CL- Oxygenation and the eye

what is the partial pressure of oxygen in the cornea when the eye is open

> approx 155mmHg at corneal surface

> same as pp of O2 in air due to cornea’s direct exposure to atmospheric oxygen

what happens when eye is closed?

> pressure can significantly decrease when eye is closed or when wearing CL

> leading to potential corneal hypoxia

Sources of oxygen of epithelium

open eye:

• Atmosphere

Closed eye:

• Palpebral conjunctiva

• Bulbar conjunctiva?

sources of oxygen of stroma?

open eye:

• aqueous humour

closed eye:

• aquous humour

Sources of oxygen of endothelium in open and closed eye?

• Aqueous humour

sources of oxygen of aqueous humour in open and closed eye?

• iris vasculature

Corneal oxygen distribution with no CL

open eye :

• 155mmHg O2 available to cornea and drops to 55mHg at endothelium

• in closed eye —> less - 55mHg available — eyelid blocks atmospheric O2

• endothelium

• homeostasis achieved without CL at 55mmHg

Corneal Oxygen distribution - various CL

• CL acts as barrier to O2 diffusion

• oxygen has to pass through CL→ Tear film→Cornea

• O2 reaching cornea depends on :

• O2 permeability of lens

• lens thickness

• area of cornea covered

O2 permeability equation

• intrinsic material property - resistance to gas flow

Permeability P=Dk

D = diffusion coefficient

k= solubility coefficient of O2 in given material

Diffusion and solubility what everybody wants/needs in CL

• want material that allows for rapid, direct and unimpeded passage of O2 through material

• want material to be extremely high in solubility

component of transmissibility —> Thickness

• myopic lens → thinner in middle

• hyperopic lens → thicker in middle

• although material has same Dk , thickness influences Dk/t

• thinner lens has higher Dk/t

• different prescriptions have different Dk/t

When does Dk/t increase?

• water content → fixed thickness

when does Dk/t decrease?

thickness - fixed water content

if CL a given thickness and more water → more O2 transmitted - hydrogel lens

when do you get a highest Dk/t

• thin, mid-water CLs

what happens if not enough oxygen to eye?

> Hypoxia leading to …

• reduction in aerobic glycolysis

• lactate accumulation (stroma)

• stromal acidosis

• osmotic imbalance —> eye will tempt to reduce , eye transports water across membranes to stroma leading to swelling

• oedema (swelling) - reduces transparency of eye - vision fuzzy

structural changes if not enough oxygen

• epithelial and stromal oedema

• decrease transparency

• microcysts + vacuoles

• striae

• folds

• endothelial blebs

• endothelial polymegathism

• vascularisation

minimum Dk/t

• 100-120 that gives full oxygenation

• at 90 → 5% of cells hypoxic

• variability in amount of Dk/t determined to be minimum

• silicon hydrogel - super Dk/t , more O2 , in excess

• Hydrogel - 18 , very low Dk/t

criterion and minimum O2% needed before we see changes in cornea

15-17% endothelial blebs

87 Dk/t → holden/mertz criteria

how are soft contact lenses categorised?

1) Low water, non-ionic

2) High water, non-ionic

3) Low water, ionic

4) High water,ionic

ionic meaning CL

• CL with ionic materials have negatively charged surface, which can attract positively charged proteins in tears

Non-ionic meaning CL

• CL with non-ionic materials have less reactive surfaces and treated to reduce their negative surface charge

• makes them less likely to attract protein deposits

how are low water and high water CL considered?

CL with less than 50% of water content considered low water

CL with more than 50% water content considered high water

what are soft CLs made from and what are the important features they have?

• hydrogel or silicone hydrogel material

Features:

comfort, wear time, O2 permeability, Handling (SiHy) easier to insert and remove , Vision

water content of hydrogel lens

• hydrogel material extremely flexible(floppy) due to its water content

• higher water content = higher Dk/t

• Hydrogel most used is poly-HEMA

• O2 permeability depends on water content

• higher water content , more O2 can pass through lens

Silicone Hydrogel lenses

• newer gen of lenses allows approx 5 times more O2 to reach eye than standard hydrogel lens

• O2 permeability depends on amount of silicone used , not water content

• earliest silicone hydrogel lens treated with coating to improve wettability → affects comfort + dryness of CL

• some companies use plasma tx or specific wetting agents → increase hydrophilic properties of lens

Wettability angle

• good indication of ability of tears to form stable layer on surface of material

• wetting angle of greater than 180 - no wetting , droplets roll of surface of lens

• ideally want angle slightly less than 90 → good wetting

Advantages of hydrogel lenses

• highly flexible

• good initial comfort

• thinner lenses

• affordable

• available in variety of modalities - daily disposable, 2-week, monthly

• available in dif designs (sphere, toric, multifocal)

disadvantages of hydrogel lenses

• low O2 permeability

• potentially higher risk for eye infections and other hypoxia-related issues

• not ideal for overnight wear

Advantages of silicone hydrogel

• High O2 permeability

• easier to handle - new CL wearers

• better durability

• extended wear + overnight wear options available

• available in variety of modalities

• available in dif designs

Disadvantages of silicone hydrogel

• early silicone hydrogel lenses sometimes had gettability challenges although newer have similar or better gettability than Hydrogel

• early SiHy stiffer materials , newer have improved flexibility similar to hydrogels

• potentially higher risk of Giant papillary conjunctivitis , newer less of a concern , low modulus monthly

• slightly higher price

What materials are eye care practitioners prescribing?

• 72% soft CL SiHy internationally

• majority wearing soft contact lenses - spherical, toric,MF