Week 6 - Contact lenses and Oxygen Flashcards
Contact lens must provide acceptable…
•Vision
•Comfort
•Usability
•Physiological response
- Mechanical or lens surface effects
- Hypoxia-related adverse responses (such limbal redness, some types of corneal staining, Microbial keratitis)
Where does the corneas oxygen supply come from?
- Atmosphere - Via tearfilm
- Aqueous Humour
- Perlimbal vasculature (+Palpebral conjunctiva during eye closure)
Why does the cornea need to breathe?
•The endothelium + Epithelium control amount of water in cornea, they need oxygen to pump water in + out
•Without oxygen, the cornea will SWELL, getting THICKER. This is called CORNEAL OEDEMA
What is oxygen permeability? (Dk)?
•Oxygen molecules must dissolve into material + travel through it.
•This is Oxygen Permeability (P)
- P=Dk
where D is diffusion coefficient
where k is solubility coefficient of oxygen in given material
What exactly is the “D” and “k” in Dk value?
•D - diffusion coefficient; represents the “Speed” at which oxygen travels through polymer. (Can be influenced by how oxygen interacts with polymer)
• k -solubility coefficient; represents how much oxygen can dissolve in the polymer
What is Oxygen Transmissibility? (Dk/t)
•The degree of which oxygen passes through a material with a particular thickness
•Equal to the oxygen permeability/thickness given as ;
Oxygen transmissibility (T)
- T = Dk/t
Dk is the oxygen permeability
t is the thickness of a material
What are the effects of lens power on transmissibility?
•Product guides quote the Dk/t to be 3.00Ds
•Any increase in lens power (both plus/minus lenses), increase thickness, therefore decreasing the Dk/t value.
• This is because less oxygen gets through, a lower oxygen transmissibility
What are the oxygen requirements to avoid signs of corneal oxygen deficiency?
• Daily wear - short days/part time - Dk/t 24
• Extended wear or continuous - Dk/t 125
Not all soft cl lenses meet these criteria;
• Bad for people that wear for long days/nap
- Therefore Dk/t contacts should be a-lot higher than 24
Why isnt Dk/t 24 always enough?
•Long days
•Napping
•Environmental (altitude, dryness etc)
What happens when eyes don’t get enough oxygen?
•If oxygen is blocked to the cornea with a low Dk/t (i.e low oxygen transmission) contact lens, over time;
- Eyes become red + blood shot
- Endothelial layer damaged; if one cell dies the neighbour cells filled gap - Polymegathism (many shapes)
- Blood vessels grow from the limbus (edge of eye) into cornea - neovascularisation
- Eye becomes vulnerable to infection
What is Neovascularisation? How is it improved?
- Hypoxia produces stromal oedema with release of vaso-stimulatory agents
- No significant problems
- Signs include new BV from limbal vessels
- more common in superior cornea due to presence of upper lid
- Manage by improving Dk/t, reducing wear time; prognosis good!
What is polymegathism and how is it managed?
•Caused by long term hypoxia structually damaging edothelial cells, with living cells filling in gap.
•No real symptoms although may develop CL intolerance with reduced VA+photophobia
•Signs: differing cell size in endothelium
•Manage by refitting with higher O2 lens, change dailies, reduce wear time;prognosis not usually good as endothelium never fully ‘heals’- long term may result in endothelial decompensation
What is Myopic Creep?
•Wear of standard Cl design lenses may change aberration profile, leading to greater degree if optically stimulated myopic increase, than if spectacles worn
- May be counterbalanced wearing high modulus silicone-hydrogels, which mechanically flatten the central corneal region
Corneal Oedema -SEAL
What is it?
•Superior epithelial arcuate lesions
•Caused my mechanical trauma due to inflexible nature of some CL
•Symptoms : FB sensation
• Signs : Arcuate staining 1mm from superior limbus
•Manage : Remove CL until clear, lubricants may be required. If problem reoccurs change lens design (flatter) and material or try RGP
Corneal Oedema - Striae
What is it?
•Often seen in diabetics or older patients+Keratoconics
•Caused by hypoxia - lactic acid accumulation in cornea causing osmotic shift in cornea/corneal oedema
•Px usually asymptomatic
•Signs include white, vertical lines in posterior stroma
•Manage by switching to dailies, increase Dk/t
