Prescribing for Astigmatism
Taming Those Torics
BY BROOKE MESSER, OD, FAAO, FSLS
Technological advancements in the production of soft toric contact lenses have dramatically contributed to their success. Their rotational stability has improved to a point at which most patients would be successful in most designs if their refraction parameters fall within the design availability. Understanding the design concepts and how the lenses interact with the eyelids and corneal shape can assist you in more efficiently selecting and fitting a design to optimize lens performance.
Stabilization Design Options
Prism-Ballast These utilize asymmetry in the superior and inferior lens thickness to align the lens on the cornea. The thicker inferior edge interacts with the upper and lower eyelid to maintain its position with each blink for consistent vision.
Early designs of prism-ballasted soft toric lenses had a base down prism in the optical zone of the lens, which could induce vertical imbalance if the design was fit to just one eye. This has been remedied in newer toric designs by placing the prism in the lens carrier, allowing for large optical zones with less prismatic effect.
These designs are considered peripheral-ballasted or peri-ballasted; they feature a wide band across the inferior lens area, with the thickest portions of the lens on either side of the 6 o’clock position for a more balanced interaction with the eyelids. Peri-ballasted lenses have thinner edges and, therefore, higher oxygen permeability in the inferior portion of the lens compared to traditional prism-ballasted lenses. Studies have shown prism- and peri-ballasted lenses to be more stable on eyes that have smaller palpebral fissures and need lower amounts of myopic correction (Young et al, 2002).
Double Slab-Off These differ from prism-ballasted lenses in edge thickness. There is a central, thicker horizontal band that will align within the palpebral fissure with each blink. This design is also known as dynamic stabilization. Multiple studies report that dynamic stabilization techniques are most stable for against-the-rule refractive corrections, as the most minus and thickest meridians of the lens align between the eyelids (Gundel, 1989; Young et al, 2009; Hanks, 1983), yet others found low myopic astigmats to be good candidates for this design (Castellano et al, 1990). Double slab-off lens designs also stay better aligned when the head positioning is away from the vertical axis (Gundel, 1989; Young et al, 2009). This could benefit patients who are highly active or those who require fine vision outside the typical workspace (e.g., a car mechanic or electrician).
Regardless of the lens design, there are anatomical factors that can affect the rotation of toric soft lenses. It would be easy to explain why toric lenses would rotate nasally, due to the motion of the lower eyelid on blink.
However, studies on the forces produced by the eyelids have shown that the upper eyelid contributes more to the position and rotation of toric lenses (Young et al, 2002).
It has been documented that when the temporal canthus is anatomically higher on the face compared to the nasal canthus, the force of the upper eyelid on blink can push a toric contact lens to rotate inferior-temporal (Young et al, 2002). In this study, 80% of eyes had an anatomically higher temporal canthus than nasal canthus.
This knowledge can be used to select a contact lens axis when the patient’s refractive axis falls between the diagnostic lens options. By applying the LARS rule (left add, right subtract) to the lens axis, evaluating eyelid anatomy, and considering patient lifestyle, we can select a lens that will perform better on eye if we anticipate potential factors causing lens rotation. CLS
For references, please visit www.clspectrum.com/references and click on document #243.
Dr. Messer practices in Minneapolis in a private optometry office focused on specialty contact lenses. She is a consultant to Precilens, has received research funding from B+L, and has received honoraria from the STAPLE program and Alden Optical.