The Scleral Lens Vault
The Scleral Lens Vault
Estimating Central Corneal Clearance
BY GREGORY W. DENAEYER, OD, FAAO
Properly fit scleral lenses completely vault the center of the cornea. Depending on the design and fitting philosophy, the amount of clearance can range between 50 to 500 microns. Efficiently
estimating the amount of clearance of a diagnostic scleral lens during the fitting process is critical to success.
How to Measure
Two ways of measuring central corneal clearance (CCC) include using biomicroscopy and optical coherence tomography (OCT). Software-generated measuring calipers can precisely measure the central corneal vault of a scleral contact lens from an OCT image. However, the disadvantage of utilizing OCT is that it is time consuming, especially when trying to incorporate measurements during an initial scleral lens fit. Additionally, not everyone has access to this technology.
Alternatively, practitioners can estimate the CCC by turning the slit beam of the biomicroscope to an angle of approximately 45º and comparing the thickness of the fluid reservoir with the known thickness of the diagnostic lens or cornea. Then, without pause or interruption, the rest of the lens fit can be evaluated.
However, is using a biomicroscope to estimate fluid reservoir thickness easy to learn, and is it accurate?
Yeung and Sorbara (2014) presented a poster at the 2014 Global Specialty Lens Symposium (GSLS) that discussed the accuracy of measuring CCC with biomicroscopy versus using an image-processing program.
Three groups of 45 subjects were asked to give CCC estimates of four scleral lens fit images with reference to known central corneal or central scleral lens thickness. The subjects had varying experience and were categorized as either novice, intermediate, or advanced. The pilot study validated the use of estimating CCC with biomicroscopy, because it was comparable with results from image processing. Performance was similar among all three groups; however, the subjects who had more experience had less interobserver variability. Overall, the trend was to underestimate CCC by about 50 microns.
Also at GSLS 2014, Dinardo et al (2014) presented a poster about practitioners’ accuracy and confidence of estimating scleral vault before and after being introduced to a scleral lens fitting guide. One hundred and fifty-six participants of varying levels of experience participated in an online survey in which they estimated the vault of slit lamp images of four fit scleral lenses. Subjects then reviewed a fitting guide that included detailed descriptions and examples of known scleral lens vaults before evaluating four different lenses. The accuracy of estimation was similar among participants, regardless of experience; the accuracy of the entire group improved after use of the fitting guide. They concluded that a detailed fitting guide is useful for beginners and experienced practitioners alike. The guide can be accessed at www.ferris.edu/ScleralLensFitScales.
Chan et al (2014) have recommended that manufacturers include the scleral lens thickness of each diagnostic lens to aid practitioners during evaluation. Even if you are an expert, referencing a fitting guide with images of known lens vaults will improve accuracy. If available, utilize OCT technology to confirm and document CCC of the final lens at follow-up visits, which can be used to monitor changes in vault secondary to long-term lens settling or corneal topographical changes. CLS
For references, please visit www.clspectrum.com/references and click on document #226.
Contact Lens Spectrum, Volume: 29 , Issue: September 2014, page(s): 46