Section four Complex lenses

Lens orientation and measuring rotation

Whichever toric lens is used, its orientation on the eye is crucial and compensation to the cylinder axis is frequently needed. In most patients 5° of lens rotation, which is more often nasal, is acceptable. Since disposable lenses are almost invariably produced with 10° axis steps, a tolerance of 5° is often unavoidable. Success is unlikely if the rotation is more than about 20°, even if consistent, and it is usually better to try a different fitting or lens type.

Apart from truncations, orientation can only be clearly observed with the slit lamp. Rotation can be measured by:

•Assessing radial markings by observation alone.

•A graticule, either internal or surrounding the slit lamp eyepiece.

•Rotating a fine slit lamp beam to align with lens engravings and reading the instrument’s external protractor scale. Horizontal markings tend to be easier to assess than vertical.

•Rotating a low power cylinder in a trial frame until its axis is aligned with the markings on the toric lens.

•Using a Javal-type ophthalmometer.3

•By over-refraction. Mislocation of the cylinder axis gives an over-

refraction in the form of a plus sphere with minus cylinder of approximately twice the power (e.g. +0.50/−1.00) at a different axis. The degree of rotation can then be calculated by computer programme or from Table 23.1.

Consistency can be evaluated by rotating the lens in either direction and observing whether the lens engravings return to the same position with blinking.

The LARS mnemonic is useful for lenses with the markings at 6 o’clock:

Rotation LEFT = ADD, Rotation RIGHT = SUBTRACT. See example for 10° Left rotation with a right eye (Figure 23.5A).

The CAAS mnemonic is useful for lenses with markings at 3 & 9 o’clock:

Rotation CLOCKWISE = ADD, Rotation ANTICLOCKWISE = SUBTRACT. See example for 10° Clockwise rotation (Figure 23.5B).

23.3 Lens designs

Numerous designs have evolved so that toric soft lenses, apart from stabilization with any of the methods in Section 23.2.2, may be of either corneal or semi-scleral diameter; use a variety of materials; and have either a front or back toric surface. Most soft torics now fitted are disposable (see Section 18.3 and Table 18.3). Conventional or custom made lenses are used mainly for very high prescriptions outside the range of mass production or where some specific design feature is required. Torics tend to have greater thickness so that silicone hydrogels are frequently the first choice because of their superior physiological properties.

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Table 23.1  Residual refractive error induced by mislocation of toric lenses

(see CD for further calculations)

Front and back surface torics

Front surface torics

Front surface torics are capable of correcting both corneal and lenticular astigmatism, up to about 4.50 D. They are successful with a wide variety of geometries, both corneal and semi-scleral. Stabilization can be by means of prism ballast, truncation or dynamic stabilization. The back surface may be either spherical or aspheric.

Back surface torics

Back surface torics have evolved logically from the fact that most of the astigmatism encountered in practice is predominantly corneal. Keratometry and

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10°

ALens rotation in left direction

B

Figure 23.5  (A) The LARS mnemonic; (B) the CAAS mnemonic

spectacle Rx give an immediate prediction of the likelihood of visual success. The back surface of the lens is essentially designed to neutralize the toric cornea by replacing it with a spherical front surface. The radii of the principal meridians are almost always predetermined by the laboratory (e.g. Ultravision Igel 58 Rx toric). It is possible, however, for the practitioner to calculate the principal meridians by using a radius to surface power conversion table.

Cylinders as high as 6.00 D can be corrected with back surface torics. Lenticular astigmatism is not theoretically correctable although, in practice, reasonable clinical results can usually be obtained. The back surface is occasionally stable enough on its own but is assisted by prism ballast, some form of dynamic stabilization or truncation.

PRACTICAL ADVICE

Most back surface torics include only the flatter meridian in the final lens specification (e.g. mark ’ennovy 4T).

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23.4 Fitting

Lenses may be either custom made to a precise Rx or selected from a simplified range of parameters predetermined by the laboratory (stock torics or now more commonly disposable lenses).

Custom made lenses

Custom made lenses permit the fitting of most prescriptions which it is technically feasible to manufacture, with a comprehensive range of lens parameters, water contents, spherical and cylinder powers, axis positions and methods of stabilization. The main disadvantages are the greater time required to obtain lenses and additional costs, particularly where changes to the prescription are necessary. Laboratories currently providing a full range of custom made toric lenses include Cantor+Nissel, CooperVision, mark ’ennovy and Ultravision International.

Disposable and stock torics

These simplified lenses enable rapid fitting from either practitioner or laboratory stock, since parameters are carefully restricted. Cylinder powers often have an upper range of about 2.75 D (this will cope with 90% of prescriptions4), and may be limited to 0.50 D or 0.75 D steps. Axes may be restricted to 20° either side of horizontal or vertical in 10° steps and oblique cylinders, which are generally more difficult to fit, are frequently omitted. Disposable lenses have largely assumed the role of stock torics since trial lenses are readily available and there is no waste of expensive prescription lenses.

No single make of standard design, even in the common range of astigmatism, can approach the accuracy of a properly calculated practitioner specification. However, by using several makes of disposable or stock toric, a standard design can usually be found to match fairly closely the correction and fitting required. This is then the preferred method of fitting.

Laboratories currently supplying stock torics include Bausch & Lomb and CIBAVision. For the much wider range of disposable torics see Table 18.3.

Fitting routine

Fitting conventional lenses is now hampered by the inability to use trial lenses of the preferred or anticipated type. Lenses can be ordered empirically according to ‘K’ readings and spectacle Rx and, if the laboratory offers an exchange or credit facility, the cost of wasted lenses is avoided. Alternatively, spherical or toric disposable trial lenses can be used, selecting a design which matches as closely as possible the probable type to be prescribed in terms of thickness, water content, power and method of stabilization.

•Assess spectacle Rx in relation to ‘K’ readings.

•For radius and diameter, fitting principles are in accordance with those given in Chapter 15. Most varieties of disposable or stock toric do not offer a choice of parameters.

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•The method of stabilization is frequently determined by the choice of lens.

•Power is determined either empirically by considering the ocular refraction in both meridians or by over-refraction with a spherical lens.

•If there is a choice between two possible cylinders, it is generally correct to select the lower power, i.e. underrather than over-correct astigmatism.

•With over-refraction, use a fully settled spherical disposable of power as near as possible to the spectacle Rx. Apart from any vertex distance considerations, this is always worthwhile to ensure that the results correlate, particularly in respect of cylinder power and axis. If there is any discrepancy, repeat with a different design of lens. Some laboratories provide plano cylinder trial lenses with the correct axis and astigmatism to permit assessment of the spherical over-refraction. With higher powers, however, the dynamics of the prescription lens may behave differently on the eye.

•Determine the lens orientation either empirically by allowing up to 5° nasal rotation of the lens base or from the angle at which the markings of a toric disposable lens settle.

•A lens examined on the eye establishes more accurately at the fitting stage whether compensation is required for the cylinder axis. The most common result is about 5° of nasal rotation. Success is unlikely if the rotation is more than about 20°, even if consistent, and it is usually better to try a different fitting or lens type. Oblique cylinders generally give less reliable results.

•The dynamics of a toric lens on the eye with similar cylinder to that anticipated are likely to be more reliable than a spherical lens. This is because the correcting cylinder which has been incorporated ensures comparable lens thickness.

•If axis compensation has been made because of lens rotation at the initial fitting, the prescription lens should also settle with exactly the same degree of rotation.

•There is little value in over-refracting with a toric lens on the eye, since the result will be that of obliquely crossed cylinders and only indirect information may be gained (see Table 23.1).

•The general nature of the over-refraction, however, may be used as a guide with the toric prescription lens, although its absolute value is not very meaningful. If cylinder is present at the original axis or at 90° to this, underor over-correction is suggested. If it takes the form of a plus sphere

85°

5°

5°

A B C

Figure 23.6  (A–C) Recording lens orientation

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