Week 5 - Correcting astigmatism SCL

Question 1

What are the different types of Lens Design?

Answer 1

• Toric back surface with a spherical front surface
• Toric front surface with a spherical back surface
• Wraps on the cornea so there is a negligible tear lens
• The design is usually simply related to the manufacturing i.e. what kind of mould is used
• All the ocular astigmatism is usually corrected just by incorporating the cyl into the lens power ordered.

Question 2

What are the different methods of stabilisation?

Answer 2

• Truncation
• Prism ballast (& peri-ballast)
• Dynamic stabilisation
• Toric back surface

Question 3

How are torics stabilised with Prism-Ballast?

Answer 3

• 1 to 1.54 D Base-down
• Stabilized by prism-induced CL thickness differences
• Decreased CL O2 transmissibility
• Discomfort with CL-lid interaction

Question 4

How are torics stabilised with Truncation?

Answer 4

• Truncation aligned with lower lid margin can -› some CL stabilisation
• Truncation can -› some discomfort
• Truncation is not always successful
• More patient visits can be required
• Seldom used now

Question 5

How are torics stabilised with Peri-Ballast?

Answer 5

• Uses thickness differences as the stabilising component, creates a base down prism effect
• Can cause Discomfort with CL-lid interaction at the thicker inferior half
• Decrease CL O2 transmissibility in thicker regions
• Thinner superiorly, thicker inferiorly
• Orientation principle similar to prism-ballast
• Similar overall thickness to spherical CL

Question 6

How are torics stabilised with Double Slab-Off?

Answer 6

• Thin zone superior and inferior
• Lid forces (upper and lower) maintain orientation
• Overall thinner CL
• CL is symmetrical
• Can exhibit decrease rotational stability if low sph, WTR astig.

Question 7

How are torics stabilised with Accelerated Stabilisation design?

Answer 7

• Thin zone superiorly and inferiorly
• Four active zones of added thickness located in the mid periphery if the lens, designed to be actively rotated into place upon blinking whenever it is misoriented and then held stable when the lens is correctly aligned

Question 8

What are the 5 steps in selecting a first lens?

Answer 8

1. Modality of wear from H&S/ slit lamp examination - as for spherical
2. Oxygen + water content considerations
3. Decision on material
4. Choose a lens as a starting point
   • BOZR/TD
   • Consider range of Rx available
   • And cost
5. Refer to manufacturer’s guidelines for fitting guidance

Question 9

When are soft toric lenses used?

Answer 9

• Refractive astigmatism
• Spherical SCLs failed to mask corneal astigmatism
• Gas Perm causes residual astigmatism
• Gas perm CLs cause discomfort

Question 10

Physiological considerations

Answer 10

• Complications such as corneal edema & corneal vascularization more likely increases CL thickness
• If problems occur, advise switch to SiHy CLs
• If problems persist, refit with Gas Perm CLs

Question 11

What important points should you remember with Steps/Axis between lenses?

Answer 11

• The power steps available can differ lens to lens
• The axis also differs lens to lens and even within one lens depending on the power,

Question 12

What are 2 specialist companies for toric lenses?

Answer 12

• Mark Ennovy:
- Extended Rx ranges (+-30 Ds with 8Ds astigmatism)
- TD of 11.50-16mm and base curve of 6.50-9.80mm
• Cantor+Nissel:

Question 13

How is the RX adjusted for BVD?

Answer 13

• CL is closed to eye than glasses
• Need to adjust as Rx increases and treat each meridian if there is a cyl
• K=F/(1-dD), where F = spec Rx in Ds, and d=BVD in metres
• Needs to be done for both meridians in toric lenses

Question 14

What 6 things are checked when assessing the fit of the lens?

Answer 14

• Centration and coverage
• Lens movement on blink/rotation*
• Lag/sag
• Push up test
• Comfort
• overall tightness

Question 15

What factors influence SCL location?

Answer 15

• Lid: tension, location, action, blink force
• Vertical palpebral aperture
• Gravity
• Water content
• Lens thickness
• Total diameter
• Tailor-made versus stock toric

Question 16

How is CL rotation measured?

Answer 16

• CL marks are for reference only, optically they are of no particular significance
• Measure rotation using:
- narrow slit-lamp beam with protractor scale
- an eyepiece with protractor-scale graticule is ideal
• Estimate magnitude of CL rotation
Important note - how many degrees on a hour of a clock rotated?

Question 17

What Mnemonics can be done to help fix CL rotation?

Answer 17

• CAAS - Clockwise, add; anticlockwise, subtract
• Adjustment should be made to spectacle Rx

Question 18

What are the 5 step procedures for fitting toric SCLs?

Answer 18

• Measure Rx and Vertex distance
• Select toric CL design
• Select CL power (BVD) to match both meridians
• Select BOZR &/or TD
• Compute final Rx, including axis compensation
- clockwise you add, anticlockwise subtract

Question 19

How must the over-refraction be done for toric SCLs?

Answer 19

• Spherical over-refraction sufficient if Va is good, and lens stable for small tweaks
- however Contact lens based on up to date spectacles so Rx change minimal
• any significant change may indicate cylinder mislocated

Question 20

How are toric SCLs recorded?

Answer 20

• Same as for spherical SCLs BUT
• Need to add a comment on the lens rotation/stability of lens

Question 21

What are some limitations of soft Toric lenses?

Answer 21

• Oblique cylinders
• Large cylinders
• Irregular astigmatism
• Lens thickness (reduced oxygen, mechanical irritation)