Educating doctors on how to fit scleral lenses is most rewarding and is one of my favorite things to do. At a recent scleral lens workshop, the topic of back surface toricity was frequently discussed. Studies have demonstrated that scleral asymmetry increases towards the extraocular muscles and that the nasal meridian is flatter than the rest of the scleral meridians.1 Additionally, anterior segment OCT and scleral profilometry studies have shown that the transition between the cornea and sclera is tangential.2 Previously, many diagnostic lenses were rotationally symmetric. Based on studies of scleral topography, rotationally asymmetric designs are frequently required to approximate the alignment of the landing zone with the sclera. Currently, many scleral lens designs are available with back surface toric designs. Some lenses also incorporate quadrant specific peripheries. If available, anterior segment topography or OCT can provide information about scleral toricity. If technology is not available, rotationally symmetric diagnostic lenses themselves can act as topographical aids. If scleral asymmetry is present, the diagnostic lens may demonstrate alignment, edge lift, compression or impingement (or both) in different meridians.
A recent observational study evaluated left eyes of 133 patients fit with a bitangential periphery scleral lens.3 The scleral lens diameter ranges from 14 mm to 18 mm with a front optic zone of 10 mm, a spherical back optic zone of 11 mm, and a midperipheral zone to vault the limbus. The transition at the midperipheral zone connects the optical zone with the landing zone. In this design, the linear landing zone is determined by an angle relative to a line perpendicular to the optical axis. This angle corresponds to the tangential angle where the lens aligns with the sclera. In a nonrotationally symmetrical design, the linear alignment zone is manufactured with two different angles in two perpendicular meridians, creating a bitangential periphery. In this lens design, the difference between these angles in the diagnostic fitting set is 6 degrees with the steeper meridian having a larger tangential angle, and the shallower meridian having a smaller tangential angle. This nonrotationally symmetrical back-surface toric periphery scleral lens is designed to fit an eye with scleral toricity. Since the lens is rotationally stable, residual astigmatism can be corrected with front surface toricity.
In this study, the most common indication was keratoconus (45%), irregular astigmatism (22%), keratoplasty (16%), ocular surface disease (13.5%), and other forms of irregular astigmatism (3%). Seventy nine percent of patients scored comfort as either a 4 or 5 (out of 5), and 82% wore lenses 12 hours or longer a day. The majority of lenses (81%) were 16 mm in diameter (median, 16 mm; range, 15.5 to 17 mm). Materials used were Boston XO2 (46%), Menicon Z (44%), Boston XO (9%), or Boston Equalens II (1%). The median corrected distance visual acuity was approximately 20/21. Fitting characteristics revealed ideal scores for centration (91%) and movement (83%). The median stabilization axis was 50 degrees. This study demonstrated that scleral lenses with a bitangential peripheral geometry may perform well both objectively and subjectively in eyes to correct irregular astigmatism, ocular surface disease or other corneal pathologies. Consider back surface toricity to heighten scleral lens success.
1. Van der Worp E. A guide to scleral lens fitting (English) Version 20 Book 2015. Available at: http://commons.pacificu.edu/mono/4
2. Visser ES, Visser R, Van Lier HJ. Advantages of toric scleral lenses. Optom Vis Sci. 2006; 83: 233Y6.
3. Otten HM, van der Linden BJJJ, Vissr ES.Clinical Performance of a New Bitangential Mini-scleral Lens. Optom Vis Sci. 2018 Jun;95(6):515-522. doi: 10.1097/OPX.0000000000001228.
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, Contamac, CooperVision, EveryDay Contacts, Johnson & Johnson Vision, Ocusoft, Paragon Bioteck, RaayonNova, ScienceBased Health, Shire, SynergEyes, and Visioneering Technologies.