I was too lazy to the read the previous section. Can you please do a recap of the main points? (posted: april 2003)

Sure.... But when you get a chance, do please go back and read Eye Anatomy, Vision and Glasses for Dummies.

What are the important parts of your eyeball? Going quickly from front to back, there’s (a) a clear tissue cover called the cornea, which, for better or for worse, is directing light into your eye and doing part of the job of focusing it, or trying to; (b) the iris or coloured part, and in the middle of that the black hole called the pupil, which regulates how much light gets into your eye; (c) the lens, which is doing the rest of the job of focusing the light; (d) the vitreous, which we don’t worry about unless something really bad happens to it; and (e) the retina, at the very back of your eye, which is where the film-type images you see actually form.

How does your vision work? Light has to pass all the way through your eye (cornea, iris, lens and vitreous) and hit the back of your eye (retina) which passes images to the brain using the optic nerves, sort like how your digital camera uploads photos to your computer through a USB port, only different. While light is coming through the eye, three important things are happening: (a) The cornea, which has a special shape just for this purpose, is doing most of the focusing work to get the light to drive straight onto the retina; (b) the pupil is getting bigger or smaller depending on the lighting conditions and what you’re trying to look at, and it’s allowing more or less light in; and (c) the lens is doing the rest of the focusing work.

What can be wrong with your vision that makes you wear glasses? There are two basic kinds of problems of which you can have only one or the other, and two problems that you might have one or both of in addition to one of the first two. (Following that?) If your eyeball grew too long, light undershoots the target, lands in front of your retina, and you’re shortsighted. If your eyeball grew too short, light overshoots the target, lands pasts the retina, and you’re longsighted. Can’t have both, you’re either one or the other. But you can also have astigmatism, which means your cornea — the clear outside part - is not nice and sphere-shaped, so it sends the light in a couple of different directions at once. (Same thing could happen to the lens, by the way, and that’s another kind of astigmatism.) And you can also have presbyopia, which just means that your lens, which starts out flexible, is getting old and stiff, and in practical terms it means you need reading glasses.

What do glasses do about it? Remember, the basic problem is your eyeball itself is too long or short (and/or the outside of it is misshapen and/or the lens muscles are getting creaky). What glasses do is they trick the light into focusing correctly on the retina in spite of the shape of the eye, by plunking a specially shaped lens in front of the eye and bending the light just so, and presto! it focuses on the retina instead of in front or behind or all over the place.

So how does laser eye surgery help instead of glasses or contacts? (posted: april 2003)

Laser surgery doesn’t do anything about the real problem any more than your glasses or contact lenses did. If your eye’s short or long, it’s going to stay that way and not all the fancy technology in the world can do a thing you can do about it.

All laser surgery does is try to replicate what your glasses/contacts did (only of course not as well and unlike glasses you can’t ever take it off). Laser surgery basically extends the concept of glasses/contacts right onto your eye itself. So instead of using a funny-shaped lens in front of your eye to trick the light into doing the right thing, let’s use the built-in clear thing on your eye - your cornea itself, by making it into a new shape! COOL! So, we vaporise enough of your cornea to mould it into a new shape, and the newly shaped cornea - provided of course that (a) we get it right, and (b) it’s nice and smooth instead of bumpy and (c) it stays that way and (d) my eyeball doesn’t get any longer or shorter in the meantime and (e) I don’t get presbyopic — well, it bends the light just the right way so I can focus correctly on everything. COOL!

Pretty nifty, isn’t it? Even if it does overlook the fact that (and please forgive me for quoting a very tired old adage, but it seems singularly appropriate here) your corneas, unlike your glasses or contacts, are not made of glass or plastic.

But isn't there more to it than that? I mean, they don't just aim the laser and shoot, do they? (posted: april 2003)

Yes, you’re right. It’s not quite as simple as that. They don’t whack molecules right off the top of your cornea, because the top is a protective layer (called the epithelium) that’s kind of pre-programmed to just grow back if something happens to it. The part they really want to hit is the big thick underneath layer that does the real work of the cornea, called the stroma. So, there’s two options: First, they can use chemicals to burn the off the top couple of layers, and then fire the laser at the thick stroma part, and just let the top layer grow back (that’s PRK), but it takes time for it to grow back and we impatient consumers don’t like waiting. We want results NOW. Enter LASIK, where instead of trashing the top layer altogether, they slice neatly through the top plus a bit of the thick part to create a nice little cap attached by a hinge, they peel it back, they use the laser to whack off the bits of the cornea they don’t want, then they fold the flap back down all nice and neat.

There’s some other kinds of surgery too, like LASEK and ALK. But basically they’re all kind of variations on the same theme: We can use blades, and we can use lasers, and we can use both, but in any event the whole point is to get past the top layer to the really juicy part of the cornea, vaporise some of it, and put the cover back on or let it grow back.

But what was that thing that made everything go black? (posted: april 2003)

Oh, I almost forgot. The microkeratome suction ring. Actually even before that they use the speculum to stretch your eyelids open and hold them still, then the suction ring to hold your eye still before they start slicing. Yeah, I know, that’s everybody’s least favourite part. It raises the internal eye pressure above that of the arterial blood supply to the retina, and basically steps on the hose of blood flow - so your retina is temporarily having a stroke.

Now, back to that lasering part.

But how do they know how much of my cornea to burn off? Is that what those scans were for? (Updated jan 2005)

Not unless you had wavefront-guided surgery. Conventional laser eye surgery is kind of a one-size-fits-all thing. They plug in your prescription (remember the three numbers, back in Eye Anatomy, Vision and Glasses for Dummies?) and the computer spits out an algorithm of some sort that is supposed to work for everybody with your prescription. If you had wavefront surgery, yes, they used the data from some wavefront aberrometry scans to programme the laser.

And why did I have to stare at the light the whole time? (Posted April 2003)

Or, "If I moved my eye, does that mean I messed it up?"

The computer plans the ablation pattern (fancy way of saying how much tissue the laser will hit and where) and does it bit by bit over all of the part of the cornea that it’s programmed to hit. If that programmed pattern is going to work properly, your eye needs to stay still. Basically, the laser needs to stop working every time your eye moves, and not start again until you’re staring right at that light. With manual lasers, the surgeon has to stop the laser when your eye moves. With lasers that have eye trackers, it should automatically stop when your eye moves.

You mean, that's IT? (posted april 2003)

Pretty much. Simple, isn’t it? Sort of like that video they showed you during the marketing presentation (oops, I mean seminar).