CLM - Keratoconus II - Week 2 Flashcards
Describe the format for specifying RGP parameters.
BOZR (BOZD), periphery: BPR (BPCD), BPR (BPCD), BPR (BPCD)
What happens to peripheral flattening as BOZR decreases?
It increases
Would apical clearance of RGPs on fluorescein staining be considered too steep or flat?
Too steep
What is the purpose of apical clearing for keratoconus? What does it minimise the likelihood of? How does it affect VA and why? What may it induce?
Central curve vaults the steepest part of the cone
Minimises the likelihood of scarring and central staining
May have reduced acuity due to lens flexure after blinking
Steep central curve mau induce corneal steepening of the thin cornea
How may steeper lenses reduce tear exchange and what is a consequence of this?
It may seal off the periphery and reduce tear exchange with reduced wearing times and comfort
What can occur with a large tear film thickness and in what kind of lens steepness can it generally be seen?
Tear film thickness >90um may develop dimple veiling in steep lenses
Is apical clearance too steep?
Yes
What is meant by feather clearance and waht else is it known as? What should the central clearance look line?
Three point touch
Point of contact to the cornea by the RGP on the superior and inferior margins (not right at the edge but close) and one more point of contact on the apex of the corneal cone
Central clearance should be slight
What is the goal with feather clearance? What does this minimise (2)?
Attempt for minimal clearance above the steepest part of the cone
Minimises mechanical stress to the epithelium
Minimises possibility of dimple veiling
What does a flatter base curve in feather clearance minimise?
Corneal moulding
What does reduced central clearance in feather clearance minimise?
Minimises lens flexure, giving more stable visiond
What is the steepness of apical bearing?
Too flat
How does apical bearing appear in contrast to feather clearance (3)?
Excessive edge clearance
Marked bearing and staining on the cone
Edge lift
Is the likelihood of eputhelial staining on the cone small or large with apical bearing? What does this increase the risk of?
Large - increasing the risk of corneal scarring
What is the concept of therapeutic treatment of keratoconus with apical bearing and does it hold weight?
Misconceived concept of retarding progression of keratoconus by holding it in place with pressure
Can apical bearing give good VA? Explain why this is so.
Occasionally better visual acuity due to hard bearing and corneal compression.
Describe the CCE acronym for RGP fitting.
Centre - feather clearance centrally
Centration - central fitting with lid attachment if possible
Edge - adequate edge clearance
What constitutes adequate edge clearance for a RGP lens?
0.6-0.8mm
For what types of keratoconus and eyes are small diameter RGPs typically selected (3)? Specify the diameter.
Advanced nipple
Smaller cones
Narrower palpebral apertures
<9.0mm
For what types of keratoconus and eyes are larger diameter RGPs typically selected (4)?
Early keratoconus
Oval and globus
Wider palpebral apertures
With what BOZR (BC) should you begin with and what diameter? What about if there is obvious central clearance (2)?
Start with 0.2mm steeper than the average K readings
Start with 9.0mm overall diameter
If central clearance, then go flatter in larger steps of 0.3-0.4mm until there is slight central touch, then 0.1mm steeper
Does better RGP centration equal better vision?
Yes
What does better centration do to flaring, especially at night and in people with what size pupils?
Less flaring at night, especially with larger pupils
What should you do if the lens keeps dropping?
Try a larger diameter
Why is adequate edge clearance important (2)?
Allows sufficient tear exchange
Creates better centration
What constitutes adequate movement of the RGP lens?
1.0 - 2.0mm
Does a tight edge often have better or reduced comfort?
Reducted comfort
List two problems with a loose edge.
May dislodge on excursions or unusual movements
Susceptible to foreign bodies
Describe what is meant by inferior tuck. Is it standard design?
Reduce excessive inferior edge clearance with a steeper inferior peripheral radii
Is standard design
Do nipple cones and advanced keratoconus need larger or smaller diameters to more accurately fit over the cone?
Smaller
What are larger diameter RGP lenses more likely to achieve?
Lid attachment
Why do larger diameter RGP lenses cause less flaring at night?
Often the BOZR is larger than the dilated pupil
When increasing the overall diameter, what needs to change in order to maintain the same sag? Does vice-versa apply? Give numbers (2).
To maintain the same sag, the base curve needs to be increased
Increase overall diameter by 1.0mm, increase BOZR by 0.1mm
Vice versa applies - decrease overall diameter by 1.0mm, decrease BOZR by 0.1mm
Approximate a 0.1mm base curve change to the equivalent change in power. Is it accurate? Provide the full formula for power change.
0.1mm base curve change - 0.50D power change
Inaccurate for moderately steep or flat base curves
Formula:
336/BOZR1 - 336/BOZR2 = power change
What is the centration and comfort of corneoscleral lenses like?
Excellent centration and comfort
Do corneoscleral lenses have consistent or varying edge clearance?
Can vary edge clearance by changing rate of peripheral flattening
Where do corneoscleral lense sprimarily rest and is there generally any movement?
On the cornea just inside the limbus
Minimal movement
What are minisclerals designed to do (2)?
Vault the cornea and land on the sclera
Are minisclerals comfortable? Does it have any lid interactions?
As comfortable as soft lens, no lid interactions
Do minisclerals dislodge or have a foreign body sensation?
Neither
Do minisclerals generally give good or poor centration?
Excellent centration
Do minisclerals generally move?
Minimal movement
What technique is best to assess lens clearance and edge profile for minisclerals?
Anterior OCT
What is an important consideration for minisclerals regarding surrounding blood vessels?
Important to avoid compression of conjunctival blood vessels at the scleral landing zone
Does central clearance usually increase or decrease as the scleral landing area settles on the conjunctiva? What percentage of this change occurs within the first 30 minutes of wear? Does the use of saline vs gel (celluvisc) have any effect on this?
It decreases by 50-100um
50% of this decrease occurs in the first 30 minutes of wear
No difference between saline and celluvisc
What happens to corneal thickness with wearing minisclerals? Compare it to overnight eyelid closure.
2% induced central corneal oedema, not clinically significant
Overnight eyelid closure induces 4% corneal oedema
Is there any association between the post lens tear vault and corneal oedema/?
No
What is the optimal central clearance for minisclerals?
100-300um
What can insufficient miniscleral lens clearance lead to? What about excessive clearance (2)?
Insufficient - may lead to bearing on the corneal apex
Excessive - may cause excessive suction and tightness causing discomfort and difficulty removing the lens
How often are minisclerals replaced generally?
6 monthly
How are minisclerals or hybrids inserted? What should be done if bubbles are present?
Insert vertically face down, and full of solution
Insert using fingers or large DMV scleral inserter
If bubbles are present, needs to be removed and reinserted
How are minisclerals or hybrids removed?
Need to break the seal by pushing under the lower lid to create a bubble to break the suction
suction cup can also be used
