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