Advanced Techniques
Astigmatism

Toric contact lenses may be indicated when residual astigmatism results in vision which is compromised. Residual astigmatism may result from a toric posterior corneal surface, a toric crystalline lens, or a partially dislocated crystalline lens.

Toric lenses may also be required when moderate to high corneal astigmatism results in either mechanical irritation or centration problems with spherical hard lenses.

Astigmatic contact lenses may be classified into the following categories and these are applicable to both hard and soft lenses.

A) Toric peripheral curves with spherical base curves
1. Spherical front surface
2. Cylindrical front surface

B) Anterior toric, with spherical back surface

C) Posterior toric base curves
1. Posterior toric only
2. Bitoric

Toric peripheral curve lenses with spherical base curves:These are rarely dispensed in this country, and when they are only as hard lenses. They are used in cases of low to medium corneal astigmatism when spherical lenses result in inadequate stability. The peripheral curves are shaped to conform to the shape of the cornea contributing to added stability.Trial lenses are required for successful fitting.

Anterior toric with spherical back surfaces: These are available in both hard and soft lenses. They are typically fit when there is a significant amount, generally more than 0.75 D, of uncorrected residual astigmatism. The correcting cylinder is put on the front surface of the lens while the diameter and posterior curves remain the same.

Posterior base curve toric lens: These are available in both har

d and soft lens materials, although they are used predominately with hard lenses. They are indicated when a spherical lens does not provide a stable fit and when there is at least 1.50 diopters of corneal astigmatism. They can also be indicated when irritation is experienced due to inadequate clearance of a spherical lens on a corneal cylinder.

A bitoric lens design is needed when the back toric surface results in enough residual astigmatism so that a front surface correcting cylinder is needed. At least 1.50 diopters of corneal astigmatism should be present.