Soft contact lens fitting Flashcards
What do we need to do before choosing which trial lenses
*Convert spectacle rx to ocular rx
*Choose TD, BVP, BOZR
When would you convert to ocular rx
If rx greater than +4.00D
When do you need to consider topic or lenticular astigmatism
If cyl is more than 0.75D
What can you do if cyl is less than 0.75D
Use mean sphere equivalent
What do you want the TD to be
2mm larger than HVID
What should you check before fit SCL
Tear film
What material should you pick if you see a lot of lipids in tears:
Hydrogel
Disadvantage of silicon hydrogen
Attract more lipids
What happens if lens fit looks too tight:
*Reduce lens movement
*Nipping of blood vessels
*Staining pattern where lens border was i.e when take lens out, can see border of lens
*This is where the lens border has rubbed the epithelial cells of conjunctiva due to tightness
When would you keep same hydrogel material:
*When px doesn’t have any problem with comfort or vision
*No problem with lens material
Which lens modality would you choose if px wants full time basis:
One month replacement modality
Daily lenses limitations:
*Rx and colours
*Expensive
What is important when fitting coloured CLs:
Pupil size in photopic light conditions
What is printed on coloured contact lenses
Iris diameter
Consequence of large pupil size in photopic conditions:
*When light dims it becomes more mesopic
*Pupil will become larger
*So want largest pupil size in the lens
What to look for lid eversion:
*Redness
*Roughness
*Papillae
*Grade these in 0.5mm
Lid abnormalities:
*Meibomian gland dysfunction
*Blepharitis
*Grade it
Centratio - slit lamp
*White light
*Look for borders of lens
*Don’t manipulate lids
Coverage - slit lamp
*Check when doing centration so white light
*Lens should have complete corneal coverage and extend at 0.5mm onto conjunctiva in all directions i.e primary, left, right, up and down
*SO SHOULD COVER WHOLE CORNEA WITH 0.5MM LESS AROUND
What will optimum fitted SCL exhibit:
*Good centration
*Moving 0.25 – 0.50mm with each blink
Tight lens:
*<0.20mm movement
*Producing conjunctival drag = conjunctiva moves with the lens and the lens movement is little to none
*When removed, the tight lens may leave an impression ring on the sclera at position of lens edge
Loose/flat lens:
*Move partly off cornea
*Move >1.00mm
Lens lag
- Loose: If lens moves more than 1.00mm
- Tight: If lens less than 0.2mm
- Ideal lens lag:
Moves up to 1.00mm
What does too much sag cause
Lens to drop
When is lag best observed:
Superior gaze
How to estimate lag:
*In primary gaze, set width of beam to distance between lens border and limbus
*BEAM = 0.5MM
Push up test
- Measure of tightness
- Px looks slightly up (lens border free from lid)
- Use finger to put pressure on lower lid
- Move lens vertically
- Allow lens to re-centre
- Judge movement (related to distance between lens border and limbus)
Expected values?
- Correct: slow to move; steady recovery; 40-60%
- Tight: Difficult to move with slow to no recovery, 65-100%
- Loose: Easy movement and fast recovery, 35-0%
When should you change the lens for PUT
When should you change the lens for push up test
Clinical significant: You must change the lens if PUT >75% or <25%
VA
- CL BVP SHOULD REPRESENT FULL OCULAR RX CAUSE IN SCL, NO TEAR LENS POWER BEHIND LENS
- But sometimes there may be “myopic creep” = slow progressive increase in myopia caused by lens induced hypoxia seen in EW/CW
Stable vision:
Good fit
- There is no liquid lens power
- Occasionally +0.50D or -0.50D shift
- Correct: stable VA
- Tight: improves after blink
- Loose: worsens after blink
BOZR
- Changes to BOZR have greatest impact on fitting
- Steeper BOR will tighten the fit -> better centration?
- Flatter BOR will loosen the fit
TD
- Changes to TD have impact on centration and movement
- Larger TD will DECREASE movement which may help centration
- But remember that they are related to sagittal depth (difficult to predict between manufacturers.
Larger TD:
*Decrease movement
*Becomes steeper
Smaller TD:
*Increase movement
*Worsen centration
Soft contact lens fitting involves
- The selection of the most appropriate lens material, dimensions and wearing modality to match the ocular characteristics and patient needs, whilst
- Giving the best fit and visual acuity with minimal physiological effects
Discontinuation SCLs reasons
Discomfort has been shown to be due to a combination of patient and lens factors:
- Patient factors include an irregular or thin tear meniscus, and an increase in conjunctival staining and palpebral roughness
- Lens factors are simply due to poorer centration, and a loose fit
- Sometimes material choice or lens edge
Underlying causes of these contact lens discontinuations is often due to poor fit or inappropriate selection i.e lens moving too much or not centered correctly
