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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
Q

what is EOP for daily wear?

A

.EOP ( daily wear ) requires more than 10% (Dk/t 24 )

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26
Q

what is EOP for extended wear ( sleep in cls)?

A

. EOP EW requires more than 18% ( DK/t 87) to prevent more than the usual 4% of overnight corneal swelling

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27
Q

what is wettability?

A

. the ability of a liquid to adhere to a surface of a contact lens

. wettability is important for vision and comfort

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28
Q

what is wettability dependent on?

A

. cohesive forces within liquid
. angle
. spread of liquid over a surface

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29
Q

what is the relationship between cohesive forces within liquid and attraction between fluid and surface?

A

. less cohesive forces within liquid= higher attraction between fluid and surface

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30
Q

what is the difference between a hydrophobic and hydrophilic lens material?

A

. 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

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31
Q

how to calculate wetting?

A

. by looking at the contact angle of fluid that sits on top of a lens material

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32
Q

what is the relationship between contact angle and wettability?

A

. lower angle = higher wettability

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33
Q

what does an increased wettability mean?

A

. the spread of liquid over a surface increased= this improves vision and comfort

34
Q

what is modulus?

A

. how well does the lens material resists deformation by pulling or stretching the CL

35
Q

what does a higher modulus mean?

A

. 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
Q

what are the principal determinants of comfort?

A

. material modulus, lens design and surface properties such as lubricity and wettability are the principal determinants of comfort

37
Q

what properties that are RGP specific?

A

. stability/hardness
. flexure
. refractive index
. specific gravity

38
Q

what is is stability and hardness?

A

. 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
Q

what is flexure resistance?

A

. 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
Q

when does flexure occur?

A

. 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
Q

what is on-eye flexure associated with?

A

. On-eye flexure is associated with increased material flexibility due to high Dk lenses

42
Q

what happens to px if on-eye flexure is high?

A

. px may experience
- residual cylinder - cause astigmatism

  • can cause fluctuating vision every time a px blinks
43
Q

how to reduce flexure as a practitioner ?

A

. reduce flexure by fitting flatter
. reduce flexure by fitting very thin lenses
. fit toric lenses when required

44
Q

what is refractive index ?

A

. refractive index changes depending on lens material

. increased refractive index allows for thinner lenses

45
Q

why do we want thinner lenses?

A

. because the px will not feel cls

46
Q

what is specific gravity ?

A

. could be used to control RGP centration on cornea

47
Q

what is relationship between specific gravity and mass?

A

. higher specific gravity = higher mass

48
Q

what to do if lens sits low?

A

. 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
Q

how much can alternating specific gravity change mass by?

A

. alternating specific gravity could change mss up to 20%

50
Q

what does focon III 2 mean?

A

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

  1. III - contains both silicone and fluorine
  2. (2) group code- indicates that the Dk range is 16-30 Dk units
51
Q

what is the advantage of PMMA RGP lens material?

A 
. great stability 
. no deposits
.  remains transparent 
. inert ( static ) and a-toxic
. low weight §- as high refractive index
52
Q

what is the disadvantage of PMMA material?

A

. zero Dk, causing hypoxia , oedema and central corneal clouding

53
Q

what are silicone acrylates?

A

. RGP lens material added on to PMMA
. silicone added to increase Dk

. e.g.

  • Boston IV. Dk 26
  • paraperm EW. Dk 56
54
Q

what is fluorosilicone acrylates?

A

. 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
Q

what is the comparison of of FSA versus SA?

A

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
Q

what is surface plasma treatment ?

A

. oxygen bombardment which chemically alters the lens surface
. this enhances cleaning process which px do at home
. improves surface wetting

57
Q

what are some issues with surface plasma treatment ?

A

. how long does this Tx last for?

. care product issues?

58
Q

when to replace RGP?

A
  • depends on how well the px looks after cls

- on average every 2-3 years

59
Q

explain RGP materials?

A

. 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
Q

which RGP material to chose ?

A

. 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
Q

what is SCL water content?

A

. 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
Q

what does lens ending in filcon mean?

A

its a SCL

63
Q

what is the problem with SCL materials?

A

. historically, poor oxygen transmission occurred in low water content hydrogel materials

64
Q

who is low water content hydrogel materials an issue to ?

A

. 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
Q

what to look out for in low water content hydrogel lens materials?

A
  • they are comfortable however start to see :

. limbal hyperaemia, stromal striae, neovascularisation, epithelial microcysts

66
Q

what happens when we increase water content in SCL materials?

A

. higher water content lenses to increase the oxygen permeability (Dk)

67
Q

what are the disadvantages of higher water content lenses?

A

. 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
Q

what does nelfilcon A II 2 mean ?

A

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
Q

what does the group suffix mean?

A
  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
Q

what is the difference between ionic and non ionic materials?

A

. 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
Q

what is the advantage and disadvantage of more water content?

A

. 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
Q

what is disadvantage of ionic material?

A

disadvantage of ionic material : more attraction of deposits

73
Q

explain the water content in SCL materials?

A

. 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
Q

what is silicone hydrogels (SiHy)

A

. 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
Q

what is the difference between hydrogel and silicone hydrogel when it comes to DK of lens material?

A

. 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
Q

how does SiHy eliminate hypoxia?

A

. 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
Q

what are the advantages of SiHy?

A

.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
Q

what are the disadvantages of SiHy?

A

. 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
Q

which SCL material to chose?

A
  • 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
Q

what is proportion of cl fits globally ?

A

. 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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