Variations in Post-Lens Tear Layer Thickness in Scleral Lens Wear
By Melissa Barnett, OD
Scleral shape studies using height terminology have demonstrated that the nasal side of the anterior sclera is more elevated than the temporal side is compared to a reference baseline chord.1 However, the ocular sagittal height nasally has a lower value compared to the temporal sagittal height (Figure 1). According to the literature, the ocular sagittal height of a normal, healthy eye that does not have disease or irregularity is approximately 3,750μm, with a maximum range of 1,000μm between the flattest and steepest portions.2,3 These measurements are valuable to provide baseline height information for a normal population to gain perspective about scleral lens sagittal height.
Figure 1. Difference in nasal appearance versus temporal appearance measured with a non-placido-based topographer. Image originally appeared in Contemporary Scleral Lenses: Theory and Application.
A recent study aimed to quantify regional variations in the post-lens tear layer (PLTL) thickness during scleral lens wear. Fifteen adults who had healthy, non-diseased eyes with normal corneas were fit with a 16.5mm rotationally symmetric scleral lens on one eye.4 After lens application, the PLTL thickness was measured across the central 5mm at 0, 15, 30, 45, 60, 90, 120, 240, and 480 minutes after lens application using a 12-radial-line scan optical coherence tomography (OCT) imaging protocol. The PLTL was divided into eight equal 45° segments and analyzed in each region.
Immediately after lens application, a tilted optic zone was observed; the largest PLTL asymmetry was between the nasal and temporal areas, with more clearance temporally (156μm ± 22μm), and between the superior-nasal and inferotemporal regions, with more clearance inferotemporally (124μm ± 12μm). The extent of lens settling in each region was associated with the initial PLTL (r = 0.59 to 0.77, P ≤ 0.02). Of interest, most regions stabilized after four hours except for the superior-nasal PLTL most distant from the pupil center, which stabilized after 90 minutes. On average, after eight hours of lens wear, the PLTL decreased by 29%, and PLTL asymmetries between opposing regions decreased by 30%.
In this study, the PLTL was thickest temporally and thinnest nasally with rotationally symmetric scleral lenses. The authors suggested that factors that contribute to regional differences may include underlying scleral elevation, eyelid forces, and lens centration. Of note, asymmetries in the PLTL diminished with lens wear, and stabilization occurred more rapidly immediately after lens application in regions with less corneal clearance.
Future studies that quantify differing amounts of lens back-surface toricity and ocular scleral toricity may be of interest. In addition, future studies on patients who have varying amounts of eyelid forces may be useful.
1. van der Worp E, DeNaeyer G, Caroline P. In Barnett M, Johns L, eds. Ophthalmology Current and Future Developments (Volume 4). Contemporary Scleral Lenses: Theory and Application. Bentham Science Publishers, 2017 Nov 16. pp.109-111.
2. van der Worp E, Hulscher J. Speed Date – CL-SAG meets OC-SAG: part I. Global Contact. 2015;70(2):22-24.
3. van der Worp E. What (about) the SAG? Global Contact. 2013;63(1):16-20.
4. Vincent S, Alonso-Caneiro, D, Collins, M. Regional Variations in Postlens Tear Layer Thickness During Scleral Lens Wear. Eye Contact Lens. 2019 Nov 8. [Epub ahead of print]
Dr. Barnett is a principal optometrist at the University of California Davis Eye Center in Sacramento, specializing in anterior segment disease and specialty contact lenses. She is the past president of the Scleral Lens Education Society. She is an advisor to and/or has received honoraria or travel expenses from AccuLens, Alcon, Alden Optical, Allergan, Bausch + Lomb, Bruder, Contamac, CooperVision, EveryDay Contacts, Johnson & Johnson Vision, Ocusoft, Paragon Bioteck, RaayonNova, ScienceBased Health, Shire, SynergEyes, and Visioneering Technologies.