EYE ADVICE: Professional Papers

(iv) Hard Contact Lenses

Correction of spherical or WRT astigmatic errors with spherical or back-surface toric hard contact lenses, often gives rise to some ATR refractive astigmatism, known as residual astigmatism. The reason for this is that most corneas have a WRT toricity which is partly or fully compensated mainly by tonic PMA. In normal eyes with no refractive astigmatism the amount of residual astigmatism is about 0.50 -1.0 D. When a hard contact lens is placed on the cornea, this corneal toricity may be neutralised and residual astigmatism is then manifest. It is important to realise that as the residual astigmatism is caused by muscular tone, a significant amount of this will reduce over time and need not always lead the practitioner into prescribing bitorics. Williams (1959) observed complete disappearance of astigmatism in a random sample of sixty hard lens patients.

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