A turning point in toric soft lens design

I mprovements in lens design over the past decade have made toric soft lenses a popular prescribing option for astigmatic patients. Prescribing of toric designs has increased around the world in recent years; one in four of new soft daily wear fits is a toric lens, compared to one in five in 2003.1 In the UK, a third of new fits were torics.2 New manufacturing techniques have brought improvements in reproducibility and made toric soft lenses simpler and quicker to fit. Daily disposable and silicone hydrogel (SiH) options are now available for enhanced physiological and comfort performance. A major advance has been the introduction of toric soft lenses utilising the Accelerated Stabilisation Design (ASD). This was developed after much research to understand what happens during blinking and the interaction between the lids and the lens. This successful design is now available in three materials and different modalities: 1-Day Acuvue for Astigmatism; Acuvue Advance for Astigmatism; and, most recently, Acuvue Oasys for Astigmatism which is now being made available across Europe. ASD lenses have shown a number of advantages over traditional designs in reducing variable vision and blur.3-6 In prism-ballasted and dual-thin zone designs, the lens can interact with the lid(s) during the blink, even when correctly aligned, which can in turn result in unwanted lens rotation. With ASD, when the lens is in the correct position there is minimal destabilising interaction with the lids. Only when the lens is misaligned, for instance, when first placed on the eye, does lid interaction have maximum effect. The upper and lower lid forces therefore continually orient and stabilise the lens to return it to its correctly orientated position. For the practitioner, achieving a stable and reliable fit while minimising chair time are key factors in successful toric soft lens fitting.3 Unstable lens orientation and variable visual acuity were among the reasons practitioners have traditionally cited for not fitting more soft torics.7 For the wearer, fast lens orientation is equally important so that they can achieve optimal vision of variable vision. Visual acuity and slit-lamp assessment of lens movement, centration and rotation may not necessarily correlate with the patient’s experience of vision quality during normal daily activities. Astigmats have high expectations, particularly with regard to vision and comfort. In a survey of 335 toric soft lens wearers a few years ago,8 prior to the introduction of ASD lenses, only 70 per cent described their habitual toric lenses as ‘excellent’ or ‘very good’, citing vision quality and stability and comfortrelated factors as the most important attribute. Of those reporting symptoms, 86 per cent said they experienced blurred vision and 57 per cent reported fluctuating vision. This compares with 82 per cent rating the overall opinion of a spherical silicone hydrogel lens as ‘excellent’ or ‘very good’.9 A recent peer-reviewed paper by Zikos et al describes novel techniques for monitoring toric lens rotational stability over a range of natural viewing conditions chosen to mimic real-world situations.4 The aim of the study was to determine whether using a more natural test procedure, allowing large versional eye movements and frequent blinking, could be used to objectively compare the rotational stability of toric soft designs. After an initial settling period, four visual tasks and stimuli were chosen – settling time, reading, visual search and large versional tasks − after each of which the lens position (degrees of rotation) was recorded in primary gaze. The tasks conducted required increasing amounts of ocular versions, and hence had the potential to destabilise lenses. First, the patient was allowed to look around the consulting room for 15 minutes while the lens settled on the eye. The patient then read a broadsheet newspaper for two minutes, held at 40cm with text extending 40° horizontally and 15° vertically. Next the patient was asked to spot a given number within the newspaper, read that paragraph, and then return to primary Figure 1 Visual search task using paragraphs highlighted in a newspaper (after Zikos4)