Poor surface wettability on a silicone hydrogel contact lens
CT, a 38-year-old female, reported difficulty tolerating monthly disposable silicone hydrogel contact lenses for the past few months. Prior to that, she had successfully worn contact lenses since the age of 18 years.
She is a moderate myope with minimal astigmatism. Examination revealed meibomian gland dysfunction and a reduced tear break up time of four seconds. Ocular health was otherwise unremarkable.
CT was instructed to apply warm compresses and was refitted to a daily disposable SiHy contact lens. While she was able to tolerate wearing the lens longer, she still complained of dryness by six hours. Examination of the lens at the six-hour mark revealed lipid deposition and poor lens wettability (above).
CT was subsequently refitted into Biotrue ONEday. She returned a week later reporting she could tolerate wearing the lens for up to 12 hours.
Daily disposables provide convenience and provide patients with fresh lenses every day. They also come with physiological benefits: lens deposition is reduced, risk of microbial and infiltrative events is significantly reduced, and solution toxicity is eliminated.
Since the advent of silicone hydrogel lenses in the late 1990s, they have become the lens of choice for many optometrists. They afford far greater oxygen than hydrogel lenses and have greatly reduced the rate of hypoxic complications.
However, silicone, being hydrophobic, brings other problems, including reduced lens wettability and increased deposition. Silicone also causes increased lens modulus resulting in mechanical problems such as papillary conjunctivitis.
Oxygen is important but it is not the only factor. While silicone hydrogel lenses have solved the oxygen problem, the rate of contact lens drop-outs remains largely unchanged. Holden and Mertz found that for safe daily contact lens wear, oxygen transmissibility of 24 Dk was required to avoid corneal hypoxia for daily wear.1
We were able to increase wearing time for CT by switching to a daily replacement schedule; however, some patients such as CT will be intolerant of or unsuitable for silicone hydrogel lenses due to excess lipid deposition and reduced lens wettability.
Biotrue ONEday (nesofilcon A) is a ‘hypergel’ material with a water content of 78 per cent to match the cornea, and mimics the lipid layer of the eye. While it is not a silicone hydrogel, Biotrue ONEday has a Dk of 42, which more than satisfies the Holden Mertz criteria for safe daily wear. It has been shown to have low dehydration on2 and off3 the eye and is a fantastic option for patients who cannot tolerate silicone hydrogels.
Holden BA, Mertz GW. Critical oxygen levels to avoid corneal edema for daily and extended wear contact lenses. Invest Ophthalmol Vis Sci 1984; 25: 1161-1167.
Schafer J, Steffen R et al. Comparing on eye dehydration and corneal staining of two high water hydrogel contact lenses in a low humidity environment. Optom Vis Sci 2011; 88 e-abstract 115540.
Cox I et al. Understanding lens shape dynamics during off-eye dehydration of various different materials with different water contents. Contact Lens Anterior Eye 2012; 35: Supplement 1. 1 Dec. e11.