Have a Safe Landing
Lynette Johns OD FAAO FSLS FBLCA
Designing a lens with toric peripheries not only improves scleral landing zone alignment, but it also has been shown to improve comfort and increase wearing time.1,2 Anecdotally, improved overall alignment reduces excessive tear exchange and, as a result, reduces the inflow of tear reservoir debris. Evaluating scleral lens landing zones using a consistent and systematic method will aid in scleral toricity pattern identification. Some manufacturers now offer diagnostic lenses with toric peripheries. How are toric diagnostic lenses incorporated in the fitting and evaluation process?
Where to Start with Toric Back Surface Designs
Even with experience, it can be confusing to begin a scleral lens fitting using a diagnostic lens that has toric peripheries. A toric diagnostic lens has two different peripheries, and neither may align to the eye. First apply a lens that has a symmetric periphery and evaluate the scleral landing zone in each of the four quadrants for compression, impingement, and edge lift. As a review, compression causes blanching of the underlying conjunctival vessels; impingement occurs when the edge is too steep and digs into the conjunctival tissue, inducing ridge-like arcuate staining. Both of these findings suggest that the landing zone is too tight. Meridional compression and/or impingement occur on the flat meridian of scleral toricity. Conversely, edge lift suggests that the landing zone is too loose, and fluorescein pooling will confirm edge lift. Toricity may occur in a with-the-rule pattern, in which the flat meridian is horizontal; the flat meridian is vertical in against-the-rule toricity. Scleral toricity may also be oblique.
Evaluating the Landing Zone
When evaluating the landing zone, it is important to view every quadrant of the periphery in primary gaze. Next, evaluate the lens with the patient viewing in peripheral gaze in each of the four quadrants. In peripheral gaze, scleral landing zones are exaggerated. When compression and/or impingement are observed in peripheral gaze, re-evaluate carefully in primary gaze. If it subsides in primary gaze, only minor modifications to loosen the fit are useful. In Figure 1 (above), a diagnostic lens with a standard symmetric periphery was applied to a patient with keratoconus. The views of the quadrants are limited in primary gaze. In peripheral gaze, the toricity pattern is much more evident. There is significant compression in the vertical meridian; however, in temporal and nasal gaze, the vessels are much less impeded in the landing zone. This pattern suggests there is against-the-rule scleral toricity in which the flat meridian is vertical and the steep meridian is horizontal.
At this point, a toric lens can be designed from the standard peripheries. If a toric diagnostic lens is available, then toricity can be confirmed. Typically, toric diagnostic lenses have one of the meridians labeled with a laser marking. Figure 2 shows a toric scleral lens applied to the same eye. The flat meridian was 90 microns flatter (more elevated) than the standard periphery at the edge, while the steep meridian was 90 microns steeper. The flat meridian was marked with a laser engraved line and is barely visible in the top part of Figure 2 (below), just above the bubble. This 16.0 mm design had a total of 180 microns of toricity. The vertical/flat meridian is improved with this flatter periphery; however, compression still remains. Here, an even flatter periphery should be ordered. When looking at the horizontal/steep meridian, there is an improvement in alignment. Careful examination revealed edge lift, so this meridian needed to be tightened.
While it seems overwhelming, fitting a back toric scleral lens design is actually less complex than it initially appears. It is like fitting an individual scleral lens in different meridians, and then combining that information into one lens. By starting with a toric scleral lens, remember that there are two completely different peripheries and they still may not align properly. Using a standard periphery first will help determine the presence of toricity and the orientation of the meridians. The toric diagnostic lens will confirm the meridians, and the axis of the toricity can be measured. The axis angle measurement is quite helpful when incorporating front optical toricity into the design so adjustments of the optical axis can be properly oriented and stabilized with the back toric design.
As stated earlier, the benefits of toric peripheries include improved comfort and wearing times. They stabilize front optical toricity so the patient can enjoy clear and stable vision for the duration of wear. The improved lens alignment with the sclera reduces significant tear exchange, resulting in less fogging and further improving the patient’s experience.
Have a go with back toric scleral lens designs – your patients will truly benefit by this customization and your expertise.
1. Visser ES, Visser R, Van Lier HJ. Advantages of toric scleral lenses. Optom Vis Sci 2006;(83):233-6.
2. Visser ES, Van der Linden BJJJ, Otten H, Van der Lelij, Visser R. Medical applications and outcomes of bitangential scleral lenses. Optom Vis Sci. 2013 Oct;90(10):1078-85.
Lynette Johns is an adjunct assistant professor and clinical attending in contact lenses at the New England College of Optometry.
She was formerly the senior optometrist at the Boston Foundation for Sight where she exclusively fit scleral lenses and managed complex corneal disease.
She is a fellow of the American Academy of Optometry and Scleral Lens Education Society as well as a fellow of the BCLA. She is a clinical and educational consultant to the GP and specialty soft lens division of Bausch + Lomb.