 2012 Dec;35:266-271.
13. Otchere H, Jones L, Sorbara L. The Impact of Scleral Contact Lens Vault on Visual Acuity and Comfort. Eye Contact Lens. 2018 Nov;44 Suppl 2:S54-S59.