By Paul Rose
DipOpt BSc FNZSCLP
Oxygen availability to the cornea with contact lens wear is one of the most important factors to consider for long-term corneal health and to reduce the risk of infection, as studies have shown that corneas deprived of oxygen are more susceptible to infection.1
Ongoing material research continues to provide contact lens practitioners with higher-Dk materials with good wetting angles that easily meet the Holden-Mertz criteria for critical oxygen transmission for daily wear. Some hyper-Dk materials meet extended-wear criteria, allowing overnight wear.2
Laboratories also endeavour to keep GP corneal lenses to a minimum centre thickness to reduce lens bulk and maximise Dk/t to ensure the lens transmits sufficient oxygen to the cornea.
The growing popularity of scleral and semi-scleral lenses raises a concern that oxygen availability to the cornea has been largely ignored. Combine this with reduced tear exchange and we have the potential to adversely affect long-term corneal health.
Total scleral system
GP corneal lenses typically have centre thicknesses of 0.1 mm to 0.16 mm, depending on the power, whereas most scleral and semi-scleral lenses are significantly thicker, anywhere from 0.25 mm to 0.7 mm. This dramatically increases the bulk of the lens, reducing the flow of oxygen to the cornea.
Oxygen transmission was an important consideration when I designed the Rose K2 XL corneo-scleral lens. I wanted to ensure that a lens that covered the cornea and had considerably less tear flow than a corneal lens would maximise oxygen transmission to avoid long-term corneal complications created by anoxia. With the tear film having a Dk of about 80, the tear layer thickness under the lens must be considered when addressing oxygen availability to the cornea, as this layer also becomes a potential barrier to oxygen reaching the cornea.
Michaud and colleagues3 looked at oxygen at the cornea, taking into account the Dk of the material, the thickness of the lens and the thickness of the tear layer under the lens. By applying the formula:
–– = –––––––––––––––
tscl (t1/Dk1) + (t2/Dk2)
where Dk/t1 refers to the lens and Dk/t2 refers to the tear layer, it is possible to calculate the overall oxygen permeability of the total scleral system (Dk/tscl).
We can calculate this number for any lens with a centre thickness of 250 microns (0.25 mm) to 500 microns and a tear layer (clearance) from 100 microns to 400 microns (Table below). Considering that most semi-scleral designs require 200 microns of tear layer under the lens, none of the lenses with centre thicknesses of 250 microns or more meets the minimum Holden-Mertz requirement of 24 × 10–9 for daily wear.
Triple the minimum
Let’s now consider the Rose K2 XL lens, using Menicon Z (tisilfocon A) with a Dk of 163 (ISO/Fatt method). Assuming a tear layer that results from a minimum corneal clearance over the highest part of the cornea of 20 microns, and a centre thickness of 140 microns (0.14 mm), we can calculate the total scleral system (Dk/tscl) is 81.8 × 10–9, which is more than three times the minimum requirement for daily wear.
When you combine these factors with a lens design that has most of the bearing zone on the cornea and some movement to prevent binding, it is apparent that this goes a long way toward ensuring some tear exchange from behind the lens.
- Goodlaw E. Risk of infection from sleeping with contact lenses on: causes of risk. Optom Vis Sci 1996; 73: 156-158.
- 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.
- Michaud L, van der Worp E, Brazeau D, Warde R, Glasson CJ. Predicting estimates of oxygen transmissibility for scleral lenses. Cont Lens Anterior Eye 2012; 35: 266-271.