Ординатура / Офтальмология / Английские материалы / Contact Lenses in Ophthalmic Practice_Mannis, Zadnik. Coral-Ghanem, Kara-Jose_2003

11

Astigmatism and Toric

Contact Lenses

Michael Twa and Saly Moreira

1. What is astigmatism?

Astigmatism is an optical distortion that results in blur that smears the point of focus (Figure 11.1). The blur is minimum at two different focal points that are separated by the distance between their focal lengths (Figure 11.2). This optical distortion is most often caused when the cornea has a toric shape. The torus has the shape of a bicycle tire and is more curved in one meridian than the other (Figure 11.3). An optical surface with a toric shape focuses light at a short distance for the more curved meridian and at a farther distance for the less curved meridian. It is important to distinguish between astigmatism, which is an optical condition, and corneal toricity, which is a toric distortion in the shape of the cornea. Although astigmatism is most often associated with corneal toricity, one does not require the other. The correction of astigmatism is an optical problem; corneal toricity is a contact lens fit consideration.

Corneal Astigmatism

Distortion of the spherical shape of the cornea is the most common cause for astigmatism. Astigmatism results when the central cornea has a toric shape. With pure corneal astigmatism, the total power of the astigmatism is equal to the corneal toricity. This can be measured using keratometry or corneal topography and can be corrected with soft or rigid contact lenses.1

Residual Astigmatism

The term residual astigmatism has several possible meanings. In general, residual astigmatism results from causes other than the shape of the anterior corneal surface and can refer to astigmatism caused by other optical elements of the eye as well as by contact lens rotation and flexure. Residual astigmatism is most commonly caused by the optical

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Figure 11.2. The optics of astigmatism.

Figure 11.3. Toric shape.

power of the lens and is sometimes called lenticular astigmatism. Other causes for residual astigmatism include a toric posterior corneal surface and tilted or misaligned optical surfaces within the eye. Residual or internal astigmatism is commonly against-the-rule (greater refractive power in the horizontal meridian) and is not correctable using spherical contact lenses but may be treated with other lens designs.

Irregular Astigmatism

Irregular astigmatism is the name given to optical distortion that is not easily described by conventional sphero-cylindrical lenses and generally refers to optical aberrations other than defocus and regular astigmatism. Unlike regular astigmatism, which is caused by a symmetric and predictable toric corneal surface, irregular astigmatism is caused by corneal distortion that is not symmetric. Videokeratography is the best way to observe an irregular corneal surface; however, distorted

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keratometry mires and/or an irregular retinoscopic reflex can also reveal irregular astigmatism. Rigid contact lens designs are most useful for the correction of irregular astigmatism, where tears between the contact lens and corneal surface can compensate for mild to moderate corneal irregularity.

Total Astigmatism

Total astigmatism is measured by a standard refraction. Total astigmatism is the sum of corneal astigmatism and residual astigmatism. In some cases anterior corneal astigmatism may be balanced by internal astigmatism.

It is important to consider how much astigmatism is caused by the toricity of the anterior cornea because this will guide the selection of both optical and shape characteristics of the contact lens during the fitting process. In cases where corneal and internal astigmatism do not cancel each other, the residual astigmatism may or may not be balanced between corneal and lenticular astigmatism.

2.What is the best way to determine residual astigmatism?

Residual astigmatism can be determined by comparing the anterior corneal toricity with the total astigmatism determined by refraction. The residual astigmatism is the difference between these two values. This is easy to interpret when the axis of corneal astigmatism matches the axis of the astigmatism measured by refraction. However, it is more difficult to interpret when the resulting cylinder powers are crossed. Another way to determine residual astigmatism is to place a rigid spherical contact lens on the cornea and perform a sphero-cylindrical overrefraction. The quantity of astigmatism that remains uncorrected is the residual astigmatism from all other optical elements including the intraocular lens, contact lens flexure, and any tear lens formed be-

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hind the lens. Predicting residual astigmatism by calculation is not exact because the effects of eyelid pressure, lens position, lens rotation, and lens flexure are unknown. The best method to determine residual astigmatism to account for these effects is by trial fitting an approximate contact lens and performing a sphero-cylindrical overrefraction.2

3.What are the indications for a toric contact lens?

There are two reasons to consider a toric contact lens:

1.Spherical lenses do not provide sufficient optical correction of total astigmatism.

2.The physical fit and alignment of a spherical lens is unacceptable because of corneal toricity.

It is common to ignore small amounts of total astigmatism when fitting contact lenses. Many patients are not bothered by uncorrected refractive astigmatism less than 1.00 D. This is partly due to the correction of refractive astigmatism through tear lens effects and contact lens flexure that can reduce residual astigmatism enough to satisfy the patient. Patients with less than 2.00 D of corneal toricity and small amounts of residual astigmatism (0.50 D) by overrefraction may find that their vision is acceptable with conventional spherical lenses. When this approach is successful, it simplifies the fitting procedure for both doctor and patient and simplifies the manufacture and reproduction of future contact lens orders. However, the needs of the patient are the most important consideration. In some cases, patients with as little as 0.50 D of astigmatism will require astigmatic contact lenses to achieve satisfactory vision.

Example

A 47-year-old woman

Occupation: Chief executive officer of a computer graphics company Goal: Wishes to wear contact lenses for presentations at business

meetings; very demanding personality. She was a previous failure with contact lenses that ‘‘did not give sharp vision.’’

Refraction: 3.00 0.75 090 20/20

Overrefraction: 0.00 0.75 090 20/25 with 3.00 D sphere soft lens

Final lens: 2.75 0.25 085 20/15

Conversely, in other cases it may appropriate to fit spherical contact lenses even for large amounts of astigmatism.

Example

A 32-year-old man

Occupation: Championship surf kayaker

Goal: Has worn lenses before that gave pretty good vision but lost

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them often. Quit wearing contact lenses because they were too expensive to replace.

Refraction: 3.75 2.25 167 20/15

Over-refraction: 0.50 1.25 170 with a 4.50 D sphere soft lens Final lens: 4.50 spherical soft disposable contact lens

4.What kind of contact lenses can be used to correct astigmatism?

Astigmatism can be successfully corrected with either rigid or soft contact lens designs. Lens selection and fitting strategy are usually guided by the patient’s prescription, corneal toricity, and personal concerns. A summary of lens categories follows; specific applications for each lens type are discussed in the following sections. Rigid contact lens types for the correction of astigmatism include:

1.Spherical contact lenses

2.Front toric contact lens

3.Back toric contact lens

4.Bitoric contact lens

Soft contact lenses for the correction of astigmatism include:

1.Prism ballast toric contact lenses

2.Thin zone stabilizing lens designs

5.What rigid toric contact lens materials are used for the correction of astigmatism?

Toric contact lenses can be made of rigid gas permeable or hydrophilic materials. To produce a predictable optical correction on the eye, rigid lens designs should not bend on the eye. Lens flexure on the eye can induce additional unwanted astigmatism. Lens flexure can be greater as corneal toricity increases.2 To guard against this source of unwanted astigmatism, one can select materials with lower oxygen permeability that are generally less elastic and therefore more resistant to lens flexure on the eye. By increasing lens thickness, one can also reduce the chances of rigid lens flexure. However, this will also further reduce oxygen transmissibility of the lens.

6.How does one prescribe a spherical rigid lens for an astigmatic cornea?

Refractive astigmatism associated with corneal toricity less than 2.00 D is often successfully corrected with a spherical rigid contact lens. In this case, residual astigmatism is corrected by a tear lens formed between the contact lens and corneal surfaces. The first step is to achieve a good alignment and position of the lens on the eye. If it is not possible to achieve a good physical fit that will not compromise the health of the

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eye, then the quality of vision is unimportant. Trial fitting the contact lens is the best way to determine the acceptability of lens fit and vision.

The same rules for fitting spherical rigid contact lenses apply to the correction of astigmatism. First, there must be adequate tear exchange beneath the lens. This is not usually a problem with a toric corneal surface where tears can easily pool beneath a spherical lens along the steep meridian. However, in a highly toric cornea, alignment of the posterior curvature of the lens with the flat meridian can cause the lens to rock to either side of this balance point, resulting in lens displacement or even ejection. The spherical lens on a toric cornea should still center well over the pupil and move adequately with a blink. Selecting a posterior lens curvature steeper than the flattest corneal meridian can help improve lens fit and centration. An acceptable fit becomes increasingly difficult to achieve as corneal toricity increases and results in discomfort, lens decentration, lens ejection, and reduced vision. If this occurs, consider fitting a contact lens with toric surfaces.

Select the posterior curvature of the initial fitting lens from the flat keratometry value. Allow the lens several minutes to stabilize on the eye before judging alignment, positioning, and movement with a blink. Reduce excessive lens movement by reducing the posterior lens radius of curvature, increasing lens diameter, or increasing the optical zone diameter.

Fitting a Spherical Rigid Lens on a Toric Cornea

Base curve

For 2.00 D of corneal toricity, select a posterior curvature slightly greater than the flat keratometry value (e.g., 0.50 D steeper than the flat keratometric reading).

As lens diameter and corneal toricity increase, select a posterior lens curvature slightly steeper than the flattest meridian by keratometry.

7.What are the indications for a front toric rigid contact lens?

Refractive astigmatism measured in an eye with a spherical corneal surface cannot be fully corrected with a spherical rigid contact lens. Although the physical fit of the lens requires a spherical posterior lens surface, the full optical correction will not be achieved. A front toric rigid contact lens design is used in this situation. A front toric rigid contact lens is designed with an astigmatic correction on the anterior

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The contact lens rotates to the 5:30 clock-hour (15 rotation, counterclockwise), and in this case 15 must be subtracted from the overrefraction’s axis of astigmatism. The final lens power is determined by combining the contact lens power with the results of the overrefraction. The final axis is calculated by correction for the trial lens rotation.

9.What is the definition of and indication for a posterior toric rigid contact lens?

A posterior toric rigid contact lens has a spherical front curvature and a toric posterior surface with toricity approximately equal to the cornea. Toricity of the posterior lens surface allows the lens to align better with the cornea, improving the physical fit characteristics of the lens. The posterior curvature of a back surface toric contact lens should align with the corneal surface but should still permit movement and tear exchange beneath the lens. There are several methods for selecting the correct amount of lens toricity to fit a toric corneal surface. Most methods allow tear exchange beneath the lens by fitting slightly flatter than alignment. A contact lens with posterior curvatures slightly flatter than the corneal toricity in each meridian orients well and aligns to the central cornea while still permitting tear exchange through the peripheral edge lift of the lens. The posterior toric contact lens is indicated for high degrees of corneal astigmatism (greater than 2.00 D of corneal toricity) especially when an acceptable fit cannot be accomplished with a spherical posterior curvature.

A toric back surface lens with a spherical front surface will induce additional astigmatism along the flat meridian of the eye due to the sphere and cylinder combination and the difference between the index of refraction of the tears and the plastic of the contact lens.1 For this reason, this lens design has limited application and is best suited for cases where corneal toricity is flatter in the vertical meridian and residual astigmatism is against-the-rule (minus cylinder, axis 90 degrees). A compensating toric surface can be placed on the front of the contact lens to address this problem. This design is known as a bitoric contact lens and will be described in the next section.