Astigmatism is a type of refractive error where in the refraction varies in different meridian.  Consequently ,the  rays of light entering the eye cannot converge to a point focus but from focal lines. Broadly there are two types  of astigmatism  :- Irregular astigmatism and regular astigmatism

Regular Astigmatism :-  when the refractive  power changes  uniformly from one meridian to another meridian.

Irregular  Astigmatism :- Where the refractive power irregularly change in different meridian.

Astigmatism is not an uncommon condition . It can be corrected by both  rigid and soft contact lenses.


  • Spherical  RGP lens  :-  sometime optometrist suggest this lens to first fit the astigmatic wearer with a single vision lens , even  if there is high corneal toricity.  Corneal astigmatism  as high as +4.00D has been reported  to be corrected effectively  with a single vision rigid lens in quite a few cases. It has been recommended in such case the lens should be made 0.2-0.3 mm smaller in diameter  than the standard routine lens  diameter.
  • RGP lens with Toric peripheral curves  :-This lens may be required in  many patients  with 1.5-2.5 D corneal astigmatism. In such patients , a spherical  posterior surface lens may lift  over the  steepest corneal  meridian.  This may lead  to poor lens centration  or frequent lens loss.

The   standard speripheral curves are used  in flattest  meridian , and in the steepest  meridian in the peripheral curves  are made  sleeper by an amount  equal to the dioptres of corneal astigmatism.  The lenses are ordered in the  usual  fashion with the notation toric peripheral curves are made sleeper by n amount  equal to the dioptres of corneal astigmatism  . This lenses are ordered in the usual fashion with the notation toric peripheral  curves and the dioptres of corneal  astigmatism.

  • Bitoric RGP lenses :-  Toric posterior  curve  lens is often necessary  to provide  an adequate  lens fit in patients in high degrees  of   corneal astigmatism,   usually  +2.5D or greater . Such a lens is usually fitted with posterior curves  the same  as the  keratometric readings.  However , when  a lens with  toric posterior curve is placed on the cornea, unlike a spherical posterior  surface lens , the lens – tear  interface becomes a toric surface with  a resultant induced astigmatism. Thus, in most situations  when a toric posterior curve is used, an additional anterior toric curve is used, an additional anterior  toric curve will be needed  to overcome the induced astigmatic error,  such a lens is called bitoric lens.
  • Front surface Toric RGP lens :-  These lenses  are required in patients having spherical  corneas with significant astigmatism.  Such an astigmatism is called  residual  astigmatism  and it usually reflects the lenticular astigmatism .  When a front  toric lens  is ordered for a spherical  corneal surface , a special change in the shape of the contact lens is necessary to prevent  it from rotating . This can be accomplished by the following methods.

A prism blast may be added to the  lens in its manufracture.  The amount of base down prism needed is usually 1.50 , al though more prism may be necessary to centre  a high minus power lens.

The lens may be truncated ,i.e . diameter  of the lens in one meridian is shortened by  cutting off the  entire  edge of the lens by 0.5-1.0 mm . Truncation  is usually used to modify  an existing lens rather than as part of the initial  lens design.


  • Spherical  Soft contact lenses :-  Spherical  soft contact lenses may correct  astigmatism of up to 1D , provided  the total  astigmatism is not more than one  third  of the spherical correction . for example a patient with – 5  DS and –  1 DC prescription may be comfortable  when  fitted with a spherical  soft contact lens .
  • Toric soft  lens :-  Toric soft lenses are required when astigmatism is more than 1 D and spherical  soft contact lenses are not able to correct it, and the patient is unable to tolerate rigid lenses.  Toric soft  lenses have different radii of curvature in opposing 90 degree meridians.



Frame as an impressive look changer


Ritika Das

Optometry student Studies at NBMC&H

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