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When single-vision spherical daily disposable soft lenses were introduced in the mid-1990s, there was a recognised need for toric lenses to fit those patients who have significant amounts of astigmatism.
From the early days of soft lens fitting, it was established that about 45 per cent of all who request contact lenses have astigmatic refractive errors of 0.75D or more.1 Yet, only 28 per cent of new soft contact lens wearers are prescribed soft torics.2 Although there are several daily disposable toric lenses available for eye care practitioners to choose from, many choose alternative fitting approaches.
Spherical equivalent, single-vision spheres or aspheric lenses are readily prescribed to attempt to reduce blurred vision associated with uncorrected astigmatism. In fact, 2008 data shows that daily disposable lenses accounted for 37 per cent of all contact lens fittings in the UK yet, only 13 per cent of all daily disposable fits were with toric lenses.3 In 2008, the percentage of fits in the UK daily disposable toric market grew only 1.1 percentage points.4 New introductions of daily disposable lenses in toric designs provide eye care practitioners with an opportunity to engage current and prospective lens wearers in discussions of new technologies to help meet their needs.
Over time, there have been numerous attempts at improving soft lens toric designs with the goal of achieving better rotational stability on the eye. It is believed that a soft toric design should ideally minimise any torsional forces that occur from the interaction with the eyelids (especially during a blink). Excellent rotational stability is an important contributor particularly with respect to vision performance and successful, uncomplicated fitting. Through the years, the most widely used designs have incorporated prism ballasting, as they have not only demonstrated predictable on-eye orientation, but have established a cornerstone for overall clinical success.
The use of prism ballasting in soft toric contact lenses evolved in the development of Bausch & Lomb's Lo-Torque design, the platform on which the SofLens Toric, PureVision Toric, and now, the new SofLens daily disposable Toric for Astigmatism have been built (Figure 1).
Design
Lo-Torque design lenses utilise a combination of features to help ensure consistent orientation and stability across the power range hence, clear and stable vision.
Prism ballast geometry
The design features a gentle transition in thickness that extends from the top of the lens through to the inferior mid-periphery. On each blink, the upper eyelid glides across this gradually changing thickness profile of the lens. The resultant forces acting on this prism ballasting geometry maintain the lens in its nominal rotational position, providing fast, stable orientation.
Comfort chamfer
The design also features a 360 degree comfort chamfer. This peripheral zone serves to taper the prism-ballast geometry to a thin and uniform edge profile that extends around the entire lens circumference. In addition, this crescent-shaped zone reduces the overall mass of lens. The resultant lens profile balances interactions with both the upper and lower lids to improve stabilisation, as well as maximising comfort.
Refined optic zones
To help ensure that optimal orientation performance is achieved for each and every lens, whatever the prescription, a third important design element employed in Lo-Torque design is the use of refined optic zones. Through careful selection of different anterior and posterior optic zone diameters depending on the lens power, it is possible to provide consistent lens thickness transitions for every prescription.
Balanced vertical thickness profile
In combination, these design elements of prism ballast geometry, 360 degree comfort chamfer, and refined optic zones act to ensure that the optimal lens profile is consistently achieved. Together, these design features enable similar thicknesses to be maintained at key points across the top, centre and bottom of the lens, regardless of prescription. The result of this balanced vertical thickness profile is uniform, predictable fitting across the entire lens range.
Aspheric optics
In addition to the Lo-Torque design features, the design of SofLens daily disposable Toric for Astigmatism, like that of PureVision Toric, seeks to further improve vision through the application of aspheric optics. Aspheric surfaces are used to fine tune the level of spherical aberration in every lens. The unique design of the SofLens daily disposable Toric for Astigmatism lens also includes a balancing of secondary astigmatism, essentially a difference in spherical aberration in two meridians, to further enhance spherical aberration control.
Unique surface moisturising effect
As in SofLens daily disposable single vision lenses, the new SofLens daily disposable Toric for Astigmatism lenses are stored in a moisture-rich packaging solution that is designed to slowly release and help enhance wearing comfort throughout the day. This ComfortMoist Technology contains the non-ionic surfactant poloxamine. Poloxamine contains a hydrophilic end group and hydrophobic centre group. While the centre group interacts with hydrophobic debris and solubilises the debris, it also attaches to the lens surface via hydrogen bonds and exposes the hydrophilic group, in turn attracting moisture to the site for continuous comfort.
Clinical performance
SofLens daily disposable Toric for Astigmatism lenses have met the high expectations set by its predecessors, the SofLens Toric and PureVision Toric lenses. The health attributes of daily disposable lenses have previously been acknowledged to be excellent, predominantly due to the high rate of compliance.5 In addition to ocular health, vision and comfort are prime attributes which practitioners must be mindful of to ensure patient success with daily disposables. In a two-week evaluation of SofLens daily disposable Toric for Astigmatism lenses, 209 subjects who had at least 0.50D of astigmatism in both eyes and an axis of astigmatism within 20 degrees of horizontal or vertical, wore the lenses in both eyes. The 12 investigational sites in this Bausch & Lomb-sponsored study had the opportunity to evaluate the subjects at a one-week and at a two-week visit.
Investigators evaluated the fit of the lenses at each visit and determined that, for centration, when rated on a four point scale (excellent, good, fair or poor) at the dispensing, one-week and two-week visits, the lenses were rated as either excellent or good in 100 per cent of eyes. Furthermore, at each of these visits, movement (rated as excessive, adequate, insufficient or adherence) was assessed as adequate in at least 85 per cent of eyes. Predictable orientation of toric lenses being a prerequisite for good visual acuity, the Lo-Torque stability features were shown to orientate the lens within ±10° of the 6 o'clock position in at least 88 per cent of eyes at each visit. Consequently, high contrast visual acuity measurements, taken at each visit, demonstrated that at least 94 per cent achieve 6/7.5 Snellen or better (Table 2) with the two axes available (90° and 180°).
In this study, investigators were asked to indicate their level of agreement with a series of statements (strongly agree = 6 strongly disagree = 1) to provide their understanding of how the lens performed for each subject. Supporting the orientation and visual acuity statistics, 87 per cent of investigator responses indicated that they were satisfied with the overall visual quality that the subject was able to attain with the available axes (90° and 180°). Furthermore, in 86 per cent of responses the investigators agreed that they were satisfied with the stability of the subject's vision. Relating to their understanding of the comfort attributes of this lens, for 85 per cent of subjects the investigators agreed that comfort throughout the day was not a problem. For 96 per cent of all subjects, the investigators agreed that the lens was easy to fit and that, overall, 87 per cent of subjects were successfully wearing the lenses.
Bausch & Lomb's significant heritage in toric contact lens design is paralleled by the knowledge and understanding of optical correction. In leveraging this, for the first time, consistent and controlled aspheric optical correction is available in a daily disposable toric lens, allowing the benefits of this technology to be available to a wider group of patients. The average eye has approximately +0.15 microns of spherical aberration (over a 6mm pupil diameter)6 which most spherical and toric contact lenses are not designed to control. These lenses have an amount of spherical aberration that varies with lens power. The controlled aspheric design of SofLens daily disposable Toric for Astigmatism lenses can systematically reduce the amount of spherical aberration in the total optical system (eye and contact lens), resulting in more crisp retinal images. The benefits of improved contrast from this control of spherical aberration are most likely to be evident in situations where the pupil is naturally larger, such as in low illumination environments.
To evaluate the spherical aberration reduction of SofLens daily disposable Toric for Astigmatism lenses across powers, a Bausch & Lomb-sponsored study was conducted with 24 adapted contact lens wearers. Each had one randomly selected eye assessed with a Zywave aberrometer (Bausch & Lomb Incorporated, Rochester, New York, USA). Baseline measurements were taken with no lenses, then subjects were re-measured while wearing three different SofLens daily disposable Toric for Astigmatism lenses (-1.00/ -0.75x180, -3.00/-0.75x180 and -5.00/ -0.75x180), inserted in random order. The Zywave wavefront acquisitions are broken down mathematically using Zernike Polynomials which can separate out elements of over- or under- correction of sphere and cylinder induced by the trial contact lenses. In this manner, spherical aberration was analysed to determine the change from baseline. The results of this investigation validated the optical design and showed that the SofLens daily disposable Toric for Astigmatism lenses significantly reduced the inherent mean spherical aberration of the subjects eyes across the power range of -1.00D to -5.00D (p<0.05) (Figure 3). This study, which recorded the mean primary gaze orientation of the -1.00D, -3.00D and -5.00D lenses as 8°, 6° and 6° from the 6 o'clock position respectively, paid further testament to the stability of the lens.
In striving to meet the demand of all those who wear contact lenses, from those who would never be without them, to those who prefer occasional use, the introduction of a new daily disposable toric contact lens adds to a strong portfolio of Lo-Torque toric lens designs. This lens has incorporated a 20-year design history that has been at the forefront in the development of quality, reproducible and affordable lenses. The SofLens daily disposable Toric for Astigmatism lens has been developed with careful attention to vision and orientation in addition to health and comfort. Strong clinical performance and positive practitioner feedback indicate that this lens can be a valuable addition when meeting the high and varied demands of clinical practice.
References
- Holden BA. The principles and practice of correcting astigmatism with soft contact lenses. Aust J Optom, 1975 58: 279-99.
- Morgan PB. UK prescribing habits. Eurolens Research 2008, The University of Manchester.
- GfK NOP, Opticians Monitor, Oct 2007-Sep 2008.
- GfK NOP, Opticians Monitor, Oct 2007-Sep 2008 v. Oct 2006-Sep 2007.
- Cairns G. Enhancing contact lens design for complete performance. Optician, Nov 2007 14-16.
- Thibos LT et al. Statistical variation of aberration structure and image quality in a normal population of healthy eyes. J Opt Soc Am A, 2002. 19:2329-2348.
? Dr Gerard Cairns is manager of medical affairs for Bausch & Lomb Global Vision Care, Rochester, New York, USA. Tim Green is manager of lens design for Bausch & Lomb, Rochester, New York, USA. Dr Gary Orsborn is director of medical affairs for Bausch & Lomb Global Vision Care, Rochester, New York, USA