CL Flashcards
The “BC” of a GP lens is actually determined based on
the front surface of the tear film
SAMFAP. For every 0.1mm of change in the BC, the power of the GP lens needs to change by
-0.50
Formula to calculate residual astigmatisms
RA= Rx - Ks
*People can tolerate up to -0.75DC of residual astigmatism. If there is more than this, the pt should be fit with a toric GP.
Javal’s Rule
Add -0.50 x 090 DC to the corneal astigmatism to account for lenticular cyl
= (1.25 x k cyl) + (-0.50x090)
Optical zone diameter
- What happens if you increase or decrease it
- for every 0.4mm change in OZD, how much should you adjust the power?
Increase OZD = Increased sag = tighter fit = need to flatten the BC
Decrease OZD= decrease sag = looser fit= need to steepen the BC
For every 0.4mm change in OZD, change the power by 0.25D.
What parameter is selected to minimize flare? (most commonly occurs when the edge of the CL is close to the edge of the pupil)
Overall diameter. Larger diameter will maximize comfort.
What parameter allows alignment between the CL edge and the peripheral cornea.
Peripheral curves. GP CLs may have 1, 2, or 3 peripheral curves in addition to the BC.
The peripheral curve has what 3 main functions
- Prevent the edge of the CL from bearing on the cornea.
- Promotes tear exchange under the CL to maintain adequate K metabolism.
- Support a tear meniscus at the edge of the CL to promote CL centration.
The ideal edge thickness to promote lid attachment is the edge thickness of a ___D GP CL
-3.00D
How to alter edge thickness with lenticular carriers
GP with more than +1.50 will have flat edges and poor LA. Add a minus carrier to promote LA.
GP with more than -5.00D will have excessive edge thickness causing the lens to ride high due to too much LA. Add a plus carrier to decrease LA.
What is edge lift and what steps should you change it in
Distance between the peripheral edge of the GP and cornea.
Make 1mm changes.
Increase edge lift–> flatten
Decrease edge lift–> steepens
Center thickness (CT)
- What role does this play in GPs
- What steps should you change it in
Influences oxygen transmissibility of the CL, flexure, and center of gravity.
Change it in 0.03mm steps.
A thinner CT has more oxygen transmissibility, better contraption due to less mass, but more flexure.
A thicker CT has less oxygen transmissibility and less flexure, but tends to drop on the eye due to increased mass.
In general, high DK lenses require a thicker CT in order to minimize flexure.
In general, high DK lenses require a ____ CT in order to minimize flexure.
Thicker
GP CLs with more anterior center of gravity will tend to drop on the eye. 4 examples
Flat BC
Small diameter
Plus lenses
Thicker CT
A lens that is moved closer to the eye becomes effectively more
MINUS.
Add more plus into the Rx
Why would you not fit each meridian exactly in a GP bitoric lens? What are the other options?
On K fit is often too tight, resulting in poor tear exchange.
Saddle fit: Equal alignment between both principal meridians.
LTS: Flat K is 0.25 FTK and steep K is 0.75 FTK to mimic a -1.00 WTR K
When can you use a back surface toric lens?
If 1.5x k cyl = refractive cyl
Usually seen in people with ATR astig
Primary disadvantage of fitting aspheric GP lenses
Decreased tear exchange due to the decrease in contact lens movement on the eye. Especially Back surface aspheric lenses.
Decentration may also induce unwanted astigmatism.
2 types of multifocal GP designs
- Simultaneous
- Aspheric MF
- Concentric BF - Translating
What characteristics will increase flexure
Thin center thickness High K cyl Large diameter High DK material Steep BC
Why does a soft lens have total flexure?
Because it completely forms to the shape of the K, resulting in a plano lacrimal lens.
RA= refractive astig.
Diff between GP flexure and warpage
GP flexure is only on the eye
Warpage is on AND off the eye. Usually due to aggressive digital cleaning.
steep CLs or high DK lenses have increased flexure, how can you counteract this?
Increase CT by 0.03mm
For every 0.4mm change in the GP diameter, how much do you need to change the BC?
By 0.25D, same with power.
Low, high, and hyper DK values
Low: 25-50
High: 50-99
Hyper: 100+
Transmissivity =
DK/ thickness (cm)
DK is gas permeability
Patients usually can be fit with a sphere soft lens if they have how much astigmatism?
0.75DC or less
Prism ballasting
Periballasting
Dynamic stabilization
Prism ballasting: BD incorporated into the bottom of the lens.
Periballasting: BD prism incorporated at the bottom of the lens OUTSIDE the optic zone.
Dynamic stabilization: Thicker horizontal meridian.
For every 10 degrees rotation in a soft toric lens, how much cyl is induced
1/3
Average K values
41-45D
Good fit characteristics of a soft CL
Centration
0.25-1mm movement in primary gaze with blink
1mm lag
Lens extends 1.5mm beyond limbus
How to find diameter of Soft CL
HVID + 3mm
Water and DK relationship based on conventional or silicone hydrogels
Conventional: As water increases, DK increases.
Silicone hydrogel: As water increases, DK decreases.
DK= permeability
DK
Permeability
The FDA classifies soft CLs in to 5 groups
1: Low water, non-ionic
2. High water, non-ionic
3. Low water, ionic
4. High water, ionic
5. Silicone hydrogels
Deposits are most likely to occur in soft CLs with
High water content, ionic (Group 4)
Rigidity of a CL is dependent on the
Thickness and modulus
Modulus =
1/stretchability
or rigidity
Describe the modulus and lens if it is low water
Low water = high modulus = more rigid = more durable = can be made thinner = easier handling
Some drawbacks to high modulus SiHy lenses
Giant papillary conjunctivitis, SEALs, mucin balls, edge fluting.
How does accommodation, magnification, and vergence change when switching a hyperope from glasses –> CLS
Accommodation decreases
Magnification decreases
Less vergence required (bc in specs, there was BO prism moving the image closer, requiring more convergence)
How does accommodation, magnification, and vergence change when switching a myope from glasses –> CLS
Increased accommodation
More magnification
More convergence (Because in glasses, BI helped move the image away so the pt wouldn’t have to converge as much)
Pre-presbyopic myope may notice near blur sooner in CLs than specs due to increased accommodation demand.
