5.3.1. Chooses and manages the fitting of toric contact lenses.

Question 1

Types of Astigmatism

Answer 1

• Regular astigmatism - Two principle meridians are at 90 deg to each other
• Irregular astigmatism - Two principle meridianrs not at 90deg eachother
• With-the-rule astigmatism - Rx error is greater in the vertical meridian (cyl axis 180 +/-30,) Ks steeper in the vertical meridian
• Against the rule astigmatism - Rx error is greater in the horizontal meridian (cyl axis 90 +/-30,) Ks steeper in the horizontal meridian
• oblique astigmatism - Two principle meridians are more than 30deg from horizontal or vertical
• Residual astigmatism - Residual astigmatism = ocular astigmatism - corneal astigmatism when sph rgp lens placed on cornea, will corect the corneal astigmatism, but not lenticular. This is residual.
• induced astigmatism - When toric back surface is placed on toric cornea, characterised by the differing refractive indices of the contact lens and tear firm beneath

Question 2

Chooses appropriate type of CL correction

Answer 2

• Consider Soft Vs RGP:

Vision —> RGP is better

Comfort —> Soft is better

Adaptation —> Soft is better

Correcting Cyl —> RGP is better & more stable. Soft is difficult once the lens you order is >2.50 toric!!

Complex Fitting —> RGPs are a go to, soft lens don’t work as well due to high cyl although something like Kerasoft IC may be quite handy

Question 3

Prism-ballasted

Answer 3

1. Prism-ballasted - first & still most common
   
   1. Thicker profile towards the base (base down prism). Tend to move less vertically
   2. Works on the watermelon seed principle - thin part of the lens goes under the upper lid which squeezes the thicker part of the lens toward the lower lid (the squeeze pressure imparted by the upper lid impels the thickest part of the lens away)
   3. Gravity aids orientation. Then making px blink a lot will accelerate it
   4. Better for oblique axis & weaker lid tension but less comfort due to thickness & less O2
   5. Peri-ballast - over the years, thickness has been minimised to improve O2 transmission & comfort by eliminating prism in the optic zone. Comfort chamfers & eccentric lenticulation (Lenticulation alters the front surface to enhance the edge shape and lens mass.) used e.g. Coopervision Bioinfinity. Stabilising prism used in periphery only

Question 4

Non Prism-ballasted

Answer 4

(double slab off, dual thin zone, dynamic stabilisation)
Dynamic - stabilisation via thin zones at top & bottom of lens which orientate beneath the lids. Lid squeeze against thickness differential to maximise stability. Works on gravity! Utilised by J&J torics
Tends to rotate less but need taut lower lid for stabilising

Learn about different markings:

(See images!)

Question 5

Soft Toric lens Fitting

Answer 5

Lens Options available & first fits:

• Daily Torics —> Linarial Torics (SiH) (Coopervision MyDay Toric), 1 Day Oasys Astig (SiH), 1 Day Moist Astig (Hydrogel)
  
  • Linarial is peri-ballast. 1 day Oasys + Moist are back surface toric eyelid stabilised design (accelerated dynamic)
  • Umere toric is prism-ballast (Coopervision Clariti 1 day)
• Reusable Torics —> Opteyes Toric & Optyeyes Toric XR (Coopervision Bioinfinity) (SiH (price sensitive), Acuvue Oasys Astig, Uvicia Plus Toric (SiH) (Coopervision Avaria Vitality)
  
  • Opteyes + Uvicia are peri-ballast, Acuvue is dynamic
  • Proclear is prism-ballast
• REMEMBER: DAILY TORICS DON’T GENERALLY COME IN >+4.00 SPHERE UNLESS LINARIALS. MAY NEED TO CONSIDER REUSABLE TORIC

Question 6

What to Fit:

Answer 6

If axis rotates 30’ or more then refit with different design

If axis stabilise between 10’ & 20’ then adapt lens ordered using LARS rule (Left Add/Right subtract)

If axis within 5’ then leave it alone!!

REMEMBER - IF VISION KEEPS FLUCTUATING ON BLINK THEN EITHER LENS NOT FITTING WELL OR UNSTABLE ROTATION

• CAUSES OF UNSTABLE ROTATION: PAPILLAE, PINGUECULA/PTERYGIUM, LOW LID TENSION FROM AGE OR PREDISPOSITION

ALSO REMEMBER: HIGHLY MYOPIC ASTIGMATICS ARE EXTREMELY PICKY & ARE BETTER WITH RGPS OR SPECTACLES!

ALSO REMEMBER: IF YOU HAVE TRIED 2 DIFFERENT LENS DESIGNS & PX STILL ISN’T HAPPY THEN EITHER SET EXPECTATIONS OR GIVE UP WITH SOFT LENSES

Question 7

IT ALL BOILS DOWN TO 3 PRACTICAL POINTS:

Answer 7

1. Try different lens design
2. Adapt fit using LARS
3. Set expectations/move on to RGPs or spectacles

Question 8

Stabilisation

Answer 8

 Prism ballast
o 1-1.5 prism BD – lens stabilised by lid force acting on prism induced thickness
o Increased thickness = possible discomfort on lower lid & reduced Dk/t
o E.g., Umere & bio infinity

 Dynamic stabilisation
o Superior and inferior thin zones, interaction between both lids for stability
o Thin lens design (high comfort & no reduced Dk/t)
o E.g., acuvue oasys

 They naturally have a nasal swing of 5-10 degrees due to pressure from the upper lid
 LARS = left add right subtract
o New lens will sit in same position – we are only compensating for axis

Question 9

Toric BVD calculations

Answer 9

 Feq = F / (1 – dF) where F = spectacle lens power, and d = BVD in metres (e.g. 0.012m)