Contact lenses materials and modality (S1 week2)

what were the first types of contact lenses, when were they made and what material

1948 - polymathy methacrylate

it was hard for oxygen to get in and became outdated by gas permeable lenses

History of Contact lenses timeline- starting from 1987

1987- lenses become a regular replacement to specs( daily disposables first)

1998- invention that allowed silicone into hydrogel material

2009- first daily disposable silicone hydrogel lens

2013- Biocompatible- mmic eye and reduce discomfort- there is more water content

2020- medical contact lenses.g myopia management , glaucoma IOP monitoring

why was adding silicone to hydrogel lenses important

significantly increased oxygen transmissibility, also means lenses now available for 24 hour wear

what are the important safety properties of a contact ( terms of manufacturing)

• oxygen permeability

• UV protecting

• Replacement schedule

• Ease of handling

what are the other properties that are important in terms of manufacturing

• Comfortability

• VA

• cost

• reproducibility

• solution compatability

• antimicrobial properties

why is wettability important

• for a smooth refractive surface

• so lens moves smoothly over eye

• avoids irritating cornea, limbal or palpebral conductive

what does a lower wetting angle mean

less hydrophobic- this is better

what is soft contact lens material made of and why

• hydrogel

• silicone

• PVA and methacryclic acid - this improves water content and binding it decreases deposition

  so oxygen permeability is increased

• alongside the silicone its treated with plasma to improve biocompatibility and comfort

Ordering soft contact lenses

• manufacturer name

• design name and material

• base curve

• diameter

• power

ordering RCL

• manufacturer

• brand

• design

• base curve

• diameter

• power

• material

• extras e.g tint

what is the units for oxygen permeability

DK

the cornea is avascular so where does it get oxygen from

Aqueous humour,

limbal blood vessels,

palepebral conjuctiva

Tear film

air

what is the equation for aerobic respiration

oxygen + glucose → Energy(ATP) + carbon dioxide + water

why is the oxygen needed (what processes)

for energy to be produced:

• epithelial cell turnover

• maintenance of endothelial pump

• maintenance of stromal hydration

where does majority of oxygen come from and how much mmHg

atmosphere and tears (150mmHG)

Aqueous (55mmHg)

palpebral conjuctiva(50mmHg)

what factors determine oxygen permeability of CLs- material factors

• permeability of materila

• thickness of lens

non material factors of oxygen permeability

• tear pump

• edge /barrier effect

• hydration

• lens deposits

definition of oxygen permeability( Dk)

rate of oxygen flow through a unit area of CL material of unit thickness, when under unit pressure difference

what does Dk stand for

D- diffusion coefficeint

k-solubility coefficient

of oxygen in that material

what are the Dk units

ISo Dk units 10^-11 cm²/s or Fatt units

does Dk increase with temeprature

yes - should be measured at 35 degrees each eye

what is Dk/t

oxygen transmissibility

what does (t) it stand for

thickness of material

definition of oxygen transmissibility

rate of oxygen flow under specific conditions when subject to thickness of contact lens under pressure difference

what can Dk/t vary with

lens power

across lens profile

thickness of lens

what do manufacturors usually quote Dk/t with

-3.00 DS and 35 degrees

how do you convert fast units to ISO

multiply by 0.75

how much oxygen is enough (Dk/t) for;

prevent oedema

normal basal epithelial function

prevent anoxia

(do for both extended and daily wear)

prevent oedema= 18 for daily wear

extended wear= 66

basal function= 17 for daily, 66.8 for extended

prevent anoxia= 26 for daily , 93.8 for extended

what issues can occur to cornea when there is low oxygen

-stromal oedema, haze,striae

-corneal staining

-limbal hyperemia

-epithelial microcsysts and vacuoles

-endothelial blebs

-

what is the relationship with Dk and water content in hydrogel lenses

the more water content the higher Dk (logarithmically increases)

max is 75%

is silicone hydrogel better and why

Dk/t is 5x higher

less dehydration

less limbal hyperaemia

reduced restriction on wearing times

no corneal oedema

what are the cons of SiH

-poor wettability

-less comfortable due to rubbery feel(modulus)

-more deposition

-solution related staining

how is the deposition solved in SiH

plasma treatment

order of classifying lenses

-prefix (brand name)

-Stem( filcon = soft lens, focon= rigid)

-series suffix

-Group suffix( soft lens groupings1-5)

-Dk

-water content

-modification code- m if surface has been modified

-Group 5 SiH- lower case ‘c’= chemically modified e.g plasma

‘w’= internal wetting agents

state the groups meanings

1- less than 50% water- non ionic

2- more than 50%- non ionic

3- less than 50%- ionic

4- more than 50%- ionic

5- for SiH enhanced

5A=ionic

5B - less than 50% - non ionic

5C- more than 50%- non ionic

what does ionic mean

more methacyrlic acid in it

its charged so more protein deposits but less lipid deposits

oppsite for non ionic

what are the groups for RCL (1-4)

GROUP 1- no silicone or flourine

group 2- silicone but no flourine

group 3-floruine but no silicone

group 4 -silicone and flourine

what is modality

appropriate wearing time and replacement pattern

examples of modality

-daily disposable

-weekly

-fortnightly

-annually

-24/2

-monthlys

-extended(overnight)

-