Clinical PErformance and Observation - Lens Fit

Kathryn Dumbleton - BSc (Hons), Uni of Wales1984 MCOptom 1985, MSc Uni of Waterloo 1988

Senior Researcher
Centre for Contact Lens Research (CCLR)
University of Waterloo, Ontario, Canada

Silicone hydrogel lens materials, as described in last month's editorial, differ significantly from conventional soft lenses. The increased stiffness of the materials lessens the degree to which they shape themselves to the cornea, compared to other soft lenses. As a result silicone hydrogel lenses are less "forgiving" to marginal fits. There may also be a tendency for the material to 'flute' (this is a buckling of the lens edge due to excessive edge lift) at the edges of the lens if the fit is not optimal.

The two types of silicone hydrogel lenses currently on the market are available in the parameters listed below (Table 1). Many myopic and hyperopic patients can be successfully fitted with these lenses regardless of whether the lenses will be worn on a daily or continuous wear basis. Currently these lenses are only available in spherical prescriptions and as silicone hydrogel lenses do not "mask" astigmatism despite their higher modulus, only patients with less than a dioptre of astigmatism should be considered.

Lens fitting characteristics should be assessed using conventional techniques and measurements. A period of 10 to 15 minutes after insertion is recommended for lens settling prior to assessment. A slit lamp biomicroscope at low magnification, employing diffuse illumination, should be used for careful examination of the lens centration and corneal coverage. Direct focal illumination with a wide illuminating beam (0.5-2.0mm) can be used to assess lens movement in primary and superior gaze and to assess alignment of the lens particularly around the periphery to ensure that there is no edge stand off. Varying the width or length of the beam can aid in measuring the lens movement or alternatively a reticule eyepiece may be utilised.

The tightness of lenses may be assessed with a push up test⁵. In this test the lens is dislodged by gently pushing upwards on the lower eyelid margin against the lower edge of the contact lens and then releasing the lid. The resistance to decentration, or tightness, is rated according to the speed at which the lens returns to a central position. The rating is a percentage scale where 0% represents the lens being held by lid tension only (that is, it falls from the cornea immediately the lower lid is released) and 100% represents no movement on attempting to decentre the lens. Thus a lens rated at 0% is too loose, and a lens at 100% is too tight, or is bound.

The general principles for a well fitting silicone hydrogel lens are no different from those used for conventional soft lenses. The lens should cover the entire cornea and limbus and remain approximately central on the cornea in all gaze positions. Sufficient lens movement is required to maximise tear exchange and to encourage the removal of debris and bacteria from under the lens.

The push-up test⁵ should give a tightness rating of 40-45% (slightly loose). Typically lens movement with silicone hydrogels is greater than observed with conventional thin soft lenses, averaging 0.3mm - 0.5mm in the primary, and up to 0.75mm in the superior gaze positions. Lenses that "flute" will cause discomfort and lens awareness. If a lens does flute, a steeper base curve or different lens type is indicated.

Lenses, which decentre, move excessively or do not provide complete corneal coverage should be avoided to prevent corneal desiccation or limbal chafing. As mentioned previously, the increased stiffness of silicone hydrogel materials makes them less prone to conform to the corneal surface, meaning that the physical lens to cornea curvature relationship is more critical to successful fitting. A phenomenon that is observed more often with silicone hydrogel lenses than conventional soft lenses is lens 'fluting'⁶. It may occur constantly or intermittently and can be most readily detected by observing the lens edge moving over the temporal limbal area near the lower lid. Lenses exhibiting fluting usually cause a foreign-body like discomfort or increased lens awareness. In extreme cases the lens will ride on the lower lid margin causing a varying degree of discomfort (Figure 1). Unfortunately fluting does not reduce with wear, the patient will not adapt to the lens awareness or discomfort induced and if observed, an alternate base curve or design must be evaluated.

Figure 1: Extreme case of lens fluting seen in a small percentage of potential wearers.

High molecular weight fluorescein dye or fluorexon may also be used to assess the static and dynamic lens fitting characteristics of silicone hydrogel lenses. A cobalt blue excitation filter and yellow barrier filter can improve the contrast of the fluorescein pattern (Figure 2). For optimal visibility the dye should be instilled on the back surface of the contact lens before insertion of the lens. Minimal dosage is of importance because higher amounts of the dye can cause stinging which results in excessive tearing and therefore an inaccurate assessment. Conventional fluorescein may be used to assess fit in a similar manner. Fluting (Figure 3), particularly if minimal or intermittent will be more easily observed with this technique.

Figure 2: Well fitted silicone hydrogel lens observed with fluorexon, cobalt blue excitation filter and yellow barrier filter.

Figure 3: Fluting is more easily observed with fluorexon, cobalt blue excitation filter and yellow barrier filter.

The PureVision lens is currently only available in one base curve - 8.7mm. This base curve has been shown to fit a wide range of corneal curvatures, with average keratometry readings of flatter than 40.00 D (8.4mm) to steeper than 48.00 D (7.0mm). The distribution of average keratometry readings for a group of 2250 eyes successfully fitted with PureVision lenses is shown in Figure 4. (Bausch & Lomb data)

Figure 4: Distribution of K readings from clinical studies: PureVision™.

The Focus Night & Day lens is available in two base curves, 8.4mm and 8.6mm. Figure 5 shows the distribution of keratometry data along the steep meridian for eyes fitted with each lens base curve (CCLR data). A clinically useful criterion that indicated the need for the 8.4 lens was a steep K of >45.50 D (<7.4mm).

Figure 5: Distribution of steep keratometry readings for eyes dispensed in 8.6 and 8.4 mm base curve lenses: Focus® NIGHT & DAY™.

In a clinical trial conducted at the CCLR, the effect of lens base curve on subjective comfort during trial fitting⁵ was investigated. Subjective discomfort during trial fit with the flatter lens was determined to be a strong indication of the need for a steeper base curve since the 8.4 lens resulted in significant improvement in comfort among subjects who reported poor comfort with the 8.6 lens. Figure 8 shows that subjects with steeper eyes among this subset had a large improvement in comfort when changing to the steeper lenses.

Figure 8: Mean "settled" subjective comfort scores during trial fitting of 8.6 mm and 8.4 mm base curve Focus® NIGHT & DAY™ lenses, for eyes requiring the 8.4 mm base curve lenses.

From this study we were able to conclude that subjective comfort was vastly improved with the steeper base curve lenses in the steeper range of corneas.

Some patients currently wearing soft lenses may experience more lens awareness, at least initially, when being refitted with silicone hydrogel lenses. It is important for the practitioner to be aware of this and advise the patient accordingly.

REFERENCES

1. Efron N, Brennan NA, Currie JM, et al. Determinants of the initial comfort of hydrogel contact lenses. Am J Optom Physiol Opt 1986; 63:819-23.

2. McMonnies CW. The critical initial comfort of soft contact lenses. Clin Exp Optom 1997;80:53-8.

3. Young G. Ocular sagittal height and soft contact lens fit. J Brit Contact Lens Assoc 1992;15:45-9.

4. Dumbleton K, Chalmers R, Bayer S, Fonn D, McNally J. Lens base curve and subjective comfort with silicone hydrogel continuous wear lenses. Optom Vis Sci 2001; 78 (12s): 227.

5. Young G, Holden BA, Cooke G (1993): Influence of soft contact lens design on clinical performance. Optom Vis Sci 70: 394-403

6. Clinical performance of silicone hydrogel lenses. In: Sweeney DF, ed. Silicone Hydrogels: the rebirth of continuous wear contact lenses. Butterworth-Heinemann, Oxford 2000; chap. 5.