Improvements in the design of toric soft lenses has made their fitting a considerably simpler process than in previous years and toric soft lenses are and should be routinely fitted for astigmatism of greater than 0.50DC.

The requirements for a good ?t are the same for a toric lens as for a spherical one, so the movement on blink, lag, sag and response to push-up test should also be the same (Figure 1).

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A lens that is too tight will not rotate properly, and the astigmatic correction will not align along the desired meridian. It may rotate progressively as the lower lid spins it and be too immobile to recover, so a lens where the reference marks are some way off their expected position should be regarded with suspicion. Those that are too loose will probably spin around randomly. A well-?tting lens should enable the various stabilisation systems to work to keep the lens on axis.

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Toric lenses generally have engraved lines along their nominal horizontal and/or vertical meridians to indicate how much the lens has rotated on the eye (Figure 2). This enables the practitioner to offset the astigmatic axis so that when the lens is on the eye, the optical correction is accurate.

Interpretation of movement

Consider the following spectacle prescription: Plano/-2.00 x 80

If the reference lines on the lens are seen to rotate by 10 degrees in a clockwise direction, the cylinder on the eye would have an axis of 70 degrees. We therefore need to change the prescription of the lens to:

Plano/-2.00 x 90

This will ensure that the axis will align correctly once the lens takes up its habitual rotated position.

  • Clockwise rotation of the lens needs to be added to the axis

  • Anticlockwise rotation need to be subtracted

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This gives ‘Clockwise Add, Anticlockwise Subtract’, or CAAS.

A common variation of the above rule is the ‘Left Add, Right Subtract’ or LARS rule.

This concept relies on the fact that most of the reference marks are found in the vertical meridian on the lower part of the lens.

  • Clockwise rotation of the lens causes the reference line to move left and we need to add the rotation to the axis.

  • Anticlockwise rotation causes the reference line to move right and we must subtract the rotation from the axis.

To summarise, the rule is Left Add, Right Subtract, or LARS. Have a look at the example in Figure 3.

Measuring lens rotation

Before we add or subtract anything, it would be a good idea to measure the lens rotation on the eye. Lenses use a variety of markings to indicate orientation (Figure 2).

A number of methods may be used:

  • An eyepiece graticule calibrated in degrees may be used on the slit-lamp (Figure 4).

  • A thin slit-lamp beam may be rotated to align with the reference lines on the lens if there is a suitable scale on the slit-lamp illumination system. Alternatively, some focusing rods are calibrated in degrees and the alignment of the beam can be measured from this.

  • Sphero-cylindrical over-refraction (SCO) may be undertaken with the lens inserted and settled. Torics tend to take longer than spherical lenses to settle, so an extended trial may be useful here, especially if the cylindrical content is large and accuracy at a premium. The spectacle prescription, trial lens power and over-refraction can be put into a programmable calculator or computer to determine the ?nal power; the software for this is widely available from manufacturers. Alternatively, the trial contact lens power and over-refraction may be placed in a trial frame and neutralised with a focimeter (lensmeter). This is the most accurate way to determine the power and axis, and should certainly be the method of choice if one is faced with an extremely expensive custom lens with a high astigmatic power.

  • The most common method used for lower powers is ‘guesstimation’. Optometrists, through practice, become rather good at estimating angles, and manufacturers often provide useful extra reference lines either side of the main one, at intervals of 10 or 15 degrees. Many disposable lenses are only available with axes at 10 degree intervals anyway, so this method is accurate enough in most cases.

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Stabilisation

Having taken the trouble to measure and calculate the axis, you do not want the lens to rotate at random, which, left to its own devices, it might. During the blink cycle, closure of the lids proceeds from the outer to inner canthus (‘the zipper effect’). The upper lid moves vertically down to close the eye, and then up again to open it. The lower lid, however, does not move vertically, so as it tightens it imparts a force on the lens that will tend to spin the lower part of the lens nasally. Some cyclo-rotation of the eye may also play a part. To counteract this tendency, several strategies are employed:

    • Back surface toricity is intended to align the lens more exactly with the cornea. However, although this is undoubtedly effective when applied to RGP lenses, it is of questionable effectiveness with modern, flexible soft lenses, since they all tend to conform to the underlying surface. It is always used with other stabilisation methods. Any effect that it has is likely to be greatest where the corneal astigmatism is high. Most disposable toric lenses have this feature.

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    • Prism ballast is commonly used. The lower lens edge is thickened (Figure 5a). When the eyes close during the blink cycle, the squeeze pressure imparted by the upper lid impels the thickest part of the lens away (Figure 5b). This is known as the ‘watermelon seed principle’. Prism ballast is effective, but it has the disadvantage of adding thickness to the lens in the area of the lower limbus. This will reduce oxygen transmission, and when low-water-content lenses were commonly ?tted, this often led to hypoxia and subsequent neovascularisation of the lower cornea (Figure 6). In addition, vertical prismatic effect may be induced, although this effect is only likely to be about one-third of the incorporated prism, and patient intolerance due to it is rare. Modern lenses may have an optic zone without prism.

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  • Truncation has been used in conjunction with prism ballast, primarily with RGP lenses. Effectively, this consists of chopping off part of the lower lens periphery so that the lower lens edge is straight or has a greater radius than the rest of the lens.

  • Dynamic stabilisation (thin zone stabilisation) also uses the ‘watermelon seed principle’, but here material is removed from both the upper and lower lens edges, so both lids contribute to the stabilising effect. The lenses have thin edges, so they tend to be a little more comfortable, and no prismatic effect is induced. Some practitioners prefer them for against-the-rule astigmatism, as they tend to thin down the areas of the lens that the cylinder would render thickest. However, prism ballast may give a slightly better visual performance post-blink. The earlier dynamically stabilised lenses had the disadvantage that the thickness differential was dependent on power, with greater stabilising effect with increasing minus. This has been addressed by designs with a central optic zone and a separately worked peripheral area containing the thin zones. This allows greater consistency of response across the power range and controls the thickness pro?le and oxygen transmission of the higher-minus lenses. A variation on the dynamic stabilisation theme is the incorporation of shaped, raised areas in the horizontal meridian of a lenticulated lens. This will also allow a thin edge, although the raised areas may cause lid sensation.

  • Lenticulation, chamfer and slab-off on the front surface of the lens may all be used to thin the edges and improve comfort. When combined with prism ballast, the idea is to produce a more uniform edge thickness. ‘Eccentric lenticulation’ produces a zone decentred upwards, to reduce the influence of the lower lid.

The geometry of modern lenses has become quite complex in the quest to combine lens stability with comfort and oxygen transmission, and the lenses in use today are far more sophisticated and effective than the ‘snowflakes’ of former times.

Troubleshooting

Soft toric lens ?tting is usually fairly straightforward these days and careful pre-assessment usually ensures success with the ?rst pair of lenses. The patient will then see you as a paragon of optometric skill, which will do your practice no harm at all. Some problems that may arise include reduced or unstable vision, poor comfort, hypoxia and staining.

Poor vision

If the visual result is disappointing, we need to determine whether it is constantly poor or fluctuates.

Constant poor vision may be associated with the following:

  • Wrong powers for spherical or astigmatic components

  • Mislocation of axis – more probable, (Figures 7a and b)

  • Both power and axis are wrong.

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A spectacle over-refraction should give an indication of the remedy, although axis mislocation can also be detected by observation with a slit-lamp as outlined earlier in this chapter. Axis mislocation produces a characteristic type of residual refractive error. The cylindrical power is twice that of the spherical component, and the axis moves in the same direction of rotation as that of the mislocating lens.

For example a lens of -2.75/1.50 x 180 that has rotated 20 degrees anticlockwise will have an over-refraction of +0.50/-1.00 x 55. Crossed cylinder calculation can be used to remedy the correction. This is often performed by the laboratory, but can be done by the practitioner if preferred.

Occasionally, every change to the lens axis to remedy mislocation results in the new lens rotating by a different amount, because the thickness pro?le of the lens has changed. This is called ‘chasing the axis’, and is normally associated with large cylindrical elements, oblique axes, and older designs. Use of a design with independent optic and peripheral zones may help, although the usual remedy to this in practice is a lucky guess.

Unstable vision may be caused by a steep or flat-?tting lens:

  • Steep lenses tend to flex irregularly, so the vision often improves just after a blink, and then deteriorates. The axis may also slowly rotate in one direction with successive blinks.

  • Flat lenses may rotate randomly.

If the ?t of the lens looks good, but the vision is unstable, the stabilisation systems incorporated in the design may not be working for this patient, so another type of design may be better. It is always a good idea to have a favourite prism-ballasted lens and a favourite dynamically stabilised one to try in these circumstances.

Some patients simply do not like the visual quality afforded by soft lenses. They may be happier with RGP lenses, but there are those who can accept no visual compromise. They will probably never be happy with any sort of lens.

Andrew Franklin and Ngaire Franklin are optometrists with Boots Opticians, and College of Optometrists assessors and examiners