Contact Lens Update

Eef van der Worp, BOptom, PhD is an educator and researcher associated with the University of Maastricht, Manchester University, the University of Montreal University College of Optometry and the Pacific University College of Optometry.

Large diameter contact lenses that have their resting point beyond the limbus are believed to be among the best vision correction options for irregular corneas; they can postpone or even prevent surgical intervention as well as decrease the risk of corneal scarring.¹ For some conditions, true clearance of the cornea is desired, without any mechanical involvement of a contact lens for conditions such as ectatic corneas.

Background

The first scleral lenses were produced 125 years ago and made of glass blown shells.² The 1936 introduction of molding techniques for glass lenses by Dallos, and the introduction of polymethyl methacrylate (PMMA) in the 1940s by workers such as Feinbloom, Obrig and Gyoffry, were important breakthroughs for the development of this lens modality, according to Tan et al..³

These lenses can now be manufactured on a lathe-cut basis and in a much more accurate manner to conform to the anterior shape of the eye. The use of oxygen permeable lenses, first described by Ezekiel in 1983,⁴ was another breakthrough, since it brought major improvements in ocular health. The development of the smaller, corneal gas permeable lenses and later soft lenses temporarily stopped further development of scleral lens fitting.

A number of scleral lens experts around the world, including Ezekiel, Pullum, Rosenthal and Visser, ^{4 5 6 7} have published the benefits of these oxygen permeable scleral lenses for patients with challenging corneas. Recently, a number of new developments have led to a revival of scleral lenses^{8 9 10}:

• improved manufacturing processes
• better designs
• greater reproducibility
• decreased costs
• technology that can more precisely image ocular topography

These developments have resulted in longer wearing times and easier lens fitting. Many scleral lens options are available to practitioners today, including front and back toric, quadrant specific and bifocal lens designs.¹

Terminology

The terminology used with respect to scleral lenses is very diverse, locally determined and oftentimes arbitrary. In the classification system proposed by the Scleral Lens Education Society, lenses are categorized based on landing area (i.e. corneal, corneo-scleral or full scleral) (Table 1). The biggest difference between the categories, apart from bearing area, is the amount of clearance that can be created underneath the central lens. In small diameter lenses the tear reservoir capacity is typically minimal, while in the large diameter scleral lenses the tear reservoir capacity is almost unlimited.¹

Indications

The indications for scleral lens wear^{11 12 13 14 15} can be categorized in the following way:

Vision improvement – Traditionally the main indication for fitting scleral lenses. The largest segment within this category is corneal ectasia, primary and secondary.

Corneal protection/therapy – Consists of a large group of exposure keratitis/ocular surface disease patients that can particularly benefit from scleral lenses because of the retention of a fluid reservoir behind the scleral lens. Sjögren’s syndrome is a common scleral lens indication, and so are conditions such as persistent epithelial corneal defects, Steven’s Johnson Syndrome, Graft Versus Host Disease, ocular cicatricial pemphigoid, neurotrophic corneal disease and atopic keratoconjunctivitis.^{16 17} Scleral lenses are also used for lagophthalmos, as in eyelid coloboma, exophthalmus, ectropion, nerve palsies and after lid retraction surgery.¹⁸

Cosmetics/sports – Painted scleral lenses have been used for cosmetic purposes in a variety of cases, often related to atrophia bulbi.¹⁹ Painted lenses have also been used to reduce glare in aniridia and albinism, although this would technically fall under “vision improvement”.²⁰ Scleral lenses have also been used to limit the effect of a unilateral ptosis.²¹

Scleral shape

The conjunctiva is the landing plane when fitting scleral lenses. Since the conjunctiva has no structure (i.e. it follows the shape of the sclera), the shape of the anterior eye beyond the corneal borders is referred to as “scleral shape”, and hence a scleral lens rather than a conjunctival lens.¹

It appears that there is roughly 7.0 mm of space between the limbus and the insertion of the eye muscle (7.0 mm temporally, 7.5 mm superiorly and 6.5 mm inferiorly) on the ocular surface, but only 5.00 mm on the nasal side. This is referred to as the Spiral of Tillaux,²² after the French physicist who first described this phenomenon. With an average corneal diameter of 11.8 mm,²³  it suggests that the maximum horizontal diameter of a scleral lens should be 24 mm to avoid interference with the eye muscle insertion, assuming the lens does not move.¹

Molds taken of the anterior segment of human eyes (in normal eyes and in eyes with keratoconus) indicate, at least in some cases, that the limbal-scleral junction has a tangential shape.[24] Using contour maps from the experimental Maastricht Shape Topographer^{25 26} that can image the limbus and part of the sclera up to an 18.0 mm diameter, it seems on a case-by-case analysis that the transition is often tangential rather than curved.

One of the few publications on limbal shape by Meier, a Swiss eye care practitioner, suggest that cornea to sclera has 5 different transition profiles, based on slitlamp observations.²⁷ However the subjective repeatability of these profiles by inexperienced observers was poor.²⁸

Optical coherence tomography (OCT) has been proposed and described as a method to image the anterior ocular shape.²⁹ By manually imaging different meridians, this technique can be used to explore what the normal limbal and anterior scleral shape looks like more accurately than possible with the subjective observer method.

In a study at the Pacific University College of Optometry (Oregon, USA), we analysed the limbal and anterior scleral area beyond the limbus from optical coherence tomography measurements (OCT, Zeiss Visante) of 96 eyes of 48 normal subjects (unpublished data). It appears that the ocular surface beyond the cornea is not rotationally symmetric (Figure 1). Typically, the nasal portion is flatter than the temporal segment (Figure 2). For smaller scleral lenses (up to approximately 15.0 mm), spherical scleral lenses should in most cases suffice, but for larger diameter lenses (>15mm, up to 20.0 mm), non-rotationally symmetric lenses such as toric and quadrant-specific shapes may be needed.¹

 

Figure 2: Nasal and temporal OCT images superimposed (Zeiss Visante) illustrating the flatter nasal scleral shape in the right eye of this subject.

The non-spherical nature of the sclera has been described previously from a clinical perspective by Visser et al..³⁰ Non-rotationally symmetric lenses may minimize localized bearing areas, which can lead to a reduction in local conjunctival blanching and improved comfort. Further, the added stability can facilitate front-cylinders and higher-order corrected aberrations such as vertical coma (a frequent finding in keratoconus).³¹

Corneal physiology

Scleral lenses are considerably thicker than corneal RGP lenses, with a typical center thickness of 250 to 400 microns. This obviously limits the oxygen transmissibility (Dk/t) of these lenses quite drastically. In addition, the lens tear reservoir can also serve as an additional barrier. The Dk of tears is believed to be around 80,³² and the tear thickness can easily be in the 300-500 micron range for the larger diameter scleral lenses.

Michaud et al.³³ made some theoretical calculations considering several material permeabilities (Dk 100–170), varying lens thicknesses (250–500micron), the known tear permeability (Dk of 80) and expected post-lens tear layer thicknesses (100–400micron), and found that most scleral lenses with clearance fitting techniques may lead to hypoxia-induced corneal swelling. The Holden–Mertz Dk/t criteria of 24 Fatt units for the central cornea³⁴ and the Harvitt–Bonanno criteria of 35 Fatt units for the limbal area³⁵ were used as reference points.

Recommendations for minimizing hypoxia-induced corneal swelling are to use the highest Dk material available (>150), a lens with a maximal central thickness of 250 microns and fitted with a clearance that does not exceed 200 microns. Peixoto-de-Matos et al. suggested that a thick post-lens tear film can act as a critical barrier for sufficient oxygen diffusion under some medium- to high-Dk lenses.³⁶ Based on these theoretical calculations, it seems advised to fit scleral lenses with:

• the highest Dk material,
• less clearance,
• a thinner central thickness.

Perhaps surprisingly, not many hypoxia-related complications are seen in scleral lens practices – based on anecdotal feedback from scleral lens practitioners, unless there is proof of scleral lens over-wear. Tear film exchange and other factors may help with oxygen delivery to the cornea, as it does in RGP lens wear.

Conclusion

Scleral lenses can have a very positive impact on people’s lives, particularly those with irregular corneas or who are suffering from severe ocular surface disease. For more information about scleral lenses, and/or about scleral lens practices worldwide, visit the website of the Scleral Lens Educational Society at www.sclerallens.org.

Acknowledgements:

Special thanks to Tina Graf for her collaboration in the Pacific Scleral Shape Study (Oregon, USA) and to the entire Pacific University contact lens team. Many thanks also to Langis Michaud of the University of Montreal College of Optometry (Canada), to Jose Gonzalez-Meijome from the Universidade do Minho in Braga (Portugal) and to Jan P. Bergmanson of the University of Houston, College of Optometry (Texas, USA).

References