International Newsletter and Forum on Rigid Gas Permeable Contact Lenses, Corneal Shape, Health and Vision
 March 2014
In This Issue
Column
Topography in Keratoconus
Bitoric (R)GP Lenses
Vaulting the Cornea
Book Review
I(n)-site-the-practice
Agenda
Join Our Mailing List
  

(advertisement)
Column
Post(-er)

Coming back from Las Vegas, I always need a little bit of time to 'digest' everything that goes on at the Global Specialty Lens Symposium (GSLS). There is simply too much going on! One thing that is really worth looking at 'post-meeting' is the posters. This year's meeting hosted 67 posters, with the bulk categorized as 'scientific' (about two-thirds) and the rest as case reports. In the case report category, Maria Walker from Pacific University (US) won first prize for her poster on A Proposed Mechanism for Scleral Lens Induced Conjunctival Prolapse (PDF). According to the authors, anterior corneal shape plays a role in where conjunctival prolapse occurs. Josh Lotoczky (Michigan College of Optometry - US) was second with his poster 'Livin' on the Edge', which looked at scleral lens edge profiles in vivo using confocal microscopy. The findings show a significant difference in edge profile among manufacturers. In the science category, Debby Yeung of Waterloo University (Canada) was the runner up with 'Scleral Lens Clearance Assessment with Biomicroscopy', comparing it to an ultrasound technique. There is an overall trend for underestimation using the slit lamp technique by approximately 50µm regardless of prior experience with scleral lens fitting, but in the intermediate and the expert group, significantly less inter-observer variability was seen. Norman Leach from the University of Houston won first prize for his poster on Midday Visual Fogging in Scleral Lens Wearers: Does Fit Matter? Apparently, fit does matter, according to the poster. Good tear quality, avoiding post-lens tear layer thicknesses larger than 300μm and avoiding tight-fitting peripheral edges appear to increase wearing time, according to their study. The posters are all posted on the GSLS website.

Eef van der Worp 

Corneal Topography
Non-Central Topographical Data in Keratoconus Fitting
 

Wishal Ramdas, Charles Vervaet and Isabel Bleyen in Contact Lens & Anterior Eye evaluate what corneal topography data is most useful in keratoconus fitting. A total of 378 eyes of 218 patients with keratoconus were included in this study. They found that including the 'K-minimum' as a non-central parameter may improve prediction of the best base curve radius for pancorneal (R)GP lenses, and this may lead to a reduction in the number of trial contact lenses needed. It is important to note that the K-minimum is not necessarily located centrally on the cornea in keratoconus. In line with other studies, suggestive evidence was found for a difference in pattern of progression of keratoconus with gender, but not with age. Photo: Randy Kojima - Pacific University

Scleral Lenses
Bitoric (R)GP Lenses
 

Also in Contact Lens & Anterior Eye, Vivian Phan et al report a study on bitoric rigid gas permeable contact lenses in the optical management of penetrating keratoplasty. Fourteen eyes satisfied the study criteria, and the average amount of total astigmatism in this study group was 10.0±5.0D compared to the control group of 3.3±2.6D. The number of diagnostic lenses used for the study group was 1.2, compared to 1.0 for the control group. The mean number of contact lenses ordered to complete the fitting for the study group was 1.9, while this was 2.1 in the control group. The study group presented for an average of 6.3 visits, whereas the control group presented for 4.9 visits. The complexity of fitting bitoric (R)GP lenses was found to be similar to fitting spherical (R)GP lenses on post-PKP eyes, and the visual outcomes were not different. A Toric GP Primer in Contact Lens Spectrum by Annie Chang and Dawn Lam is a nice article regarding the 'comprehensive examination of optimal toric GP candidates, contact lens design, and modalities'. 

Scleral Lenses
Vaulting the Cornea
 

'Scleral contact lenses (diameters of 15mm to 24mm) by definition are fit to vault over the cornea and rest on the sclera/bulbar conjunctiva, enabling them to hold a fluid reservoir,' according to Greg DeNaeyer in Contact Lens Spectrum. Typically, scleral lenses are fit to vault the central cornea by 100 to 400 microns, he writes. The variables involved in creating vault are covered in this article, namely: back-surface curves (where the base curve has the most influence on sagittal depth versus any one peripheral curve), sagittal depth, lens diameter (sagittal depth increases with increasing contact lens diameter) and lens design. Settling of scleral lenses needs to be taken into account, as scleral lenses rest on the bulbar conjunctiva, which is spongy in nature. Among the clinical considerations mentioned, the oxygen issue is highlighted. Using theoretical calculations, Langis et al (2012) concluded that to avoid corneal swelling, the central corneal vault of a scleral lens should not exceed 200 microns when using hyper-Dk materials (>150) and limiting central thickness to 250μm. However, another possibility is that the reservoir acts as an oxygen depot that is continuously renewed by atmospheric oxygen that permeates the lens (Sonsino and Mathe, 2013; Caroline, 2013). This could explain, according to DeNaeyer, why clinically significant corneal edema isn't observed when the above hypothetical criteria are not met in eyes that have a healthy corneal endothelium.

Greg DeNaeyer - Contact Lens Spectrum, Jan 2014  

Book Review
Clinical Ocular Anatomy and Physiology
 
Jan Bergmanson of the Texas Eye Research and Technology Center at the University of Houston, College of Optometry (US) just published the 2014 version of his book on clinical ocular anatomy and physiology - its 21st edition. In 28 chapters, the book covers everything from eyelids to the cornea, crystalline lens and vitreous to cranial nerves and extra-ocular muscles. The whole scope from embryology to clinical applications such as corneal shape and corneal topography is explored in this 300-page book. It also contains 10 separate pages of clinical anatomy pictures and includes 259 sample multiple choice questions (with answers). Visit http://tertc.opt.uh.edu/book.html for information how to obtain the book (ISBN 978-1-4675-9029-7).

Texas Eye Research and Technology Center

I(n)-site-the-practice   
Can Scleral Lenses be an Alternative to a Tarsorrhaphy?
  

Extreme and longstanding corneal exposure from either mechanical or neurogenic lagophthalmos may require tarsorrhaphy for corneal protection. A tarsorrhaphy is a surgical procedure that narrows the palpebral fissure by partially suturing the eyelids together. It can be performed in a variety of ways. While the procedure is therapeutic, it may limit peripheral visual field and also have cosmetic implications for the patient. Prior to tarsorrhaphy, a variety of protective measures are utilized including aggressive and copious lubrication, punctal occlusion, bandage soft contact lenses, lid weights and even amniotic membranes. If left untreated, exposure and neurotrophic keratopathy can potentially lead to persistent epithelial defects, corneal ulceration and perforation. The use of scleral lenses in cases of exposure and neurotrophic keratopathy can lubricate and protect the exposed cornea as well as provide adjunctive therapy and, in some cases, an alternative to a tarsorrhaphy.

International Agenda
Upcoming Events
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. Its editor Eef van der Worp, optometrist, PhD, FAAO, FBCLA, FIACLE, FSLS is a lecturer for a variety of industry partners, but is not related to any specific company. Please contact us at: i-site@netherlens.com.