The Importance of Scleral Shape
Scleral mapping technology and current knowledge of scleral shape affect modern lens design.
By Sheila D. Morrison, OD, MS
Innovations in contact lens lathing technology and the availability of GP contact lens materials have greatly improved the performance of therapeutic scleral lenses. Despite these incredible gains, the optimization of contact lens design is likely still in its infancy. A new understanding of the importance of scleral shape has emerged, which is imperative to successful fitting of specialty contact lenses.
The story of scleral lenses has roots dating back to the 19th century, when they were the first invented contact lens modality originally created from glass blown shells (Wollensak, 2004; Pearson, 2007). Similarly, keratometry has been used since the late 1800s (Figure 1) to evaluate the shape of the central cornea. As such, it is the earliest method for gaining some form of topographical data of the eye (Kojima et al, 2013).
Figure 1. Image of a 19th century keratometer taken from The History of Ophthalmology by Julius Hirschberg (1899).
Beyond The Keratometer: Corneal Topography
Although the keratometer is capable of providing some information on corneal shape using a single placido ring system, it is unable to measure beyond the central 3mm (Kojima et al, 2013).
Measurement of the entire corneal surface is an important part of clinical diagnosis and management of irregular corneas. Since its introduction, videokeratography (corneal topography) has become the gold standard for measuring the front surface of the eye (Kojima et al, 2013; DeNaeyer and Eiden, 2012).
Corneal topography has historically been classified into two categories: 1) Reflection-based corneal topography is most widely used and captures data using a placido-ring imaging system; and 2) Projection-based corneal topography measures both the anterior and posterior cornea using either a rotating slit scan imaging system or Scheimpflug imaging (Shibayam, 2016).
Several recent clinically documented studies at Pacific University have shown that it may be possible to utilize new software attributes to extrapolate out and predict a starting sagittal height of a scleral lens based on corneal topography data. However, it is commonly understood that there are inherent limitations of corneal topography in measuring chords greater than 10mm and in calculating the sagittal depth of the anterior segment for fitting larger-diameter lenses beyond the cornea (DeNaeyer and Eiden, 2012).
Today, with so many modern lens options available, determining the initial lens design (i.e., soft, corneal GP, or scleral) often depends on topographical data of the cornea. Some corneal topographers utilize a reference sphere to produce an elevation map of the cornea and predict potential areas of clearance or bearing for corneal GP lenses (DeNaeyer and Eiden, 2012). Corneal GP lenses (including orthokeratology) can be empirically created using state-of-the-art lens design software to custom-build lenses based on individual curves and elevations measured by corneal topography (Figure 2).
Figure 2. Process of empirical corneal GP lens design: corneal mapping (left) to empirical lens design using contact lens software (middle) to the predicted lens on the in vivo eye (right).
Logic would propose that scleral mapping could be a valuable asset in the initial lens selection, troubleshooting, and empirical lens design of any rigid or soft contact lens that lands on the sclera—and we do see this to be the case through clinical and academic research at multiple centers around the world in the area of scleral shape and optimal lens design.
Scleral Mapping Tools
Scleral mapping tools make it possible to measure the topography of the eye beyond the limbus, where the majority of lenses (greater than 12mm) land. Knowledge about the shape of the eye beyond the cornea to predict success with different lens modalities and designs is possible using the following instruments.
Anterior segment optical coherence tomography (OCT) achieves views of the anterior segment in 360° cross sections and uses calipers to measure distances, depths, and angles across the anterior eye (van der Worp et al, 2010). With the increasing use of both rigid and soft lenses that land beyond the limbus, practitioners and researchers are beginning to understand the importance of the optimal fitting relationship between contact lenses and the central cornea, peripheral cornea, limbus, and sclera. Anterior segment OCT not only measures the sagittal height of the anterior eye, but also images a lens on the eye and quantifies the space between the posterior contact lens surface and the anterior cornea for diagnostic purposes.
Scheimpflug imaging is a form of projection-based topography and can be used to map both the anterior and posterior surface of the cornea and lens, allowing for a three-dimensional reconstruction of the anterior chamber (Belin and Ambrosio, 2013).
Profilometry (Figure 3) scleral lens mapping techniques require the application of sodium fluorescein to the anterior eye and utilize a reflection-based system with projected pattern sequences to map the corneal and scleral surfaces. With lid retraction or eye movement in different gaze positions, profilometry techniques are capable of imaging beyond a 20mm scleral chord (Kinoshita et al, 2016).
Figure 3. Data acquisition images with novel profilometry scleral lens mapping instruments.
Similar to the determination of initial lens parameters for corneal GP lenses as imaged in Figure 2, some scleral topographers have incorporated sophisticated software that is capable of predicting the sagittal height and fit of scleral lenses (Figure 4). Clinically and through empirical scleral lens fit studies, this has been effective at predicting well-fitting lenses, even in the absence of placing a diagnostic lens on the eye (Morrison et al, 2016). This technology could also potentially be used to design custom soft lenses.
Figure 4. Process of empirical scleral fitting: scleral mapping with lens design software display (top) to empirically placing a digital scleral lens with fluorescein beneath it (bottom left) to the predicted lens on the eye (bottom right).
Recently, there has been a shift in the nomenclature used to describe corneal astigmatism as not only a dioptric power, but also as a micron height differential between steep and flat meridians (van der Worp et al, 2010). The description of sagittal depth in microns is also utilized in scleral mapping. This is especially important because, unlike corneal topography, there is no globally accepted standard to convert a micron height differential to diopters in the sclera.
The use of terminology such as “toricity” (the difference in height between the two principle meridians) versus “asymmetry” (the difference in height between the eight different principle meridians) to describe topographical data of the sclera is debatable. Presently, the trend appears to be the predominant use of the word “asymmetry.” Evolution of scleral mapping technology and further research in the topic should aim to clarify this for the industry and for practitioners.
Identification of Norms and Demographic Scleral Shape Data
In the literature, there are several decades of publications documenting normative and demographic data related to corneal topography (Bogan et al, 1990; Riley et al, 2001; Goto et al, 2001; Rabinowitz et al, 1996). Databases are currently in progress to begin to characterize scleral shape of “normal” eyes. Reports of the presence or absence of scleral shape patterns in patients who have pathologic corneas could also be of future interest. In addition to race, gender, and age norms, there is a need to understand the relationship between corneal and scleral shape.
Scleral Shape and Mapping Technology…Why Do We Care?
The reality is that while our current technology seems to allow practitioners to successfully improve vision and ocular health in most patients who have an indication for some form of contact lens, there are pressing issues that demand our attention.
Does the amount and orientation of corneal toricity extend across the limbus onto the sclera? The first study to investigate whether scleral shape and corneal toricity have a relationship was performed at Pacific University College of Optometry by Kinoshita and colleagues (2016). The study compared sagittal height measurements of the cornea taken with placido-based corneal topography (Medmont E300 Corneal Topographer, Medmont International Pty Ltd) and of the sclera taken with profilometry-based scleral topography (sMap3D, Visionary Optics).
In the study, 10% of subjects who had low (<0.75D) with-the-rule (WTR) astigmatism maintained a WTR orientation of the sclera at a 15mm chord, whereas 0% of the subjects maintained that orientation at a 17mm chord. Thirty percent of subjects who had high (>0.75D) WTR astigmatism maintained a WTR orientation of the sclera at both 15mm and 17mm chords.
The average amount of height differential between the principle meridians increased from the cornea to the sclera, meaning that it would appear that scleral asymmetry starts at the more symmetrical limbus and increases in asymmetry further into the peripheral sclera. The orientation of the steep and flat meridians was variable at 15mm and 17mm chords for both high and low astigmatism corneas. In conclusion, the study confirmed that scleral shape is highly asymmetric and that the asymmetry is not often of the same magnitude as the corneal toricity (van der Worp et al, 2010).
As practitioners, could we achieve more efficient and appropriate scleral lens fits? I believe the answer to this is a resounding yes. Scleral lens fitting, regardless of design, is based on the sagittal height of the anterior chamber (Figure 5), and quadrant-specific lens modifications are now possible. Through scleral mapping technology, we have the ability to determine the toricity and asymmetry in all eight quadrants of the eye.
Figure 5. Image of varying sagittal height options available in the same scleral lens design, to be selected based on the depth of the anterior chamber as measured by OCT.
Is empirical scleral lens design/ordering possible? Is there a need for eliminating fitting sets? In clinical practice, many irregular corneal conditions require the physical placement of a diagnostic lens to achieve the appropriate base curve and power of the rigid lens, especially in eyes for which reliable keratometry or refraction is not possible due high irregularity. However, there are countries and government institutions that limit or prohibit the use of diagnostic fitting sets, thereby necessitating a more reliable way to predict not only the initial fit success but also power of scleral lenses (Morrison et al, 2016).
As we mature in our understanding of scleral shape and continue to improve scleral mapping technology, it seems to be theoretically possible to empirically design and order scleral lenses. It has been proposed that a single platform instrument could potentially be used to collect accurate topographical data of both the cornea and sclera and also to measure the axial length of the eye. This data could then be combined in a model to empirically predict a reasonable starting lens—in some cases, an excellent starting lens (Morrison et al, 2016).
It is now considered “accepted knowledge” that scleral elevation impacts successful contact lens fitting. Can we continue to alleviate scleral lens fogging and end-of-day discomfort by optimizing lens design based on individual scleral shapes? In the highly referenced study by Visser et al (2006), not only was lens stabilization documented with back-surface toric scleral lenses, but improved comfort and wearing time in patients who were fit with this design were also reported. Nearly a decade later, we are finally starting to understand this; with recent advances in scleral mapping technology, our objective findings related to scleral shape parallel these earlier clinical findings.
It has been reported that the sclera has an approximate differential of 120 microns between the steep and flat primary meridians. The industry has responded, and a variety of scleral lens designs are now available in standard back-toric designs (Woo and Messer, 2016).
End-of-day discomfort is prevalent in some scleral lens wearers. It could be postulated that an etiology for this discomfort is unequal lens bearing on the asymmetrical sclera; therefore, the increased use of back-toric scleral lenses could help to more equally distribute pressure where the lens lands. Some reports indicate that changing patients from a spherical to a toric back surface may also decrease fogging (Visser et al, 2006).
Should all scleral lenses have a back-toric design? Current research suggests that the answer to that question is no (Kojima et al, 2013; Woo and Messer, 2016). The limbus is generally more symmetrical relative to the cornea and sclera, and this is the area on the eye where smaller-diameter scleral lenses (<15.0mm) land. Unlike the general trend of creating toric back surfaces on standard large-diameter scleral lenses, We have found that mini-sclerals are usually more successful with spherical designs.
Additionally, although the majority of patients have some degree of scleral asymmetry, there are exceptions. Some patients do have a spherical scleral shape, and some patients have one eye with a high degree of asymmetry while the fellow eye has a spherical shape (Figure 6), thus highlighting the need for comprehensive scleral mapping to most efficiently determine the best lens design.
Figure 6. Topographical data showing significant scleral asymmetry in the right eye and minimal asymmetry in the left eye.
Can we use scleral shape to our advantage to manage bumps, lesions, shunts, and difficult-to-correct refractive error? Today, while scleral lenses are being used on both regular and irregular corneas, the primary indication for scleral lenses continues to be conditions that prohibit the use of traditional soft or corneal GP lenses due to a highly irregular corneal shape or ocular surface defect.
A scleral lens can vault the irregular corneal surface, with the full weight of the lens resting on the sclera. Fitting specialty contact lenses on irregular corneas is further complicated by abnormalities on the sclera and bulbar conjunctiva (Morrison et al, 2015). Rotational stability in scleral lenses can be achieved without prism ballast by using a back-surface toric rigid lens to fit an asymmetrical sclera (Figure 7). This technique can also be used to place microvaults, utilizing front-surface toric optics similar to soft contact lenses, which also tend to consistently land in the place of least resistance (Woo and Messer, 2016).
Figure 7. Notching made possible by achieving rotational stability, using a toric-back haptic to act as a “lock-and-key” on the asymmetrical sclera of a patient who has advanced keratoconus and a raised pinguecula.
Can knowledge of scleral shape be applied to lens designs other than scleral lenses? What is very important to understand is that scleral shape affects all lens designs that interact with the corneo-scleral junction and beyond. Soft lenses decenter temporally on the eye due to differences in the elevation of the nasal and temporal sclera: the nasal sclera is higher/flatter, and the temporal sclera is lower/steeper (Figure 8).
Figure 8. Anterior segment photography (top) showing typical temporal decentration of a traditional soft lens due to scleral anatomy (bottom).
In contrast, the anatomical location of the pupil is slightly nasal. This creates the perfect storm with regard to delivering the multifocal optics required to manage presbyopia and myopia. Clinical findings of blurry and poor quality vision result from a disconnect between a temporally decentered soft lens and the nasally located pupil.
Multifocal soft lens fits can be challenging in some patients due to fluctuating visual quality. Inevitably, the intended optics for myopia control may not be optimized, and peripheral defocus not fully delivered, for myopia control. Ongoing studies at Pacific University College of Optometry continue to explore the benefit of decentered optics in custom soft multifocal lens design (Zheng et al, 2016).
At the forefront of research today is an emphasis on scleral lens design, myopia control, and the placement of multifocal optics. All of these have incredible potential to evolve by pushing our knowledge of scleral shape forward and utilizing this knowledge to design better specialty contact lenses. CLS
For references, please visit www.clspectrum.com/references and click on document #247.
All images used with permission from the Pacific University Contact Lens Team.
Special thanks to: Patrick Caroline, FAAO; Randy Kojima, FAAO, FBCLA, FSLS, FIAO; Beth Kinoshita, OD, FAAO; Matt Lampa, OD, FAAO; Mark André, FAAO; Eef van der Worp, BOptom, PhD, FAAO; Markus Ritzmann, MS; Greg DeNaeyer, OD, FAAO; Maria Walker, OD, MS; Shane McDonald, OD; and Frank Zheng, OD.
Dr. Morrison is the Cornea & Contact Lens resident at Pacific University College of Optometry. Her main areas of research interest are contact lens design, scleral shape, and contact lens solutions.