Contact Lens Division Astigmatism by benbenzhou


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        Medicals International SARL

        Contact Lens Division
.   .       .     .   .   .   .   .                  .                  .
        May 2001.

        Issue 5


   Astigmatism is the condition caused by the differential refraction of light
   rays, making it impossible for light to focus on the retina. Most
   astigmatism is the result of the cornea being spherocylindrical rather than


    Regular astigmatism:

    In regular astigmatism the two principal corneal meridians are
   perpendicular to each other. Regular astigmatism is correctable with
   spherocylindrical lenses. This type of astigmatism can be divided into two
   types also:
   1. With-the-rule astigmatism, where the vertical meridian is steeper than
      the horizontal one.
   2. Against-the-rule astigmatism, where the horizontal meridian is steeper
      than the vertical one.

   P.S.: Regular astigmatism that has principal meridian other than 90º or
   180º is oblique astigmatism.

   Irregular astigmatism:

    Irregular astigmatism is the case of a cornea that is scarred, inflamed or of
   irregular shape so it will focus light onto multiple points on the retina. e.g.

   Residual astigmatism:

Residual astigmatism is astigmatism that remains after corneal astigmatism
has been corrected. Most of residual astigmatism is the result of a toric
crystalline lens and is referred to as “Lenticular astigmatism” or an internal

        It has been estimated that more than a quarter of contact lens patients have
        astigmatism in excess of 1.25 D. To correct astigmatism with contact
        lenses, the practitioner can choose from the following lens types:

        1. Spherical soft lenses.
        2. Toric soft lenses.
        3. Spherical rigid lenses.
        4. Toric rigid lenses.
        Each of these lens types has a place in correcting astigmatism.
        Historically, the descision of choosing a soft toric lens Vs. Single vision
        design was based on the “4 to 1” rule, which says that if the astigmatic
        component of a patient’s refractive error is less than 25 percent of the
        spherical component (in minus cylinder form), then the cylinder may be
        left uncorrected.

        Mild astigmatism (< 1,00D) is often masked by spherical soft contact
        lenses, or when the astigmatism is mild to moderate, the choice is often
        between a toric soft lens and a spherical RGP lens. In general, RGP lenses
        provide crisper vision, are easier to care for, last longer. Soft lenses are
        more comfortable, require less adaptation, and can be worn intermittently
        (for sports or social occasions).



         It should be noted that the best method available today for correcting
        refractive astigmatism is the soft toric lens. With superior manufacturing
        techniques, including toric generated curves, slaboff thin zones, edge
        chamfers, and periballasting, today’s soft toric lenses provide good
        comfort, crisp optics, and a high degree of reproducibility. The use of
        diagnostic fitting sets makes the fitting of such lenses relatively simple. In
        addition, the large range of stocks lens parameters available, including
        correction of as low as 0.75D cylinder errors, is ideal for the correction of
        residual astigmatism present with spherical lenses.

         Furthermore, the ability of the practitioner to obtain customized soft toric
        lenses in round-the-clock axes and in almost unlimited refractive powers
        allows the use of such lenses for the correction of high corneal and
        refractive astigmatism, as well as for use in post surgical applications.

Soft Toric Contact Lens Design and Orientation Systems

        Axis orientation for toric soft lenses:

              a) For front - surface cylinder (“front toric”):

                     i. Prism ballast: a prism is added to weight the bottom
                        edge of the lens.

                     ii. Truncation: the bottom surface of the lens is cut off
                         and made flat (truncated): the resulting “corners” hold
                         the lens in place, and the flat edge makes possible
                         alignment with the lower lid.

                     iii. Thin zones (“dynamic stabilization”; “Double slab
                          off”) thin zones at the top and bottom of the lens allow
                          pressure from the eyelids to hinder rotation.

                     iv. Custom.

              b) For back - surface cylinder (“back toric”):

                     2. Prism ballast.

                     3. Thin zones.

         Choosing an orientation system for toric soft lenses:

                a) Prism ballast: works best with flat corneas, tight lids, or
                   oblique astigmatism, use a prism of 0,75 to 1,50 prism diopters.
                b) Truncation: is an effective stabilization system, but truncated
                   lenses can be uncomfortable. Truncated lenses should have
                   well-beveled edges to maintain comfort. Since truncation
                   loosens the fit, steepening the base curve may help tighten the
                c) When the astigmatism is corneal, complementary back surface
                   toricity in the lens will promote stability but may be too tight
                   with minimal lens movement.
                d) Thin zones offer the most comfort but the least resistance to
                e) Optimum stability is obtained by using prism ballast combined
                   with beveled truncation. The tradeoff is comfort.

         Toric soft lens tips:

                f) Trial lens set: A trial lens set is highly desirable. The lens set
                                   Should have as many cylinders in as many axes
                                   As possible. At the least, the lens set should have
                                   All available cylinders in the 90º and 180º axes.
                                   In addition, one should have an assortment of
                                   Sphere powers e.g.: +3.00;Plano; -3.00.
                g) A hand-held cross cylinder is very helpful.
                h) Over refract with a trial frame not a phoropter.
                i) Over refract with sphere only; spherocylindrical over refraction
                   are not helpful and can be confusing.

Steps in fitting toric soft lenses:

         Trial set fit:

                1. The trial lenses:
                   a. Base curve: it should be slightly flatter than the flatter
                      keratometric (K) reading.
                   b. Diameter: The lens should be 2,0 to 2,5 mm larger than the
                      visible iris diameter.
                   c. Sphere power: The sphere power closest to the patient’s
                      Rx. Adjust the vertex distance for Rx> -4,00
                   d. Cylinder power: should be less or equal to the patient’s
                   e. Cylinder axis: should be the closest to the patient’s RX.

                2. Place the trial lens for at least 20 minutes before evaluation.

                  3. Check for comfort.
                  4. Check for the fit:
                      a. Adequate movement.
                      b. Good centration.
                      c. Stable and consistent orientation (no more than 10 seconds
                          for the lens to return to its initial stable orientation).
                  5. Verify the base curve:
                      a) If too much movement, steepen the base curve.
                      b) If too little movement, flatten the base curve.
                  6. Check the lens rotation: Check the lens rotation by means of
                  laser marks at 3 and 9 o’clock or at 6o’clock. Record the amount of
                  rotation and the direction of the rotation. If the lens rotates to the
                  practitioner’s left direction, the amount of deviation in degrees is
                  added to the spectacle axis (forget about the axis of the trial lens
                  and take into consideration the prescription’s axis to use in adding
                  or subtracting). If the rotation is to the practitioner’s right, the
                  amount of deviation in degrees is subtracted from spectacle axis
                  (The LARS rule).

P.S.: One clock hour = 30 degree.

                  7. Over refract with sphere if all the other parameters are
                  acceptable. The order lens should orient itself on the eye like the
                  trial lens did. The final position, rotation on blinking should be less
                  than 5 degrees.
                  8. Final lens selection:
                       a) Base curve: same as the trial lens.
                       b) Lens diameter: 2.0-2.5mm larger than the visible iris
                           diameter (VID)
                       c) Sphere power: add spherical over refraction to sphere
                           power of trial lens; correct for vertex distance.
                       d) Cylinder power: same as trial lens cylinder.
                       e) Cylinder axis: use “LARS = Left Add; Right Subtract”.
                                   1.      If rotation is to the left, add the amount of
                                           trial lens rotation to the axis from the
                                           spectacle refraction. (Do not add it to the
                                           trial lens axis).
                                   2.      If rotation is to the right, subtract the amount
                                           of rotation from the axis obtained in the
                                           spectacle refraction.
                           e.g.. Spectacle Rx at the plane of the cornea:
                                 Contact lens (trial set):
                               Lens rotation: 20° to the left
                               Appropriate lens power for the patient:

          The L.A.R.S. technique is a viable way to compensate for on-eye
          lens rotation.
          However, it is important to remember that this technique does not
          correct for residual errors that may be present secondary to:

                         1. Cylinder masking
                         2. Vertex distance error.
                         3. Tear effects (FAP flatter add plus, SAM steeper
                             add minus).
                         4. Lens flexure effect.
          Resultant: Cross cylinder effect.
          Requires: Sphero-cylinder over refraction.

          Therefore, to identify both lens rotation and lens power
          requirements, the use of the S.C.O.R and L.A.R.S. technique is
          important. The S.C.O.R. technique is used when the cylinder is
          higher than 3.00/4.00 D.

Empirical fit:

          1. Convert the spectacle Rx to minus cylinder form.
          2. When the sphere power is –4,25D or greater, compensate for
             vertex distance effects on both sphere and cylinder power.
             Example: spectacle lens Rx:           -5,00 –2,25 x 180º
                       Vertex compensated power: -4,75 –1,75 x 180º
          Spectacle lens Rx cross

              Using the vertex table –5.00 will be converted into –4.75 and
              –7.25 into –6.50
          Vertex compensated power cross will be

             So vertex compensated power is:        -4.75 –1.75x180°
             P.S.: -1.75 = -6.50-(-4.75)

              3. Select the vertex compensated sphere power.
              4. Select the vertex compensated cylinder power. (If not available
                 choose the next smaller power).
              5. Select the cylinder axis closest to the axis in the spectacle Rx.
              6. Order the lens.

       Dispensing the lens:

              1. Place the lens on the patient’s eyes.
              2. Allow to equilibrate (20 minutes).
              3. Record visual acuity.
              4. Verify the lens fit and orientation with a slit lamp. Fitting
                 criteria should demonstrate:
                     a) Acceptable visual acuity.
                     b) Corneal coverage with 0.5 to 1 mm of movement with
                          straight-ahead gaze.
                     c) Stable orientation of laser mark.
              5. Allow patient to wear the lens for one (1) week if visual acuity
                 And fit are acceptable.

       CLARITY T 43

Design and Specifications

       The Clarity T 43 lens is especially designed to fit all astigmatic patients
       with less chair time and at first time trial.
       The Clarity T 43 is stabilized with a double system to assure minimal lens
       rotation on the corneal surface and more visual stability.
                       a)     The prism-ballast is included on the posterior
                              surface of the lens, and is factored by:
                              a. A gradual increase in thickness towards the lower
                              b. A squeezing force of upper lid pushes down to
                              the inferior lid.
                       b)     A double slab-off or two thin zones on the anterior
                              surface of the lens:
                                i. Half-moon shaped zones.
                                ii. Zones position at 12 and 6 o’clock.
       The large diameter of 14.4 mm assures minimum lens rotation and the
       back surface toricity of Clarity T 43 provides aberration control and
       enhances astigmatic correction.
       The Clarity T 43 is manufactured of glycerin-coated tetrafilcon A with
       43% water content.
       The tetrafilcon A is a non-ionic polymer that is a deposit repellant and is
       an excellent remedy for dry eye patients.

     Clarity T Parameters

                                          Clarity T 43
                       Material               Tetrafilcon A (43% water)
                      Base curve                         8.6 mm
                       Diameter                          14.5 mm
                     Sphere powers            - 0.50 to -8.00 (in 0.50D steps)
                                              +0.50 to +3.00 (in 0.50D steps)
                    Cylinder powers                      -1.25, -1.75, -2.25
                     Axis degrees             10, 20, 70, 80, 90, 100, 110, 160,
                                              170, 180
                      Axis marking                One laser mark at 6 o'clock
                    Handling tint                           Light blue
                   Wearing schedule                         Daily wear


                     To fit the Clarity T 43, we just have to take the spectacle refraction of
                     any astigmatic eye, convert the vertex distance if needed, and order the
                     final lens.


This Bulletin is researched and prepared by Paul Baakliny      Medicals International LLC/Dubai


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