cones and parameters

Question 1

What can excessively flat BOZRs lead to? (2)

Answer 1

corneal scarring/distortion
incr discomfort

Question 2

What can too much apical clearance lead to? (2)

Answer 2

corneal molding
transient compromise - e.g. corneal oedema, staining, epi lens imprint from an immobile lens

Question 3

Which diameter lens is most appropriate for nipple or centred cones and early oval cones?

Answer 3

Small diameter lens (8.5-9.3mm)

Question 4

What is the advantage of smaller diameter lenses in nipple cones? How?

Answer 4

better centration + avoid bubbles around cone
-> smaller lens to match the cone as it gets steeper to minimise the gap above and below the cone where tears would pool

Question 5

Which diameter lens is most appropriate for progressing oval cone? Why?

Answer 5

Medium diameter lens (9.4-9.9mm)
- to avoid excessive clearance and bubbles around cone

Question 6

Which diameter lens is most appropriate for advanced cones/keratoglobus?

Answer 6

Large TD corneal (10-12.8mm) and corneal-sclerals (12.9-13.5mm)

Question 7

What is the location of the touch in 3-pt touch dependent on?

Answer 7

the location of the apex of the cone

Question 8

In an ideal fit, where is the touch in a nipple cone?

Answer 8

central

Question 9

In an ideal fit, where is the touch in an oval cone

Answer 9

decentred inferiorly or inferior-laterally

Question 10

In an ideal fit, where is the mid-peripheral touches typically located?

Answer 10

along the flattest meridian

Question 11

Which corneal meridian is typically flattest in keratoconus?

Answer 11

the horizontal meridian

Question 12

Which corneal meridian is typically flattest in PMD?

Answer 12

the vertical meridian

Question 13

Is minimal/no edge clearance acceptable?

Answer 13

no, in any diameter, no

Question 14

What might minimal/no edge clearance lead to

Answer 14

Peripheral impingement causing:
- corneal molding, staining, reduced/no lens movement
–> leading to entrapment of tears/debris
–> oedema and infl response

Question 15

What might excessive edge clearance cause? (3)

Answer 15

irritation and FB sensation
Lower lens edge to sit above lower eyelid (threatening to eject from cornea on extreme lateral gaze)
Upper eyelid may also draw lens up to a lid attachment position (also not desirable, since will cause regional flattening over apex of inf displaced cone)

Question 16

What can you do if >2-3mm central touch?

Answer 16

Steep BOZR in 0.1mm steps until central fit acceptable

Question 17

What can you do if central bubbles?

Answer 17

Flatten BOZR in 0.1mm steps until central fit acceptable

Question 18

What can you do if bubbles around cone?

Answer 18

Reduce size of BOZD and steepen BOZR to maintain same fit

Question 19

if central fit is great but peripheral is not, what can you do?

Answer 19

Steepen periph curve if excessive edge clearance or flatten if insufficient.

(NB: If BOZD <6mm, lab will make adjustments to maintain same overall sag)

Question 20

When might you want to increase BOZD (and maintain same BOZR)?

Answer 20

When cone diameter has increased, to better align with topography

Question 21

When might you want to decrease BOZD (and maintain same BOZR)?

Answer 21

When there are bubbles over the pupil, to reduce the clearance around the cone (apical clearance) and better align with the size of the cone. In these cases, adjust the power of the lens due to change in lens sag and thus tear film

Question 22

How will a steepening of BOZR affect overall power?

Answer 22

Requires an increase in minus power (and vice versa for flattened)

Question 23

By how much should you change BOZR to maintain the fit if you change BOZD?

Answer 23

For every 0.5mm incr BOZD (or TD), you should 0.05mm incr BOZR to maintain same fit

Question 24

How will increasing BOZD affect edge clearance?

Answer 24

Inverse relationship, Increasing BOZR reduces edge clearance

Question 25

How will increasing BOZD affect apical clearance?

Answer 25

Increases apical clearance.

Question 26

How much larger should the BOZD be compared to pupil size in normal lighting to avoid flare?

Answer 26

1mm larger at least

Question 27

How does a change of BOZR from a trial affect overall power (approximation)?

Answer 27

0.05mm BOZR change equivalent to 0.25D change in power

Question 28

Define SAM FAP

Answer 28

Steeper Add Plus, Flatter Add Minus

Question 29

When might increasing lens diameter be helpful? (3)

Answer 29

When:
- Cone diameter has increased
- lens begins to decentre
- evidence of 3 and 9 o’clock staining

Question 30

When might you decrease lens diameter? (2)

Answer 30

if lens edge approaches/sits on edge of limbus
if you want to reduce the weight of the lens

Question 31

What might result from a lens edge sitting directly on limbus?

Answer 31

irritation

Question 32

When would you adjust AEL?

Answer 32

Only adjust AEL if central NaFl is ideal but peripheral fit is not

Question 33

How should BOZR be modified if you increase AEL?

Answer 33

BOZR should be steepened (usually by 0.1mm) and the power adjusted to include the appropriate amount of increased minus power with small optic zone diameters
- Lab should do this in Rose K, I believe

Question 34

Do toric peripheries affect the fit?

Answer 34

no

Question 35

When might you consider a sectoral AEL adjustment in PMD?

Answer 35

In PMD, when the inferiorly displaced cone results in the lower edge of the lens causing irritation to the lower lid and where a larger diameter/larger BOZD did not allow the lower lens edge to sit under the lower lid