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Oxygen: are your corneae getting enough? C-20102 O/CL.

As contact lens materials have evolved, the amount of oxygen able to pass through them has increased and clinicians seldom see the severe hypoxia-related changes that were evident in the days of PMMA and thick low water content hydrogel contact lenses. Hyperpermeable rigid lens materials have been available for many years and silicone hydrogel lenses appear to more than meet the corneal oxygen demand. However, more often than not it is the oxygen transmissibility at the centre of a -3.00D lens that is quoted--this is highly unlikely to be clinically applicable to every patient, so how can practitioners interpret this information?

Dk, Dk/t, flux

"Dk" or "permeability" is a property of the lens material, which governs how easy it is for a gas (oxygen in the case of contact lenses) to pass into, through, and out of the material. It is made up of the diffusion coefficient (D) which indicates the amount of a particular substance that diffuses across a unit area in one second, under the influence of a gradient of one unit, and k, the coefficient of solubility from Henry's Law, which is an expression of how easy it is for substances to dissolve into a material. The permeability of a material is independent of thickness so gives no indication of how much oxygen might reach the cornea when a contact lens is manufactured.

Along with the permeability of the lens material, the thickness of the contact lens (t) influences the amount of oxygen transmitted through a finished lens. Manufacturers' data give the centre thickness of a -3.00D contact lens, by convention, as a comparator (Table 1). Obviously this has limited value in that the thickness of a -3.00D contact lens will vary across the lens and that the majority of contact lenses fitted to patients are not -3.00D in power. The maximum and minimum thickness of a lens will depend on its power, front surface design (eg size of front optic zone) and the back surface design. Dk/t or transmissibility, is the ease with which oxygen will pass through the finished contact lens. Clinically this is a more important measure than Dk. Relying on the Dk/t at the centre of a -3.00D contact lens, as this article will show, may not be the best way to judge the likely clinical performance of a contact lens.

Oxygen flux is a mathematical model (3) of how well the amount of oxygen passing through the contact lens matches the corneal oxygen demand. If the oxygen flux is too low, the cornea will become hypoxic, whereas once the flux equals the corneal oxygen uptake then the cornea will metabolise normally. If the oxygen flux exceeds the oxygen demand of the cornea there is no advantage.

How much is enough?

In an ideal world, it could be argued that the amount of oxygen reaching the cornea through a contact lens would be the same as if there was no lens on the eye. Practically, it could be said that the corneal tissues have been receiving 'adequate' oxygen as long as there are no clinically observable changes in the ocular tissues over the lens wearing life of in individual. There are difficulties in both these approaches. In the first case, the amount of oxygen available is not constant--there is considerable reduction in the amount of oxygen available as we climb from sea level. Furthermore, there are considerable differences in the oxygen demands between individuals both independent of previous contact lens wear and dependent on whether they have experienced hypoxia. Holden and Mertz described, in the 1970s, the criteria for achieving lens wear that lead to no more than 4% corneal oedema (the amount produced by normal lens-free sleep). (4) A more recent study by Papas suggests a higher threshold is necessary to avoid zero hypoxic stress (125x[10.sup.-9]). (5) Fonn and Bruce6 reviewed the Holden-Mertz criteria in light of the development of silicone hydrogel materials and the research that had been undertaken to establish the corneal response to overnight wear of very high Dk/t lenses. They concluded that for extended wear the critical Dk/t should be revised upwards to 125 x [10.sup.-9], in agreement with Papas. (5)

Even if practitioners donot advocate extended wear to patients, it ie important to remember that almost half of wearers report sleeping in their lenses on occasions, so en suring that they have adequate oxygen supply during those times is important. It has also been pointed out that limbal hypoxic stress is more influential on hyperaemia than central hypoxia stress. (7) This is particularly important to remember when fitting hyperopic contact lenses, which may be more than three times thicker centrally than myopic contact lenses but which will be relatively thin in the periphery. If limbal hyperaemia is used to judge hypoxic stress then it will give a false picture of oxygen conditions under the plus powered lens.

How much oxygen are the lenses we fit providing?

The industry standard is to state the oxygen transmissibility at the centre of a -3.00D lens. As such, practitioners will be excused for not knowing what is happening with other powers or across the whole of the lens diameter where there are thickness variations. Also the design of the lens will impact on oxygen transmissibility. For example, in toric lenses the stabilisation zones will be thicker so there maybe localised hypoxia. If a lens material has a low modulus, in order to enhance handling manufacturers may increase peripheral thickness, which in turn reduces limbal Dk/t. Ideally practitioners need mans of Dk/t across the entire lens diameter and over the whole power range to determine the oxygen performance of a lens. A technique that uses diffraction patterns to accurately mea sure lens thickness non-invasively and calculate oxygen maps is now mailable, as described below.

A new technique for measuring contact lenses

The Phase Focus Virtual Lens

The Phase Focus Virtual Lens is a new method, known in the scientific literature as ptychography, which analyses scattered light reflected by or transmitted through an object. The scattered light, or 'diffraction pattern; can be captured on a standard detector such as a CCD (charge-couple device) camera. The Virtual Lens works by mathematically processing the multiple diffraction patterns that are collected while overlapping areas of the object are sequentially illuminated. The data redundancy introduced by the illumination overlap allows solution of the well-known 'phase problem': even though only the intensities of the diffraction patterns can actually be measured, by using an iterative phase retrieval algorithm the Virtual Lens can recover both the amplitude and phase of the wave after it has been scattered! by the object. (8,9) The technique has been demonstrated at visible light, electron and X-ray wavelengths, and is being used in a wide range of applications include surface topography and stain-free live cell imaging as well as ophthalmic lens metrology. (10) The nature of the method results in high quality images that often eliminate aberrations and artefacts induced by conventional microscope lenses. In addition, the images produced a re truly quantitative, allowing them to be used as a direct basis for measurements.

Application to ophthalmic lens metrology

Post-processing of the output from the Virtual Lens is used to provide application-specific analytical information which, in the case of contact lenses, can include whole-lens maps of axial and radial thickness, oxygen transmissibility, and refractive power. Lens diameter is also obtained. Therefore, these outputs require knowledge of the refractive indices of the lens material and saline, as well as (if an oxygen transmissibility map is also required) the lens material's oxygen permeability (Dk). The back curvature of the lens is required in order to convert optical thickness measurements into axial and radial thickness values. The lateral (side-to-side) resolution of the output data points across the lens is typically 7[micro]m. However, a lens thickness measurement with very high lateral resolution (better than 1[micro]m) is acquired over at least one lens edge area for the purposes of absolute thickness calibration. Relative thickness measurements are typically provided with a sensitivity of [+ or -]0.1[micro]m.

[FIGURE 1 OMITTED]

Method

Lens measurements are acquired by placing a contact: lens in a cuvette containing saline solution onto the stage of the Phase Focus Lens Profiler instrument (Figure 1). Prior to insertion into the cuvette, each lens is allowed to equilibrate overnight in saline solution at a temperature of 23 [+ or -] 1[degrees]C. The contact lens is mapped by moving the stage automatically with respect to the stationary illumination to a pre-determined series of positions from which diffraction patterns are acquired. Following data processing using) the Virtual Lens algorithm, maps of thickness and power are output as required. Dk/t maps can be calculated directly from the measured radial thickness maps using manufacturer's values for material Dk.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

A review of oxygen maps

The Phase Focus Virtual Lens was employed to measure several popular silicone hydrogel contact lenses in both -3.00D and +6.00D powers. The lenses chosen were Air Optix (Alcon) (Figure 2), Biofinity (CooperVision) (Figure 3), Acuvue Oasys (Johnson & Johnson) (Figure 4) and Clariti (Sauflon) (Figure 5).

Upon review of the oxygen maps it can be seen that in the -3.00D powers, the amount of oxygen passing through! the peripheral part of the lens is significantly reduced compared to the centre (as we would expect due to greater peripheral lens thickness), an effect that can now tie quantified. In the +6.00D powers there are marked differences in the maps from lens to lens. As expected, the oxygen transmission is lowest centrally but it is in the periphery end mid-periphery that the differences are revealed, with both the Acuvue Oasyn (Johnson & Johnson) and the Aim Optix (Alcon) lenses having an annulus of reduced Dk/t in this area. This is presumably an area of increased thickness that is created to help make the lens easier to handle. The Biofinity lens (CooperVision) has a much more uniform thickness in the periphery. Meanwhile, the Clariti lens (Sauflon) has a low Dk/t across the entire diameter, with the majority of the lens appearing to struggle to meet the Holden and Mertz criteria of Dk/t of 24 for daily wear.

Clinical implications

Practitioners are constantly bombarded by information from manufacturers about the latest developments in lens ranges and new scientific studies are often difficult to interpret in a clinical setting, especially if they seem to contradict previous findings. Practitioners need to meet the demands of patients for mere economical lenses while ensuring

they remain happy and healthy lens wearers who do not suffer adverse events from lens wear. It is difficult to see major differences in the lenses at first glance especially when, compared to hydrogel lenses, all silicone hydrogel contact lenses seem to have very good Dk/t values. However, oxygen maps provide a more holistic view of the cornea's life under the contact lens and clearly demonstrate the differences between different contact lenses. Simply fitting a lens because it is a silicone hydrogel may not be providing the cornea with optimum oxygen supply. Practitioners therefore need to be mindful of this when selecting which lens to fit for a patient. Patients who have high oxygen demands, eg long wearing times, signs of hypoxia/early limbal engorgement, and high myopes/high hyperopes, will need to be fitted with a lens that truly provides the desired amount of oxygen across its whole diameter.

Other factors

Of course the key to successful contact lens wear is not purely down to oxygen supply (if it were, everyone would be wearing hyper-permeable RGPs!). There are many other factors involved such as lens wettability, ease of handling, comfort (on application, end of day and end of wearing period), quality, of vision, and deposit resistance. Many of these factors are subjective and the wearer ca n easily tell you if they prefer the vision and comfort with lens A or lens B. Practitioners know that corneal metabolism suffers if there is not enough oxygen supply: microcysts, polymegethism, weakened tight epithelial junctions and new vessel growth are just some of the clinical manifestations. All of these have a long) and insidious time for development; some are reversible and some are not. The only way to avoid them is to make sure that patients are fitted with lenses that meet the corneal oxygen demand across the entire diameter of the lens and across Ss wide at power range as possible. Oxygen maps are a very good way of understanding the oxygen supply to the cornea and no doubt they will be referred to increasingly in future contact lens development.

Module questions Course code: C-20102 O/CL

PLEASE NOTE There is only one correct answer. All CET is now FREE. Enter online. Please complete online by midnight on December re, 2012--you will be unable to submit exams after this date. Answers to the module will be published on www.optornetry.co.uk/cet/exam- archive. CET points for these exams will be uploaded to Vantage on December 24, 2012. Find out when CET points will be uploaded to Vantage at www.optometry.co.uk/cet/vantage-dates

1. Which of the following statements about the Dk/t of silicone hydrogel contact lenses, when considered for overnight wear, is TRUE?

a. The central Dk/t of all lenses is adequate to meet the cornea's oxygen demand

b. The peripheral Dk/t of all lenser is adequate to meet the cornea's oxygen demand

c. The central Dk/t of a single power of a particular lens is representative of whether all lenses meet the cornea's oxygen demand

d. The Dk/t across the whole lens diameter for each power must be taken into account to decide if the cornea's oxygen demand is met

2. What is the benefit of fitting a contact lens with at Dk/t that meets the Holden/Mertz criteria for extended wear?

a. There will be no corneal oedema expected after overnight wear

b. There will be 4% corneal oedema expected after overnight wear

c. There will be an acceptable level of corneal oedema after overnight wear

d. All patients will have the same amount of corneal oedema after overnight wear

3. Which of the following statements about the Phase Focus Virtual Lens is TRUE?

a. It is a computer-based model for estimating lens thickness

b. It measures oxygen permeability

c. It uses the detection of diffraction patterns to create images

d. It is not dependent on the refractive index of the material

4. Which of the following statements about the Phase Focus Lens Profiler instrument is FALSE?

a. It provides a Dk/t profile across the whole lens

b. It provides a thickness profile across the whole lens

c. It provides a power profile across the whole lens

d. It allows a Dk/t profile to be calculated across the whole lens

5. What are the clinical implications of lens oxygen maps?

a. They are of limited use as all silicone hydrogel lenses provide adequate oxygen flow to the entire cornea irrespective of power

b. They could aid clinicians in deciding if a lens will provide adequate oxygen to a wearer

c. They provide no more information than that already provided by manufacturers

d. They are only relevant to people with prescriptions over [+ or -]6.00D

6. What do the oxygen maps presented in this article demonstrate?

a. Manufacturer-quoted Dk/t values accurately describe oxygen performance for all powers

b. All silicone hydrogel lenses have a similar oxygen profile across the diameter

c. Differences in lens designs can be identified from their respective oxygen maps

d. Lenses with the highest Dk will always have the best oxygen transmissibility in all areas

References

See www.optometry.co.uk/clinical. Click: on the article title and then on 'references' to download.

Andrew J Elder Smith, MSc, FCOptom, DipCLP, FBCLA

Andrew J Elder Smith is an optometrist at Contact Solutions Consultants, providing training and consultancy services to the profession and industry, and provides Professional Services Management at Alcom. Andrew regularly provides CET lectures on contact lenses (multifocals, toric fitting and the benefits to practices of communicating contact tenses to patients). In 2011 he was invited to address the BCLA annual clinical conference on 'Introducing and fitting RGP contact lenses in your practice'. Andrew would like to thank Gary Gibson of Phase Focus Ltd for his help in the preparation of this article.
Table 1
Manufacturers' stated oxygen permeability
(Dk) and lens centre thickness (tc),
with calculated oxygen transmissibility
(Dk/t)

Lens Dk tc Dk/t

Acuvue Oasys (1) 103 0.07 147
Air Optix (1) 110 0.08 138
Biofinity (1) 128 0.08 160
Clariti (2) 86 0.07 123
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Author:Elder Smith, Andrew J.
Publication:Optometry Today
Geographic Code:4EUUK
Date:Nov 16, 2012
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