Orbscan

We’re not going to talk a lot about Orbscans because, well, first, we don’t know a whole lot about them, and, second, as far as we know they aren’t much good at what standard topographers do, but what they are good at is corneal thickness.

Corneal thickness is a very important piece of information for diagnosing certain complications from refractive surgery. And this is so important that they measured it before surgery, probably with an Orbscan (which you should ask for a copy of, incidentally), in order to make sure your corneas were thick enough to do surgery safely in the first place. Now, after surgery, the Orbscan can still indicate whether the cornea was made any thinner than intended, particularly important if you are considering further surgical treatment.

It should be noted that Orbscans do not do this flawlessly (they are simply calculating averages over certain areas) and even if they did, for LASIK patients it would be of limited use, because all it really tells is the total thickness including the LASIK flap. Remember that now there are two distinct parts of the cornea: the flap, and everything underneath.

The part you might have a reason to worry about is the thickness of the stroma under the flap. (Although come to think of it, equally, if your flap was really sloppy, and is thicker in some parts than others, you’re going to have reasons to worry about it too.) That is because if it gets too thin, the pressure in your eye may cause bulging, which does not do nice things to your vision and may be a progressive condition called corneal ectasia. So how can you measure the stroma under the flap? Most don’t. They simply take the total thickness at the thinnest point (as indicated on the Orbscan), say, 420 microns, and subtract the thickness the flap was supposed to be, say, 160 microns. That leaves 260 microns. But suppose the flap is thicker? There could be less stroma. Or if thinner, more stroma. The flap can be analysed in better detail using VHF digital ultrasound (Artemis). That is not widely available, but you can get it in London.