CLM - Soft Contact Lens Fitting I - Week 6 Flashcards
List the 8 parameters for contact lenses.
Back optic zone radius
Total diameter
Back vertex power
Water content
Dk
Material
Design
Thickness
Define BOZR.
Back optic zone radius - radius of curvature of the back surface of the lens (mm)
Define Dk.
Amount of oxygen passing through a contact lens meterial over a set amount of time and pressure difference
Define Dk/t.
Amount of oxygen passing through a contact lens of specified thickness over a set amount of time and pressure difference
By how much is the BOZr generally flatter than the average K-reading?
0.6 to 0.8mm
What does a BOZR that is too flat result in?
Poor centration
What does a BOZR that is too steep result in?
Reduced lens movement
Does changing the BOZR of disposable soft lenses greatly alter the lens movement?
No
Can lens designs in the same BOZR have very different movement profiles?
Yesd
Do disposable soft contact lenses generally have many BOZRs?
No, generally just one or two
What is the average K reading (mm and D)? What BOZRs are mostly expected to fit?
7.8mm
43.00D
Expected that most 8.6/8.7mm lenses will fit
What is considered a steep K reading (mm and D)? What BOZRs are mostly expected to fit?
<7.5mm
>45.00D
Few brands will have steep BOZRs of 8.3/8.4mm
To try first, what BOZR should be chosen first and why?
Choose the steepest BOZR first to minimise mechanical issues
How large is the total diameter of a soft lens compared to the iris?
Usually 2.0mm greater than the horizontal visible iris diameter (HVID)
What effect does a large total diameter have (2)?
Reduced lens movement and inadequate tear exchange
What effect does a small total diameter have (3)?
Excess movement, poor centration, and poor comfort
Do disposable soft lenses typically have one or many total diamters?
Usually just one
In what two ways can the back vertex power of a contact lens be measured?
Calculated from the spectacle refraction
From over-refraction with a trial contact lens
What is the spherical equivalent? What about if its a toric lens?
Sphere + cyl/2
If toric, the BVP must be calculated separately in each meridian
For what powers does vertex distance need to be considered when converting between spectacle and contact lenses?
≥±4.00D
What is the clinical pearl regarding the choice between contact lenses and spectacles for myopes and hyperopes?
Myopes - CLs < GLs
Hyperopes - CLs > GLs
What four factors will the choice of lens material depend on?
Desired Dk/t
Deposition profile
Wettability/dry eye issues
Modality
What is an advantage of thinner lenses? List 4 disadvantages.
Advantage
Maximises oxygen transmissability
Disadvantage
Poor handling by the patient
Lens life is shortened
Thinner lost-lens tear flim, therefore lower movement with blinks
Dessication staining of the cornea
Which are thicker, plus or minus lenses? What about sphere vs toric?
All plus lenses and toric lenses are relatively thicker
What mechanical issues can SiH lenses cause?
GPC
Is there a substitute for trialling a soft contact lens on the eye?
No
What 4 eyelid factors affect the fit of a contact lens?
Position
Tension
Blink strength
Surface regularity
What three eye factors affect the fit of a contact lens?
Topography of the anterior corneal surface
HVID
Corneal size
What 4 lens factors influence the fit of a contact lens?
Sag properties of the lens
Lens prescription/thickness
Lens material
The design - spheric/aspheric, edge profile, peripheral curves etc
Whenever possible, what contact lens material should be chosen?
SiH
What is the desirable BOZR for a contact lens?
0.6 to 0.8 flatter than the average K reading
Define the 2C-MAP mneumonic.
Centration
Coverage
Movement
Acuity
Push-up
What two conditions will satisfy lens centration as optimal?
If the lens edge shows uniform and symmetrical overlap onto the sclera in all meridians
If the lens is decentred (as with the pushup), it should regain optimal centration in less than a second
Define lens lag and the three categories and their cutoffs.
Amount by which the lens trails the movement of the eye
<0.2mm - insufficient
0.3mm - 0.7mm - acceptable
>0.7mm - excessive
What can cause excessive lag and no lag?
Excessive - poor centration
No lag - lens adherence
What four factors should be considered with poor lens centration?
BOZR
Comfort
Visual acuity
Lens diameter
What is the ideal and acceptable coverage of the cornea by a soft contact lens (in all gazes)?
Ideal - ≥1mm
Acceptable - ≥0.5mm
Unacceptable - <0.5mm
List three reasons why complete corneal coverage is important.
Mechanical - prevents corneal, limbal, and conjunctival trauma
Physiological - minimises corneal exposure/dessication/tear film disturbance
Comfort
What two factors should be considered with poor lens coverage?
Total diameter
BOZR
What is lens movement important for?
Promoting post-lens tear film exchange and mixing
How is lens movement quantified? List the three categories for it and the measurement.
Vertical change in lens position before and after blinking
Inadequate - <0.2mm
Excessive movement - >1mm
Well-fitting lens - 0.25mm to 0.5mm
What is the pushup test? How does a well-fitting lens respond?
Examiner pushes the lens superiorly using the inferior eyelid
A well-fitting lens is able to e easily moved and will rapidly return to centre location
List four important factors about the post-lens tear film.
Antibacterial
Foreign body and debris removal
Drives epithelial exfoliation
Epithelial nutrition
Where is the pre- and post-lens tear flim?
Post - found between the cornea and the lens
Pre - found over the front surface of the contact lens
List two important factors of the pre-lens tear flim.
Lubrication of the lids (comfort)
Optics (vision quality)
How can the post-lens tear film be observed using the slit lamp (mag, angle, slit width) and how will an optimal fit appear? What about if there is reduced tear exchange?
Specular reflection, high mag, high angle, narrow slit
Optimal fit - aqueous (non-coloured)
Reduced - coloured/patterned flim
In what kind of lenses is lens edge alignment important?
Stiffer higher modulus SiH lenses
What is the optimal lens comfort?
Only the slightest awareness of the lens being present
What can no sensation of the contact lens be due to?
Lens adherence (too tight)
How can lens surface quality be assessed?
Using keratometry or corneal topography
Can an abnormal pre- and post-lens tear film distort keratometer mires?
Yes, but only pre-lens
What can cause lens settling?
Tear flim trapped behind the soft lens appears to be squeezed out by the initial blinking leading to a reduction in lens movement
Is movement immediately upon lens application representative of normal movement?
No, wait 2-3 minutes
