Lecture: Biometry: Basics and Practical Review

Objectives of this presentation are:

  • Define biometry as it applies to the eye
  • Describe the advantages of optical interferometry over ultrasound
  • Name 2 models of optical biometer in use today
  • Name at least 3 intraocular measurements used for determination of IOL power

Lecturer: Ms. Kimberly McQuaid, Dartmouth-Hitchcock Clinic, Lebanon, NH, USA


(To translate please select your language to the right of this page)

MS MCQUAID: Hello, everybody. My name is Kim McQuaid, and I’m from New Hampshire, USA. What I’m gonna do is go through just a little basic review of some different types of biometry that we often use for preoperative cataract measurements. So my objectives for this lecture are that, upon completion, you should be able to, first of all, identify what biometry is, exactly. I’d like you to be able to describe some differences between a couple of types of biometry. One is called optical interferometry. And the other is ultrasound. I’d like you to be able to name two types of optical interferometer biometers that are in common use worldwide today. And I’d also like you to be able to name at least three — there are many structures that we measure — but I’d like you to name at least three intraocular measurements that we find really important for IOL calculations. So biometry, as a definition, is the practice of applying mathematics to biology. And of course, I’m gonna focus my conversation with you on ophthalmic biometry. But if you look down in the corner of this slide, fingerprints have a biometry all their own as well. So, for example, it’s fingerprint biometry that is applying mathematics to the biology of your fingerprint, that allows us to get into our smartphones and our iPads and our laptops. With regard to ophthalmology, there are several systems that we use for making measurements of ophthalmic structures. Those include A-scan ultrasound and B-scan ultrasound and pachymetry. OCT machines, which use a low coherence interferometry. Is a form of biometry. And then the laser interferometry, which you’ll find in use with the IOLMaster and the Lenstar machines. So all of these systems that I just mentioned use mathematics as they apply to biology to measure things like axial length, keratometry, pachymetry, and retinal thickness, among many other things. But typically, in an ophthalmology practice, when you hear the word biometry, it typically means — or is referring to the preoperative measurements that we do before… For IOL calculations. You know, before we do cataract surgery on a patient. So there are many factors that can affect the refractive state of a patient after an implant, an intraocular implant, has been put in. And forever, for the longest time, the axial length of the eyeball and the curvature of the cornea, the keratometry, have been considered the predominant factors in the refractive outcome. And it turns out that measurement errors of either of these two things in particular can radically alter the postoperative refraction, usually leading to unpleasant, unexpected surprises. So as our surgeons have gone along over the years, they’re getting a lot better. The cataract surgery itself has advanced. The researchers and the cataract surgeons themselves have identified a whole bunch of other ocular measurements that actually play into that final postoperative refraction. So some of those other things that matter — a little bit, anyway — corneal thickness. Anterior chamber depth. Lens thickness. And corneal diameter. And while these remaining items, each individually, maybe won’t have a very large impact, at the end they can have quite a large cumulative impact. So whatever it is that you are tasked with measuring for these preoperative cataracts, just keep in mind that little mistakes — they do add up. So I think most of the people here are using A-scan ultrasound for their biometry. And A-scan ultrasound uses a sound wave frequency of about 10 megahertz, which is a high frequency probe that allows for minimal depth of penetration, yet excellent resolution of the structures in which you’re looking at. So just to put that in a little perspective, that’s a little bit different than the ultrasound that we do for OB stuff, for checking on babies. Those probes have very low frequency. 1 or 2 megahertz. The sound waves penetrate very deeply. Because the fetus is usually quite a bit inside the body. And the resolution of the pictures is not nearly as fine as the resolution of an eye with ophthalmic ultrasound. So in a nutshell, the A-scan ultrasound just emits this one single pencil beam of sound, which strikes each surface of the eye as it goes through and gets echoed back into the same probe that emits it. And that’s different from a B-scan ultrasound. A B-scan ultrasound probe emits a fan — you know, it has a oscillating probe tip, and emits a fan of sound waves, to give you a two-dimensional image. And so what the A-scan biometry is doing — it’s measuring the time it takes the sound wave to travel from one structure within the eyeball to the next, and back again. So there are two common methods for obtaining A-scan. The first is the contact A-scan, where the probe is in direct contact with the cornea. And then the second way to do an A-scan is through an immersion technique. And that’s where the probe is separated from the cornea and is actually in a water bath. And there are just some examples of different A-scan ultrasound machines there on the right. So optical interferometry, on the other hand, has only been used for ophthalmic biometry since about 1999 or year 2000. And the IOLMaster was the first optical interferometer. We do happen to have an IOLMaster on the plane. So when I’m done, if anybody has any interest in seeing how it works, I’m happy to take you back there and show it to you. So I find that I’m not very good at optical physics, and I’m not very good at explaining them. So I’m gonna keep this part of how it works really brief. Okay? The gist of it is: A beam gets split into two. And one beam goes straight on to its end target, while the other beam travels through the ocular tissues. So what happens is that the laser beam that’s going through all those ocular tissues gets slowed down, compared to the other beam that goes straight to the end target. So both beams end up on the target, but they end up there at different times, and create what’s called an interference pattern. Which you see on the far right there. And that’s really all I’m gonna say about that. It does use a 780 micrometer infrared light wave that has 8 times the resolution of a 10-megahertz sound wave. So the measurement of the axial length and the structures within the eye is very, very precise, compared to ultrasound. And actually, because you’re not touching the cornea, you’re not manipulating anything over the cornea, a lot of the operator variation is eliminated. So from me to you to you to you to you, we’re all gonna get the same results with this laser interferometry method. So as I said, the IOLMaster, which came about in about year 2000, that’s the original optical biometer. This is pretty much what it looks like, even still today. And the printouts that you get look like this, on the right. And there have been many improvements in it, in the last 17 or 18 years. The most recent improvements, the most recent models of IOLMaster do allow us to do a better job getting through dense cataracts, which was always one of the drawbacks of IOLMaster. And one thing to keep in mind — that while the IOLMaster is considered laser interferometry, it actually still uses slit imagery for some of its measurements. So not all of the measurements are from optical interferometry. So next up came a machine called the Lenstar, about 10 years later, in 2009. This is a machine from Haag-Streit. And it uses laser optic measurements for every section of the eye. Every section of the eye that it measures. And the nice thing about the Lenstar is that it has integrated all of the latest formulas that surgeons are using for determining IOL power. And has integrated the specific measurements and the formulas that are used for the fancy lenses. You know, the multifocal lenses and the toric lenses, for example. So the axial length itself — that part of biometry has long been considered the most important measurement that we can get on a patient. And we’ve been using A-scan biometry since about the 1970s. And prior to using ultrasound to measure axial length, lens implanters — and it’s important to distinguish that not all cataract surgeons were actually implanting lenses. So lens implanters would just typically use a standard +18 diopter IOL in everybody. So I think we can all agree that not everybody is average, and maybe a +18 worked for a lot of people, but it’s like a bell curve. Didn’t work for half on either end, either. So there was a lot of unhappy people, in spite of using this +18 across the board. So when we are doing A-scan ultrasound, or finding the axial length, it’s important to remember that for every 0.33 millimeters — that is, for every 1/3 of a millimeter — there’s approximately a 1 diopter postoperative error at the end. So if you’re doing your A-scan ultrasound on a patient, and some of your readings are 24.00, and some of your readings are 24.50, you have to remember — you could be over 1 diopter off in the end, if you don’t get it right. So I do encourage you to look at your scans as you’re doing them. Try to see if they’re making sense. Compare the scans with the axial lengths that you’re getting with what you maybe already know about the patient. You know, and I’ve said this over and over, these last few weeks. You are smarter than the machine. So use your head to analyze these scans that you’re doing. Okay. Back to talking about axial length a little bit. Every time the sound wave encounters a new change in media, a spike is generated. So that’s how we ended up with these spike patterns that everybody’s familiar with. Sound waves travel more quickly through solid materials than they do for gas, for example. And when it comes to the eye, sound waves are gonna travel more quickly through a dense cataract than they are through vitreous. So each structure within the eye has a different sound velocity. That is, the sound waves travel at a different speed through each structure. But it turns out it’s not too bad. Because the tissue that makes up cornea and the tissue that makes up lens is pretty similar. They have a pretty similar sound velocity. And the sound velocity between aqueous compared to vitreous is also almost the same. So what the biometry machines do is they take a total average velocity, to determine the final axial length. And so that’s why the settings on your machine is very important, to look at those settings before you start out and to pay attention to them. For a phakic eye, an eye with its own human lens, the sound velocity is gonna be set at about 1550. So for an aphakic eye — now take away the lens — once you take away the human lens, the sound velocity actually slows down a little bit, because the sound is traveling much more through aqueous and vitreous than any solid structures. If the patient happens to have an IOL in that eye already, and we’re planning on doing something with that, the sound velocity actually goes up. The exact amount depends on the IOL. What the IOL is made of. But the sound velocity speeds up. Just a note on the difference between the immersion and the contact biometry. It’s important to set your machine to the right setting for that, because the immersion A-scans allow for that water bath in front of the cornea. Okay? So as I’ve said, a spike is generated each time the sound velocity changes. So this is the probe. This is the immersion A-scan. This is the probe right here. This is where the sound wave starts coming out and enters the water bath. Then the sound strikes the front of the cornea. And exits the cornea. Into the aqueous. And if you have a nice scan, you’ll actually see two little spikes here on this corneal spike. That’s the front. And then the back of the cornea. And then it travels slightly through the aqueous, with not much going on, before striking the anterior lens capsule. Then the sound wave travels through the lens itself. If there are significant lens opacities, it’s very common to see some spikes in between the anterior and the posterior of the lens. The sound waves exit the posterior of the lens — so this spike is generated when it goes from lens material to vitreous. And then the sound wave has pretty typically an uneventful pass through the vitreous. So not much going on there, until it strikes the retina. The internal limiting membrane. Okay? Then there’s typically a little bit of spike in here. That would be choroid. And then the next big spike is sclera. And this, which we call the grass here, growing on the end — that’s just orbital fat behind the globe itself. So while A-scan ultrasound is fine for most people that we’re doing biometry on, it does have some drawbacks. The first is that using the contact method for the A-scan usually always causes some corneal compression, and that artificially shortens the axial length of the eye. The immersion method of doing A-scan eliminates that corneal compression, but it is much harder to determine if you’re actually on the axis or not. Another drawback for A-scan ultrasound is it actually does require some skill and experience and some practice. It’s not just something anybody can do. The person doing it really needs to have a little background information to get it right. And then another drawback to the A-scan ultrasound is that it only measures to the internal limiting membrane. So if we go back to our retinal anatomy, and we think about the 10 layers of the retina, we know that the internal limiting membrane is the innermost layer. That’s the layer that’s in contact with the vitreous. Then there’s an important 6 or 7 or 8 different types of cells, and then there’s the photoreceptors. You know, the rods and the cones. And those are in the outer retina. And then the bed of the outer retina, you know, is the retinal pigment epithelium. So since the A-scan ultrasound only strikes the internal limiting membrane, it’s actually not measuring down to the photoreceptor layer. It’s not measuring that extra 200 or so microns of retinal thickness. So the manufacturers of the machine, they know this. So they just tack on an extra 200 microns, because that’s about what the average retinal thickness is, at the fovea. So the problem with this is: Not everybody’s average. So retinal thickness can fluctuate. Anywhere from 140 to 400 microns. The ultrasound also measures along the anatomic axis of the eye, rather than the optical anxious. So, for example, in highly myopic eyes or eyes with a staphyloma, it may read the eye — well, you’re gonna want to take the longest measurement. But the longest measurement may not land you on the fovea. Okay? So this is a horizontal axial-oriented B-scan. The macula is located over here. But the A-scan ultrasound is measuring all the way to here. And I think you can see that there is — there would be a difference from here to the macula, versus here to the axial pole of the eye. This diagram here on the right is kind of an explanation of the difference between the A-scan and the optical biometry. So in using the optical interferometry method for measuring A-scan, first of all, there’s no contact with the cornea at all. So there’s no chance of any kind of corneal compression. And optical biometries are actually said to be accurate to within 0.01 millimeters, versus an A-scan, which is typically accurate to 0.1 millimeters. The way they work is the patient fixates on a target, while the measurement is in progress. So this is allowing the optical biometry to measure to the fovea. The optical axial length. Not the anatomical axial length. And when it comes right down to it, don’t we want to focus the image on the fovea? And the optical biometry, as I mentioned, measures to the level of the RPE. All the way down here, where the photoreceptors are. Rather than just using an average 200-micron retinal thickness. So in spite of all of these advantages of optical biometry, it too has some drawbacks. First, it tends to be quite expensive, when compared to ultrasound, for example. Secondly, if the patient cannot fixate, you’re not gonna be able to get a measurement. This guy here, with the exotropia, you’re gonna have a real hard time measuring the length of that eyeball. Because it’s not aimed straight ahead. And although there have been some improvements with the newer models of these optical biometers, dense cataracts still remain a problem. So the next second most important measurement that we do, that’s part of optical biometry, that’s called keratometry. And the readings you get when you’re checking keratometry correlate to the postoperative refraction on a 1 to 1 basis. That is: If you make a 1-diopter mistake doing keratometry, you’re gonna leave the patient with a 1-diopter mistake after the cataract surgery. So some different ways of getting keratometry — of course, if you’re doing A-scan ultrasound for your biometry, you’re gonna have to get your keratometry from a different method. Either an auto refractor or an auto keratometer, or a manual keratometer. So you get that reading from someplace else, and you plug it into the A-scan machine to do the final calculations. The IOLMaster was great when it first came out, because we found it actually does keratometry readings from 6 spots on the cornea. And it measures a very fine area of the central corneal zone, where our central optical axis is. 2.35-millimeter central zone. As compared to manual keratometry, which measures only two points, within a 3.2-millimeter ring. So yeah. The IOLMaster comes along, and we’ve got much better keratometry already. Then the Lenstar came along, and now it’s measuring from two zones. A 2.35 and a 1.65-millimeter circle. And it averages those 32 points together to give you your keratometry. So anterior chamber depth is… Anterior chamber depth is considered the third most important measurement that we can get in IOL calculations. Now, maybe you haven’t really thought much of ACD all of these years. But with the IOLMaster and the Lenstar coming around, and the advanced calculations that I measured, the advanced formulas, anterior chamber depth is becoming more important. Because it’s one of the remaining causes of residual refractive error. Why do we even care about anterior chamber depth? It’s actually because it directly relates to the effective lens position. And therefore it’s got a critical role in those types of IOL calculations. Because if the intraocular lens implant is closer or further from the cornea, or closer or further from the retina, that’s actually gonna alter its effective power. And some studies have shown that for every 1-millimeter measurement error of anterior chamber depth, you’ve introduced 1.5 diopters of a refractive error. So the A-scan ultrasound does measure — you can see the anterior chamber depth with A-scan ultrasound. The IOLMaster — this is one of the measurements that the IOLMaster does, where it actually uses slit imagery. So just optical slit imagery, light, regular light, to get this measurement. And then the newer generation Lenstar actually does use optical interferometry to measure the anterior chamber depth. Lens thickness. Another part of biometry. Another thing that we can measure. So preoperative lens thickness can be related to the development of the cataract itself. As we age, our crystalline lens — you know, as it becomes a cataract — tends to get thicker. And as it gets thicker, it tends to make our anterior chamber a little shallower. So there is a connection there. To make a long story short, knowing what the crystalline lens thickness is helps us with IOL prediction using these latest generation IOL calculation formulas. And some of those newer generation formulas — the Holladay formula and a formula called the Olsen formula — and the Olsen formula, just as an interesting aside, has introduced a C-constant, they call it, which uses lens, the crystalline lens, measurements to account for the true physical dimensions of the eye’s optical system. So it’s interesting, because it uses exactly the same physics that are employed in the design of telescopes and camera lenses. But what it comes right down to is that the C-constant relates to where in the capsular bag the IOL is going to have its final resting place. Lens thickness is measuring with A-scan ultrasound, and it is measurable with the Lenstar. It is not a measurement that’s picked up with the IOLMaster versions prior to 2014. Another biometric measurement is corneal diameter. Also called white to white. Corneal diameter is… Its role in IOL calculations is related to the anterior chamber depth. To tell you the truth, I was a little murky on the exact connection and didn’t want to get bogged down with it. So I didn’t look into it too closely. But suffice it to say that there is a connection, somehow, between corneal diameter and anterior chamber depth. And that it does have its place. This measurement does have its place, again, in those more advanced IOL formulas. So to just kind of summarize what we’ve talked about, optical or ophthalmic biometry includes the precise measurements of all the structures of the eye. And A-scan ultrasound has been the gold standard for measuring axial length in particular since about the 1970s, but optical interferometry instruments are proving to be superior in most cases of measuring. So each type of biometer, whether you’re talking ultrasound or optical interferometry, has its strengths and its weaknesses. And if you’re in a position to acquire one of these instruments, you should kind of think about what your needs are. You have to factor in things like the portability of the instrument, for example. An A-scan ultrasound is really quite portable. So you can move it from room to room, or even clinic to clinic. Whereas an IOLMaster or a Lenstar, you know, you’re gonna have a lot of trouble moving that around. You want to consider the cost of these items. Again, A-scan ultrasound tends to be much less expensive than these optical interferometers. You want to think about the skill level of your technicians and nurses. The people that are doing these tests. Because A-scan ultrasound, while inexpensive and portable, requires quite a bit of skill and practice. So there’s that trade-off. But if you throw caution to the wind and decide you want an optical interferometer, a Lenstar or an IOLMaster, you have to keep in mind again your patient population. You know, I know a lot of the patients that we’re doing surgery on this week and last week — very dense cataracts. And we worked a little bit this afternoon on a couple of our patients that are having surgery today, and our IOLMaster was unable to actually get a reading through the dense cataracts. So even if you do have an IOLMaster or a Lenstar, you may want to consider or keep that A-scan handy for those people with very dense cataracts and the people who are not so easy to seat at these machines. I forgot to ask these questions before we started. But maybe we could just look at them briefly now. Do we have a minute? So true or false — you don’t have to answer these out loud. Just maybe think about it here. Is this statement true or false? Lens thickness is considered the most important measurement for determination of IOL power after cataract surgery? False. Yes. It’s false. What is the most important measurement? Axial length is the most important measurement. Very good. Which of the following in this list — which of the following uses optical interferometry for measuring the intraocular structures? F. Right. Both C and D. The Lenstar and the IOLMaster. Those are examples of optical interferometry. True or false? Is this statement true or false? The IOLMaster is better suited than ultrasound for measuring axial length in eyes that have a dense cataract or other media opacity? False. IOLMaster is not better suited for dense cataracts. Optical biometry measures axial length from the apex of the cornea to the level of… D. That’s correct. Optical biometry measures all the way down to the retinal pigment epithelium. Passes through all these structures on its way there. And that’s all I have.

June 7, 2017

Last Updated: October 31, 2022

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