When we’re discussing contacts, there are three major categories: soft contact lenses, hard contact lenses, and hybrid contact lenses. In today’s post, I’m going to dive into the lens everyone knows and loves – the soft contact lens!
Soft Contact Lenses
Soft contact lenses, for a simple definition, are flexible lenses that sit on the eye to provide refractive correction. These can be divided into several different categories, based on material and wear time. Let’s start with material.
Soft contact lenses can be hydrogel or silicone hydrogel.
What’s the Difference?
Hydrogel lenses are made from a gel-like plastic substance that is very good at holding water (hence hydro-gel). This is what the very first soft contact lenses were made of, and makes for relatively comfortable lenses that allow some oxygen to pass through.
Silicone hydrogel lenses are similar to hydrogel lenses, however, they have an additional component – silicone. Adding silicone to the initial hydrogel material allowed for improved oxygen flow through the lens, however, it decreased the ability of the lens to hold water.
Why Does It Matter?
With hydrogel and silicone hydrogel lenses, there are two main factors that we’re looking at – water content (how much water the lens can hold), and oxygen permeability (how much oxygen can pass through the lens). Both are important for maintaining the health of your eye.
First up, water content.
Okay, I’m gonna start this part out a little weird.
Stop what you’re doing, and look at your skin.
Is there water on it? (The answer should be no unless you just got out of a body of water or are actively sweating..)
Is it uncomfortable? Chances are the answer is also no.
Because the skin (or the outer layer of it anyway) that covers your arms, legs, etc, is keratinized – which just means that it has a material (keratin) in it that helps to form a hard(er) protective barrier
Now, take a look at your eyes – they obviously look quite a bit different, right?
Though there’s a lot more to it, one of the major differences is that the outer layer of tissue in your eyes doesn’t have keratin! It’s missing part of that protective barrier!
While this is a great thing for vision (keratinized skin doesn’t exactly promote sight…), it means that your eyes need something else to stay happy (comfortable) and healthy (alive and transparent) – moisture!
From that point, the water content (how much moisture it holds) of your contact lens is important, as it helps determine the comfort, clarity, and consistency of your vision!
With this information in mind, it would probably be easy to say: give me the contact lens with the highest water content!
Unfortunately, water content and oxygen permeability are inversely proportional: the more water is in your lens, the more difficult it is for oxygen to pass through.
But why does oxygen matter?
Let’s go back to high school or college biology for a bit.
Your body is made up of cells – which are made up of even smaller components called organelles (tiny organs). Each of these organelles has a role in the functioning of the cell factory. You’ve got the foreman (nucleus), the assembly line (endoplasmic reticulum), the packaging plant (golgi apparatus), even the janitor/maintenance crew (lysozome). But how is this all powered? Oh yeah, by the energy source: the mitochondria.
Keep your thinking caps on, we’re not done yet – now we get to fast forward a few lectures to cellular respiration day.
Cellular respiration is the process by which cells (well, specifically the mitochondria) take simple sugars (glucose) and break them down to create energy (ATP). This process can be broken down into 4 (not so simple) steps:
- Glycolysis (Glucose -> pyruvate, ATP, NADH)
- Pyruvate Oxidation (Pyruvate -> Acetyl CoA, NADH, CO2)
- The Krebs/Citric Acid Cycle (Acetyle CoA -> ATP, CO2, NADH, FADH2)
- Oxidative Phosphorylation (NADH + FADH2 -> H2O + ATP)
The catch? The last three of these steps require oxygen.
Which is pretty much to say: for your cells to (efficiently) produce energy, they need oxygen! And that oxygen is carried by the blood.
Take a look at your eyes again – paying particular attention to your (clear) cornea). Do you see any blood vessels?
Hopefully the answer is no.
To promote the transparency that allows for vision, the cornea is naturally avascular: it doesn’t have blood vessels! This means that it has to get its oxygen from somewhere else – the air. However, when you put a contact lens on, that ability to take in oxygen from the air is decreased.
Without proper levels of oxygen, the cornea can swell or, in cases of chronic oxygen depletion, form new, leaky blood vessels that may eventually cause permanently reduced vision.
So then, what’s the solution?
Honestly, it’s all about finding the balance that works best for your eyes, as determined by your prescription, wear time, and pre-existing conditions (like dry eye), and not being afraid to try a new lens if your current ones aren’t working!
In the grand scheme of things, today’s discussion barely scratches the surface of all there is to know about contacts, so be sure to stay tuned for a later post to learn more about your lens options!
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