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Toric contact lens

The use of contact lenses for the correction of visual acuity is well known. Contact lenses having a toric optical zone (commonly referred to as "toric contact lenses") are commonly used to correct refractive abnormalities of the eye relating to astigmatism. Astigmatism may be associated with other refractive abnormalities, such as myopia (nearsightedness), and hypermetropia (farsightedness), presbyopia and the like and therefor toric contact lenses can be prescribed with one or more spherical corrections. A toric lens for astigmatism and to correct myopia, hypermetropia, presbyopia and/or one or more other refractive abnormalities will have prescription variables not only for spherical correction but also for toric correction and axis of toric correction. The toric optical zone provides cylindrical correction to compensate for the astigmatism. Since astigmatism requiring vision correction is usually associated with other refractive abnormalities, such as myopia (nearsightedness) or hypermetropia (farsightedness), toric contact lenses are generally prescribed also with a spherical correction to correct myopic astigmatism or hypermetropic astigmatism. The toric surface may be formed in either the posterior lens surface (back surface toric lens) or in the anterior lens surface (front surface toric lens). A toric lens has both spherical and cylinder correction and accordingly must maintain a fixed and predetermined orientation during use. A normal spherical lens tends to rotate in the eye as it floats on tears and is subject to normal blinking and eye movement. The problem of preventing rotation of a toric lens has been addressed in several ways. Toric contact lenses are manufactured with a selected relationship (or offset) between the cylindrical axis of the toric optical zone and the orientation of the ballast. This relationship is expressed as the number of degrees (rotational angle) that the cylindrical axis is offset from the orientation axis of the ballast. A toric surface is characterized by a pair of orthogonal curvatures with different radii, and is typically formed by superimposing a toric deviation upon a spherical base curve.

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