5.1.1 Flashcards
What is the recommended back optical zone radius (BOZR) for soft lens fitting?
0.6-1.0mm flatter than average keratometry readings
Limited use in soft lens fitting but important for comparative purposes.
How do you calculate the total diameter (TD) of a soft lens?
Add 2.0-2.5mm to horizontal visible iris diameter (HVID)
This is to allow full corneal coverage.
How is back vertex power (BVP) calculated?
Mean spherical equivalent / (1 - vertex distance(m) x mean spherical equivalent)
BVD is crucial for powers above 4.00DS.
What materials are commonly used for soft lenses?
- Hydrogel
- Silicone hydrogel
- Silicone+ Hydrogel
- Water gradient lenses
What happens to hydrogel lenses as the day progresses?
They become dehydrated, tightening on the eye - minus worse than plus.
Hydrogel lenses have low oxygen transmissibility and poor wetting.
What is a key benefit of silicone hydrogel (SiH) lenses?
They allow 5x more oxygen to reach the cornea than regular hydrogel lenses.
What are the characteristics of silicone hydrogel (SiH) lenses?
- Higher modulus for good handling
- Keeps shape which is beneficial for new wearers
- Can cause rubbing underneath the eyelid
What improvements do silicone+ hydrogel lenses offer over SiH lenses?
More curved profile on the lens edge, thinner, and much softer.
What distinguishes water gradient lenses?
Increased oxygen, better shape, 100% water on both back and front of the lens for minimal friction.
Examples include MyDay and Dailies Total 1 lenses.
True or False: Silicone+ hydrogel lenses have a higher modulus than regular silicone hydrogel lenses.
False
Fill in the blank: The total diameter (TD) of a soft lens is calculated by adding _______ to the horizontal visible iris diameter (HVID).
2.0-2.5mm
How to assess optimal CL fit
Centration
Corneal coverage
Upgaze blink = 0.2-0.6mm
Horizontal lag = 0.2-0.4
Lower lid push up test/tightness = 50%
Comfort rating
How to change lens parameters of a steep fitting lens
This means the lens is tight and needs to loosen so , flatten by increasing BOZR by 2mm or reduce TD
How to change lens parameters of a flat fitting lens
This means lens is too loose and needs to tighten so reduce BOZR by 2mm or increase TD
What is the relationship between water content and Dk/t?
The higher the water content, the higher the Dk/t, leading to more oxygen transmissibility to the cornea.
Dk/t is a measure of how well a contact lens transmits oxygen to the eye, with higher values indicating better oxygen flow.
What are the drawbacks of high water content in contact lenses?
High water content leads to:
* Lens instability
* More deposits
* Low refractive index
These issues can negatively affect the comfort and performance of contact lenses.
What material is used to solve the problems associated with high water content?
SiH (Silicone Hydrogel)
SiH materials are designed to provide better oxygen transmissibility while reducing the issues related to high water content.
What is the effect of high modulus in contact lenses?
High modulus (stiffer lens) can cause discomfort from edge interaction with the lid and results in contact lens associated papillary conjunctivitis (CLAPC).
CLAPC is an inflammatory response that can occur with certain types of contact lenses.
What are the benefits of low modulus in contact lenses?
Low modulus (softer lens) is:
* More forgiving to fit
* Lower squeeze pressure
* Tends to move better
Softer lenses generally provide a more comfortable experience and are easier to fit.
What the brand name/Dk/t value,material and BC/dia values that easyvision linarials come in
MyDay
Dk/t = 100
SiH
BC avail = 8.4 and 8.6
Dia avail = 14.2
What the brand name/Dk/t value,material and BC/dia values that easyvision umere come in
Clariti 1 day
Dk/t = 86
SiH
BC avail 8.6(toric) (+9.0 for normal)
Dia avail 14.4 (toric) 14.2(normal)
What the brand name/Dk/t value,material and BC/dia values that easyvision optyes come in
Biofinity
Dk/t = 160
SiH
BC avail 8.6 and 9.0 for toric and normal
Dia 14.2(normal) 14.6(toric)
What the brand name/Dk/t value,material and BC/dia values that easyvision myday come in
MyDay
Dk/t = 100
BC avail 8.4 and 8.6 for normal and toric
Dia avail 14.2 normal, 14.5 toric
Why do toric lenses have a bigger diameter than sph only lenses
Because to ensure stability and alignment on the eye. It prevents lens from rotating and shifting. Wherease sph only lenses don’t have an added need for rotational stability
What is average pupil diameter and corneal diameter
Average pupil in light = 3-4mm
Average pupil in dark = 6-8mm
Average HVID = 11.5-12mm
Average VVID = 10.5-11mm
How would you alter contact lens rx if spec rx has sph < 4D and cyl < 1D
Add half the cyl to sph and give sph only lenses
When would you give toric lenses
If cyl > 0.75D you give the full cyl value
When would you change sph value of cl rx from spec rx
If sph> 4D you if myopia then reduce rx by 0.25-0.50 and if hyperopia then increases rx by 0.25-0.50
What are the ingrediants of Specsavers multi purpose soft lens solution
PHMB - preservative, antibacterial for disinfecting lenses
EDTA - chelating agent to remove dirt/debris
Sodium chloride - match solution to natural tears
Boric acid - maintains PH of solution
HPMC - lubricant to keep lens moist and comfortable
What to do if someone is allergic to preservatives in the soft CL solution
Switch to hydrogen peroxide solution as it’s preservative free
How is hydrogen peroxide solution used
Remove lenses and place in solution with neutralising disc. This neutralises the hydrogen peroxide. The disc is at the bottom of the case. The process takes 6.8hrs to neutralise. The lens goes in the lens holder of the case. Once done remove lens and rinse with saline water.
