If this is the dawning of the age of silicone hydrogels, it is also the coming of age for toric soft lenses. The emergence of well designed, reproducible, frequent replacement lenses has brought toric soft lenses into mainstream contact lens practice from their former position as specialist lenses used only with care and trepidation. Although astigmats represent approximately one-third of potential contact lens wearers, the proportion of toric soft lenses prescribed has been much lower than this. In Europe we have seen an encouraging increase in the number of astigmats fitted with toric lenses. In the UK, Morgan et al have reported a doubling in the use of toric soft lenses to 27% of new fits over a five year period [1,2].
It is a natural progression to use silicone hydrogel materials for toric soft lenses, particularly as silicone hydrogels have increasingly come to be used for daily wear. Since, by their nature, toric soft lenses are thicker than spherical lenses, the benefits of increased oxygen transmission are even more compelling than with spherical lenses.
In a previous article, I speculated that prism ballasted designs might be unsuitable for silicone hydrogel torics [3]. I suggested that silicone hydrogel materials might require new, special toric designs in order to overcome some of their limitations and to maximise oxygen transmission and reduce mechanical trauma. However, I must confess to having made this statement prior to the benefit of first hand experience. In the event, both conventional and new designs have been used in recently launched silicone hydrogel torics and, in both cases, to good effect.
Bausch & Lomb’s PureVision Toric uses a prism ballast design similar to that of the Soflens 66 Toric with a few minor differences [4]. The PureVision Toric incorporates a similar diameter and edge profile to the PureVision sphere with the aim of allowing low astigmats to be easily upgraded from PureVision to PureVision Toric. We have found that low astigmats (i.e. 0.75 or 1.00DC) gain nearly one line of visual acuity when fitted with torics and most enjoy similar comfort to their previous spherical lenses [3].
The PureVision Toric is claimed to provide more stable orientation due to increased prism ballast. However, the stiffer modulus of balafilcon A compared to the Soflens 66 material (alphafilcon A), in my experience, also seems to aid stability and centration. Brennan and Coles have shown that corneal swelling with PureVision Toric is similar to that of the spherical version ( see poster [6]). My previous calculations of reduced transmissibility had overlooked the effect of lateral diffusion.
In a break from convention, Vistakon’s silicone hydrogel toric is branded ‘Acuvue Advance for Astigmatism’. Perhaps this follows the cosmetic industry’s recent trend of naming products in plain English so that consumers understand what they actually do; but I like to think that it’s intended to shame recalcitrant practitioners into admitting to patients that astigmatism is not a contra-indication for soft contact lens wear. The Acuvue Advance ‘toric’ utilises a novel design (Accelerated Stabilization design) which, in contrast to previous horizontally symmetric designs, places the stabilization zones in the inter-palpebral zone in order to take full advantage of the forces generated during blinking [7].
The imminent launch of Ciba’s O2 Optix Toric will provide a third option which one hopes will enable an even wider range of ocular topographies to be successfully fitted.
Both of the marketed designs show clinical performance characteristics that improve on older products. Since this is an editorial, I am not obliged to provide hard data to support my statements and, in any case, clinical reports often poorly characterize the performance of toric soft lenses. With all soft lenses, we are interested in efficacy as determined by visual acuity and safety in relation to slit lamp findings and complications but, with toric soft lenses, we are also interested in orientation which can be characterized in terms of predictability, stability, settleability, and repeatability. How quickly does the lens reach its orientation position, how consistent is the orientation position on a range of eyes and how stable is the lens once settled? I propose that we use more consistent measurements to monitor improvements and draw comparisons in toric soft lens performance (see panel).
Suggested measures of toric soft lens performance:
Predictability: Proportion of lenses orientating within 5° of the zero position
Stability: Proportion of lenses maintaining orientation within ±10° of settled position during normal range of eye movements
Settleability: Proportion of lenses orientating within 5° of the final settled position within 3 minutes of insertion
Repeatability: Proportion of lenses of similar specification orientating, when settled, within 5° of the median position |
Thirty years after the birth of toric soft lenses, it is heartening that real strides are being made in the design and usage of toric soft lenses. Advances in frequent replacement torics have already changed the way we view the fitting of toric lenses. The advent of silicone hydrogel toric lenses extends the benefits of high Dk soft lenses to the sizeable astigmatic population.
References
- Morgan P, Efron N. Trends in UK contact lens prescribing 1999. Optician 1999; 217(5700): 43-44.
- Morgan P, Efron N, Woods C, et al. International contact lens prescribing in 2004. CL Spectrum 2005; 20(1):34-37.
- Young G. Toric contact lens designs in hyper-O2 materials. CLAO J 2003; 29:S171-173.
- Robinson R. Soft toric lenses design matters. CL Spectrum 2005; 20(7):52-54.
- Bayer S, Young G. Fitting Low astigmats with toric soft contact lenses - What are the benefits and how easily is it achieved? Poster, American Academy of Optometry Meeting, San Diego, Dec. 2005.
- Brennan N, Coles MC. Overnight corneal swelling with a silicone hydrogel ( balafilcon A) toric lens. Optom Vis Sci 2004; 81(12S):
- Szczotka-Flynn LB. Toric CLs: the next phase of silicone hydrogel materials. CL Spectrum 2005; 20(4):23.
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Graeme Young MPhil PhD FCOptom DCLP FAAO