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OV2006 contact lenses > properties of CLS > Flashcards

properties of CLS Flashcards

1
Q

why is oxygen permeability important?

A

. we need to determine if cls provide patient with sufficient levels of oxygen

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2
Q

what is oxygen permeability (Dk) ?

A

. amount of oxygen passing through a contact lens material over a set amount of time and pressure difference

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3
Q

what does the D in DK stand for ?

A

. D=diffusion coefficient: dissolved molecules move within the material

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4
Q

what does the K in DK stand for ?

A

. K=solubility coefficient: number of oxygen molecules dissolved in the material

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5
Q

what is the relationship between DK and temperature ?

A

. DK varies with temperature: the higher the temperature= higher DK

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6
Q

what is the unit of DK?

A

. DK in ISO unit

  • 75% of Fatt unit
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7
Q

why is oxygen transmissibility Dk/t more relevant than Dk alone?

A

. because we are determining the amount of oxygen that passes through a contact lens but of a certain thickness over a set time

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8
Q

what is oxygen transmissibility DK/t?

A

. 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

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9
Q

what is the relationship between thickness and DK?

A

. the higher the thickness= lower DK

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10
Q

what is relationship between DK and temperature ?

A

. the higher the temp= higher DK

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11
Q

what is the relationship between DK/t and thickness?

A

. oxygen transmissibility decreases with increasing thickness of the lens
- especially important in toric and high positive

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12
Q

why is it important to consider oxygen transmissibility with toric lens ?

A

toric lens vary significantly in thickness

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13
Q

what happens when a lens is thinner compared to when thicker?

A

. when a lens is thinner, more oxygen passes through

. when a lens is thicker, less oxygen passes through

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14
Q

what can a low Dk/t result in ?

A 
low Dk/t results in corneal changes:
. oedema
. microcysts
. polymegathism
. blebs
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15
Q

what should the Dk/t for daily wear cls be?

A

. 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

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16
Q

what is oxygen flux?

A

. 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

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17
Q

what is the oxygen flux in an eye wearing no cls?

A

100% with normal amount of atmospheric oxygen entering the eye

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18
Q

what is oxygen flux dependent on ?

A

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

  1. 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

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19
Q

why don’t we have cls with very high value of Dk/t such as 1000?

A

. 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

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20
Q

what is the natural upper limit of oxygen that can enter the eye?

A

. 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

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21
Q

why doesn’t doubling the Dk/t lead to doubling of oxygen that enters the eye?

A

. 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

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22
Q

what to use to measure oxygen flux?

A

. oxygen flux is difficult to measure

. manufactures use EOP ( equivalent oxygen percentage )

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23
Q

what is EOP?

A

. level of oxygen at surface of the cornea under CL

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24
Q

what is normal EOP?

A

. normal available 20.9% in open eye and 8% in closed eye ( sea-level)

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25
what is EOP for daily wear?
.EOP ( daily wear ) requires more than 10% (Dk/t 24 )
26
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
27
what is wettability?
. the ability of a liquid to adhere to a surface of a contact lens . wettability is important for vision and comfort
28
what is wettability dependent on?
. cohesive forces within liquid . angle . spread of liquid over a surface
29
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
30
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
31
how to calculate wetting?
. by looking at the contact angle of fluid that sits on top of a lens material
32
what is the relationship between contact angle and wettability?
. lower angle = higher wettability
33
what does an increased wettability mean?
. the spread of liquid over a surface increased= this improves vision and comfort
34
what is modulus?
. how well does the lens material resists deformation by pulling or stretching the CL
35
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
36
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
37
what properties that are RGP specific?
. stability/hardness . flexure . refractive index . specific gravity
38
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
39
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
40
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
41
what is on-eye flexure associated with?
. On-eye flexure is associated with increased material flexibility due to high Dk lenses
42
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
43
how to reduce flexure as a practitioner ?
. reduce flexure by fitting flatter . reduce flexure by fitting very thin lenses . fit toric lenses when required
44
what is refractive index ?
. refractive index changes depending on lens material . increased refractive index allows for thinner lenses
45
why do we want thinner lenses?
. because the px will not feel cls
46
what is specific gravity ?
. could be used to control RGP centration on cornea
47
what is relationship between specific gravity and mass?
. higher specific gravity = higher mass
48
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.
49
how much can alternating specific gravity change mass by?
. alternating specific gravity could change mss up to 20%
50
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 3. III - contains both silicone and fluorine 4. (2) group code- indicates that the Dk range is 16-30 Dk units
51
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 ```
52
what is the disadvantage of PMMA material?
. zero Dk, causing hypoxia , oedema and central corneal clouding
53
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
54
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
55
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
56
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
57
what are some issues with surface plasma treatment ?
. how long does this Tx last for? | . care product issues?
58
when to replace RGP?
- depends on how well the px looks after cls | - on average every 2-3 years
59
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
60
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
61
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
62
what does lens ending in filcon mean?
its a SCL
63
what is the problem with SCL materials?
. historically, poor oxygen transmission occurred in low water content hydrogel materials
64
who is low water content hydrogel materials an issue to ?
. poor oxygen transmission of low water content hydrogel materials cause problems in those who 1. over-wear their lenses overnight 2. wear relatively thick lenses due to a high spherical prescription or being a toric design
65
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
66
what happens when we increase water content in SCL materials?
. higher water content lenses to increase the oxygen permeability (Dk)
67
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
68
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
69
what does the group suffix mean?
1. hydrogel< 50% water content , non-ionic 2. hydrogel, >/50% water content, non-ionic material 3. hydrogel, <50% water content, ionic 4. hydrogel , >/50% water content , ionic 5. silicone hydrogel- enhanced oxygen permeable materials
70
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
71
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
72
what is disadvantage of ionic material?
disadvantage of ionic material : more attraction of deposits
73
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
74
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
75
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
76
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
77
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
78
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
79
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
80
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

OV2006 contact lenses (11 decks)

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