Section

Complex lenses four

Toric rigid lenses 22CHAPTER

Toric rigid lenses are used either to improve on the fitting of a spherical lens or to give better visual acuity with an astigmatic eye.

22.1 Residual and induced astigmatism

Residual astigmatism

Residual astigmatism is the uncorrected astigmatism found by refraction when a spherical contact lens is placed on the cornea. It derives from the crystalline lens and is usually against-the-rule. It is predictable where the spectacle cylinder and ‘K’ readings do not correlate. The corneal astigmatism is neutralized by a spherical rigid lens leaving the lenticular astigmatism uncorrected.

Residual astigmatism = Ocular astigmatism − Corneal astigmatism

Example 1:

Spectacle Rx: −3.00/−1.50 × 95 ‘K’ 7.90 mm × 7.85 mm Corneal astigmatism = 0.05 mm ≡ 0.25 D

Residual astigmatism = 1.25 D

©2010 Elsevier Ltd, Inc, BV

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

Section four Complex lenses

Example 2:

Spectacle Rx: −2.00/−1.00 × 180 ‘K’ 7.60 mm (along 180) 7.80 mm (along 90)

Corneal astigmatism (against-the-rule) = 0.20 mm ≡ 1.00 D Residual astigmatism = 2.00 D

Induced astigmatism

The liquid lens beneath a rigid contact lens neutralizes only nine-tenths of the anterior corneal astigmatism because of the difference in refractive indices.1 Induced astigmatism is created when a toric back surface is placed on a toric cornea and is against-the-rule if the corneal and spectacle astigmatism is with- the-rule. It is a characteristic of the lens because of the different refractive indices of the lens material and the tears. Modern rigid gas-permeable materials tend to give less induced astigmatism than PMMA, which has a higher refractive index.

Ocular refraction

The ocular refraction at the surface of the eye is obtained from the dioptric  power of the spectacle refraction by compensating for the measured back vertex distance (BVD) in millimetres. The ocular refraction is required when the  spectacle powers are greater than ±4.00 D in either principal meridian. It is always has a value that is less minus or more plus than the spectacle prescription.

22.2 Patient selection

22.2.1 Indications and contraindications

Indications

To improve the physical fit

•The difference between principal meridians is greater than 0.6 mm.

•A spherical lens is unstable.

•A spherical lens decentres, usually along the steeper meridian.

•A spherical lens gives unacceptable bearing areas due to the corneal toricity.

•The bearing areas minimize tears exchange which may give rise to midperipheral corneal staining along the flatter meridian.

•A spherical lens produces corneal moulding and unacceptable spectacle blur.

•The cornea becomes significantly more toric towards the periphery.

•Poor comfort with a spherical lens due to rocking.

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To give optimum visual acuity

•Residual astigmatism is greater than about 1.00 D.

•Lens flexure due to a steeply fitting spherical lens.

•Induced astigmatism.

Contraindications

•Where a spherical lens gives a satisfactory result.

•Sensitive eyes where increased thickness causes discomfort or reduces transmissibility to an unacceptable level.

•Critical visual needs where acuity may be unstable.

•With a toric cornea but spherical spectacle Rx; a thin spherical soft lens should be the first choice (see Section 5.4).

•The greater expense of toric lenses.

•A toric lens is unnecessary with an amblyopic eye.

22.3 Lens designs

There are several possible designs for the correction of astigmatism of which not all are toroidal.

22.3.1 Non-toric lens forms

Small spherical lenses

Lenses with very small TDs are often successful. They fit only the central area of the cornea, which may be more spherical than the periphery. Fitting sets with narrow axial edge lift (typically 0.10 mm) are used to avoid excessive edge clearance or stand-off in the steeper meridian.

PRACTICAL ADVICE

•Lenses may sometimes be as small as 7.50 mm.

•Centration is important to avoid flare.

•Use a centre thickness in the region of 0.12 mm.

Aspheric lenses

Most aspheric designs also have narrow edge lift and give reduced clearance along the steeper meridian. In some cases, they can mask up to about 4.00 D of astigmatism. They are usually fitted in alignment or flatter to avoid flexure.

22.3.2 Toric lenses

Back surface toric

Both central and peripheral radii are toric with a spherical front surface. The final peripheral radius is sometimes spherical for ease of manufacture and to

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assist tear flow. Stabilization is unnecessary since the lens radii should correctly follow the principal meridians of the cornea. The fluorescein appearance should look identical to that of a spherical lens on a spherical eye.

Bitoric

Both central and peripheral back surface radii are toric, combined with a front surface cylinder to correct induced astigmatism. Stabilization is usually needed for correct orientation of the cylinder axis because its position is important.

Front surface toric

A spherical back surface with a front surface cylinder to correct residual astigmatism. Stabilization is necessary to maintain the correct cylinder axis.

Toric periphery

A spherical BOZR with toric back peripheral radii. Used to give stability and to fit corneas where peripheral toricity is significantly greater than central. The front surface is usually spherical.

22.4 Methods of stabilization

Prism ballast

Prism ballast usually employs 1.5 (with an upper limit of about 3.0 ). The weight differential between the top and bottom of the lens should cause it to orientate with the prism base downwards. It is usually marked to assist observation. There is a tendency for a 5–10° nasal rotation of the prism base on blinking due to lid tension and eyelid position.

Truncation

Truncation may be used either on its own or in conjunction with prism ballast and can be single or double. A chord of 0.50–1.00 mm is removed from the lens edge and the optimum effect is achieved when the truncation sits on the lower lid. It is therefore ineffective with small diameters and if the lower lid is below the limbus. To assist stability, compared with the optimum trial lens diameter, the TD should be increased by 0.50 mm for single truncation and 1.00 mm for double.

22.5 Fitting back surface torics

22.5.1 Toric fitting set

•A diagnostic lens is selected to have the flatter meridian the same as flattest

‘K’ and the steeper meridian 0.1 mm flatter than the steepest ‘K’. The meridians are therefore not exactly matched (e.g. ‘K’ 8.00 × 7.40; BOZR 8.00 × 7.50).

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•The ideal fluorescein pattern should look the same as an alignment spherical fit.

•Over-refraction should be carried out with minus cylinders. The spherical component is the power along the flatter meridian.

•The laboratory will produce the cylinder along the steeper meridian.

•The possibility of induced astigmatism must always be considered.

22.5.2 Spherical fitting set

•A spherical lens is fitted in alignment to establish the flatter meridian.

•The steeper radius is determined from the ‘K’ readings.

•The spherical power is determined by over-refraction with minus cylinders.

•The cylinder is obtained by calculation. Example:

The astigmatism is entirely corneal.

Diagnostic lens BOZR giving alignment along 180 = 8.00 mm

By fluorescein assessment the toric lens needed is:

 r1 = 8.00 mm along 180

 r2 = 7.55 mm along 90

BVP calculation:   F = n − 1/r

where refractive index of tears n = 1.336

Along 180 BOZR of 8.00 mm gives anterior surface power to the tear lens in air of:

  336/8.00 = +42.00

Along 90 BOZR of 7.55 mm gives anterior surface power to the tear lens in air of:

  336/7.55 = +44.50

Along 180 BVP = −2.00D (from over-refraction with trial lens) Along 90 BVP = −2.50D

Final prescription:

8.00 : 7.008.60 :8.0010.50 : 9.00 7.55 8.15 10.00

−2.00/−2.50 × 180

The rule-of-thumb 0.1 mm ≡ 0.50 D gives useful confirmation, since 8.00 mm −7.55mm is 0.45 mm ≡ 2.25 D.

22.5.3 Fitting by calculation

A lens can also be ordered for the patient by theoretical calculation using the ocular refraction, ‘K’ readings and refractive index of the material. BOZD, TD

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and edge clearance are nevertheless determined from clinical assessment. The toric difference of the lens radii can be chosen to neutralize the corneal astigmatism but the induced astigmatism must also be calculated.2

Explanation:

Ocular refraction:  Sphere (S)  Cylinder (C) Induced astigmatism (I):

I = n − n′ − n − n′ r1 r2

where

n = refractive index of tears

n’ = refractive index of the contact lens material r1 = steeper radius of curvature, in mm

r2 = flatter radius of curvature, in mm However, other factors have to be calculated.

The lens astigmatism induced by the back surface in air (A):

A = 1− n′ − 1− n′ r1 r2

The BVP along the flatter meridian is S. The BVP along the steeper meridian is S + A

To correct the induced astigmatism this becomes S + A − I along the steeper meridian.

Induced astigmatism: = −7.70144 − −8.00144

= (−18.70) − (−18.00) = −0.70 D

This almost exactly corrects the residual astigmatism and a front surface cylinder is unnecessary in this case.

BVP along the flatter meridian: = −2.00 D

BVP along the steeper meridian needs to be calculated: Lens astigmatism in air (back surface lens astigmatism)

=(−62.33) − (−60.00)

=−2.33 D

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