5.3.1. Chooses and manages the fitting of toric contact lenses

Question 1

Soft contact lenses astgiramtism?

Answer 1

• > 0.75DC = toric CL
• <0.75 = correct with sphere + half cyl
• Low myopes = less tolerance to uncorrected astigmatism
• Oblique axis astigmatisms = more problematic than WTR & ATR
  *

Question 2

Stabilisation

Answer 2

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

**Dynamic stabilisation **
* Superior and inferior thin zones, interaction between both lids for stability
* Thin lens design (high comfort & no reduced Dk/t)
* 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
• New lens will sit in same position – we are only compensating for axis

Question 3

Toric BVD calculations

Answer 3

 Feq = F / (1 – dF) where F = spectacle lens power, and d = BVD in metres (e.g. 0.012m)
 Must correct for both meridians in toric fits
o Cyl = difference between each meridian

Question 4

First choice lenses

Answer 4

• Opteyes dk/t 160
• Opteyes toric dk/t116
  (Bioinfinity Monthly LOM 6 o’clock marking 48% )
• Umere dk/t 86
• Umere toric dk/t 57
  (Clariti 1 day Daily LOM 6 o’clock marking, 56%)
• Acuvue oasys dk/t 147
• Acuvue oasys (astig) dk/t 129
  (1 week – EW 2 weekly replacement LOM 6 & 9 marking 38%)

Question 5

Relationship between Ks & Rx

Answer 5

• 0.1 difference = 0.50DC i.e. difference in Ks x5 = corneal astigmatism
• High number = flatter cornea
• Average 7.80mm; radius ranges 7.4 – 8.8mm
• Example
  7.80 @ 180, 7.40 @ 90 = +2.00 DC x 90
• WTR
  1. Vertical meridian steepest a 90 (smallest number is at 90), flattest 180
  1. +ve cyl at 90 / -ve cyl 180
• ATR
  1. Horizontal stays close to 180 & is steepest, flattest at 90
  1. +ve cyl at 180 / -ve cyl at 90

Question 6

Difference between back surface and front surface

Answer 6

• Back surface looks at neutralising the corneal astigmatism
• Front surface takes care of the rest (lenticular astigmatism & rest of Rx)
• </= 2.50DC = spherical back surface (tears will fill gaps)
• > 2.50 DC = back surface toric
• Spherical back surface & front surface toric = if lenticular astig, prism ballast to stabilise
• Bitoric/ FSTx +BSTx = if both corneal & lenticular astigmatism = stable