5.3.1 Chooses and manages fitting of toric CLs

Question 1

What is with and against the rule astigmatism?

Answer 1

• With the rule astigmatism: the flattest K is at 180
• Against the rule astigmatism: the flattest K is at 90

Question 2

What type of Rx needs toric lens?

Answer 2

Types of Astigmatism that require correction:
* Up to 0.75: BVS (Sph + ½ Cyl)
* 0.75 and above: Exact rx

Question 3

What are the stabilisation methods of soft lenses and which lenses?

Answer 3

• Prism Ballast – 1-1.5 Prism Base Down, lens stabilised by lid force acting on prism induced thickness, Discomfort
  along lower lid margin, Copes well with oblique cyls, simple production. Reduced Dk/t due to increased thickness
  (DACP, Uvicia, Umere, Clarison)
• Dynamic Stabilisation – Superior and inferior thin zones, interaction between both lids for stability, thin lens
  design (higher comfort no reduced Dk/t), optic free prism, no top or bottom to the lens (Acuvue Oasys)
• Accelerated Stabilisation Design – active only when misaligned- active zones imbalanced under lids causes lids to
  move lens back into place.
• CV Optimised Toric Lens Geometry: Stabilises lens
  1. Uniform horizontal ISO thickness
  Improves lens stability and reduces rotation for better visual acuity
  2. Smooth, continuous ballast
  Maximizes comfort during eyelid interaction
  3. Larger toric optic zone - Provides clear visual acuity
  4. Optimized ballast toric design - Provides a stable, comfortable fit
• Alcon SmarTears Technology: - Release phosphatidylcholine (PC) - phospholipid found naturally in tears that helps stabilise lipid layer of tear film
• 8/4 stabilisation tecnology
• Prism ballast

Question 4

Methods of RGP stabilisation?

Answer 4

Prism Ballast: Lower lens edge is thickened.
* As px blinks, the lid pushes the thickest part of the contact lens away and so
downwards.
* Disadvantage: Added thickness reduces oxygen transmissibility and therefore
increases neovascularisation.
Dynamic Stabilisation: Material is thinned at upper and lower lens edges, so both lids
now contribute to stabilising the lens.
* No prismatic effect is induced.
* Preferred for against-the-rule astigmatism as they tend to thin down the areas of
the lens that the cylinder would usually make the thickest
* Note: Higher minus rxs have a better stabilising effect than lower minus.
Truncation: Part of the lower lens is sectioned off so that it lies flat on the lid margin.
Often combined with Prism Ballast.

Question 5

Describe front-surface toric RGP?

Answer 5

• This lens design is used when there is over 0.75DC with a spherical or near
  spherical cornea (lenticular astigmatism).
• This lens is ordered with a cylinder power and axis.
• To prevent the lens from rotating, the lens is ordered with base down prism to act
  as a weighted ballast or stabilizer. o usually 1.5Δ base down
• The lens is also dotted at 6 o’clock for diagnostic purposes when dispensing and
  for evaluations at later visits

Question 6

Describe back-surface toric RGP?

Answer 6

• This lens design is rarer and is used when there is a highly toric cornea (3.00D or
  more) with a spherical correction.
  Where corneal astigmatism exceeds 2.50DC, a back surface toric is required to achieve a
  perfect fit
• On the back surface, the back surface of the lens matches the curvature of the
  anterior corneal surface and if alignment, the tear lens is plano.

Question 7

Describe bi-toric RGP?

Answer 7

• Used when front surface or back surface toric result in unacceptable level of residual astigmatism
• Toric BS for physical fit
• Toric FS for full astigmatic correction
• Rotational stability