RGP Fitting 3

Question 1

How do we define an aspheric lens?

Answer 1

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

Question 2

What are the advantages of aspheric lenses?

Answer 2

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

Question 3

How do elliptical aspheric lenses need to be fitted?

Answer 3

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

Question 4

How can polynomial and variable eccentricity aspheric lenses be fitted?

Answer 4

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

Question 5

Describe the structure and fit of a fully aspheric lens.

Answer 5

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]

Question 6

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

Answer 6

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]

Question 7

What are polynomial aspheric lenses also known as?

Answer 7

“Spherical optic/ aspheric periphery lenses”

Question 8

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

Answer 8

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.

Question 9

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

Answer 9

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

Question 10

What are the advantages of aspheric lenses?

Answer 10

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

Question 11

What are the disadvantages of aspheric lenses?

Answer 11

• 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

Question 12

What are advantages of multicurve lenses over aspheric lenses?

Answer 12

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

Question 13

What are advantages of aspheric lenses over multicurve lenses?

Answer 13

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

Question 14

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?

Answer 14

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

Question 15

When should a lens be lenticulated?

Answer 15

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

Question 16

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

Answer 16

By changing the front optic zone diameter (FOZD)

Question 17

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

Answer 17

Reducing the FOZD in a positive lens reduces centre thickness

Question 18

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

Answer 18

Reducing the FOZD in a negative lens reduces periphery thickness.

Question 19

What type of edge carriers are the following?

Answer 19

A- plano

B- Positive

C- Negative

Question 20

What is high riding and which lenses display this quality?

Answer 20

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]

Question 21

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

Answer 21

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

Question 22

Why is a negative edge carrier used?

Answer 22

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).

Question 23

Why is a positive edge carrier used?

Answer 23

To reduce thickness in the periphery

Question 24

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

Answer 24

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

Question 25

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

Answer 25

Better vision Better comfort Easier adaptation to the lens by the px A more stable lens position

Question 26

What does a plano edge carrier do?

Answer 26

Miantains lens thickness

Question 27

Do positive lenses naturally high or low ride?

Answer 27

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).

Question 28

How does centre of gravity affect position of a lens?

Answer 28

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.

Question 29

What is the relationship between steepness and centre of gravity?

Answer 29

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.

Question 30

What is the relationship between diameter and centre of gravity?

Answer 30

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.

Question 31

Why is the edge profile important?

Answer 31

It is important for patient comfort.

Question 32

In general to edge profiles need to be adjusted?

Answer 32

No - unless patient complains about 'feeling the edge of the lens'

Question 33

What should an edge profile always be?

Answer 33

It must always be rounded, smooth and well finished.

Question 34

What are the different type of edge profiles?

Answer 34

A. Posterior B. Central C. Anterior D. Blunt E. Sharp

Question 35

Why may a tint be allocated to contact lenses?

Answer 35

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]