Modern GP scleral lenses are specialty medical devices designed to aid vision for patients with corneal irregularity, ocular surface disease, high refractive error, and presbyopia.1 An increasing number of practitioners are using scleral lenses to treat relatively normal eyes or to help patients cope with exposure to dust or other irritants.
Every eye is different, and scleral lens designs may vary dramatically to accommodate individual ocular shape, size, and depth characteristics (Figure 1). While there are multiple paths to fitting GP scleral lenses, all are intended to reach the same endpoint: an appropriately fitted lens that provides the best visual outcome.
Scleral lens fitting strategies may be based on either trial and error or measurements of the sclera and overlying conjunctiva (Figure 2).
Trial and error typically involves the use of a diagnostic lens or lens fitting set. During this process, a number of different lenses are applied to the ocular surface to determine the best-fitting lens.
To measure the shape of the ocular surface before designing or placing a scleral lens, a practitioner starts with a known diagnostic lens and evaluates the interaction of the lens with the ocular surface beneath the landing zone. Fluorescein applied externally or beneath the lens may help determine the ocular shape (Figure 3).
Custom molding and scleral topography measure the ocular shape in the absence of a lens, allowing for GP scleral lens fitting without compression of the conjunctiva or other elevations. Scleral topography can be helpful for troubleshooting problematic fits and gaining insight into the structural differences between the left and right eyes. It is also used to identify any disparity in the fitting relationship between the surface of the eye and the lens.
Understanding scleral/conjunctival shape is essential to optimize the GP scleral lens fitting process, as modern lens design affords multi-complex curves. The curves of the ocular surface are asymmetrical, and corneo-scleral junctions vary individually.2
Specific demographic differences exist, but exact classifications are yet to be fully elucidated by peer-reviewed research. The best way to gain an understanding of these individualized curves is to measure them.
Custom molding and scleral topography enter the realm of virtual lens design, which theoretically eliminates the guesswork from fitting scleral lenses by using computer technology to create lenses that are custom designed to each individual eye shape. Currently, this technology is being applied to manufacture GP scleral lenses, but it may have utility for custom soft lenses in the future.
Lens Geometry and Scleral Shape
The breadth of varying lens designs can be overwhelming, because various designs and manufacturers use unique characteristics and terminology. Nonetheless, all scleral lenses have several zones in common (Figure 4): an optic zone, a midperipheral zone, an intermediate zone, and a landing haptic zone.3 The landing/haptic zone of a scleral lens makes the most contact with the anterior eye and, therefore, is the most pertinent to understand in relation to the shape of the sclera and the overlying conjunctiva.
The conjunctiva overlies the sclera; therefore, any measurement of the sclera by ocular surface mapping tools is inherently a measurement of the conjunctival shape. Because the more rigid sclera provides the underlying shape, scleral/conjunctival shape is commonly referred to simply as scleral shape. The bulbar conjunctiva is continuous with the limbus and the cornea.2
Ocular Surface Anatomy
Understanding the surface on which we place a lens is imperative to effectively fit any soft or GP scleral contact lens. Each ocular surface is unique. Partially because of a lack of demographic databases to characterize corneo-scleral shape, variations significant to the successful fitting of soft and GP scleral lenses can be unpredictable for practitioners. A consequence of this lack of predictability is that scleral lens fitting may take more time and be more costly if the correct initial lens is not selected.
When fitting scleral lenses, a basic review of ocular surface anatomy is warranted and is a foundation upon which our research community could continue to expand our understanding of scleral shape.
Two primary outcomes depend on scleral shape and are the most significant predictors of a successful GP scleral lens fit: habitual position of the lens on the eye (centration) and the fitting relationship between lens and eye (landing).
Characteristics That Affect Lens Positioning
Differences in shape and elevation of the corneo-scleral junctions and other structures have been widely accepted to explain the etiology of lens decentration and positioning on the eye. When a lens is applied to the ocular surface, the laws of physics usually cause it to consistently come to rest at the place of least resistance. This is an important concept to understand, because it explains how to use scleral shape to achieve a lock-and-key fit and gain rotational stability of a GP scleral lens. Rotational stability is necessary to place a toric, prismatic, or wavefront-guided optic in the lens or to strategically place a notch or microvault.
Soft and GP scleral lenses commonly decenter temporally, owing to a more elevated (flatter) ocular surface and a less elevated (steeper) ocular surface (Figure 5A). This may be related to the structure of the extraocular eye muscle insertions (Figure 5B) where the medial rectus inserts closer to the limbus (heaping up closer to the limbus) than the lateral rectus.2,4
Gravity may also contribute to the slight inferior lens decentration we often see clinically in patients wearing scleral lenses. Major inferior decentration may not be tolerated in some patients, as it can result in distorted vision and lens awareness or discomfort.
One problematic implication of the decentration of any contact lens on the eye is that this may inadvertently cause failure of accurate placement of optics over the pupil. Specifically, lenses designed to deliver multifocal or wavefront-guided optics suffer from disconnect between the placement of the optic on the lens and where that optic habitually rests over the pupil.4 One possible solution for this is the intentional decentration of the optic in the equal and opposite direction to compensate for the physical decentration of the lens. This is another example of lens customization for which rotational stability must be present.
The sclera and overlying conjunctiva is usually an asymmetrical surface, which increases in asymmetry as chords extend farther away from the limbus. For the purpose of fitting GP scleral lenses, we can measure and classify the scleral/conjunctival shape as spherical, toric, or asymmetrical. Most eyes are asymmetrical, and the anterior shape of right and left eyes is usually different enough to require a unique lens fit for each eye.5,6
Smaller-diameter scleral lenses may require less toricity or asymmetry, whereas larger-diameter scleral lenses may require more height disparity between different meridians. Scleral mapping technology has demonstrated that, on average, most eyes would benefit from a toric haptic, which is consistent with findings by Visser and colleagues more than a decade ago.7 This is not always true, however, because of the variability in eyes. Therefore, the clinician must keep an open mind to all lens designs and treat each eye individually and objectively.
New Technology Optimizes Lens Design
Advances in imaging and measurement put more opportunities for customization in the hands of clinicians.
- Globe Impressions. This process involves making an impression of the globe (Figure 6) from which a digital model of the eye is created using a 3D scanner. A scleral lens is then designed with proprietary software based on ocular elevations measured by the impression mold.
- Advanced Computer Software, Curves, and Channels. Some scleral designs utilize computer software to achieve meridian-specific curves and customized channels to avoid any atypical elevations that would interfere with a successful GP scleral lens fit. Many other GP laboratories are beginning to manufacture quadrant-specific and custom lathe-cut GP scleral lenses. Several can use data directly from corneoscleral topographers to create sophisticated lenses.
- Anterior Segment OCT. Anterior segment OCT has been an incredibly important technological advantage for the evolution of scleral lens design and the optimization of scleral lens fitting. OCT provides views of the anterior segment in 360-degree cross-sections and uses calipers to measure distances, depths, and angles across the ocular surface.12 Early pioneering work outlining the use of OCT data to design scleral lenses was first published in 2008.13 Other studies have further investigated the anterior surface of the eye using OCT, with respect to the scleral shape of 8 meridians.14 With the advent of new corneo-scleral mapping tools that offer relatively simple data collection to map the ocular surface beyond the limbus, one mainstay use for the OCT in clinical practice is to quantify and evaluate lens clearance in microns.
- Scheimpflug Imaging. A form of projection-based topography, Scheimpflug imaging can be used to map both the anterior and posterior surfaces of the cornea and lens, allowing for a 3D reconstruction of the anterior chamber.15
- Profilometry. Profilometry scleral lens mapping techniques require the application of fluorescein to the ocular surface and the use of a reflection-based system with projected pattern sequences to map the corneal and scleral surfaces. With lid retraction or eye excursion, profilometry techniques are capable of imaging beyond a 20 mm scleral chord.16
Case #1: Early Diagnosis of Interocular Scleral Shape Differences
A 71-year-old patient with keratoconus reported for contact lens evaluation. He experienced discomfort with corneal GP lenses and was interested in being fitted with scleral lenses.
Scleral elevation maps from corneo-scleral topography revealed a spherical scleral shape for the right eye and against-the-rule scleral toricity of the left eye. A sphero-cylindrical over-refraction revealed significant residual astigmatism in both eyes. For the right eye, software from corneo-scleral topography designed a 16.0 mm scleral lens with a spherical back-surface haptic design that would vault the central cornea by a pre-settled amount of 300 microns. Double slab-off prism was added to rotationally stabilize a power of –3.50 –0.75 x 076. For the left eye, fitting software designed a 16.5 mm scleral lens with customized back-surface toric haptic that would vault the central cornea by a pre-settled amount of 300 microns. Back-surface toricity was used to rotationally stabilize a power of –3.75 –1.25 x 095.
At the dispensing visit, both lenses had adequate corneal vault and haptic alignment and were properly positioned for the front-surface toric correction. The patient was happy with the comfort, and he was able to pass his driver’s license examination.
This patient has significantly asymmetric scleral shapes between the right and left eyes. The right eye was appropriately fit with a spherical haptic back surface. The customized toric scleral lens for the left eye not only fit well but also stabilized the front-surface toric power.
Case #2: Quadrant-specific and Other Custom Lathe Designs
A 36-year-old patient with keratoconus was fitted for a scleral lens for his left eye. Corneoscleral topography revealed asymmetric scleral geometry. A 16.5-mm scleral lens with back-surface toricity was dispensed, but the patient reported midday fogging. Slit lamp examination showed debris accumulation that originated at the inferior temporal quadrant, correlating with the sclera’s steepest area.
Another lens was designed with a multi-meridian design, matching an asymmetric back surface haptic precisely to the eye’s scleral shape. The customized lens was comfortable to wear, and the patient no longer had reservoir debris accumulation.
Sagittal height data from measurement enables practitioners to design haptic surfaces that match the asymmetry of the sclera, which can improve comfort and performance.
Today, software is capable of generating lens-fitting simulations with virtual fluorescein patterns and providing a full spectrum of lens design options that could be demonstrated with the computer-generated simulations. A particularly helpful feature for novice or high-volume fitters is the option to send corneo-scleral topographical data directly to a laboratory and have scleral lenses designed empirically based on the data. This computer-generated empirical system mirrors that of GP corneal lenses, which are routinely ordered empirically from corneal topography data.
There is an element of practicality for clinicians when considering what works best in clinical practice to measure scleral shape. When considering investment in technology, factors to consider should be inclusive of (but not limited to) ease of data collection, time required to collect data, the ability to train technicians to collect data, reimbursement potential, safety, accuracy of the instruments used, and cost of the instruments versus reimbursement. Table 1 outlines the pros and cons of profilometry-based scleral mapping in clinical practice.
|• Avoid costly (time & money) mistakes||• Requires investment in equipment|
|• Good for troubleshooting||• Empirical design relies heavily on quality of measurement|
|• May result in final lens faster||• Somewhat invasive (requires application of fluorescein)|
|• Technology is impressive to patient||• Does not completely replace diagnostic lens fit (still often requires placement of GP lens to determine power)|
|• Ability to virtually design scleral lenses|
|• Billable ICD-10 Code|
|Table 1. Advantages and disadvantages of profilometry-based scleral mapping.|
Understanding the shape of the surface on which a scleral lens will be placed results in greater fitting success. The future of corneoscleral topography, custom molding, and virtual lens design is not limited to GP scleral lenses, as custom or conventional soft lens design may also have potential applications in the future.
Special thanks to the Scleral Lens Education Society, Jesus Martinez, OD, Langis Michaud, OD, Markus Ritzmann, Patrick Caroline, Randy Kojima, Beth Kinoshita, OD, Matt Lampa, OD, Mark Andre, Eef van der Worp, BOptom, PhD, Maria Walker, OD, Jan Bergmanson, OD, PhD, and Kimberly Thompson.
- van der Worp E. A Guide to Scleral Lens Fitting, Version 2.0. Forest Grove, OR: Pacific University; 2015. Available at http://commons.pacificu.edu/mono/10/
- Bergmanson JPG. Clinical Ocular Anatomy and Physiology. 24th edition, Houston, Texas Eye Research and Technology Center, 2016.
- Barnett M and Johns L. Contemporary Scleral Lenses: Theory and Application. In press.
- Zheng F, Caroline P, Lampa M, Kojima R, Kinoshita B, Andre M, Morrison, S. Visual effects of centered and decentered multifocal optics. Poster presented during the Global Specialty Lens Symposium 2016 in Las Vegas.
- Kojima R, Caroline P, Graf T, et al. Eye shape and scleral lenses. Contact Lens Spectrum; April 2013.
- Kinoshita B, Morrison S, Caroline P, Kojima R, Lampa M. Corneal toricity and scleral asymmetry... are they related? Poster presented during the Global Specialty Lens Symposium 2016 in Las Vegas.
- Visser SV, Visser R, Van Lier HJ. Advantages of toric scleral lenses. Optom Vis Sci. 2006;83(4):233-236.
- Bogan SJ, Waring GO, Ibrahim O, Drews C, Curtis L. Classification of normal corneal topography based on computer-assisted videokeratography. Arch Ophthalmol. 1990;108(7):945-949.
- Riley AF, Grupcheva CN, Malik TY, Craig JP, McGhee CN. The Auckland Cataract Study: demographic, corneal topographic and ocular biometric parameters. Clin Exp Ophthalmol. 2001;29(6):381-386.
- Goto T, Klyce SD, Zheng X, Maeda N, Kuroda T, Ide C. Gender and age-related differences in corneal topography. Cornea. 2001;20(3):270-276.
- Rabinowitz YS, Yang H, Brickman Y, et al. Videokeratography database of normal human corneas. Br J Ophthalmol. 1996;80(7):610-616.
- van der Worp E, Graf T, Caroline P. Exploring beyond the corneal borders. Contact Lens Spectrum; June 2010.
- Gemoules G. A novel method of fitting scleral lenses using high-resolution optical coherence tomography. Eye Contact Lens. 2008;34(2):80-83.
- Ritzmann M, Morrison S, Caroline P, Kinoshita B, Lampa M, Kojima R. Scleral shape and asymmetry as measured by OCT in 78 normal eyes. Poster presented during the Global Specialty Lens Symposium, 2016.
- Belin MW, Ambrosio R. Scheimpflug imaging for keratoconus and ectatic disease. Indian J Ophthalmol. 2013;61(8):401-406.
- Iskander DR. The clinical utility of the Eye Surface Profiler. Invest Ophthalmol Vis Sci. 2013;54:533.