Properties of CLs

general properties of SCL and RGP

O2 permeability

O2 transmissibility

wettability

modulus

RGP specific properties

stability and hardness

flexure

RI

specific gravity

SCL specific properties

water content

what is Dk

oxygen permeability through CL material over a set amount og time and pressure difference

what is D

diffusion coefficient

dissolved molecules move within the material

what is K

solubility coefficient

number of oxygen molecules dissolved in the material

what does DK vary with

describe the relationship

temperature

higher temp higher Dk

what are the units of DK

Fatt

ISO

what is Dk/t

amount of oxygen passing through a CL of specified thickness over a set amount of time and pressure difference

what is t

thickness of lens

describe the relationship of Dk and thickness

higher thickness lower Dk

what can a low Dk/t result in

corneal changes:

oedema

microcysts

blebs

what number Dk/t is good to prevent corneal swelling

35

what is O2 flux

volume of oxygen passing through specified area of a CL over a set amount of time

what is O2 flux dependent on

sea level - higher level = lower pressure

eyelid - closed eye = lower pressure

because oxygen flux is too difficult to measure, what do we use instead

EOP

equivalent oxygen percentage

what is EOP

level of O2 at surface of the cornea under CL

what is the EOP for daily wear in Dk/t

24

what is the EOP for extended wear in Dk/t

87

to prevent less than 4% overnight swelling

what os wettability

the ability of a drop of liquid to adhere to a solid surface

state the relationship between angle and wettability

lower angle higher wettability

therefore more spread of liquid over a surface --> improved vision and comfort

what is modulus

how well does the material resist deformation by pulling or stretching

what does a higher modulus mean

stiffer and better resistance to deformation

but px can complain of a foreign body sensation

what are the principle determinants of comfort

material modulus

lens design

surface properties: wettability / lubricity

disadvantages of lenses being of higher stability and hardness

breakage in production process

distortion whilst wearing in ast corneas

what is flexure resistance

number of flexing cycles before fracture

should return to its original form after deformation

what is flexure

increased material flexibility due to high Dk lenses

what can on-eye flexure cause

residual cylinder (induce ast)

poor fluctuating vision

higher Dk/t = softer material --> lids cause pressure and edges are pushed together

how would you solve the problem for on-eye flexure

reduce flexure by fitting flatter lens

reduce flexure by fitting v thin lens

what does a higher RI allow

thinner lenses

good bc then px doesnt feel the CL

what is the RI for PMMA

1.49

what is the RI for silicone acrylates

1.471-1.48

what is the RI for fluorosilicone acrylate

1.453-1.471

what can specific gravity be used to control

RGP centration on the cornea

what is the relationship between specific gravity and mass

higher sg higher mass

what material is PMMA

RGP

advantages of PMMA

stability

no deposits

transparent

low weight

disadvantages of PMMA

0 Dk

hypoxia

oedema

advantages of FSA vs SA

(fluorosilicone acrylates vs silicone acrylates)

increased O2 permeability

less hydrophobic

less protein deposists

disadvantages of FSA vs SA

(fluorosilicone acrylates vs silicone acrylates)

less stable

increased breakage

higher modular weight

more prone to lipid deposits

describe how SCL have higher water content

O2 transported through H2O

increased H2O increases Dk

healthier bc more O2

signs of poor O2 transmission

limbal hyperaemia

stromal striae

disadvantages of high H2O content

dehydration - towards end of the day

mechanical weaker material - low water content

deposits

advantages of SiHy

less hypoxia therefore reduced:

limbal injection

corneal edema

vascularization

better comfort

disadvantages of SiHy

max O2 permeability but not always max comfort

increased modulus

possible foreign body sensation

if more O2 is needed which SCL to choose

SiHy