ALL about SCL Flashcards
indication for SCL
- Spherical cornea with spherical refraction
- Toric cornea with astigmatism refraction
- Toric cornea with spherical refraction
FDA grouping and its example
1 e.g soflen38 - Non ionic, low water 2 e.g biotrue - Non-ionic, high water 3 - -Ionic, low water 4 e.g biomedic- Ionic, high water 5 e.g acuvue oasys - SiHy
Advantage of hydrogel
- Excellent initial comfort
- Minimal adaptation time
- Occasional wear possible
- Disposable
- Low incidence of oedema, lacrimation, flare, photophobia
- Long wearing times
Disadvantage of hydrogel
- Reduced va with uncorrected astig due to lower modulus as compared to SiHy
- Deposit prone
- Oxygen transmission with hydrogel is lower than those of SiHy and rgp
- lens water content have to be maintained
- Increase risk of bacteria contamination
- Corneal vascularization with thicker or low water content lenses
Advantage of SiHy
- Excellent high oxygen permeability
- Very low risk of vascularization
- Suitable for extended wear
- Less hydration because lower water content
disadvantage of SiHy
- Greater incident of arcuate staining
- More liable to lipid deposit
- More expensive
Comparing SiHy and hydrogel
SiHy :
- higher DK
- water content 33-56
- higher modulus
- less wettable (attract lipids )
Hydrogel
- Lower dk
- water content 30-78
- lower modulus
- more wettable
5 property of SCL
- Dk and Dk/t
- water content
- ionic content
- wettability
- modulus
what is dk and dk/t
- Dk is the oxygen permeability of the material (relating to the material)
- Dk/t is the oxygen transmissibility (relating to lens thickness)
what is water content
- the higher the water content the higher the Dk
* the higher the water content = higher amount of tear quantity is needed to maintain the lens
what is ionic content and what is proteins and lipid
- Nonionic – less protein and lipid attracted
- Ionic – more protein and lipid attracted
- Protein and lipid; lysozyme, lactoferrin and lgA
what is wettability and what is SiHy and Hyrdrogel respective wettability (high/ low)
how quickly water can spread over the surface
- SiHy – less wettability – more lipid deposit
- Hydrogel – more wettability – more protein deposit
- High contact angle = low wettability
what is modulus
ability of lens material to align to the ocular surface and resist deformation under tension.
• Low = softer, harder to handle, more comfortable
• High = stiffer, easier to handle, less comfortable
what kinds of SCL is there
daily, biweekly, monthly
what is keratometry
- measure shape and power of cornea
what is total ocular astigmatism
Total ocular astigmatism = corneal astigmatism + lenticular astigmatism
what is the product detail formula
product name/ BC/ DIA / POWER
production selection for trial lens
decide between SiHy and hydrogel , frequency of wear, patient expectation , tear quality and quantity etc.
selection of trial lens bc and diameter
conventional:
bc = flattest K + 8 (in 0.3 steps)
diameter : HVID +2 (in 0.5 steps)
disposable:
bc and diameter = one size fits all
how do we select trial lens power
convert subjective rx into corneal plane - select closest power (power, before 6 is in 0.25 steps and after 6, is in 0.5 step )
- astig= 0.75 and above then use astig lens
what do we use for astig less than 0.75
SE = sphere + 1/2 astig
Cyl and axis value in toric SCL
-0.75 ,-1.25, -1.75,-2.25
axis in 10 degree step
Cyl and axis value in toric SCL
-0.75 ,-1.25, -1.75,-2.25
axis in 10 degree step
trial lens evaluation
a) Comfort
b) Vision
c) Corneal coverage (should be adequate in all direction): incomplete coverage: too loose/ too small lens
d) Centration – decentration can cause blur vision and decrease lens comfort
e) Movement – should move about 0.5mm. inadequate movement = tight fit(does not allow adequate tear exchange, cause a build up of debris and metabolic waste behind the lens )
f) Lag on up gaze
g) Push up test
