June 2021
Column
Ep. 2: The One With the Scleral Lenses
As reported in the May 2021 episode of I-site newsletter (featuring ortho-k), the BCLA recently delivered a global consensus report on evidence-based guidance to the profession on all aspects of prescribing and fitting contact lenses to allow for best patient care. The Contact Lens Evidence-based Academic Reports (CLEAR) were published in the April 2021 issue of Contact Lens and Anterior Eye and consist of 11 different reports ranging from anatomy and physiology to all types of lens modalities. This issue of I-site newsletter is ‘the one with the scleral lenses,' directed by Melissa Barnett: a 17-page paper, with a reference list of 355 citations. With regard to scleral lenses, 'clear' resonates in a double way: the primary benefit of sclerals is that they can (and should) stay clear of the cornea, with the space between the lens and cornea traditionally marked as clearance, although the new and official term is Fluid Reservoir Thickness, according to the official guide to scleral lens terminology, adapted too, of course, in this paper. All clear on sclerals? Well - some items, such as the potential IOP rise during scleral lens wear, remain unclear to some degree - but everything that we know about sclerals is covered and well balanced in this paper. For 'a reunion' with FRIENDS of the CLEAR papers: see the on-demand BCLA Clinical Conference 2021 - June 13+14.
Scleral Lenses
The last decade in the specialty lens industry marks the decade of scleral lenses, judging by the spur of publications in peer-reviewed plus other journals and implications in practice. A number of factors are responsible for this.
Ocular Surface Shape
Maybe one of the key components of the gradual increase in success of scleral lenses is the parallel increase in understanding of the shape of the eye beyond the corneal borders. Hence, justifiably, a good chunk of the scleral CLEAR paper is devoted to this. Three different types of technology – two types of profilometers using fluorescein projection, one Scheimpflug-based corneal-scleral profiler and a number of OCT instruments – are capable of imaging the ocular surface shape over the area where the scleral lens lands (at least in the horizontal plane). The corneo-scleral junction is more often tangential in shape rather than the traditional textbook understanding of a marked transition of two matching curves. Also, the sclera certainly is not spherical in nature by default: only about 5-6% can be considered 'spherical', but even those have some irregularity. Most eyes are either 'toric' (two meridians) or are asymmetrical/irregular in nature. Typically, the amount of irregularity increases more toward the periphery. All of these findings on shape have spurred development of new lens designs and better fitting techniques. The paper mentions that toric, quadrant-specific or customized landing zones can improve scleral alignment, which provides numerous advantages such as a reduction in lens decentration, lens flexure, excessive debris, the formation of air bubbles, conjunctival prolapse, localized conjunctival vessel blanching and lens impingement. For example, patients fitted with toric landing zones report improved comfort, increased wearing time and overall satisfaction, and better optical and visual outcomes. Image: Birmingham Museums Trust
Fluid Reservoir Thickness
When trying to stay clear of the cornea - what is the actually suggested space behind the lens? The optimal initial central post-lens fluid reservoir thickness is one that does not adversely affect corneal physiology or optical performance throughout long-term lens wear. That is, the lens has sufficient clearance that the posterior lens surface does not bear on the corneal epithelium toward the end of the day or in the long-term but not an excessive amount that results in lens tilt or decentration. The recommended target central fluid reservoir thickness varies among lens manufacturers, ranging from approximately 300–500μm immediately after application to 100–300μm after settling. For modern sealed scleral lens designs, the central fluid reservoir decreases by ~100-200μm over 8h as the lens settles into the conjunctiva. The reduction in central vault follows an exponential decay, with ~50% of the total settling observed after 30 min of lens wear, which stabilizes after ~2−4h. The magnitude and time course of lens settling does not vary with the fluid used to fill the scleral bowl (e.g., preservative-free saline or a viscous gel). Similarly, the posterior lens surface must vault the limbus during lens wear to avoid mechanical insult. Limbal settling of ~50-120μm (70–84%) has been reported after 3h of lens wear for a range of initial central and limbal reservoir thickness values, which resulted in a settled limbal vault of 10−50μm in the end. In another study, limbal settling was found to be similar for lenses fitted with low (~160μm) or high (~200μm) initial limbal vault after 2h of wear (both ~34μm or a 20% reduction on average in absolute terms), and greater comfort was reported for lenses fitted with greater limbal vault. Image: Marija Zaric
Oxygen
One of the biggest challenges and subjects for debate since the early days of this modality has been 'oxygen'. But it seems that the evidence is settling on that topic too. In essence: the ideal lens material will have a high Dk and a low wetting angle to optimize wettability and oxygenation to the cornea. New insights show that increasing the Dk of a sealed scleral lens beyond 100 has minimal effect on edema in young healthy eyes, although it may improve comfort. Scleral lens materials range in Dk value from approximately 88–180, with center thickness from 250μm to greater than 500μm. Total oxygen delivery through the lens system is dependent on the balance between lens thickness, Dk and fluid reservoir thickness behind the lens. Regarding the lens thickness, it should be noted that the actual center thickness of a scleral lens can vary by ±100μm from the ordered thickness and still be within the manufacturing tolerance. Highly oxygen-permeable rigid materials provide increased oxygen transmission and result in less bacterial adhesion to the corneal epithelium following overnight lens wear, but higher-Dk materials are less scratch resistant and therefore may require more frequent replacement. And small variations in optical quality with high- versus low-Dk materials have been measured in-vitro, although the magnitude of these variations in higher-order aberrations would have minimal impact on visual performance compared to other factors during lens wear such as lens centration or wettability. Studies have shown that corneal edema is primarily stromal in nature, and on average modern scleral lenses induce ~2% corneal edema in healthy or keratoconic eyes and approximately double in post-graft corneas. Image: Nonsap Visuals
Scleral Lens Challenges
In general, scleral lenses provide similar visual outcomes to rigid corneal lenses. But decentration is an issue, as scleral lenses typically decenter infero-temporally, and inferior decentration can induce a small base-down prismatic effect (can be problematic for unilateral lens wearers) and ca create residual higher-order aberrations. These optical effects can be minimised by fitting a toric or customized landing zone and reducing apical clearance. Fluid reservoir debris (midday fogging) is commonly encountered in 26–46% of patients. The exact etiology and composition of this particulate matter is unknown, but it has been linked to the accumulation of leukocytes, lipids and external tear film debris. Midday fogging must be diagnosed through biomicroscopy, OCT, or Scheimpflug imaging rather than through symptoms alone, since corneal edema or a non-wetting anterior lens surface can also cause visual symptoms. The condition is currently managed by lens removal and reapplication, fitting modifications (improving landing zone alignment or reducing reservoir thickness), using a more viscous application fluid and treating underlying ocular surface disease including allergies and dry eye disease. Filling solutions that closely mimic the composition of the natural tears can result in improvements in comfort and subjective visual quality, but no significant change in visual acuity or particulate matter in the post-lens fluid reservoir has been found. A small number of case series suggest that microbial keratitis (MK) does occur with scleral lens wear, but the overall frequency may be low due to the relatively small but growing population of scleral lens wearers. The most significant risk factors for MK may be poor hygiene, overnight lens wear, and exposure to tap water. As said in the introduction, not all is clear: the true change in IOP during scleral lens wear and especially its clinical significance still require further investigation. Image: Neora Aylon
CLEAR - Scleral Lenses
Melissa Barnett, Claudine Courey, Daddi Fadel, Karen Lee, Langis Michaud, Giancarlo Montani, Eef van der Worp, Stephen J. Vincent, Maria Walker, Paramdeep Bilkhu, Philip B. Morgan

I-site is an educational newsletter that is distributed on a monthly basis and provides an update on rigid gas permeable-related topics (scientific research, case reports and other publications worldwide). I-site is objective and non-political. Disclosure: I-site's editor Eef van der Worp, optometrist PhD FAAO FBCLA FIACLE FSLS, receives educational grants from a number of industry partners but is not related to any specific company.