CLM - Soft Contact Lens Fitting II - Week 6 Flashcards

1
Q

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

A

-2.25D or less

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2
Q

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

A

≥ 0.75DC

~50% have this in at least one eye

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3
Q

List three advantages of soft toric contact lenses.

A

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

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4
Q

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

A

Centration
Movement
Stability
Rotation

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5
Q

List 2 additional considerations for soft toric contact lenses.

A

Lens comfort

Increased cost

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6
Q

In what two ways is the astigmatic correction made?

A

Toroidal back surface with spherical front

Sperical back surface with toroidal front

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7
Q

List 5 stabilisation techniques for toric soft contact lenses.

A
Prism ballast
Dynamic stabilisation
Per-ballast
Truncation
Back surface toricity
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8
Q

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

A

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

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9
Q

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

A

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

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10
Q

What structure is the dominant stabilising component?

A

Superior lid

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11
Q

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

A

Minus lens carrier with increased thicknes sinferiorly

It is a prism-free optic, and acts similar to the prism ballast

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12
Q

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

A

Dynamic stabilisation

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13
Q

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?

A

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

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14
Q

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

A

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

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15
Q

Explain 2C MAPRS for soft toric lenses.

A
Centration
Coverage
Movement
Acuity
Push-up
Rotation
Stability
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16
Q

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

A

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

17
Q

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

A

Empirically
Using spherical trial lenses
Using toric trial lenses

18
Q

Describe the empirical method to calculating BVP for trial lenses.

A

Calculated according to specatcle prescription, taking into account vertex distance

19
Q

Are tear lens calculations required for soft contact lenses?

A

No, thank fuck

20
Q

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

A

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

21
Q

What is the preferred method for fitting custom toric lenses?

A

Using spherical trial lenses

22
Q

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?

A

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

23
Q

What is the effect of lens rotation on astigmatic power?

A

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

24
Q

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

A
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
25
Q

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

A

Nasally

26
Q

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

A

No

27
Q

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

A

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

28
Q

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

A

5-10 degrees of nasal rotation

29
Q

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.

A

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

30
Q

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

A

<10 is acceptable

≥ 10 - need to compensate

31
Q

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

A

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

32
Q

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

A

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

33
Q

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

A

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

34
Q

What is meant by stability for soft toric lenses?

A

The degree of rotation is constant between blinks

35
Q

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

A

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

36
Q

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

A

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

37
Q

List two physiological problems with soft toric lenses.

A

Generally thicker

Can result in reduced Dk/t and increase mechanical irritation