December 2021
Column
Micron Management
At the end of this turbulent year, there is more than enough to reflect on. In our field, scleral lenses continued to thrive and to grow, and ortho-k and certainly myopia management are hotter than ever. In addition, different types of corneal and ocular surface shape topographers are gaining ground in the field with regard to applying them to improve lens design. What all of these have in common is that we increasingly are using 'micron' as the dominant terminology: a sign of evolution that we should embrace going into the future. The corneal structures are all in microns, of course: starting with the epithelium being 50 microns (and the change we aspire to obtain with ortho-k is about 6-7 microns centrally and mid-peripherally). The corneal thickness in general (between 540-560 microns) becomes increasingly important when deciding on a refractive surgery technique or on corneal cross-linking. The cornea typically needs be 400 microns thick minimally in the latter case. But also in contact lens practice, microns rule: more than 30 microns of elevation difference on the corneal surface may mean that we need to consider a toric corneal or a toric ortho-k lens. More than 350 microns of difference (in the meridian of greatest elevation change) on a cornea means that we need to consider a scleral lens. And if a concentric 16mm circle on the sclera is drawn, more than 300 microns of elevation difference in that circle is indicative of an irregular surface, and a toric or other adjustment to the haptic zone may be needed. To top it off, scleral trial lenses and the fluid reservoir thickness all are defined in microns these days. Even soft lenses can be described now in overal sagitttal height (in microns). And in myopia management, the yearly change in axial length of a child between the 10th and 13th year of age is about 90 microns. This may sound better, and potentially more convincing, than 0.09mm does to parents. Everything is in microns these days. By the way, as 2021 will always be the year of the coronovirus, the minimum size of a respiratory particle that can contain SARS-CoV-2 is calculated to be approximately 4.7... microns. Let’s move on. Let’s use micron.
Sclerals
Prolific Prolapse Probing
Conjunctival prolapse refers to the drawing up of conjunctival tissue beneath the peripheral region of a scleral lens adjacent to the limbus. The exact cause of conjunctival prolapse is unknown, but it seems to at least partly arise from the pressure or fluid forces generated behind the scleral lens. It often occurs in a location of excessive limbal clearance (a thicker post-lens tear reservoir). Damien Fisher, Michael Collins and Stephen Vincent from Queensland University of Technology (QUT) in Australia prolifically probed the incidence and peak elevation of conjunctival prolapse. Ten young, healthy adults wore a scleral lens with an initial central post-lens fluid reservoir thickness defined as low (144±23 microns), medium (487±63), and high (726±55) for 90 min. The incidence of conjunctival prolapse was 37% across all fluid reservoir thickness conditions, with 80% of participants exhibiting conjunctival prolapse at least once. Prolapse was observed more frequently nasally (73%) than temporally (27%). The peak prolapse elevation (the height of the elevated conjunctival tissue during prolapse - measured in microns, indeed) did not vary with central fluid reservoir thickness condition or anatomical location, but eyes with conjunctival prolapse had greater initial limbal clearance (97±38 microns compared to 43±34 in the low-fluid-reservoir thickness condition). The peak elevation of the conjunctival prolapse was associated with the extent of limbal settling, but not with landing zone tissue compression or with fluid reservoir thickness asymmetry. Photo by Patrick Caroline - Pacific University OR (US)
Radial Radar
The total number of incisions in radial keratotomy (RK), a surgical procedure that is defined as a radial pattern of partial-thickness corneal incisions to flatten the central cornea to correct myopia, typically varies between 4 and 16 depending on the magnitude of myopia, with deeper and more central incisions producing a greater flattening effect. In this case series, the magnitude and regional variation in scleral lens–induced corneal edema in post-RK eyes was examined. Scheimpflug imaging was used to measure corneal thickness across the central 6mm before and after scleral lens wear in nine post-RK eyes. Variations in corneal edema were examined as a function of distance from the corneal center (six 0.5mm annuli) and from 0° to 360°. The median central corneal edema was 2.19% and increased in magnitude and variability further from the central cornea (from 1.30% in the central 0–0.5mm zone to 3.12% in the 2.5–3.0mm zone). The edema was greater in magnitude further from the corneal center and at the approximate location of corneal incisions. In conclusion, high-Dk/t scleral lens wear can induce central corneal edema in post-RK eyes, which increased in magnitude further from the corneal center and at the approximate location of corneal incisions in this study. It seems imperative that practitioners have oxygen delivery to the cornea on their radar in post-RK eyes and should aim to minimize the fluid reservoir thickness to optimize this, while taking care to maintain corneal clearance to avoid mechanical contact with the anterior corneal surface. Photo by Tom Arnold-Memorial Eye Center Sugarland TX (US)
IFSLR Roundup
For a research update on sclerals, consider looking at the International Forum for Scleral Lens Research (IFSLR) recent roundup in Contact Lens Spectrum. Maria Walker, Jan Bergmanson, Muriel Schornack and Daddi Fadel provide a summary of cutting-edge research from the forum's research meeting earlier this year. Among the topics discussed is the potential of intraocular pressure (IOP) increase with sclerals. Collectively, there is sufficient evidence to state that moderate (3 mmHg to 5 mmHg) changes in IOP may be occurring in some scleral lens wearers. It is recommended to monitor IOP in all scleral lens wearers, and it is prudent to carefully monitor at-risk populations, such as patients with glaucoma or ocular hypertension. On a different topic: it was stated that scleral lenses are remarkable and safe devices for irregular corneas; however, when scleral lenses are prescribed for disease management, complications related to the disease state may inevitably occur. Diseases must be carefully monitored, and special considerations should be given to highly diseased corneas such as in corneal transplants and in post-herpes zoster/-simplex ophthalmicus cases. The topics of oxygen delivery and scleral shape are looked at too in this paper. In the latter category: work by Piñero et al indicates that more scleral asymmetry is present in eyes that have keratoconus and that there is some level of correlation among the corneal and scleral data. Work by Consejo et al found correlations between flat and steep keratometry values and scleral asymmetry. Anterior corneal astigmatism showed poor correlation with the level of scleral irregularity, while posterior corneal curvature and corneal thickness were not correlated with scleral asymmetry.
Scleral Shape vs Eye Length
Bringing scleral lenses and myopia management together, two recent studies have found a relationship between axial length and corneo-scleral topography. In a study by Niyazmand et al from QUT in Australia, high myopes exhibited a different anterior eye surface shape than emmetropes did: myopes showed greater sagittal height in the nasal corneal periphery and on the nasal anterior sclera. There was also less nasal-temporal asymmetry of sagittal height and axial radius of curvature in high myopes than in emmetropes. Asymmetric growth of the eye associated with myopia development may be the underlying reason. This may have implications for design of soft and scleral lenses, the authors state. Bataille et al from Spain found too that there is a correlation between anterior sagittal height measurements of the eye and axial length. Specifically, differences between nasal and temporal sagittal heights of the anterior eye correlated significantly with axial length in their study. Potentially, the axial length might be predicted with acceptable levels of precision by means of a linear equation considering refractive, corneal, and corneo-scleral variables, it was stated.
Education
Advanced Technology
The GPLI hosted hosted webinars on gpli.info on two interesting topics, tying in with the previous items in this newsletter of using elevation data (in microns) and of using this information to further refine and optimize lens design in specialty lens practice. In October, Jason Jedlicka looked at software applications for specialty lens designs, in which he discussed the evolution of contact lens fitting based on technology. Software can improve the contact lens fitting process by analyzing corneal and scleral shape data and creating a lens that matches those shapes precisely and by allowing modification using tools in the software that also show simulations of those modifications. Ordering lenses directly from the software at the time of design often results in less initial chair time and better initial outcomes in many cases as compared to diagnostic fitting. Greg DeNaeyer, in the November 16 webinar, goes beyond the basics of sclerals and focuses on advanced scleral lens design and fitting, again discussing how to use technology to better define the ocular surface and how to use the resulting information for improved scleral lens designs.
International Agenda
Global Specialty Lens Symposium
(Early Bird Pricing Ends 15 December)
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.