cones and parameters Flashcards
What can excessively flat BOZRs lead to? (2)
corneal scarring/distortion
incr discomfort
What can too much apical clearance lead to? (2)
corneal molding
transient compromise - e.g. corneal oedema, staining, epi lens imprint from an immobile lens
Which diameter lens is most appropriate for nipple or centred cones and early oval cones?
Small diameter lens (8.5-9.3mm)
What is the advantage of smaller diameter lenses in nipple cones? How?
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
Which diameter lens is most appropriate for progressing oval cone? Why?
Medium diameter lens (9.4-9.9mm)
- to avoid excessive clearance and bubbles around cone
Which diameter lens is most appropriate for advanced cones/keratoglobus?
Large TD corneal (10-12.8mm) and corneal-sclerals (12.9-13.5mm)
What is the location of the touch in 3-pt touch dependent on?
the location of the apex of the cone
In an ideal fit, where is the touch in a nipple cone?
central
In an ideal fit, where is the touch in an oval cone
decentred inferiorly or inferior-laterally
In an ideal fit, where is the mid-peripheral touches typically located?
along the flattest meridian
Which corneal meridian is typically flattest in keratoconus?
the horizontal meridian
Which corneal meridian is typically flattest in PMD?
the vertical meridian
Is minimal/no edge clearance acceptable?
no, in any diameter, no
What might minimal/no edge clearance lead to
Peripheral impingement causing:
- corneal molding, staining, reduced/no lens movement
–> leading to entrapment of tears/debris
–> oedema and infl response
What might excessive edge clearance cause? (3)
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)
What can you do if >2-3mm central touch?
Steep BOZR in 0.1mm steps until central fit acceptable
What can you do if central bubbles?
Flatten BOZR in 0.1mm steps until central fit acceptable
What can you do if bubbles around cone?
Reduce size of BOZD and steepen BOZR to maintain same fit
if central fit is great but peripheral is not, what can you do?
Steepen periph curve if excessive edge clearance or flatten if insufficient.
(NB: If BOZD <6mm, lab will make adjustments to maintain same overall sag)
When might you want to increase BOZD (and maintain same BOZR)?
When cone diameter has increased, to better align with topography
When might you want to decrease BOZD (and maintain same BOZR)?
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
How will a steepening of BOZR affect overall power?
Requires an increase in minus power (and vice versa for flattened)
By how much should you change BOZR to maintain the fit if you change BOZD?
For every 0.5mm incr BOZD (or TD), you should 0.05mm incr BOZR to maintain same fit
How will increasing BOZD affect edge clearance?
Inverse relationship, Increasing BOZR reduces edge clearance
How will increasing BOZD affect apical clearance?
Increases apical clearance.
How much larger should the BOZD be compared to pupil size in normal lighting to avoid flare?
1mm larger at least
How does a change of BOZR from a trial affect overall power (approximation)?
0.05mm BOZR change equivalent to 0.25D change in power
Define SAM FAP
Steeper Add Plus, Flatter Add Minus
When might increasing lens diameter be helpful? (3)
When:
- Cone diameter has increased
- lens begins to decentre
- evidence of 3 and 9 o’clock staining
When might you decrease lens diameter? (2)
if lens edge approaches/sits on edge of limbus
if you want to reduce the weight of the lens
What might result from a lens edge sitting directly on limbus?
irritation
When would you adjust AEL?
Only adjust AEL if central NaFl is ideal but peripheral fit is not
How should BOZR be modified if you increase AEL?
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
Do toric peripheries affect the fit?
no
When might you consider a sectoral AEL adjustment in PMD?
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
