CLM - Soft Contact Lens Fitting II - Week 6

Question 1

95% of astigmatic patients have a cyl prescription of what D or less?

Answer 1

-2.25D or less

Question 2

What is generally considered clinically significant astigmatism? What percentage of patients have this in at least one eye?

Answer 2

≥ 0.75DC
~50% have this in at least one eye

Question 3

List three advantages of soft toric contact lenses.

Answer 3

Potential to deliver enhanced VA and contrast sensitivity
Improved longer-term retention of CL wearers
Range of lens designs/modalities now available

Question 4

What lens fitting characteristics are more critical with soft toric lenses (4)?

Answer 4

Centration
Movement
Stability
Rotation

Question 5

List 2 additional considerations for soft toric contact lenses.

Answer 5

Lens comfort
Increased cost

Question 6

In what two ways is the astigmatic correction made?

Answer 6

Toroidal back surface with spherical front
Sperical back surface with toroidal front

Question 7

List 5 stabilisation techniques for toric soft contact lenses.

Answer 7

Prism ballast
Dynamic stabilisation
Per-ballast
Truncation
Back surface toricity

Question 8

What is the underlying principle of a prism ballast and what does it do to the lens inferiorly? What power prism is typically used?

Answer 8

Balance the forces acting on the lens in order to stabilise it
Increased lens thickness inferiorly
1-1.5^BD prism used

Question 9

Describe how the watermelon seen principle applies to the prism ballast.

Answer 9

The squeezing pressure of the upper lids causes the lens to be expelled in a direction away from the apex, pushing the prism into BD position

Question 10

What structure is the dominant stabilising component?

Answer 10

Superior lid

Question 11

What is a peri-ballast and how does it compare to a prism ballast?

Answer 11

Minus lens carrier with increased thicknes sinferiorly
It is a prism-free optic, and acts similar to the prism ballast

Question 12

What is the most common method of soft toric lens stabilisation?

Answer 12

Dynamic stabilisation

Question 13

Explain how dynamic stabilisation works. Note whether it uses prism in its design, whether or not it is symmetric, and how its thickness compares to other means of stabilisation. What happens to lens with this design when blinking?

Answer 13

The superior and inferior zones incorporate a thickness differential
It is prism-free, symmetrical, and thinner lens design
Lens moves back against the lids during blink to maintain orientation

Question 14

What is truncation for toric lens stabilisation? Is it successful? What is lens comfort like?

Answer 14

The prism ballast lens can be truncated inferiorly to align with the lower lid margin
Has limited success and is uncomfortable

Question 15

Explain 2C MAPRS for soft toric lenses.

Answer 15

Centration
Coverage
Movement
Acuity
Push-up
Rotation
Stability

Question 16

What is a good toric fit in terms of coverage, movement, and rotation (3)?

Answer 16

1.5-2mm coverage is ideal
0.25-0.5mm with blink at primary is ideal
Good fit - quick return to axis if lens is mislocated
Tight fit - slow return to axis
Loose fit - orientation unstable

Question 17

What are the three ways of determining the BVP to trial a lens with?

Answer 17

Empirically
Using spherical trial lenses
Using toric trial lenses

Question 18

Describe the empirical method to calculating BVP for trial lenses.

Answer 18

Calculated according to specatcle prescription, taking into account vertex distance

Question 19

Are tear lens calculations required for soft contact lenses?

Answer 19

No, thank fuck

Question 20

How can spherical lenses be used to calculate BVP for toric lenses?

Answer 20

Use a spherical trial lens to determine fitting characteristics, then measure sphero-cyl over-refraction
BVP is then BVP of the trial lens + over-refraction

Question 21

What is the preferred method for fitting custom toric lenses?

Answer 21

Using spherical trial lenses

Question 22

Are toric trial lenses commonly used? What is the selection of BVP based on? What is done after to get the correct final BVP? What may need to be done to determine this?

Answer 22

Commonly used as they are readily available
Choose BVP based on spectacles, with vertex distance accounted for
Measure sphero-cyl over-refraction
To determine the final BVP, may need to do matrix calculations of oblique cyls

Question 23

What is the effect of lens rotation on astigmatic power?

Answer 23

The contact lens only has to rotate a small amount to generate a large amount of cyl at a different axis

Question 24

What three factors influence the extent of lens rotation? Explain each if applicable.

Answer 24

Lid anatomy
-tighter lids have a greater effect
Fitting relationship
-steep lenses may not stabilise if tightly adhered to the eye
Lens thickness profile
-influenced by the refractive error

Question 25

Lid action moving inward causes the lens to rotate in what direction?

Answer 25

Nasally

Question 26

Does the scribe mark of a toric lens represent actual cyl axis?

Answer 26

No

Question 27

What is the direction of rotation of a toric lens taken with respect to?

Answer 27

The inferior axis of the lens - 6 o’clock position

Question 28

How many degrees of rotation is assumed during fitting and delivery?

Answer 28

5-10 degrees of nasal rotation

Question 29

How is on-eye rotation accounted for? Give the acronym to help remember this rule (hint tekken). Explain how this compensation works by giving an example.

Answer 29

LARS
Left Add
Right Subtract
If the lens moves more than 10 degrees in either right or left, this number of degrees needs to be added or subtracted from the prescription
ie 10 degrees to the right means a 10 degree subtraction from the prescription

Question 30

What is typically the threshold for degree rotation above which you would need to compensate for?

Answer 30

<10 is acceptable
≥ 10 - need to compensate

Question 31

What is the over-refraction method for compensating lens rotation? What about if VA fluctuates during this process?

Answer 31

Perform sphero-cyl refraction over the lens
If VA improves, prescription may need adjustment
The over-refraction can be used to calculate the degree of rotation (axis misalignment)
If VA fluctuates, reassess contact lens fit

Question 32

In what two ways can the degree of misalignment of a toric soft contact lens be calculated? What is required to facilitate this?

Answer 32

The patient’s ocular refraction
Refraction over the mislocating lens
Matrix optics are used for this

Question 33

If you have adjusted for on-eye lens rotation, will the lens still appear to rotate? Explain.

Answer 33

Yes it will still appear to rotate, but this isnt a concern as this rotation has already been compensated for

Question 34

What is meant by stability for soft toric lenses?

Answer 34

The degree of rotation is constant between blinks

Question 35

What are three possible reasons if lens stability with soft toric lenses is low?

Answer 35

Stabilisation methods arent working
BOZR is too flat
Diamter is too small

Question 36

In what four cases are soft toric lenses for difficult to fit?

Answer 36

Low spherical components
Oblique cyl (poorer stability)
Large cyl
Irregular astigmatism

Question 37

List two physiological problems with soft toric lenses.

Answer 37

Generally thicker
Can result in reduced Dk/t and increase mechanical irritation