Mainly two toric lense are popular , soft toric lens and rigid gas permeable lens . 

Toric RGP lens  (Front surface toric Rigid gas permeable lens ) :-

These lenses are required in patients having spherical corneas  with significant astigmatism . This type of 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 .It can be  accomplished  by the following methods :-

A prism blast may be added to the lens in its manufacture .  The amount of base down prism is needed  is usually 1.5 D, although 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  of 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

Toric soft lens  :-   Toric soft lenses are required when astigmatism  is more than 1D and spherical soft 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 . A wide variety of lenses are available with fitting guidelines provided  by the manufacturer .  All forms of soft toric lenses need to be stabilized  so that the toric optics of the lens can be maintained in the desired orientation so as to correct ocular  astigmatism. The aim is to minimize  rotation  from the ideal in –eye orientation . The orientation of a soft  toric  lens on the eye must be predictable and consistent , otherwise  suboptimal  vision will result . The development of the technique of  dynamic stabilization in the 1980s has resulted in an overall improvement  in the performance of  soft toric lenses .

Toroidal  back surface :-  Soft toric lens with a toric back surface  will generally locae better than a front surface toric lens , because that the back toric surface is more likely to align, or ‘look on’, to the matching toroidal corneal  surface . However , experience has shown  that  a toroidal  back surface  alone is insufficient  to achieve lens stabilization.

Prism ballast :- Prism ballast is that base – down  prism is incorporated into the lens so that the lens will be heavier at the prism base.  Gravity  then acts to cause  the prism base to locate  inferiorly. A prism of between 1.00 to 1.50 D is ground base down into the lens . However , greater amounts of prism may be  needed  for patients with particularly tight  lids,  flat corneas , or oblique axis astigmatism . The lens will tend to rotate so that the  base of  the prism is oriented inferiorly . The added thickness of the lens along the prism base can reduce oxygen  permeability  through  that portion of the lens resulting in possible hypoxic disturbance in the inferior zone of the cornea.

Peri- ballast :-  This is one of the common stabilization techniques . This  method  of lens stabilization  features a lens with a minus carrier , with the carrier being thicker inferiorly . in other words , the prismatic thickness profile changes are confined  to the lens carrier , where the carrier is thicker inferiorly. The design is fabricated simply by removing the high – minus lenticular carrier from the superior  portion of the lens . In effect , it is similar to prism ballast except that with peri – ballast all the prism is outside the region of the optic zone.

Truncation :-  Truncation refers to the techniques of slicing off the bottom  of the lens , so as to form a ‘shelf’ that will rest  upon – and therefore align with – the lower lid. This is a reasonably successful method  of stabilizing lenses with thick  edges,  especially when combined with prism ballast.  Either a single lower truncation , or a double truncation  can be used.  With the former , the truncated section of the lens that is removed can be anywhere  between  a sag of 0.5 and 1.5 mm.

Dynamic  stabilization :-  Dynamic stabilization was initially developed by Fanti and this is currently the most commonly used method of stabilization for soft toric lenses. With this technique, the dominant lens orientation effect is achieved by pressure from the upper lid and  the lower lid . Hanks used  the analogy of the ‘water melon seed’ to  illustrate how dynamic stabilization works.  Simply put, pressure applied to the thin  end of a water melon seed by the fingers that the pressure exerted on a thin zone of a lens between the upper lid and  globe causes the watermelon seed  to move away  from the fingers that the causes the lens to orient away from the squeezing force of the eyelid and globe .  Hanks demonstrated that the effect of gravity is insignificant , and that the effect of thickness profile interaction with the upper lid.




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Ritika Das

Optometry student Studies at NBMC&H

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