The clinical performance of senofilcon A

This article is best viewed in a PDF Format.

Since the launch of Acuvue Oasys (senofilcon A), there have been several peer-reviewed publications examining the lens performance, including investigations into the comfort of the lens in potentially challenging environments. Eye care practitioners will often base lens selection initially on manufacturers' literature and subsequently on their own experience of product performance.

The most credible sources of information on lens performance are study results published in peer-reviewed journals, where the study design and results will have demonstrated that they follow good clinical practice guidelines. The aim of this article is to review the peer-reviewed literature relating to the clinical performance of Acuvue Oasys with Hydraclear Plus.

Comfort and clinical performance in challenging environments

Dryness and discomfort are the most common complaints of contact lens wearers, with anywhere between 37 and 73 per cent of wearers complaining of dryness during their normal lens wearing period.1,2 In addition, dryness and discomfort are the most commonly cited reasons for dropping out of CL wear, with one study finding that 51 per cent of previous lens wearers blamed discomfort as the main reason for giving up lens wear.3 There have been several papers examining patient comfort with lenses made with senofilcon A material.

Riley, Young and Chalmers4 estimated a prevalence of problems in current wearers of soft CLs from a population of 1,092 existing lens wearers. Soft CL wearers from across the US and Canada were interviewed about current symptoms including dryness and discomfort, and asked to complete a dry eye questionnaire. They were also asked about their typical average wearing time and period of comfortable wear. If comfortable wearing time was two or more hours less than actual average wearing time, this was considered to be a significant indicator of less than successful wear. A slit-lamp examination was carried out and the lens wearers were then classified as 'problem' wearers or 'problem-free'.

Fifty-two per cent of the wearers were classified in the 'problem' group, with reduced comfortable wearing time (31 per cent) and dryness (28 per cent) the most often reported symptoms. A subset of these 'problem' wearers were refitted with senofilcon A lenses. Refitting was found to be very effective at reducing symptoms of dryness and discomfort after a two-week period, with 73 per cent of wearers reporting a decrease in dryness symptoms from 'frequent' or 'constant' to 'infrequent' or 'never'. A large proportion of wearers (88 per cent) reported some improvement in symptoms of discomfort. Signs of limbal and bulbar hyperaemia and corneal staining were all reduced. These results are summarised in Figure 1. Although comfortable wearing time still remained a problem in some cases, the average comfortable wearing time for these lens wearers increased from 10.4 (± 3.3) to 11.6 (± 3.8) hours (Figure 2). The authors concluded that refitting problem patients with a new generation SiH, such as with senofilcon A CLs, can reduce some of the problems experienced by many soft CL wearers.

The increased oxygen delivery to the cornea associated with SiH lenses when compared to hydrogel lenses reduces signs of limbal and bulbar hyperaemia. The lower water content of SiH materials may result in lower on-eye dehydration rates compared with traditional hydrogel lenses.5 In particular, the lubricious and highly wettable senofilcon A lenses may increase patient comfort by reducing the disruption of the post lens tear film and reduce lid sensation during blinking.4,6

Young et al7 carried out a study looking at the range of challenging environments experienced by hydrogel lens wearers in everyday tasks and the lens comfort in these situations. The study then looked at the effect on comfort of refitting these lens wearers with one of three second-generation SiH CLs (senofilcon A, galyfilcon A and lotrafilcon B). The type of situation that can cause discomfort during lens wear includes computer use, demanding task content such as driving at night or reading, environments such as air-conditioning or heating, low humidity, smoky, dusty or polluted environments and napping in lenses. The most common challenging situations reported in the study were driving at night, using a computer and being in a heated or air-conditioned car.

Following a refit with the SiH CLs, many wearers reported some increase in comfort in the reported challenging situations. However, those refitted with the senofilcon A lenses reported a statistically significant improvement in comfort in all of the challenging situations, including computer use.

The most recently published study looking at the effect of environmental factors on the ocular discomfort of soft CL wearers was carried out by Ousler et al.8 They recognised the need to control the variation in environmental and behavioural differences among subjects to enable more accurate evaluation of study lenses. They used the Controlled Adverse Environment (CAE) model to investigate dryness during lens wear and in particular the effect of ocular drying symptoms during CAE exposure when wearing senofilcon A lenses compared with the wearers' habitual lenses.

The CAE model is an evaluated model used in dry eye studies and is designed to exacerbate dry eye symptoms in a repeatable, controlled manner. It uses closely regulated humidity, temperature, airflow and lighting conditions as well as visual tasks, in a way that is representative of the adverse environmental conditions encountered in the daily life of contact lens wearers. The randomised, cross-over, double-masked design included 11 pre-existing lens wearers. The patients were exposed to the CAE for 75 minutes on three occasions, once wearing no lenses, once wearing their habitual lenses and once wearing senofilcon A lenses. There were no statistically significant differences in the slit lamp findings after each of the CAE exposures. The discomfort scores increased with the amount of time spent in the CAE as would be expected. There were statistically significant improvements in discomfort scores with senofilcon A lenses across all time points of CAE exposure compared with the wearers' habitual lenses and also in comparison with no lens at all (Figure 3). The authors concluded that wearers experience less subjective discomfort when wearing senofilcon A lenses than when wearing either no lens or their habitual contact lens in a controlled adverse environment.

Physiological performance of Senofilcon A

All SiHs have a greater capacity to allow oxygen to reach the eye when compared to hydrogel lenses, whether in the open- or closed-eye environment. Efron et al9 published independent estimates of the oxygen permeability (Dk) of several commercially available SiH materials, including senofilcon A. They found that the measured Dk of senofilcon A (107.4 ± 7.4) was in very good agreement with the manufacturer's quoted value (103) and the claimed Dk value fell within the 95 per cent confidence interval of the experimentally measured value.

Steffen and Schnider10 studied the corneal swelling associated with overnight wear of SiH CLs. The normal cornea swells by approximately 2-4 per cent overnight in a closed-eye environment, but then quickly returns back to normal after eye opening. Any CL wear will potentially reduce the amount of oxygen available to the cornea and the original Holden and Mertz criteria suggested that a lens should have a Dk/t of 87 x 10-9 cm s-1 mLO2 mL-1 mmHg-1 to limit overnight corneal swelling to normal physiological levels.11

Steffen and Schnider compared the corneal swelling associated with senofilcon A to two other SiHs approved for continuous wear, as well as a mid-water content traditional hydrogel (etafilcon A) and no lens at all. They found that after eight hours of closed-eye wear, the corneal swelling associated with senofilcon A was 1.4 ± 3.6 per cent. There was no significant difference between this value and the corneal swelling associated with no lens at all (0.8 ± 3.4 per cent). There was also no significant difference between senofilcon A and the other SiHs tested. However, there was a significant yet expected difference when compared with the hydrogel etafilcon A, which showed an overnight corneal swelling of 5.8 ± 3.8 per cent.

Hamano et al12 looked at the oxygen performance of SiH CLs compared with two well-established daily disposable lenses. They studied the impact of dozing or napping in lenses on corneal thickness increase and endothelial bleb development. The number of lens wearers who opt for continuous wear is relatively small, however many wearers may doze in their lenses for short periods of time. In this study, test lenses were worn for seven hours, one hour of which was spent with closed eyes to simulate napping. After the closed-eye period the results showed that there was no significant difference in corneal thickness increase after 'napping' in senofilcon A lenses compared with 'napping' in no lens. Similarly there were no significant differences between the different SiHs. However the corneal thickness increases noted with the two daily disposable, hydrogel lenses were significantly higher than both the control (no lens) and the SiHs.

The same study also looked at endothelial bleb formation. Blebs are an acute endothelial response to hypoxia and can therefore be considered a sensitive index of the oxygen performance of a lens (Figure 5). The results showed that there was no significant difference in the bleb formation after the closed eye period in SiH CLs compared with no lens. Bleb formation was noted after the closed eye period in each of the two daily disposable lenses tested and this was significantly greater than that with the SiHs.

UV protection

Ultraviolet (UV) radiation is well known to damage ocular tissue. The large diameter of a soft CL and its position on the cornea mean that it can reduce the level of UV radiation reaching the ocular surface, protecting the cornea and internal eye structures. Kwok et al13 describe an effect known as peripheral light focusing (PLF). PLF occurs when the extreme corneal periphery refracts obliquely incident light to focal areas of concentrated light inside the eye. This is postulated to be a mechanism for UV-related ocular pathology such as pterygium and cortical lens opacities. Although sunglasses protect against PLF to a degree, they do not protect against the oblique radiation that can occur from the side and from behind the eyes' frontal plane. UV-blocking CLs were found to be effective in protecting against obliquely incident light and PLF.

The amount of UV absorbed and transmitted by a soft CL depends on the brand of lens. Senofilcon A lenses were the second of the SiH materials to incorporate a UV-blocking agent in the material, the first being galyfilcon A. Senofilcon A lenses have the highest UV-blocking characteristics of any soft lens currently on the market, blocking 96 per cent of UVA rays and 100 per cent of UVB. Galyfilcon A and senofilcon A are the first contact lenses to receive the World Council of Optometry's global seal of acceptance for their UV protection.

Moore and Ferreira14 carried out an independent study that examined the UV attenuating properties of various lenses including senofilcon A lenses and galyfilcon A lenses. Their results showed that senofilcon A had the lowest UV transmittance of all the lenses tested (8.36 per cent), meeting the ANSI standard15,16 for UV blocking. There was a statistically significant difference in the UV transmittance of senofilcon A and galyfilcon A compared with the other SiHs tested. Moore and Ferreira also calculated a protection factor for each of the test lenses. This protection factor is designed to quantify the UV protection afforded by a CL in a similar manner to the protection factor on a bottle of sunscreen. Higher protection factors indicate superior levels of UV blocking. In this study senofilcon A was found to have a superior UV protection factor (11.96) to the other SiHs tested.

Summary

It is claimed that Acuvue Oasys with Hydraclear Plus has improved performance, especially for patients in challenging environments who may have issues with lens comfort and dryness. The peer-reviewed literature discussed in this article supports the performance benefits claimed. The enhanced wettability and lubricity of the senofilcon A material have been shown to increase levels of reported patient comfort compared with habitual lenses, even in adverse environments. The material has been shown to allow an increase in comfortable wearing time in a population of lens wearers for whom comfortable wearing time had previously been a problem. It has also been shown that senofilcon A lenses can increase patient comfort in specifically challenging situations such as computer use and driving at night.

The lens has FDA and EU approval for daily wear and overnight wear for up to six nights and studies confirm minimal levels of corneal swelling and signs of hypoxia. More importantly, the oxygen performance has benefits for daily wearers. Not only is there a reduction in bulbar and limbal conjunctival hyperaemia, but patients who wear their lenses for many hours a day, seven days a week, and may doze or nap in their lenses, are at much lower risk of suffering from changes in their corneal physiology as a result of hypoxia. Indeed it has been demonstrated that napping for one hour while wearing senofilcon A lenses results in no significant increase in corneal thickness or endothelial bleb development compared with napping for one hour in no lens at all. The studies reviewed in this article highlight the beneficial clinical performance measures of senofilcon A lenses, which will give confidence to practitioners prescribing the lens, as they and their patients experience the benefits first hand.

References

1. Begley CG, Caffery B, Nichols KK, Chalmers R. Responses of contact lens wearers to a dry eye survey. Optom Vis Sci, 2000 77(1):40-6.
2. Vajdic C, Holden BA, Sweeney DF, Cornish RM. The frequency of ocular symptoms during spectacle and daily soft and rigid contact lens wear. Optom Vis Sci, 1999 76(10):705-11.
3. Young G, Veys J, Pritchard N, Coleman S. A multi-centre study of lapsed contact lens wearers. Ophthalmic Physiol Opt, 2002 22(6):516-27.
4. Riley C, Young G, Chalmers R. Prevalence of ocular surface symptoms, signs, and uncomfortable hours of wear in contact lens wearers: the effect of refitting with daily-wear silicone hydrogel lenses (senofilcon a). Eye & Contact Lens, 2006 32(6):281-6.
5. Morgan PB, Efron N. In vivo dehydration of silicone hydrogel contact lenses. Eye & Contact Lens, 2003 29(3):173-6.
6. Osborn K, Veys J. A new silicone hydrogel lens for contact lens related dryness. Optician, 2005: 229:6004: 39-41.
7. Young G, Riley CM, Chalmers RL, Hunt C. Hydrogel lens comfort in challenging environments and the effect of refitting with silicone hydrogel lenses. Optom Vis Sci, 2007 84(4):302-8.
8. Ousler, GW, Anderson RT, Osborn KE. The effect of senofilcon A contact lenses compared to habitual contact lenses on ocular discomfort during exposure to a controlled adverse environment. Current Medical Research and Opinion, 2008 24(2):335-341.
9. Efron N, Morgan PB, Cameron ID, Brennan NA, Goodwin M. Oxygen permeability and water content of silicone hydrogel contact lens materials. Optom Vis Sci, 2007 84(4):328-37.
10. Steffen RB, Schnider CM. The impact of silicone hydrogel materials on overnight corneal swelling. Eye Contact Lens, 2007 33(3):115-20.
11. Holden BA, Mertz GW. Critical oxygen levels to avoid corneal edema for daily and extended wear contact lenses. Invest Ophthalmol Vis Sci, 1984 25(10):1161-7
12. Hamano H, Maeda N, Hamano T, Mitsunaga S, Kotani S. Corneal Thickness Change Induced by Dozing While Wearing Hydrogel and Silicone Hydrogel Lenses. Eye & Contact Lens: Science & Clinical Practice, 2008 34(1):56-60.
13. Kwok LS, Kuznetsov VA, Ho A, Coroneo MT. Prevention of the adverse photic effects of peripheral light-focusing using UV-blocking contact lenses. Invest Ophthalmol Vis Sci. 2003 44(4):1501-7.
14. Moore L, Ferreira JT. Ultraviolet (UV) transmittance characteristics of daily disposable and silicone hydrogel contact lenses. Cont Lens Anterior Eye, 2006 29(3):115-22.
15. ANSI/Z80.3 - 1996 (R1999) Non-prescription sunglasses and fashion eyewear - requirements.
16. ISO 8599:1994 Optics and optical instruments - contact lenses - Determination of the spectral and luminous transmittance.

? Dr Karen French is an optometrist working in private and hospital practice in Cambridgeshire. She has previously carried out research in the area of contact lens material properties