Section

Complex lenses four

Toric soft lenses 23CHAPTER

23.1 Patient selection

Toric soft lenses are prescribed not to improve the physical fitting, as with rigid lenses (except with very high degrees of astigmatism), but to provide good visual acuity where spherical lenses are unable to achieve this.

Indications

•Vision is unsatisfactory with a spherical soft lens.

•Astigmatism is 0.75 D or greater.

•Tolerance is poor with a rigid gas-permeable lens.

•Keratometry and optical considerations indicate that a rigid lens requires a much more complex, bitoric design (see Section 22.6).

Contraindications

•Astigmatism is purely corneal and rigid lens tolerance is good.

•Existing rigid lens wearers.

•Irregular astigmatism.

•Monocular patients.

©2010 Elsevier Ltd, Inc, BV

DOI: 10.1016/B978-0-7506-7590-1.00011-X

Section four Complex lenses

23.2 Stabilization

23.2.1 Influences on lens behaviour

The main influences on lens orientation are the method of stabilization and the lids. Eyelids are important in respect of:

•Position of lower lid.

•Lid angles, whether sloping upwards or downwards.

•Size of vertical palpebral aperture.

•Lid tension.

•Force of blink.

•Direction of movement on blinking.

In cases of with-the-rule astigmatism, the thickest portions of the correcting toric lens lie at the top and bottom. The normal action of the lids is to rotate the lens 90° off-axis to bring the thickest parts into the horizontal meridian. The action of the lids on the lens edge has been compared to squeezing a watermelon seed, so that they control the ultimate lens position even with the head inverted.1

There are several other factors that have some effect on lens behaviour:

•Gravity.

•Water content.

•Material elasticity.

•Lens thickness.

•Hydrostatic pressure.

23.2.2 Methods of stabilization

Various techniques are possible, either on their own or in combination.

Prism ballast

Prism ballast usually employs 1 or 1.5 base down. The upper limit is approximately 3 . Modern refinements of lens design reduce the thickness previously associated with this method and give improved comfort and physiological response:

•Prism-free optics to incorporate the stabilizing prism only in the peripheral areas of the lens (e.g. CooperVision Omniflex toric).

•Slab-off prisms to give equal thickness at both the base and 3 and 9 o’clock positions.

Where a toric is required for only one eye, in theory the spherical lens for the other eye should also include base-down prism to prevent binocular imbalance. This is not often necessary in practice (not at all with prism-free optics) and the spherical lens is usually ordered without prism but with subsequent assessment of binocularity. Conversely, it is sometimes effective to employ a prism ballast toric specifically to reduce a vertical imbalance.

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Toric soft lenses 23 Chapter

Advantages

•Good comfort.

•Cosmetically good.

•Works with oblique cylinders.

•Can be used with disposable lenses.

Disadvantages

•Careful slit lamp observation required to assess lens orientation.

•Not always successful at stabilization.

•Lenses may be thicker.

•Greater risk of oedema with low Dk materials.

Dynamic stabilization

The term dynamic stabilization may be used for those techniques which depend on the interaction between the lids and front surface of the lens.

Thin zones

Top and bottom portions of the lens are chamfered to reduce the thickness where the stabilization zones fit beneath the lids (Figure 23.1). The optic portion is the central band which lies within the palpebral aperture. Lenses usually have the DS axis marked in the 3 and 9 o’clock positions. The thickness differential is related to the lens BVP: low powered lenses therefore do not stabilize as reliably as high minus powers. A refinement of thin zones is to have a smaller independent optic which gives greater consistency across the power range.2

Disadvantages

•Careful slit lamp observation required to assess lens orientation.

•Limitation to the amount of cylinder (approximately 4.00 D).

•Stabilization depends on lens power.

Advantages

•Lens remains thin overall.

•Good comfort with thin edges.

•Good cosmetic appearance.

•Unimportant if lens is upside down.

•Can be used with disposable lenses.

Thin zone

Ds axis

Thin zone

Figure 23.1  Thin zones

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Section four Complex lenses

Figure 23.2  ASD – courtesy of J&J Vision Care

Accelerated stabilization design (ASD)

Lenses employing the accelerated stabilizaton design (e.g. the Acuvue range of toric lenses, Johnson & Johnson) have four stability zones (Figure 23.2). This construction is claimed to provide more rapid settling on the cornea, within 1 minute, and achieve correct orientation within 5° of the anticipated position in 90% of cases.

Other methods

•Lens elevations (orientation cams) in the 3 and 9 o’clock positions (e.g. Lunelle, CooperVision).

•Wing ballasting to increase thickness along the horizontal meridian.

•Specially shaped, non-circular lenses.

Truncation

Truncation is now rarely used with soft lenses as it does not lend itself to the mass production moulding methods required for disposable lenses. At the same time, other methods of stabilization have improved. Truncation is usually single, removing a 1.00–1.50 mm chord from the lower edge of the lens (e.g. Ultravision Rx-T, Ultravision International). Oblique truncations (up to 20°) are feasible  and could be used with angled lids. Some designs of front surface toric have employed double truncation.

Advantages

•Excellent stability.

•Easily observed and measured on the eye.

•Thinner lenses can be used.

Disadvantages

•Less comfortable.

•Buckling of the lower edge if too flat or the vertical corneal meridian is very steep.

•Cosmetically more noticeable.

•Less satisfactory with oblique cylinders.

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Toric soft lenses 23 Chapter

•Increased deposits along truncated edge.

•Very few lenses now available.

•Not used with disposable lenses.

Toric back surface

A toric back surface has a natural stabilizing effect when placed in apposition to an equivalently toroidal cornea because least elastic distortion occurs when the lens is correctly aligned. A much more stable result, however, is achieved when used in conjunction with prism ballast or truncation. The toroidal optic zone is ellipsoidal in shape, the dimensions depending upon power and radius. It is larger with a lower cylinder and vice versa.

23.2.3 Assessing lens rotation

Lens markings

In order to assess any rotation on the eye, it is essential that lenses are marked with reference points, except for truncations which are easily measurable. The following methods are commonly used:

•Radial engravings at the base of the lens. These give an assessment of rotation on the eye and are usually separated by either 15° (e.g. Z6T) or 30° (e.g. Purevision Toric) (Figure 23.3).

•A single vertical line at the base of the lens (e.g. Biofinity).

•Vertical lines in the 6 and 12 o’clock positions (e.g. 1 Day Acuvue for astigmatism).

•A single dot, usually at the base of the prism (e.g. mark ’ennovy 4T).

•Horizontal lines in the 3 and 9 o’clock positions (e.g. Focus) (Figure 23.4).

•Horizontal elevations in the 3 and 9 o’clock positions (e.g. Lunelle).

•A combination of horizontal and base markings (e.g. Proclear).

•Dots in the 3 and 9 o’clock positions (e.g. Focus Dailies Toric).

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