properties of CLS Flashcards
why is oxygen permeability important?
. we need to determine if cls provide patient with sufficient levels of oxygen
what is oxygen permeability (Dk) ?
. amount of oxygen passing through a contact lens material over a set amount of time and pressure difference
what does the D in DK stand for ?
. D=diffusion coefficient: dissolved molecules move within the material
what does the K in DK stand for ?
. K=solubility coefficient: number of oxygen molecules dissolved in the material
what is the relationship between DK and temperature ?
. DK varies with temperature: the higher the temperature= higher DK
what is the unit of DK?
. DK in ISO unit
- 75% of Fatt unit
why is oxygen transmissibility Dk/t more relevant than Dk alone?
. because we are determining the amount of oxygen that passes through a contact lens but of a certain thickness over a set time
what is oxygen transmissibility DK/t?
. amount of oxygen passing through a contact lens of specified thickness over a set amount of time and pressure difference
- higher DK means more oxygen passing through the CL
. D= diffusion coefficient
. k= solubility coefficient
. t= thickness of lens or sample of average lens thickness -3.00D
what is the relationship between thickness and DK?
. the higher the thickness= lower DK
what is relationship between DK and temperature ?
. the higher the temp= higher DK
what is the relationship between DK/t and thickness?
. oxygen transmissibility decreases with increasing thickness of the lens
- especially important in toric and high positive
why is it important to consider oxygen transmissibility with toric lens ?
toric lens vary significantly in thickness
what happens when a lens is thinner compared to when thicker?
. when a lens is thinner, more oxygen passes through
. when a lens is thicker, less oxygen passes through
what can a low Dk/t result in ?
low Dk/t results in corneal changes: . oedema . microcysts . polymegathism . blebs
what should the Dk/t for daily wear cls be?
. holden and mertz found that contact lenses for daily wear requires Dk/t = 24
. harvitt and bannano found that Dk/t of 35 is needed to prevent corneal swelling - this ensure delivery of oxygen to the basal epithelial cells under open eye conditions
what is oxygen flux?
. calculates the amount of oxygen that passes through lens on eye and enters the cornea
. volume of oxygen passing through a specified area of a contact lens over a set amount of time
what is the oxygen flux in an eye wearing no cls?
100% with normal amount of atmospheric oxygen entering the eye
what is oxygen flux dependent on ?
1 . sea level - because at sea level the partial pressure of oxygen in the atmosphere is a 150mmHg which is the natural upper limit
. when comparing that to countries above sea level where partial pressure drops to 122mmHg, the same contact lens causes less oxygen to reach the cornea
- depends on eye lid ( if eyes are open or closed )
- contact lenses worn overnight , higher dk/t properties as the partial pressure is low under the eye lid
. open eye: 159mm Hg
. closed eye: 59mmHg
why don’t we have cls with very high value of Dk/t such as 1000?
. there is a natural upper limit for Dk/t
. there is also reasons to use materials that have lower DK/t as there are advantages for these materials
what is the natural upper limit of oxygen that can enter the eye?
. the amount of oxygen that can enter the eye has a natural upper limit because of the partial pressure of oxygen in the atmosphere which is 159mmHg at sea-level
why doesn’t doubling the Dk/t lead to doubling of oxygen that enters the eye?
. because of that natural upper limit , beyond a certain level doubling the Dk/t doesn’t lead to doubling of the oxygen that enters the eye
.if you move from a dk/t of 60 to one that is 3x greater , only results in extra 3% of oxygen entering the cornea
what to use to measure oxygen flux?
. oxygen flux is difficult to measure
. manufactures use EOP ( equivalent oxygen percentage )
what is EOP?
. level of oxygen at surface of the cornea under CL
what is normal EOP?
. normal available 20.9% in open eye and 8% in closed eye ( sea-level)
what is EOP for daily wear?
.EOP ( daily wear ) requires more than 10% (Dk/t 24 )
what is EOP for extended wear ( sleep in cls)?
. EOP EW requires more than 18% ( DK/t 87) to prevent more than the usual 4% of overnight corneal swelling
what is wettability?
. the ability of a liquid to adhere to a surface of a contact lens
. wettability is important for vision and comfort
what is wettability dependent on?
. cohesive forces within liquid
. angle
. spread of liquid over a surface
what is the relationship between cohesive forces within liquid and attraction between fluid and surface?
. less cohesive forces within liquid= higher attraction between fluid and surface
what is the difference between a hydrophobic and hydrophilic lens material?
. if material is hydrophobic, the fluid will sit like a droplet on top of the lens
. if a material is hydrophilic, the fluid spread over lens material, there is good wetting
how to calculate wetting?
. by looking at the contact angle of fluid that sits on top of a lens material
what is the relationship between contact angle and wettability?
. lower angle = higher wettability
what does an increased wettability mean?
. the spread of liquid over a surface increased= this improves vision and comfort
what is modulus?
. how well does the lens material resists deformation by pulling or stretching the CL
what does a higher modulus mean?
. a higher modulus MEANS LENS Material is very stiffer and has better resistance to deformation
. but the px may complain of a foreign body sensation, due to the stiffer lens material
what are the principal determinants of comfort?
. material modulus, lens design and surface properties such as lubricity and wettability are the principal determinants of comfort
what properties that are RGP specific?
. stability/hardness
. flexure
. refractive index
. specific gravity
what is is stability and hardness?
. resistance of the material to penetration
. has to do with the breakage in the production process, as well as distortion while wearing cls
. has to do with flexure resistance
what is flexure resistance?
. how many times you can flex cls before fracture
. how may times we can squeeze the lens and push edges together before the lens can break
. incomplete recovery causes distortion
when does flexure occur?
. flexure may occur either as the lens tries to assume the shape of a toric cornea or as lens becomes distorted with age and handling
what is on-eye flexure associated with?
. On-eye flexure is associated with increased material flexibility due to high Dk lenses
what happens to px if on-eye flexure is high?
. px may experience
- residual cylinder - cause astigmatism
- can cause fluctuating vision every time a px blinks
how to reduce flexure as a practitioner ?
. reduce flexure by fitting flatter
. reduce flexure by fitting very thin lenses
. fit toric lenses when required
what is refractive index ?
. refractive index changes depending on lens material
. increased refractive index allows for thinner lenses
why do we want thinner lenses?
. because the px will not feel cls
what is specific gravity ?
. could be used to control RGP centration on cornea
what is relationship between specific gravity and mass?
. higher specific gravity = higher mass
what to do if lens sits low?
. decrease material’s specific gravity if lens sits too low
-the upper eyelid is not able to pull lens up after blinking- every time px blinks lens will sit low- px can’t see well as it is not looking through optic centre of the CL
-decrease the specific gravity of lens material making the lens lighter - less mass
. only if steeper or larger diameter is not option.
how much can alternating specific gravity change mass by?
. alternating specific gravity could change mss up to 20%
what does focon III 2 mean?
1- focon - RGP material
2- A - indicates the revision
level of the chemical formula
A is original; B is the second
- no letter if there is only one formulation
- III - contains both silicone and fluorine
- (2) group code- indicates that the Dk range is 16-30 Dk units
what is the advantage of PMMA RGP lens material?
. great stability . no deposits . remains transparent . inert ( static ) and a-toxic . low weight §- as high refractive index
what is the disadvantage of PMMA material?
. zero Dk, causing hypoxia , oedema and central corneal clouding
what are silicone acrylates?
. RGP lens material added on to PMMA
. silicone added to increase Dk
. e.g.
- Boston IV. Dk 26
- paraperm EW. Dk 56
what is fluorosilicone acrylates?
. fluorine added on to PMMA and increases Dk
. higher SA content improves Dk, but often reduces wettability
. improved wettability
examples:
. Boston ES. Dk 31
. fluoroperm 60 DK 60
. Boston XO. Dk 100
what is the comparison of of FSA versus SA?
ADVANTAGES of FSA
- increased oxygen permeability
- less hyrdophobic
- less protein deposits
DISAVANTAGES
- less stable
- increase breakage
- higher modular weight
- more prone to lipid deposits
what is surface plasma treatment ?
. oxygen bombardment which chemically alters the lens surface
. this enhances cleaning process which px do at home
. improves surface wetting
what are some issues with surface plasma treatment ?
. how long does this Tx last for?
. care product issues?
when to replace RGP?
- depends on how well the px looks after cls
- on average every 2-3 years
explain RGP materials?
. PMMA
- no longer prescribed
- need to fit these px with low Dk values
. silicone acrylates
- attract more proteins than lipids
- generally lower Dk values
- poor wettability
. fluorosilicone acrylates
- improved wettability
- little protein deposits but more prone to lipid deposits
- hyper Dk available
. surface ( plasma ) coatings which have improved wetting performance
which RGP material to chose ?
. first choice: pre-fixed by lens design e.g. aspheric
. recommended by manufacturer
. patient always worn it
. own/supervisor’s favourite
- good performance/price
what is SCL water content?
. oxygen in a SCL lens is transported through material by water content of lens material- high water content - more oxygen it can transport - higher Dk/t - lens will be thinner
. more oxygen; healthier
what does lens ending in filcon mean?
its a SCL
what is the problem with SCL materials?
. historically, poor oxygen transmission occurred in low water content hydrogel materials
who is low water content hydrogel materials an issue to ?
. poor oxygen transmission of low water content hydrogel materials cause problems in those who
- over-wear their lenses overnight
- wear relatively thick lenses due to a high spherical prescription or being a toric design
what to look out for in low water content hydrogel lens materials?
- they are comfortable however start to see :
. limbal hyperaemia, stromal striae, neovascularisation, epithelial microcysts
what happens when we increase water content in SCL materials?
. higher water content lenses to increase the oxygen permeability (Dk)
what are the disadvantages of higher water content lenses?
. cause significant corneal staining, especially in thin lenses
. a hydrogel lens looses water during cls wear, when contact lens is placed on open eye, the water from cl will evaporate into the air due to temperature and humidity , the cl will loose its shape, the cl will suck up some tears from patient
-dehydration of CL causes staining
-common in thin CL
-more discomfort
what does nelfilcon A II 2 mean ?
1 - filcon- hydrogel material; means the material contains >10% water by mass
2- A - indicates the revision level of the chemical formula
- A is the original ; B is the second
- no letter if there is only one formulation
3-II - water content>50%, non-ionic material
4- 2 Dk range 16-30 Dk units
what does the group suffix mean?
- hydrogel< 50% water content , non-ionic
- hydrogel, >/50% water content, non-ionic material
- hydrogel, <50% water content, ionic
- hydrogel , >/50% water content , ionic
- silicone hydrogel- enhanced oxygen permeable materials
what is the difference between ionic and non ionic materials?
. ionic hydrogel - tend to have a negatively charged surface which makes them sensitive to changes in PH and osmolarity and more likely to attract proteins which are positively charged
. non-ionic: attract more lipids, treated to remove negative charge
what is the advantage and disadvantage of more water content?
. advantage :higher water content good for oxygen
. disadvantage of more water content , more dehydration which causes more staining particularly in thin lenses
. material is weaker in more water content
. attracts more deposits
what is disadvantage of ionic material?
disadvantage of ionic material : more attraction of deposits
explain the water content in SCL materials?
. the water content of SCLs is important for oxygen to pass through the material to the cornea
- advantage: higher water content is great for the eye
- disadvantage:
. lens feels dry towards the end of the day due to dehydration
. material is mechanically weaker compared to low water content
. it attracts more lens deposits
- preference is non-ionic materials
. but expensive to manufacture
what is silicone hydrogels (SiHy)
. introduced in 1999( europe) and 2001 (USA)
. development focused on meeting ( or surpassing the oxygen transmissibility requirements for overnight wear( EW and CW)
. si component supports transports oxygen
. superior health benefits of these materials for continuous wear - with similar advantage with daily wear of sihy
what is the difference between hydrogel and silicone hydrogel when it comes to DK of lens material?
. when water content increases, the Dk of material also increases
. silicone hydrogel have very very high Dk- let a lot more oxygen compared to normal hydrogel lenses
how does SiHy eliminate hypoxia?
. the cornea depends on oxygen to maintain its thickness and transparency
. CLs reduce the amount of oxygen reaching the cornea, creating a hypoxic environment
. there is a strong negative correlation between corneal swelling and oxygen transmissibility
. low-Dk hydrogel lenses cause 7-15% corneal swelling overnight
. this is less when cl is made of SiHY material
. corneal swelling with SiHy materials (2-5%), which is similar to overnight ( closed-eye) corneal swelling without contact lens wear
what are the advantages of SiHy?
.less hypoxia causes reduced
- hyperemia
- limbal injection- no vessels dilating
- vascularization
- corneal edema
- myopic creep
. maybe better comfort??
- lower water content of SiHy materials cause slower dehydration
- SiH materials are treated to enhance wettability and comfort to make up for the hydrophobic nature of the silicone component
what are the disadvantages of SiHy?
. maximum oxygen permeability, but not always maximum comfort
. if cl has lots of silicon there is increased modulus
- high modulus: stiff material: discomfort
- low modulus- Low water content: poorer handling: reduced durability- when you remove- more likely to break/rip the lens
. possible foreign body sensation due to a stiffer material
which SCL material to chose?
- first choice: pre-foxed by lens design
. if more oxygen needed: SiHy
. if EW or CW required: SiHy - EW - extended wear CW- continuous wear
. if tear deposits ( Hy) this is because Hy attracts less lipids
what is proportion of cl fits globally ?
. RGP fitting deceased- 20% fitted with RGP
. daily disposable SCL and daily disposable SiHy were introduced in 2005
. high amount of people using reusable SI-H
. less than 10% fitted with soft EW
