RGP Fitting 3 Flashcards

1
Q

How do we define an aspheric lens?

A

A lens where either one or both surfaces are non-spherical.

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2
Q

What are the advantages of aspheric lenses?

A

They produce a visual advantage of best form lenses and a cosmetic advantage of thinner, flatter and lighter lenses.

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3
Q

How do elliptical aspheric lenses need to be fitted?

A

They generally need a flatter fitting than spherical lenses– To provide alignment across the corneal surface fit them 0.05mm flatter than flattest K

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4
Q

How can polynomial and variable eccentricity aspheric lenses be fitted?

A

These CAN be fitted in the same way as spherical lenses. - On the flattest K

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5
Q

Describe the structure and fit of a fully aspheric lens.

A

It has a completely aspheric back surface (which theoretically fits perfectly along the cornea). [They may be uncomfortable to some patients as lens edge presses into cornea]

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6
Q

Describe the structure and fit of a mainly aspheric/part sphere.

A

It has a mainly aspheric back surface with a spherical peripheral curve. This provides it with: – Good edge clearance – and as a result Prevents edge pressing into cornea – Allowing Assist in tear exchange [However, edge lift is sometimes insufficient]

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7
Q

What are polynomial aspheric lenses also known as?

A

“Spherical optic/ aspheric periphery lenses”

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8
Q

Describe the structure of polynomial aspheric lenses and how they may be fitted.

A

They have a central spherical portion (BOZ) with an aspheric peripheral zone area. These are typically fitted 0.05 mm flatter than the flattest k.

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9
Q

When altering total diameter of an aspheric lens what do we need to remember?

A

That nothing else needs to be tweaked. No change should be made to the radius or the power.

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10
Q

What are the advantages of aspheric lenses?

A

Advantages AS • More comfortable as Achieves true alignment – because Fit is more closely to corneal topography • Pressure more evenly distributed over cornea • Less lid sensation • Useful in higher astigmatism • More gradual FP (centre to periphery) • They are easier to fit – as Fewer parameters to consider and changing a parameter such as total diameter doesn’t require change to other parameters such as BOZR or power

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11
Q

What are the disadvantages of aspheric lenses?

A

• In general these lenses need to be fitted flatter – in order to provide an acceptable amount of movement • Larger lenses are needed to help centration which in turn reduces amount of oxygen getting to the peripheral cornea. • The lens edge may press into peripheral cornea • Some aspheric lenses e.g. Bi-aspheric lenses can induce residual astigmatism – And aberrations • There are less Variations available in design -which makes it harder to customise for px that are not entirely getting on with their lenses. – Manufacturer’s info should be read

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12
Q

What are advantages of multicurve lenses over aspheric lenses?

A

More control over individual lens parameters, e.g: – BOZD – Flatter/steeper periphery – Adjust edge clearance [Essentially you can give the patient a customised fit]

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13
Q

What are advantages of aspheric lenses over multicurve lenses?

A

They tend to have a more successful first fit ( as you are only really thinking about three parameters; BOZR, TD and BVP) They tend to have a uniform fluorescein pattern - as there are no blended junctions thus more comfortable for the px

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14
Q

What would you do if you had a px with a multi-curve lens for which everything is perfect, however, the edge clearance ONLY is too large?

A

Consider refitting with an aspheric lens. (because edge clearance is thinner in an aspheric as a pose to a multi-curve).

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15
Q

When should a lens be lenticulated?

A

When the BVP is greater than -6.00D or +4.00D

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16
Q

How can we change centre and edge thicknesses of a lens?

A

By changing the front optic zone diameter (FOZD)

17
Q

How does reducing the Front Optic Zone Diameter affect thickness in a positive lens?

A

Reducing the FOZD in a positive lens reduces centre thickness

18
Q

How does reducing the Front Optic Zone Diameter affect thickness in a negative lens?

A

Reducing the FOZD in a negative lens reduces periphery thickness.

19
Q

What type of edge carriers are the following?

A

A- plano

B- Positive

C- Negative

20
Q

What is high riding and which lenses display this quality?

A

Negative lenses - high riding is when the lens sits too high this happens because negative lenses are thicker in the periphery and so show natural lid attachment occurs holding the lens up

[Plus lenses aren’t thick enough in the periphery for the lid to hold onto it]

21
Q

What type of edge carrier would you suggest for negative lenses?

A

You would suggest a positive edge carrier for negative lenses ( to reduce edge thickness).

22
Q

Why is a negative edge carrier used?

A

To increase thickness in the periphery ( thus allowing it to be thick enough in the periphery for lid addtachment so the lens can be lifted up).

23
Q

Why is a positive edge carrier used?

A

To reduce thickness in the periphery

24
Q

What type of edge carrier would you suggest for positive lenses?

A

A negative edge carrier for positive lenses ( to improve lens centration - as it will lift the lens up)

25
Q

What are the possible pay-offs of selecting the right edge carrier for the lens?

A

Better vision

Better comfort

Easier adaptation to the lens by the px

A more stable lens position

26
Q

What does a plano edge carrier do?

A

Miantains lens thickness

27
Q

Do positive lenses naturally high or low ride?

A

Posiitve lenses naturally low ride ( thus it is important to consider a negative edge carrier to lift the lens up - thus improving centration of the lens).

28
Q

How does centre of gravity affect position of a lens?

A

The more anterior the centre of gravity the lower it rides e.g. with positive lenses the centre of gravity is inside the lens ( i.e. more anterior than with minus lenses) thus positive lenses ride lower.

29
Q

What is the relationship between steepness and centre of gravity?

A

In steeper lenses the centre of gravity is further back (more posterior) and as a result there is better centration as it doesn’t fall down as much.

30
Q

What is the relationship between diameter and centre of gravity?

A

The centre of gravity is further back in larger diameter lenses - which means lens is less likely to fall and so we achieve better centration.

31
Q

Why is the edge profile important?

A

It is important for patient comfort.

32
Q

In general to edge profiles need to be adjusted?

A

No - unless patient complains about ‘feeling the edge of the lens’

33
Q

What should an edge profile always be?

A

It must always be rounded, smooth and well finished.

34
Q

What are the different type of edge profiles?

A

A. Posterior

B. Central

C. Anterior

D. Blunt

E. Sharp

35
Q

Why may a tint be allocated to contact lenses?

A

For the sake of identification. i.e. gRey for the Right Eye and bLue for the Left Eye.

This is called a handling tint.

[In some people with very light iris colour it may enhance eye colour]

36
Q
A