Toric Lens Performance: Insights Into Orientation Stability
Gerard Cairns, PhD, MCOptom, FAAO, Paul China, OD, BS, FAAO, Tim Green, MS, Bill T Reindel, OD, MS
Bausch & Lomb Inc., Rochester, New York, USA
The technology used to create soft toric contact lenses for the treatment of astigmatism has advanced remarkably in the last two decades, bringing more options and variety for astigmatic patients. The ability to provide a wide range of correction is important as astigmatism is a fairly common vision disorder. In the general population, the prevalence for spectacle astigmatism of 1 diopter or more is ~30%, as seen in several studies, such as a U.S. study of more than 2500 children.1 Other works have estimated that in the contact lens-using population, the prevalence of astigmatism of ≥0.75D was 45%.2 Despite the variety of options available, however, a vast number of astigmatic patients do not wear soft toric contact lenses.3 With such a large pool of potential soft toric contact lens wearers, it is important that lenses not only correct vision properly, but that they do so to a degree that results in high patient satisfaction.
SOFT TORIC CONTACT LENSES
To effectively neutralize astigmatic refractive error, a toric lens aligns its axis of cylinder correction to the axis of astigmatic error. Different stabilization designs keep the cylinder axis aligned with the eye. Some of these designs include: prism ballast, thin zones (also known as double slab-off), posterior toric, chamfering, truncation and combinations that incorporate different design features into a single lens.4
Since soft toric contact lenses were introduced, a number of techniques have been developed to assess rotational stability. In addition to a simple orientation assessment of a well settled lens on an eye looking straight ahead (primary gaze orientation [PGO]), evaluating the rate or degree of orientation recovery is another essential means to assess soft toric contact lenses. The ability of a lens to return to its original position following misorientation, for whatever reason, can have a profound effect on the vision correction the lens provides.
Rotational Recovery Method of Evaluation
Patient-masked clinical studies were conducted to evaluate two soft toric lens designs, Lo-Torque® (Bausch + Lomb, Rochester, N.Y.) and Accelerated Stabilization (AS) design (Vistakon, Jacksonville, Fla.).5,6 The orientation recovery of the designs was assessed across eight power and axis configurations. Institutional review board approval was obtained for the studies and all patients signed an informed consent form prior to participation.
In the first study, the Bausch + Lomb Lo-Torque design of PureVision® Toric was compared with the AS design of Acuvue Advance for Astigmatism. In the second study, the Bausch + Lomb Lo-Torque design of PureVision Toric was compared with the AS design of Acuvue Oasys for Astigmatism.
In each study, 32 habitual contact lens wearers (64 eyes) were randomly assigned a Lo-Torque and an AS design parameter-matched lens pairing on each eye. The sphere component of the lens power was −1.00D or −5.00D; for each spherical power, the cylinder component was −0.75D or −2.25D, and the axis of the cylinder was 90° or 180°, making a total of eight lens power/axis combinations.
The first lens was applied and allowed to settle for 3 minutes before the fit was assessed. Rotational recovery was assessed 1 minute after the lens was manually displaced 45° temporally from the primary gaze orientation position. To assess rotational recovery, the measure of interest was the angular difference between PGO and the orientation to which the lens recovered after 1 minute (Figure 1). The lens was then removed, the next lens applied and the procedure repeated.
Figure 1. A) To assess rotational recovery, the lens was manually rotated temporally by 45° using a surgical sponge. B) A lens demonstrates good rotational recovery when it returns to the primary gaze position within 1 minute. Note: These images are for demonstration only. Study lenses were not artificially marked.
Rotational Recovery Results
The studies revealed the mean (± SD) orientation recovery was 5.8° (± 7.3°) and 4.3° (± 4.3°) for the PureVision lenses and 10.7° (±13.5°) and 7.7° (± 7.8°) for the Advance for Astigmatism lenses and the Oasys for Astigmatism lenses, respectively. The results demonstrated statistically significant difference in favor of the PureVision Toric lens in both studies for rotational recovery (T-test; P<.05 in both studies). Post-hoc analysis showed no significant interaction with sphere power, cylinder power or cylinder axis (Tukey's HSD, P>.05). Additionally, significantly less variability was observed with the PureVision lens (Levene's Test, P<.05 in both studies).
Furthermore, in both studies a significantly greater proportion of PureVision Toric lenses returned to within 10° of PGO (χ2 Test, P<.05 in all cases). The percentage of Advance for Astigmatism lenses and Oasys for Astigmatism lenses that did not recover to within 10° of PGO was 30% and 23%, respectively. This value for the PureVision Toric lenses in both studies was 12% and 3%, respectively (Figure 2).
Figure 2. Proportions of lenses returning to within 10° of primary gaze orientation within 1 minute.
To provide consistent vision correction, soft toric contact lenses must keep a stable orientation. Effective cylinder power is reduced with rotation of the lens on the eye, and at 30° misorientation, the cylinder of a soft toric lens is almost entirely ineffective. When a toric contact lens rotates from the PGO, lens design characteristics can play an important role in realigning the lens to the PGO.
Figure 3 illustrates the schematic of a dual thickness design and a prism-ballasted design that have been rotated in a temporal direction. The arrows highlight the direction of the lid motion and the resulting lid forces during the blink. As the lids close over the misaligned lens, the upper lid moves down and nasally in a rapid motion. This is followed by a temporal motion and slower return to the open position. It is believed that return to the PGO by a rotated dual thickness lens can be delayed, as the thicker ballast region can be caught by the upper lid. The illustration highlights that the return to the PGO by a rotated prism-ballast lens is less likely to be caught by the lid forces of the upper lid. The results of these two studies demonstrated that the Lo-Torque design lenses had a greater proportion of lenses return to within 10° after 60 seconds when compared with the AS design lenses.
Figure 3. Schematic of rotated lens design interaction with eyelids. A) Dual thickness design. B) Vertical thickness tapered design. Note: These images are for demonstration only and may not represent the actual thickness profiles of the study lenses.
In the brief time practitioners have to assess individual toric lenses during the fitting process, an evaluation of rotational recovery can provide insights about lens stability and vision consistency when the patient has left the office. Careful attention to patients' symptoms associated with various activities, such as eye rubbing, could point to difficulties due to a lens with poor rotational recovery.
In choosing a toric contact lens, practitioners should consider the ability of a lens to recover to the PGO following rotation. Further work related to the lid force and lens design interactions should establish what factors contributed to the Lo-Torque design demonstrating a greater propensity to return the lens to its original position.
1. Kleinstein RN, Jones LA, Hullett S et al. Refractive error and ethnicity in children. Arch Ophthalmol. 2003;121:1141-1147.
2. Holden BA. The principles and practice of correcting astigmatism with soft contact lenses. Aust J Optom. 1975;58:279-299.
3. Morgan PB. Trends in UK contact lens prescribing. Optician. 2009;June 5:20-21.
4. Russell CH et al. The correction of astigmatism with soft contact lenses. Ophthalmol Clin North Am. 2003;16:353-358.
5. Cairns G, China P, Green T, Reindel WT. Differences in Toric Lens Performance: Lens Orientation and Orientation Recovery. Paper presented at: Annual Meeting of the American Academy of Optometry; Nov. 11-13, 2009; Orlando, FL.
6. Cairns G. Toric Lens Stability: Insights and Assessments. Presentation at: Annual Meeting of the American Academy of Optometry (Breakfast Seminar); Nov. 11-13, 2009; Orlando, FL.