Section ONE Preliminaries

1.2.7 Cell apoptosis

Under normal conditions, the shedding of cells affects 1% of those along the epithelial surface. In contact lens wear, fewer cells are shed and the highest incidence is at the centre of the cornea. Hypoxia is known to be an important factor in inhibiting epithelial shedding which may explain the increased incidence of microbial keratitis in contact lens wearers.

1.2.8 Corneal sensitivity

One of the first, important effects of hypoxia, of which the patient is unaware, is a drop in corneal sensitivity.6 Reduced corneal sensitivity is most obvious when a long-term wearer of PMMA (polymethyl methacrylate) is refitted with a rigid gas-permeable material. One of the side effects is a period of lens and foreign body awareness that has not been present before. This indicates a rise in corneal sensitivity which eventually settles to the expected norm with an increase in available oxygen.

1.2.9 Closed eyelid conditions during sleep

The following changes are induced:

•Increase in temperature.

•Hypotonic shift in tear osmolarity as a result of increased evaporation.

•Slight acidic shift in tear pH as a result of retardation of carbon dioxide efflux from the cornea.

•Corneal oxygenation reduced from 155 mmHg (open eye) to 55 mmHg (closed eye).

1.3 Physical properties of materials

1.3.1 Oxygen permeability, oxygen transmissibility and equivalent oxygen percentage

Oxygen permeability

The oxygen permeability of a material is generally referred to as the Dk. In this nomenclature, D is the diffusion coefficient – a measure of how fast dissolved molecules of oxygen move within the material – and k is a constant representing the solubility coefficient or the number of oxygen molecules dissolved in the material.

The Dk value is a physical property of a contact lens material and describes its intrinsic ability to transport oxygen. It is defined as:

the rate of oxygen flow under specified conditions through unit area of contact lens material of unit thickness when subjected to unit pressure differences.7

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Table 1.1  Comparison of Dk and Dk/t for traditional (Fatt) and ISO units

After Hough.9

It is not a function of the shape or thickness of the material sample, but varies with temperature. The higher the temperature, the greater the Dk.8 The traditional units are 10−11 cm2/s ml O2/ml × mmHg (usually referred to as Fatt units), now also known as barrers. They are often omitted for convenience. When the international standard (ISO) unit of pressure, the hectopascal, is used instead of mmHg the units of Dk are 10−11 cm2/s ml O2/ml hPa. To convert to ISO units multiply by 0.75 (Table 1.1):9 to convert from hPa to mmHg multiply by 1.33322.

Oxygen transmissibility

Oxygen transmissibility is referred to as Dk/t, with Fatt units of 10−9 cm/s ml O2/ ml × mmHg and ISO units of cm/s ml O2/ml hPa (see Table 1.1). Here, t is the thickness of the lens or sample of material, and D and k are as defined above.

The Dk/t for a particular lens under specified conditions defines the ability of the lens to allow oxygen to move from anterior to posterior surface. The value of t is generally an average lens thickness for powers between ±3.00 dioptres

(D). Outside of this range, it is necessary to apply a nomogram.10 Oxygen transmissibility is not a physical property of a contact lens material, but is a specific characteristic related to the sample thickness.

The methods employed to undertake measurement of oxygen permeability are:

•The polarographic method. Developed by Fatt during the 1970s for rigid and hydrogel materials – ISO 9913-1 (1996).

•The coulometric method. Developed by ISO during the 1990s for use with highly permeable non-hydrogel materials – ISO 9913-2 (2000).

Surface effects

High Dk materials do not always give the oxygen performance on the eye that would be expected from laboratory results. The corneal swelling is equivalent to

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Section ONE Preliminaries

that of a lens with a Dk only 55% of the measured value.11 This barrier effect is due to an intermediate water layer used in measurement.

Edge effect

There is also an edge effect due to oxygen flow around the periphery of the sample, since, in the laboratory, flat as opposed to curved test pieces are generally used.12

Boundary effects

The boundary effect (or boundary layer effect) is important for rigid gas-  permeable materials as there is resistance to oxygen permeation at the boundary between the tears and polymer surface when measurement is made under water/water conditions. For a clean lens, the boundary effect is constant whether the lens is thick or very thin. The effect therefore has a relatively greater influence on a thin lens. This means that a particular Dk/t value with a lens, for example, of thickness 0.35 mm will not be significantly improved compared with a thin 0.10 mm lens. For thin lenses, the boundary effect becomes more important in determining the dissolved oxygen permeability as the Dk of the material increases. There are virtually no boundary effect implications for PMMA and low Dk materials.

PRACTICAL ADVICE

Consistently reliable comparisons of various materials can be made only by the same person using the same instrument under identical conditions. Care is therefore required in comparing Dk measurements from different sources.13

Equivalent oxygen percentage

The equivalent oxygen percentage (EOP) refers to the level of oxygen at the surface of the cornea under a contact lens. For the uncovered cornea exposed to the atmosphere at sea level, the amount of oxygen available is 20.9%, whereas with the eye closed the cornea receives 8%. The EOP obviously varies with altitude. A particular value for a material is that oxygen percentage on a scale of 0 to 21 which, in a goggle experiment, produces an oxygen demand after steady state has been reached, equivalent to that produced by a test contact lens worn for the same period of time. With lens wear, to avoid oedema, the EOP should be over 10%14 (Dk/t = 24.1); for no overnight swelling it needs to be as high as 18% (Dk/t = 87). All EOPs are lens thickness dependent. An EOP profile (Figure 1.1) for a lens of known material and thickness shows whether it can provide enough oxygen to avoid corneal oedema.

Oxygen flux

•Is a measure of the actual amount of oxygen available to the corneal epithelium.

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•Is expressed in l O2/cm2/h.

•Decreases with contact lens thickness and hydration.

•Requires the tear pump mechanism to make up any deficiencies.

•There is a rapid increase in oxygen flux with increasing Dk/t. However, as Dk/t continues to increase, a gradual levelling off of oxygen flux occurs at a Dk/t of approximately 30 Fatt units for the open eye and 80 Fatt units for the closed eye. This means that increases in Dk/t beyond certain values have less impact in terms of oxygen utilization.

•With the advent of silicone hydrogel materials, the large differences in critical Dk values published for various studies are in reality small when using oxygen flux values instead of Dk. Different studies are in general agreement as to the minimum oxygen requirements in terms of flux values (Table 1.2).

Custom Sperical RGP : 7.80 : 9.50 : -4.25 Material: Paragon HDS 100

Tear pump contribution = 2.00%

Figure 1.1  Equivalent oxygen percentage profile

Table 1.2  Differences in critical Dk values

compared to oxygen flux

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Section ONE Preliminaries

1.3.2 Water content and water uptake

The water content is the amount of fluid taken up by a lens material as a percentage of the whole under specified conditions:

Water is lost by evaporation when a hydrogel lens is worn on the eye. This is in part caused by a rise in temperature and is accompanied by a tightening of the fit (see Section 21.4.2).

Water balance ratio

This represents the water retention capability of a lens material (Table 1.3).

A high ratio indicates a material with stable hydration characteristics.

Table 1.3  Comparison of water balance ratios

1.3.3 Wettability

Wettability is the ability of a drop of liquid to adhere to a solid surface. The lower the cohesive forces within a liquid, the greater the attraction between the fluid and surface. Thus, superior wettability enhances the spread of liquid over a surface.

Contact angle is a measure of the hydrophilicity of a surface. The contact angle may be measured in a variety of ways:

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B

Material

Figure 1.2  Sessile drop method (A, advancing angle; B, receding angle)

*From manufacturers’ details; PMMA, polymethyl methacrylate; CAB, cellulose acetate butyrate.

•Sessile drop method: measures the tangent to a drop of liquid placed on a sample surface (Figure 1.2).

•Captive bubble method: measures the tangent to an air bubble formed on the surface of an immersed sample.

•Wilhelmy balance method: a sample is immersed or withdrawn vertically from a liquid.15

•Direct meniscus method.15

Both the advancing and the receding angles are measured. These are formed when liquid is added to or removed from the controlled liquid drop used for measurement (see Figure 1.2).

The lower the contact angle, the more wettable the surface (Figure 1.3). Typical values are given in Table 1.4, which demonstrates the great inconsistency between different methods. Comparisons can therefore only be made when the same method has been employed.

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