Stability is an important quality in toric lenses

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Historically, constraints such as limited parameter availability, lens rotation, and unreliable stabilization methods meant that some optometrists avoided prescribing soft toric contact lenses. The perception was that soft toric contact lenses took more chair time than they were worth and did not deliver visual outcomes that outperformed spherical options, especially for patients with low amounts of astigmatism. For some, a few bad experiences with toric lenses 10 or even 20 years ago may have left a negative impression that persists.

With the modern soft toric options we have available to us, however, most toric soft lenses stabilize easily on most eyes, and there is no appreciable difference in chair time to fit them.¹ Toric parameter availability has improved exponentially in recent years. We can now offer most patients a toric soft contact lens option with the optimal cylinder power and axis, even in a daily disposable modality. Not surprisingly, toric lens fits have increased in the US and around the world,² but there is still room to expand toric soft contact lens prescribing and offer these lens options to more patients.²

Patients with astigmatism of 0.75 diopter (D) or greater in 1 or both eyes benefit from correction with an astigmatic contact lens. Depending on the amount of astigmatism, patients can gain 1 to 2 lines of visual acuity when switched from soft spherical to soft toric lenses.^3,4 Even if patients with astigmatism can achieve acceptable visual acuity in a spherical lens, studies have shown that visual clarity improves with toric lenses.^1,4 Toric correction improves astigmatic patients’ near contrast acuity and digital device task performance compared with spherical lenses.⁵ Patients report lower levels of ocular fatigue and significantly greater satisfaction with visual quality in toric lenses.⁴ Perhaps most importantly, 80% to 85% of patients subjectively prefer toric lenses to spherical options when given the choice.^4,5

For all these reasons, refitting new and established patients who have historically worn spherical lenses with a more appropriate toric lens is a great way to enhance your patients’ visual experience and demonstrate the importance of annual eye exams to ensure they are wearing the best option for their visual needs.

Stabilization technologies

Rotational stability is a crucial characteristic of a toric lens. Because toric soft contact lenses are designed to correct astigmatism on a particular axis, they must be correctly oriented on the eye. Contact lens manufacturers use various combinations of prism ballasting and thickness distribution to allow toric lenses to stabilize on the eye after application and throughout the day with blinking and eye movements. Stabilization technologies continue to evolve and improve with each new product that hits the market.

Prism ballast and peri-ballast designs are weighted in the optical zone or the periphery of the lower portion of the lens to take advantage of gravity when orienting the lens. Although this is an older stabilization technique, it has been continuously modernized for contemporary lenses. Alcon’s approach, for example, is the Precision Balance 8|4 design, which redistributes the ballast from the traditional 6-o’clock position to 4 o’clock and 8 o’clock to improve stability and reduce lower lid interaction.

Johnson & Johnson Vision’s toric lenses rely on Blink-Stabilized Design (BSD). With BSD, 4 unique stabilization zones work with the eyelids to quickly stabilize the lens and realign it naturally with every blink. Harnessing the power of the blink in this way helps keep the lens in the correct position during head tilts, blinks, and extensive eye movements. Because BSD does not have prism in the optical zone, as prism-ballast and peri-ballast styles do,6 patients wearing a toric lens in just 1 eye will not experience “image jump” or headache due to vertical prism disparity.

Assessing stability

Most toric lenses will stabilize on most patients, but it is also important to recognize that no single toric lens design will work for every patient. Sometimes, it is apparent in-office that a particular design is not working, but most patients need to trial the lenses during their everyday activities to determine whether vision stability is acceptable. Although objective in-office visual acuity measurements are important when evaluating a len’s visual potential, it is important to remember that visual acuity alone does not tell us the full story about a patient’s visual experience.⁷ Factors such as versional eye movements, peripheral targets viewing, and activities that change head posture (eg, lying on a couch watching TV) can impact lens stability and visual performance.^7,8 When trialing a toric lens for the first time, educate patients on what they might experience if the lens is not stabilizing well, and let them know that you can troubleshoot any visual issues they experience during the trial period.

When assessing the stability of a toric lens in the office, the lens should first be allowed to settle for several minutes. Rotation and stability can first be assessed by considering vision. After checking objective visual acuity, ask patients to subjectively assess vision and stability of visual clarity when they look at the eye chart and look away to a target in the periphery. These diagonal or versional movements challenge a toric lens’ stability and may give you and the patient a sense of how stable the lens and vision will be during everyday activities.

Rotation can be assessed and quantified in the slit lamp. Knowing that most lenses stabilize on most eyes, if you see large amounts of rotation early on, you might want to reach for a brand utilizing a different stabilization design category. Generally, addressing rotation depends on the magnitude and stability of the rotation (Table).

With most modern lens designs, it is rare to have to alter the axis significantly based on rotation. If you do find yourself needing to modify axis based on rotation, however, use the rule of LARS (“left add, right subtract”) where you add or subtract axis degrees from the manifest refraction axis based on the direction and magnitude of rotation you are viewing in the slit lamp.

Every soft contact lens wearer with 0.75 D or more of astigmatism should wear a soft toric design. We don’t commonly replace astigmatic correction for the spherical equivalent option when writing glasses prescriptions, so we shouldn’t sacrifice vision quality in that way for our soft contact lens wearers. In a market where patients can go online to update their contact lens prescriptions, offering a toric upgrade can emphasize to the patient the value of checking in annually with their eye care provider. About half of all soft contact lens wearers have 0.75 D or more of astigmatism in at least 1 eye, which means most eye care providers could upgrade many patients in their practice to a toric option. By showing patients you are considering their individual and unique refractive needs, you can underline the value of yearly contact lens eye care, prevent contact lens drop out, and create loyal patients for years to come.

References:

1. Cox SM, Berntsen DA, Bickle KM, et al. Efficacy of toric contact lenses in fitting and patient-reported outcomes in contact lens wearers. Eye Contact Lens. 2018;44(suppl 1):S296-S299. doi:10.1097/ICL.0000000000000418

2. Morgan PB, Efron N. Global contact lens prescribing 2000-2020. Clin Exp Optom. 2022;105(3):298-312. doi:10.1080/08164622.2022.2033604

3. Richdale K, Berntsen DA, Mack CJ, Merchea MM, Barr JT. Visual acuity with spherical and toric soft contact lenses in low- to moderate-astigmatic eyes. Optom Vis Sci. 2007;84(10):969-975. doi:10.1097/OPX.0b013e318157c6dc

4. Chaudhry M, Sah SP, Sharma IP, Mondal S. Does offering only the spherical contact lens trial to the low astigmats mislead practitioners? Int J Ophthalmol. 2021;14(8):1281-1284. doi:10.18240/ijo.2021.08.21

5. Logan AM, Datta A, Skidmore K, et al. Randomized clinical trial of near visual performance with digital devices using spherical and toric contact lenses. Optom Vis Sci. 2020;97(7):518-525. doi:10.1097/OPX.0000000000001540

6. Sulley A, Hawke R, Lorenz KO, Toubouti Y, Olivares G. Resultant vertical prism in toric soft contact lenses. Cont Lens Anterior Eye. 2015;38(4):253-257. doi:10.1016/j.clae.2015.02.006

7. Chamberlain P, Morgan PB, Moody KJ, Maldonado-Codina C. Fluctuation in visual acuity during soft toric contact lens wear. Optom Vis Sci. 2011;88(4):E534-E538. doi:10.1097/OPX.0b013e31820ea1ea

8. McIlraith R, Young G, Hunt C. Toric lens orientation and visual acuity in non-standard conditions. Cont Lens Anterior Eye. 2010;33(1):23-26. doi:10.1016/j.clae.2009.08.003