Professor Lyndon Jones, long time professor at the University of Waterloo where he is director the Centre for Ocular Research & Education (CORE), has been at the forefront of contact lens material research over the 20 years since silicone hydrogel was first introduced.
In the second of his recent lectures in Birmingham, part of a busy worldwide education tour organised by CooperVision, Jones asked the audience a simple question – ‘why silicone hydrogel?’ Though the material has known health benefits and 1-day silicone hydrogel contact lenses have been available for around 10 years, only half (52%) of the daily disposable fits recorded in the 2017 international prescribing report used silicone hydrogel materials.
The background for the talk included data from the 2018 ‘1 Day Silicone Hydrogel Contact Lens Report’ organised by CooperVision (available from the CooperVision UK website as a free download) which aimed to ascertain reasons for the low uptake of sihi as a daily lens material.
Oxygen
Jones began by reminding the audience that hypoxia is no longer a problem with sihi material lenses. He was reassured that audience members were able to cite some of the ‘magic numbers’ relating to oxygen transmissibility (such as the Holden Mertz values of 24 x 10-9 for daily wear and 87 x 10-9 for overnight wear if hypoxia is to be avoided, though 125 x 10-9 is argued if subtler hypoxic changes are to be avoided).
Jones pointed out a couple of reasons why these exact figures need reviewing. Firstly, the transmissibility figures are usually cited for a set lens power (usually -3.00DS) and thickness. When one considers lenses with variable thickness across their profile, and also variable oxygen requirements of the cornea from centre to limbus, a much better representation of the oxygen performance would be represented by individual oxygen maps. These are easily available for different lens designs and powers on the CooperVision website and two examples are shown in figure 1.
A second point from Jones was that ‘patients vary. One study looked at overnight wear for many patients with highest dk/t lenses study swelling and found a variation of 9% in their
individual response.’ So different corneas have different oxygen needs.
A third important, though often forgotten, point relates to the behaviour of daily lens wearers. Those in the audience had to agree that it is not unusual to nap in the lenses and a few even admitted, on occasion, to sleeping for hours in them. Jones cited research suggesting that ‘between 23% and 50% of wearers sleep in their daily lenses, and twice this amount will nap in them’. A study by Hamano et al1 has shown significantly greater corneal swelling after just a one hour nap when conventional hydrogels are worn instead of sihis.
Figure 1b: oxygen map for a range of toric lenses
Comfort and Health
‘Daily wear reduces the risk of infiltrates by 12.5 times,’ Jones stated. While the risk of infiltrates with reusable lenses is two times greater with sihi, there is no difference between materials when worn on a daily wear modality.
Addressing another reason for possible under-prescribing of sihi for daily wear, Jones cited three studies which have all shown that comfort levels are comparable between the two materials.2-4 As he explained, ‘Many patients who have issues with silicone hydrogels do so because of tear film issues that can often be overcome by switching to a daily disposable silicone hydrogel.’
So Professor Jones’ conclusion? ‘1 day silicone hydrogel lenses provide excellent levels of comfort and physiological response.’
References
1 Hamano H, Maeda N, Hamano T et al. Corneal thickness change induced by dozing while wearing hydrogel and silicone hydrogel lenses. Eye Contact Lens 2008; 34;1: 56-60.
2 Wolffsohn JS, Mroczkowska S, Hunt OA, et al. Crossover Evaluation of Silicone Hydrogel Daily Disposable Contact Lenses. Optometry and Vision Science 2015;92:1063-8
3 Diec J, Tilia D, Thomas V. Comparison of Silicone Hydrogel and Hydrogel Daily Disposable Contact Lenses. Eye & Contact Lens 2018; 44 Suppl 1, S167-S172
4 Woods J, Ng A, Luensmann D, et al. Short-term comfort comparison of two daily disposable contact lenses of different material and modulus. Invest. Ophthalmol. Vis. Sci. 2018; 59(9):1753.