Oxygen transmissibility within the optic zone of soft toric lenses
Andrew Elder Smith and Tony Hough explain how new high-precision thickness measurement can help practitioners to better understand how oxygen delivery varies with power
Since the early 1980s there has been extensive discussion about how much oxygen gets through contact lenses to the cornea. With the introduction of silicone hydrogels this discussion has intensified, possibly due to increased funding from the major manufacturers to promote wider awareness of the clinical benefits of materials having enhanced permeability.1-4 Lens manufacturers quote oxygen transmissibility (Dk/t) values that appear to be more than adequate to meet the oxygen demand of the cornea in open eye conditions. However, the quoted values are universally based on the centre thickness of a -3.00D lens. One interesting and little-mentioned outcome of this is that the reference values published by manufacturers for the Dk/t of torics and spherical lenses are very similar as shown in Table 1.
Soft torics: Do design features impact on zonal Dk/t?
Soft toric fitting as a percentage of total fits has continued to increase over the past 15 years or so. The confidence shown by practitioners in this modality may be due to greatly improved lens design and manufacturing technology; each of the major manufacturers owns their soft toric design and has invested very considerably in protecting and enhancing that design through their intellectual property portfolio. As a result, there is now a wide range of successful silicone hydrogel toric lenses available to eye care practitioners which can provide wearers with a choice of material benefits and design features.
For soft toric contact lenses to give optimum visual performance they must maintain stable on-eye orientation during eye movements and on blink. All soft torics have zones of increased thickness to enable on-eye orientation and stabilisation; such zones are normally arranged fairly statically on the eye. The location of the thick zones will depend on lens design; often they will cover the limbal area, with the associated risk of hyperaemia and new vessel formation. Should the thick zones of the lens produce areas of localised low level oedema then potentially there may be disruption to the optics of the cornea. This is particularly significant given that many wearers admit to sleeping or napping in their contact lenses.5
It has been noted that the swelling response of the peripheral and central cornea are different when covered by the thicker portion of toric soft lenses.6 This may be due to the peripheral cornea being less able to swell than the central cornea due to a tighter meshwork of collagen fibres. This and other clinical indications of corneal response together with the industry policy just described have led to the current situation where the transmissibility properties of complete contact lens product ranges are quantified by a single value; the Dk/t of a -3.00D lens at its geometric centre.
The objective of this study is to address the following questions:
1 For toric soft lenses, does the published value of Dk/t correctly reflect the oxygen available to the central cornea with a lens in place?
2 What is the influence of the toric design (stabilisation mechanism) on the Dk/t in the central optic zone?
Thickness profiles of current silicone hydrogel torics
There is very little information given by manufacturers as to whether the variation in peripheral thickness caused by the orientation mechanism carries through to the optic zone, or to what extent it may cause unusual thickness variation across the optic zone.
Here we look in detail at the overall and zonal thickness of four current silicone hydrogel lenses; Acuvue Oasys for Astigmatism (Johnson & Johnson Vision Care), Air Optix for Astigmatism (Alcon Eye Care), Biofinity Toric (CooperVision) and PureVision Toric (Bausch + Lomb). Of these, three achieve on-eye stability by having a prismatic structure – including some variations on the precise design of the prism – which would generally place the thickest zones in a 6 o’clock direction. The Acuvue Oasys product has dynamic stabilisation which should place the thick zones at 3 and 9 o’clock.
Historically, it has been difficult to measure the thickness of any soft contact lens. The only technique recognised as an ISO-approved method is the low force gauge for the measurement of centre thickness in which a simple electronic thickness gauge is fitted with a probe that is lowered gently until it makes contact with the lens surface. The thickness value is then read from the digital gauge. The instrument has a sensor to detect surface contact in order to minimise errors due to indentation.
Recently, new measurement technology has been developed in the UK by Sheffield based Phase Focus Limited (www.phasefocus.com) which can map the overall thickness of any soft lens, including lenses having complex geometry such as soft torics. The technology is non-contact with the lens immersed in saline during the measurement. The instrument together with its associated software is known as the Lens Profiler Mapping System, shown in Figure 1. The system provides a complete map of lens thickness. As well as having micron level accuracy and precision, the instrument has high spatial resolution; a typical measurement will provide values of lens thickness every 7 microns across the lens surface.
In this study 16 silicone hydrogel soft torics, identified in Table 2, were measured with the Lens Profiler. The lenses were selected to provide a representative range of current silicone hydrogel torics and also to cover a clinically useful range of lens powers.
Figure 2 shows the thickness profiles of four soft torics, one from each manufacturer, all having labelled powers of -2.75/-2.25×20. The thickness scales have been standardised so that the colour code is the same for all lenses to enable easy visual like for like comparison of overall thickness profiles and the location and relative thickness of the stabilisation zones.
In this study we will attempt to quantify the Dk/t in the central optic zone. A central area corresponding to a typical effective optic zone having a diameter of 7.50mm is shown schematically in Figure 3.
The radial thickness for the central 7.50mm was extracted from the overall lens thickness map for each lens at 58 micron intervals to provide just over 13,000 thickness data points for each of the (7.50mm diameter) optic zones. These data were then used to calculate the average Dk/t inside the optic zone for all 16 of the lenses identified in Table 2. The results, together with the manufacturers’ published value of Dk/t, are shown in Figure 4.
The data in Figure 4 show that the average value of oxygen transmissibility within the optic zone is significantly less than the reference value published by lens manufacturers.
Expected and design-dependant thickness variation
For all of the lenses measured, the toric geometry is located centrally on the back surface contained approximately within the optic zone. Cylinder size will therefore influence thickness over the optic zone, with higher cylinder powers having greater thickness variation.
The average thickness within the optic zone of the prismatic lenses (Air Optix, Biofinity and PureVision) increases much more rapidly with increasing positive power than does the dynamically stabilised Acuvue Oasys lens, confirming that the individual design of lens stabilisation does have an influence on Dk/t in the central zone. The increased thickness due to stabilisation zones of some of the lenses impinge on the optic area and lie within the pupillary zone. In wearers with large pupils this could decrease the quality of the image formed in addition to any corneal shape changes induced by the differential oxygen transmissibility.
However, current product trends indicate that prismatic structure is likely to continue to be the preferred stabilisation mechanism for the foreseeable future; the challenge for lens manufacturers is to maintain lens stability during wear while reducing thickness especially within the more sensitive central optic area.
Results and summary
This study shows that the widespread policy of stating Dk/t based on the centre thickness of a -3.00D lens does not generally provide a useful indicator for clinicians as to the overall transmissibility on the eye. The values of average thickness within the central optic zone based on lens design and sphere power, as set out in Table 3, provide a better indicator of the transmissibility of soft torics.
Does lens design influence Dk/t within the optic zone as power varies? Yes. This study confirms that the design of the on-eye stabilisation mechanism does influence the Dk/t within the central optic zone. Prismatic lenses have a higher average thickness over the central 7.50mm compared to dynamically stabilised lenses. Cylinder size will also influence average thickness.
Modern silicone hydrogel toric contact lenses offer a reliable mode of vision correction and provide excellent oxygen transmissibility to the cornea. Practitioners should be aware of the variations in design and how that has potential to impact on corneal metabolism and the quality of the wearer’s experience. By better understanding the design principles of the different brands practitioners will be in a better position to resolve clinical complications which may arise in the medium and longer term due to localised oxygen deficiency. ●
● Andrew J Elder Smith is a contact lens consultant and a clinical tutor at the University of Bradford School of Optometry and Vision Science. Tony Hough is a director of Cambridgeshire design consultancy CLS and is a past president of the BCLA