SILICONE HYDROGEL TORICS
Fitting and Vision Characteristics of Two Silicone Hydrogel Torics
This study compared the performance of two silicone hydrogel lens designs for correcting astigmatism.
By Bill Reindel, OD, MS, & Gary Orsborn, OD, MS, FAAO
Dr. Reindel is director of medical affairs for Bausch & Lomb.
Dr. Orsborn is director of medical marketing for Bausch & Lomb.
While prospective contact lens wearers indicate that appearance is the main reason (51 percent) they are interested in contact lenses, a significant number of prospective wearers (29 percent) are interested because of the potential for better vision that contact lenses offer. Unfortunately, 24 percent of spectacle wearers who believe they might have a problem that would prevent contact lens wear cite astigmatism as the potential deterrent.
With the introduction of modern soft toric lenses in both silicone hydrogel and hydrogel designs, eyecare practitioners have an opportunity to engage prospective and current contact lens wearers with discussions of new technologies available to help meet their needs.
Today's toric contact lenses offer advanced designs to help provide patients with better vision. Adding silicone hydrogel materials to the toric mix has been a welcome addition for patients and practitioners alike. According to information from Health Products Research (HPR), of the approximately 36 million lens wearers in the United States, 20.1 percent of them were prescribed toric lenses in the past year. HPR data through Q2 2007 indicates that the usage of silicone hydrogel toric lenses is clearly on the rise, with silicone hydrogel toric lens prescribing increasing 37.3 percent. There is still a segment of the astigmatic population that relies on hydrogel torics for their vision correction; however, many new toric patients are bypassing hydrogel torics altogether and moving directly into silicone hydrogel toric lenses. This discussion will focus on two silicone hydrogel offerings for astigmatic patients.
Studying Two Silicone Hydrogel Toric Designs
Two prominent silicone hydrogel toric designs include the dual thin zone profile used in the Acuvue Advance for Astigmatism (galyfilcon A, Vistakon) lens or the balanced vertical thickness profile used in the PureVision Toric (balafilcon A, Bausch & Lomb) lens. Both designs use the natural force of the lids to orient and center the lens during and between blinks. The PureVision Toric has also integrated aspheric optics into the lens design to help reduce the average amount of positive spherical aberration of the eye and to help improve retinal image quality in low light conditions.
Recognizing that lens design and material properties can influence fitting characteristics and vision, this evaluation compared the fitting and vision characteristics of these two silicone hydrogel toric lenses.
Eighteen practitioners participated in this open-label, prospective, multi-center study. At the time of enrollment, subjects were established Acuvue Advance for Astigmatism wearers. To ensure that the subjects were successful with their toric lenses, they had to have worn the Acuvue Advance for Astigmatism lenses for at least one month. Patients' habitual Acuvue Advance for Astigmatism lenses were assessed for fit/orientation and visual acuity at an initial visit.
Practitioners refitted patients into PureVision Toric lenses and performed equivalent fit and vision assessments at the initial/dispensing visit (V-1) and after two weeks of PureVision Toric lens wear (V-2).
A total of 107 patients were refitted from Acuvue Advance for Astigmatism lenses to PureVision Toric lenses and completed the two-week follow-up evaluation. Of these, the majority of subjects (57.9 percent) had successfully worn their Acuvue Advance for Astigmatism lenses for more than six months (Table 1). The mean spherical power from the refraction was –3.00D. The mean cylinder power was –1.25D.
Figure 1. Orientation of lenses by evaluation.
Figure 2. Centration of lenses by evaluation
Orientation At the initial visit, rotation of the Acuvue Advance for Astigmatism lenses ranged from 0 degrees to 5 degrees for 81.1 percent of the lenses. Rotation of 0 degrees to 5 degrees was reported for 84.9 percent and 77.9 percent of PureVision Torics at the V-1 and V-2 visits, respectively (Figure 1). A comparison of the proportion of lenses that had an orientation of 0 to 5 degrees demonstrated that there was no significant difference between Acuvue Advance for Astigmatism and PureVision Toric lenses.
Centration Practitioners rated lens centration as fully centered (Excellent), slight decentration with no corneal exposure (Good), decentered with slight corneal exposure (Fair), or incomplete corneal coverage or edge lift (Poor). The proportion of eyes reported as fully centered was 77.5 percent for the Acuvue Advance for Astigmatism lenses, 88.0 percent for the PureVision Toric lenses at V-1 and 90.6 percent for PureVision Toric at V-2 (Figure 2). The difference in proportions was statistically significant (p<0.05), with PureVision Toric lenses being fully centered more frequently at both V-1 and V-2.
Movement Practitioners rated lens movement as adequate, excessive (>0.6mm), insufficient (<0.2mm) or adherent. The proportion of Acuvue Advance for Astigmatism lenses reported to have adequate movement was 92.0 percent. The proportion of Purevision Toric lenses reported to have adequate movement was 97.5 percent at V-1 and 96.1 percent at V-2. Only the PureVision Toric V-1 movement results demonstrated a greater proportion of adequate movement and were statistically different from Acuvue Advance for Astigmatism (p<0.05).
Visual Acuity The proportion of eyes exhibiting 20/20 or better visual acuity was 74.5 percent for the Acuvue Advance for Astigmatism cohort and 85.9 percent at V-1 and 85.2 percent at V-2 for PureVision Toric lenses (Figure 3). At both V-1 and V-2, there were significantly more eyes wearing PureVision Toric lenses that demonstrated 20/20 or better visual acuity (p<0.05). At V-2, 55 eyes (27.7 percent) improved one line or more in acuity, whereas only 21 eyes (11.8 percent) decreased one line or more. The number of eyes with improved acuity was statistically more favorable (p<0.05).
Both lens design and eyelid dynamics play important roles in providing astigmatic contact lens patients with crisp, stable vision. As manufacturers develop silicone hydrogel toric options, they have reported design improvements over their previous hydrogel toric lenses.
The PureVision Toric's Advanced Lo-Torque design features a number of attributes that help the lens orient toward 0 degrees and help minimize rotational forces applied by the lids during the blink. These attributes include a 360-degree comfort chamfer, which reduces mass at the base of the lens and helps equalize the thickness around the lens periphery; and a refined optic zone, which helps minimize variations in thickness profile with power by adjusting the anterior and posterior optic zone diameters. In addition, the PureVision Toric lens has an aspheric front surface that is designed to reduce the inherent positive spherical aberration of the eye.
The Acuvue Advance for Astigmatism lens design utilizes four active zones near the midperiphery of the lens to help minimize lens rotation. These stabilization zones have a thicker profile compared to the rest of the lens. The stabilization zones are designed to be located within the interpalpebral aperture.
While this evaluation demonstrated that lens orientations were similar, results also demonstrated that the PureVision Toric contact lens provided better visual acuity. Therefore, other design and lens/eye fit factors are likely contributing to these better visual outcomes.
Results demonstrated that lens centration was different between the two products. The PureVision Toric had a greater proportion of eyes for which the lens was fully centered. Disparities in the proportion of eyes with fully centered lenses could indicate that the lid forces are interacting with the lens geometry differently. Proper centration plays an important role in aligning the lens optics with the visual axis.
Other factors, such as material and surface characteristics, can play a role in the overall optics and contribute to better vision outcomes. For example, the PureVision Toric lens design offers an aspheric anterior surface to help reduce positive spherical aberration, and the anterior surface asphericity is adjusted across the range of powers. Purposely controlling the spherical aberration of a contact lens may reduce the average amount of positive spherical aberration and improve the retinal image quality in low light and/or large pupil conditions.
Figure 3. Visual acuity of lenses by evaluation.
Reconsider Toric Lens Options
Advancements in contact lens materials and designs provide eyecare practitioners with an opportunity to discuss the benefits of contact lenses with spectacle wearers. This refit study demonstrated that factors such as lens design and lens centration may play a contributing role in improving visual outcomes. CLS
This study was funded by Bausch & Lomb.
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