[Music] okay 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 diameters that are in common use worldwide today and I'd also like you to be able to name at least three there are 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 IO L calculations so biometry as a definition is the practice of applying mathematics to biology and of course I'm going to focus my conversation with you on off thalmic 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 easkey an ultrasound and pick Emma tree o CT 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 IOL master in the lens star machines so all of these systems that I've just mentioned used mathematics as they apply it to biology to measure things like axial length teratoma tree Tech Emma tree 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 and inter ocular implant has been put in and for forever for the longest time a axial length of the eyeball and the curvature of the cornea the carrot AMA tree 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 post-operative 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 post-operative refraction so some of those other things that matter a little bit anyway corneal thickness anterior chamber depth lens thickness and corneal diameter and well and while these remaining items each individually maybe won't have a very large impact at the end they can have a quite a large cumulative impact so whatever it is that you are tasked with measuring for these preoperative cataracts just keep in mind that that little mistakes they do add up so I think most of the people here are using a scan ultrasound further biometry and a scan ultrasound uses a sound wave frequency of about ten megahertz which is a high frequency probe 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 a very low frequency one or two 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 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 and gets echoed back into the same probe that emits it and that's different from a bee scan ultrasound a bee scan ultrasound probe emits fan you know it has an oscillating probe tip and emits a fan of sound waves to give you a 2-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 in 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 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 IOL master was the first optical interferometer we do happen to have an i/o l master on the plane so if 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 the gist of it is 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 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 the both beams end up on the target but they end up there at different times and created what's called an interference pattern what 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 any anything over the cornea a lot of the operator variation is eliminated so from 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 IOL Master which came about in about your 2000 that's the original optical biometry 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 provements in it in the last 17 or 18 years the most recent improvements the most recent models of IOL master do allow us to do a better job getting through dense cataracts which was always one of the drawbacks of IOL master and one thing to keep in mind that while the IOL master 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 lens star about 10 years later in 2009 this is a machine from hog straight and it uses laser optic measurements for every section of the eye in every section of the eye that it measures and the nice thing about the lens star is it has integrated all of the real the latest formulas that surgeons are using for determining IOL power and has integrated the specific measurements in 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 since about the 1970s and prior to using ultrasound to measure axial length lens implants and it's important to distinguish that not all cataract surgeons were actually implanting lenses so lens implant errs 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 plus 18 worked for a lot of people but you know it's like a bell curve didn't work for for 1/2 on either end either so there was a lot of unhappy people in spite of using this plus 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 point three three millimeters that is for every one third of a millimeter there's approximately a one diopter post-operative error at the end so if you're doing your a scan ultrasound and on a patient and some are your readings are twenty four point zero zero and some of your readings are twenty four point five zero you have to remember you could be over one 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 this 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 Soundwave encounters a new change in media a spike is generated so that's how we end up with these spike patterns that 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 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 in the final axial Liang 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 fecit guy and I with its own human lens the sound waves the sound velocity is going to be set at about fifteen hundred and fifty so for an a fecit guy 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 IO L 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 i/o l what the IO L 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 the a spike is generated each time the sound velocity changes so this 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 skin you'll actually see two little spikes here on this corneal spike that's the front and then the back of the cornea 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 wave exit the posterior of the lens so this spike is generated when it goes from lens material to vitreous and then the Soundwave has pretty typically an uneventful pass through the vitreous so not much going on there until it strikes the 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 um orbital fat behind the globe itself so while a scan ultrasound is fine for most people that were doing biometry on it does have some drawbacks the first is 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 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 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 interior internal limiting membrane so if we go back to our retinal anatomy and we think about the ten 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 six or seven or eight different types of cells and then there's the photoreceptors you know the rods and the cones and those are in outer retina and then the the bed of the outer retina you know is the retinal pigment epithelium so since the ACE can 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 at the fovea okay so the problem with this is not everybody's average so retinal thickness can fluctuate from anywhere from 140 to 400 microns the ultrasound also measures along the anatomic axis of the eye rather than the optical axis so for example in highly myopic eyes or eyes with a staff Aloma it may read the eye well it's it's 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 bee 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's there would be a difference from here to the macula versus here to the axial pull of the eye this diagram here on the right is kind of an explanation of of the difference between the a scan and the optical biometry so using 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 by ometer czar actually said to be accurate to within point zero one 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 going to 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 by ometer 's 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 cara Tama tree and the readings you get when you're checking cara Tama tree correlate to the post-operative refraction on a one-to-one basis that is if you make a one diopter mistake doing carrots hama tree you're gonna leave the patient with a one diopter mistake after the cataract surgery so some different ways of getting carrots hama tree of course if you're doing a scan ultrasound for your biometry you're gonna have to get your carrots hama tree from a different method either an auto refractor or an auto carrot ometer or a manual carrot ometer yep so you get that meeting reading from someplace else and you plug it in to the a scan machine to do the final calculations the element the IOL master was great when it first came out because we found it actually does carrot AMA tree readings from six spots on the cornea and it measures a very fine area of the central corneal Zone where you know our central optical axis is about two point three five millimeter central zone as compared to manual carrots hama tree which measures only two points within a 3.2 millimeter ring so yeah the IOL master comes along and we've got much better carrot amma tree already then the lens star came along and now it's measuring from two zones a 2.35 and a one point six five millimeter circle and it averages those 32 points together to give you your carrot amma tree 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 a CD all of these years but with the IOL master and the lens star 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 it's it's got a critical role in those types of IOL calculations because if the intraocular lens implant is 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 one millimeter measurement error of anterior chamber depth you've introduced 1.5 diopter of a refractive error so the a scan ultrasound does measure you can you can see the anterior chamber depth with a scan ultrasound the IOL master used this is one of the measurements that the IOL master does where it actually uses slit imagery so just optical slit imagery light regular light to get this measurement and then the newer generation lens star 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 tends to make our anterior chamber a little shallower so that 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 a holiday formula and a formula called the Olsen formula and the Olsen formula just as an interesting aside has introduced a see constant they call it which uses lens the crystalline lens measurements to account for the true physical dimensions of the eyes 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 it the C constant - where in the capsular bag the IOL is going to have its final resting place lens thickness is measurable with a scan ultrasound and it is measure measurable with the lens star it is not a measurement that's picked up with the IOL Master 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 looking 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 by amateur 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 what your needs are you have to factor in things like the portability of the instrument for example and a scan ultrasound is really quite portable so you can move it from room to room or even clinic to clinic whereas a an IO L master or a lens star you know you're gonna have a lot of trouble moving that around you want to come that are 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 on an optical interferometer a lens star or an IO L master 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 weekend 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 IOL master was unable to actually get a reading through the dense cataracts so even if you do have an IO L master or a lens star 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 we have a minute so true or false you don't have to answer these out loud just maybe think about 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 lens star and the IOL Master those are examples of optical interferometry true or false is this statement true or false the IOL master is better suited than ultrasound for measuring axial length and eyes that have a dense cataract or other media opacity false IOL master 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