Prescribing for Astigmatism
Tips and Tricks for 360º of Scleral Lens Fitting Success
BY TIFFANY GATES, OD; HEIDI G. MILLER, OD; & TIMOTHY B. EDRINGTON, OD, MS, FAAO
Blanching is a choking off of the conjunctival blood vessels close to the limbus, while impingement occurs at the edge of the lens (Figure 1). Our literature search revealed no studies that quantified what amount of blanching or impingement is detrimental, but practitioners agree that this should be addressed.
Figure 1. Post-penetrating keratoplasty patient with blanching at 3 o’clock (left), and pellucid patient with impingement at 7 o’clock (right). Click here to see a video of blanching.
Toric Peripheral Curves
Because the anterior ocular surface is typically not symmetrical in shape beyond the cornea, the sclera may have certain areas and meridians that are steeper or flatter compared to others. Focal areas of blanching are inevitable when a spherical lens is placed on a toric sclera; this often leads to sectoral areas of compression at 3 o’clock and 9 o’clock along with areas of excessive edge lift at 6 o’clock and 12 o’clock. This is especially the case for scleral lens diameters greater than 15.0mm.
Scleral toricity increases further away from the limbus, making a toric peripheral curve system more of a necessity. Toric or quadrant-specific lenses are now available to improve the overall fit of a scleral lens. The landing zone is generally the only toric portion unless otherwise specified.
Toric peripheral curve systems more equally distribute pressure over the sclera, enhancing lens stability, comfort, and anterior ocular surface health (Visser et al, 2006; Visser et al, 2007). Some patients report less accumulation of reservoir debris in the tear lens and, therefore, less clouding of vision during wear. With the increased stabilization provided by toric peripheral curves, a front-surface toric design can also be incorporated for patients requiring correction for residual astigmatism.
Let’s Poll the Crowd
How often is customization of toric peripheral curves put into practice? Polled contact lens laboratory consultants and practitioners who regularly fit scleral lenses reported that 10% to 20% of finalized scleral lenses utilized toric peripheral curves. They said that these lenses should be used to reduce blanching, impingement, and edge lift; eliminate bulbar redness; stabilize a front-surface toric lens; create a mismatch between lens and sclera to reduce seal-off and allow for easier lens removal; reduce flexure without changing lens center thickness; or increase wear time and comfort.
Our respondents recommended to design scleral lenses based on trial lens fitting (as opposed to topography); prescribe a toric periphery more often for larger diameters (>15.0mm); and start with 2.00D to 3.00D of peripheral toricity on the initial lens order.
Lastly, in dispensing to patients, it may be beneficial to dot the lens to evaluate the amount of rotation as well as indicate to patients the correct orientation for lens application. CLS
We would like to thank the practitioners and laboratory consultants for their contributions to this article.
For references, please visit www.clspectrum.com/references and click on document #227.
Dr. Gates and Dr. Miller are the cornea and contact lens residents at the Southern California College of Optometry (SCCO) at Marshall B. Ketchum University (MBKU). Dr. Edrington is the cornea and contact lens residency coordinator at SCCO. He is also a Fellow of the American Academy of Optometry and a Diplomate in their Cornea, Contact Lens, and Refractive Technologies Section.