hello and welcome to Insight of thology I'm Dr Amrit welcoming you to another comprehensive lecture today we are going to talk about in detail the anatomy of limbus so what is limbus the term limbus basically denotes a border between two different types of tissues and when we talk about the limbus of the eye we are basically referring to a border zone between the cornea and the Scara now why is this limbus important despite its small size the limbus actually excites our interest because it demarcates the optical clear cornea from the conjuntiva and opaque Scara it also maintains nourishment to the peripheral cornea now we know that our cornea is basically a vascular and the limbus is highly vascular and therefore through the limbus the peripheral part of the cornea receives its nourishment as you go ahead in the video you will know that the limbus actually is a side for and it actually contains the pathway for Aquis drainage internally it is also a side for various incisions in the Cataract and glaucoma surgeries and this is actually going to be the highlight of this lecture today the limbus also consist of the Palisades of V which actually Harbor these stem cells right so like any geographical border Zone around the Earthly Globe there is no uniform agreement about the precise boundary lines of the limit lus as well it is actually a very busy place and often quite confusing and therefore we brought out this video for you so well then in this video we shall be talking about the gross anatomy of the limbus with important clinical nuggets we shall try to understand the limbal Zone not just from outside but also from inside we will understand the term anatomical limbus the histology of the limbus the various external landmarks of the limbus what is meant by Surgical limbus and what is the relationship between various limbal incisions and the structures of the anterior chamber angle okay so let's get started I hope you will like this video so externally if you see if you have a look at the limbus it is basically formed by the junction between the corneal epithelium and the conjunctival epithelium so actually speaking it is a gradual transition at the limbus from the stratified non- caratini cinar epithelium of the conjunctiva to the stratified non- kenite squamous epithelium of the Cora and if You observe carefully the limbus is actually about 7 to 10 cell layers thick okay now the limal epithelium If You observe carefully here is actually thrown into these various FS with intervening tissues of subconjunctiva okay so the cells which are present at the basil part of of the epithelium or in the basil layers of the epithelium of the limbus they seem to be actually stacked on one top of each other right now this stacking pattern of the cells is known as Palisades and in limbus this is known as the Palisades of w right now another important point to remember over here is that the presence of Palisades of v and their quantity is quite inconsistent and it varies from one person to another right now the Palisades of vog they are important because they actually contain the coral stem cells which have regenerative capacities and they help in the corial wound healing processes the Palisades also consist of some amount of pigmented cells which are known as the melanoides and these melanoides are actually responsible for the brownish appearance of the Palisades that you see in the second picture over here in our video on the anatomy of Cora we already discussed about the XY Z hypothesis of the limbal cell proliferation but for your benefit I shall summarize that now the cells basically are proliferating in this uh Crypts okay and as they start proliferating from the basil layer they start stacking on top of the each other and this stratification of the basil cells on top of each other basically is set to happen along the xais now after they have multiplied these cells will start migrating centripetally towards the cornea and this happens along the Y AIS and now as the cells start growing moving centripetally and stacking on top of each other the older cells basically will start to Slough basically from the U from the cornea into the T and this actually happens along the Zed axis okay so that is your XYZ hypothesis of coral cell proliferation now before we understand the anatomy of limbus in detail it is very important to understand the concept of scleral and conjunctival overlap over the cornea okay so first let us try to understand this diagram here because using this diagram I'm going to try to um um explain the various landmarks of the limbus as well when seen from outside the limbus seems to be basically the junction between the epithelium uh between the coral epithelium and the conjunctival epithelium that you can see over here right so this is the conjuntival epithelium this is the epithelium and the limbus is basically the junction over here you can actually note that the conjunctival epithelium is actually overlapping the cornea so the cornea is up to here whereas the cornal epithelium is only till this level and the conjunctiva along with the substantia propr that the subconjuntival tissue is actually overlapping your cornea considerably right so that marked over here is the substandard propria so there the gray color part is a Scara this is the cornea the Scara basically forms an internal Groove okay and this internal Groove is known as the scleral sulcus now the scleral sulcus ends posteriorly with the spur or projection which is called a scleral Spur now the sulcus is basically covered by this green color triangular tissue in this cross-section okay so that is your tabular measure and external to the trepic mesh work is your Slim's gal right so the trapic mesh work basically covers your scleral sulcus and converts it into a slems canal so that is important now let us go further inside and try to understand the limbus from inside before that you can observe over here this yellowish line so that yellowish line is nothing but it is the Bowman's membrane so I hope by now everyone knows what are the layers of cornea if not I'm going to provide the link to anatomy of cornea in the description box now we know that the epithelium basically sits on the basement membrane which is known as the Bowman's membrane and this over here in yellow color is the Bowman's membrane then we have the stroma and then we have the desmet's membrane the termination or The prominent posterior end of the desperate membrane basically in the cornea is actually known as the Schall Base Line right then you can see from there what is beginning this is the travic mesh work right and it is very important important to note that the coral endothelium does not continue over the tricular measure workk so that is very important point and that also brings you to a clinical nugget that we'll discuss is that during the embryological development the tricular mesh work is also covered by the endothelial lay which is uh confluent with the coral endothelium it is only at the late in the latest stages of the third trimester when this membrane starts to getting when this membrane starts retracting back towards the cornea okay now what really prevents the cornal endothelium respect this border is not really understood but occasionally what happens is that the coral endothelium will actually start to grow towards the tabular mes workk and then what happens is this endothelial cells of the uh cornal EP Coral endothelium which are growing on the tripic mesh work they basically are replaced with a more epithelial like cells and these epithelial like cells have this migratory characteristics they start migrating towards the tricular measure work and they don't just limit to tular measure work they also migrate on the surface of the iris right apart from that they have this contractor properties so now when they're going to contract it will lead to formation of the holes in the iris it will actually pull the iris forward and closing this angle leading to the sin formation this is called peripheral anterior sin and ultimately it can lead to the secondary angle closure gloma right so so proliferation of these similar endothelial cells has been described in the ocoro endothelial syndromes also known as the I syndromes it is also seen after tabular trauma and it is also seen sometimes after the laser tabic plasty let us talk about a very important topic of discussion and that is the difference between the anatomical limbus and the surgical limbus okay so first let us talk about the anatomical limbus the pathologist basically Define a li the anatomical limbus as a block of tissue which is bordered anteriorly by a line joining the peripheral extremes of the Bowman's membrane and the desmit membrane so as you can see over here drawn in orange color is your Bowman and drawn in green color is your Des Smith's membrane right so a line joining these two will form the anterior border of the anatomical limbus and the posterior border of the anatomical limbus will be basically formed by a line which is drawn from the scaros spur and this line should be perpendicular to a tangent which is drawn at the external surface of the globe okay now this makes the limbus basically an oblique transition zone right now if you carefully observe these two pictures you can see that externally the the width of the limbus is different in these two pictures so what what I'm trying to convey with these diagrams is that we know that the conjuntiva basically overlaps your cornea to a significant extent and and at the termination of the conjuntival epithelial overlap the Bowman's membrane basically is terminated right now what exactly happens at 12:00 and at 6:00 meridians that is at the vertical meridians of the eye what happens is that the conjunct I extends more anteriorly over the cornea leading to an earlier termination of the Bowman's membrane now since the Bowman's membrane is ending prematurely in the upper and the lower quadrant of the eye therefore the border of the cornea the width of the limbus is also more in the upper and in the lower quadrant of the eye okay so in the upper border that is 12:00 and 6:00 the limbus is broader compared to the nasal side and the temporal side that is in the horizontal Meridian now let us talk about the histology of the lympus now at this point I would like to tell you about the various changes that occur at the limbus the first change occurs with respect to the epithelium we know that the coral epithel ium is basically stratified non-keratinized sorous epithelium that epithelium changes to the uh cinar stratified non-keratinized conjunctival epithelia what about the Bowman's membrane which is sitting right next or below the corneal epithelium the Bowman's membrane will basically terminate at the lipus what about the substantia propria the substantia propria of the conjuntiva over here you can see it is basically fusing with the conun I it is f fusing with the episclera and the tenons capsule right so substantial propior fuses with the episclera tenons and conjuntiva at the limbus and this actually provides a very firm grip if you hold the eyeball using your forceps during the surgery right so it helps in stabilization of the globe apart from that the regular corneal collagen fibers which are present in the stoa of the cornea they change into the irregularly arranged scleral collagen fibers the desperate membrane is also going to terminate over here and the end of the desate membrane in the cornea is known as the Schwalbe line the corneal endothelium also terminates over here and at the limbus now it reaches the tabular mesh work and it basically wraps around the tricular meshwork beams okay so what I tried what I'm trying to tell you over here is that the cornal endothelium does not blindly covers the tricular mesure work because if it does that there will be no filtration occurring through the tricular measure work right instead the endothelium is going to wrap around those uh perforations or the beams that we have in the tabular mesh a little bit more about the collagen fibers so in the cornea the fiber size is less it is about 700 anrom the fibers are regularly arranged and because of this regular arrangement we basically get that blue appearance of the cornea or the transparent appearance of the cornea whereas if you look at the scera the Scara basically has collagen fibers which are about 700 to th000 angstroms and they're irregularly arranged and this makes the Scara basically opaque again one more point that I would like to you to observe is that the scleral fibers are actually extending more anteriorly EX on the external side compared to the internal side of the limbus that means that Scara is basically also overlapping the cornea along with the conjuntiva now if you look at the interface between the Scara and the Cora it is not a straight line instead it is actually forming this curvy linear diagonal because of the conable overlap of the Scara over the cornea right and the importance of this curv linear diagonal is that because of the curaline diagonal that you have these zones where the cornea and the sclera coexist and there will be a different there will be a differential scattering of light that occurs because of this overlap because of the presence of the curval linear diagonal interface between the cornea and the Scara and this is something which is responsible for the appearance of the surgical limbus so we'll talk about the surgical limbus in a while right the same can be observed here as well on the anterior segment OC you can see that this is actually your cornea the cornea Lam being regularly arranged they do not reflect the light much and therefore they appear hypo reflective compared to the Scara and this over here the whsh part is actually the Scara and over here this is the iris right so you can see this exactly here this line which is ending is basically a shoulder like however the most consistent Landmark that you find on anterior segment OC is basically your scar spur so you have to trace basically this is the iris and this ex this is where the Scara is actually changing its curvature so this is your scleral spur and from the scar spur then you can actually uh trace your other structures at the angle example your tabular measor so tabular mesor basically looks like a triangular structure anterior to the scaros spur on the anterior segment o right so moving ahead let's talk about the cells at the limbus the important cells of the limbus are basically present at the Palisades of work that we discussed and also they are located in the anterior part of the tricular measure workk so over here somewhere here you have some important cells residing now these cells the tular cells which are present in the anterior part almost about 60% of the newly dividing cells are concentrated in this most anterior region of the tabular measure right these cell actually have a phagocytic capability and in fact they can actually detach themselves from the tricular beams in a very macras like manner whenever the eyeball is exposed to foreign material right or whenever tular measure work is exposed to foreign material right now as they as they basically um detach themselves from the tubular beams this denudation of the tubular beam actually is a contributor to the outflow obstruction that occurs in some form of glaucoma now the these uh these denotation of the cells or this Detachment of the tabular cells and they converting into phagocytic cells has also been seen post laser trepic plasty so post laser tripic plasted has been seen that these cells will basically detach themselves and they will become phagocytic and then these dividing cells will migrate to the laser burn areas right so that is one important point that you must remember so now let us talk about the surgical limbus so what exactly is surgical limbus so don't worry we will be talking in detail about the surgical limbus when we talk about the limble boundaries and landmarks in a while but to make things easier as you can see in the first picture we have this Blue Zone which is present all around the cornea okay now this Blue Zone which is present around the cornea is known as the surgical limbus okay now we know that the cornea is also it has some amount of conjuctival overlap right now because of that conjuntiva present over the cornea sometimes it becomes very difficult to appreciate this Blue Zone but in most of the cases after you do the uh per peritomy that is the reflection of the KCA from the cornea the Blue Zone becomes quite appreciable okay the second picture is that of a trap and that something that I'll explain to you in a while now what about the limbal boundaries and landmarks okay so to understand that let us take this diagram so you can clearly observe that we basically have this Zone over here around the cornea which is called the Blue Zone and which is which can also be called as the surgical limbus right now around that Blue Zone you can see another Zone and this zone is known as the white zone now what are the boundaries of these zones so just at the junction over here of the cornea and the Blue Zone you have the anterior limble border then you have another line over here between the Blue Zone and the wide Zone which is referred to as the mid limble line now some people like to call it as the posterior border of the surgical limbus right it quite makes sense because Blue zone is also a surgical limbus so some people try to call it as the posterior border of the surgical limbus okay but here we would like to call it a mid limble line then we have the posterior limble border which is the posterior limit of the white zone so in this diagram you can actually observe this Blue limble Zone okay so this is is the Blue limble Zone then over here you can observe that actually uh the phenomena of sclerotic scatter so if you don't know what is sco scatter we have a video on the slit lamp examination technique so I would put the put down the link in the comment in the comment section or maybe in the description box okay so you can check that out so the scerotic scatter is important uh because it helps us in seeing the white zone okay so blue zone is quite visible as you can see over here but the white zone is is difficult to appreciate because the Scara itself is white in color right so if you carry out the sclerotic scatter the posterior limit of the sclerotic scatter basically forms the posterior end of your white zone okay so using sclerotic scatter one can actually appreciate the wide zone of the limbus now cner basically popularized the correlation between these external landmarks of the limbus and the internal structures of the angle and understanding these landmarks is immensely important because it helps the surgeon in planning the incision during various types of intraocular surgeries okay so let us see how so over here we have the anterior limble border and you can see that the anterior limble border basically corresponds with the termination of the Bowman's membrane the mid limal border which is the posterior part of the surgical limbus basically correlates with the termination of the desmat membrane now at this point we can see that the the Blue Zone which is here is basically a Zone which is formed by the termination of The Bowman membrane and the termination of the despit membrane and is basically consisting of what it is consisting of the cornea and as you reach the desate membrane as you go posterior part of your Blue Zone you have slight amount of overlap by the Scara as well then the posterior limit of the wide zone or the posterior limble border is basically corresponding to the scleral spur okay so you might want to pause the video over here and put these points in your head clearly okay you might want to repeat it that the anterior limble border corresponds with the termination of the Bowman's membrane the mid limble line corresponds with the termination of the Desmet membrane and the posterior limble border or the posterior limit of the sclerotic scatter basically corresponds roughly with the scleral spur okay so again in this picture after you can see that in the first picture there's Cataract and probably the patient is undergoing cataract surgery after the limbal peritomy you can see that this is the Blue limbal Zone over here okay so the Blue limbal zone is basically a bluish translucent zone between the anterior limble border and the mid limble line This is the anterior limble border this is the mid Liber line okay and this corresponds to the part of the cornea between the termination of the Bowman's membrane and the desid membranes as I explained to you okay it can be seen after the conjuntiva and tons have been reflected after a perido okay now the extent of Blue zone is basically quite variable it's variable because of the variable overlap of the cornea with the conjuctiva as I explained to you previously okay so the Blue limble Zone in the superior quadrant is about 1 mm in the inferior quadrant is about 0.8 mm in the temporal and in the nasal quadrants it is about 0.4 mm and it is very important for you to know that the Blue zone is nothing but your surgical limbus okay so this diagram actually represents the various extents of the Blue limbal Zone in different quadrant so you can see superiorly the limble zone is broader the question is why do you see surgical limbus as blue in color as I told you the interface between the cornea and the Scara is not straight instead it's curv linear diagonal and that causes a differential scattering of light to that oblique interface leading to U giving it an appearance of bluish grayish or bluish translucent appears right now the first picture is that of a tectomy surgery so you can see over here that a scar flap is basically being made and as the flap is basically inverted you can see the blue part or the Blue Zone more clearly right now basically why is this happening we know that there's a considerable amount of scleral overlap over the Cora right and in the surgery as you actually raising this flab you raising the ceral flab the blue cornea which is underneath is becoming more apparent or visible right and this is exactly the diagrammatic representation of the same so you can see the flap has been raised and the Blue Zone becomes more prominent here so this is your Blue Zone about 1 mm and that is the white zone about another 1 mm now it's very important to understand the relationship between the limble incisions and the structures that you're going to cut in the anterior chamber angle okay so let us try to understand them first let us talk about the incision at the anterior limble border okay so when the incision is given at the anterior limble border that means that you are basically trying to give the incision at the end of the Bowman's membrane over here right that means you are if you continue that incision you will enter your anterior chamber and this incision will be quite anterior to the termination of the desate membrane which is over here and you're going to pass straight into the anterior chamber passing through your Des membrane right so I hope that is clear now since the incision is actually passing through the desate membrane there's actually a risk of desm membrane stripping and Detachment and this basically applies to all the incisions that are actually taken um at the anterior limble border or even the incisions which are taking in the Blue Zone Okay the reason is because even if you take an incision say this is the point this is called the mid limble line or the posterior border of your blue zone right we know that corresponds to the end of the desmid membrane right so idly speaking this is bman this is desmet's ending and this is your blue zone so anywhere here if you take an incision you are passing to the desm membrane and there's a risk of desm Detachment right so these incisions are called as the anterior limbal incisions which are basically taken within the Blue Zone now it is talk about another incision and this incision is taken at the mid limble line This is called a mid limble line incision or an incision at the posterior border of the surgical lbus here also when you take an incision you will enter the anterior chamber so this is the anterior chamber which is present between the cornea and the iris okay and here the entry is actually uh through the ending of the des's membrane however this entry you can see is anterior to all the angle structure so this is your tricular measure this lenss canal so when you enter like this all the uh structures of the angle are basically posterior as a matter of fact the Slims Canal is about 0.5 mm from this mid limble line and since the structures are posterior to your incision this entry is actually considered a safe entry into the anterior chamber right so that is your mid limble line incision or a posterior border of the surgical limbus now what about if we take an incision at the posterior limal border or the posterior limit of the sclerotic scatter here don't get confused between the posterior border of the Surgical limbus and the posterior limbal border which is the posterior limit of the sclerotic scatter okay so here we're trying to talk about the posterior limit of the wide Zone which is the which is the posteri limit of the sclerotic scatter as well so since I told you that that posterior limit of the scerotic scatter basically corresponds roughly to your scal spur and when you actually cut internal when you actually give an incision here you are somewhere opening at the scaros spur and sometimes even behind it and in such a condition you will not actually enter your anterior chamber and instead you will enter the supraciliary space okay which is located about5 mm behind the scleral sper okay so that is something which is very important that at the posterior limit of the scero scatter if you take an incision and make an entry into the eye you will enter the supraciliary space which is a space located above your sary body now at this point I would like to ask you what is meant by a clear Coral incision okay so as you can see over here the incision is actually given anterior to the anterior limble border so this is your anterior limble border okay and if the incision is given anterior to the anterior limble border that is called a clear Coral incision something what you can see over here in this surgery the blade is being entered somewhere here okay so this is actually your F incision so fitions are usually clear Coral that means you are entering anterior to your termination of the woman's membrane right the only problem with these uh incisions is that if not performed carefully there's a greater risk of desmat Detachment and also they induce high amount of astigmatism because the incision is right away into the cornea now as a rule the more anterior the incision is the more chances of getting astigmatism the more posterior and incision is the more chances of bleeding but less chances of fastic matism now what about the wide Zone incision so here I'm trying to talk about about an incision which is taken between the mid limble line and the posterior limit of the scerotic scatter so the mid limble line corresponds to your desm mits and the posterior limit of the scerotic scatter corresponds to your scar spur so in between that is your white zone so if you take an inition at this level basically you are going to enter the tular me you are going to basically enter the anterior chamber passing through the tricular meas work right so white zone inis will basically cut through the travic mesure work into the anterior chamber so I hope that is clear now there are another important landmarks also in relation to the surgical limbus that you must remember surgical limbus is nothing but your Blue Zone and it is B it is basically bordered by the anterior limber line and the mid limble line right now first let's talk about the root of the iris so root of the iris is nothing but where the iris attaches to your Scara the root of iris is basically situated at a distance of about 1.5 mm from the surgical limbus so your surgical limbus will be somewhere here that is about 1.5 mm so if you have an iodo dialysis and you want to repair an iodo dialysis okay then in those cases the suture has to be the iris basically has to be sutured at a distance of about 1.5 mm from the Blue limble zone right so you can see over here because the root of the iris is 1.5 mm so this is an AO dialysis you can see a suture double arm suture has been passed through the iris and it has been sutured under a flap onto the scera at a distance of about 1.5 mm so that is one important Landmark that you must remember another important Landmark is that of a celery sulcus first you should know what is meant by a celery sulcus the celery sulcus is basically a space between the iris and the uh Parts P part of the selary body so This Groove over here is known as the celery sulcus this illustration is that of an AB internal iOS fixation the iel is basically being fixated with the Scara so you can see that the haptics are tied with the sutures and then the sutures are being brought out from inside to outward Direction and that's why it is known as an AB internal scal fixated iell okay now the question is that at how much distance from the limbus are you going to bring your needle or your sutures out so that the I is nicely secured in the cery sulcus right now this was actually answered by this paper I would actually give the link in the description box now this paper actually suggested that whenever you are carrying out an AB internal cery sulcus fixation the suture uh which is fixing the eyel and if you're using a straight needle the needle basically should emerge around at a distance of about 2.5 mm from the mid limble line or the posterior surgical limus now this paper act suggested that although the celery sulcus which is here it is located at a distance of about 0.4 to 0.5 mm and the width of the par pet is another 2 mm okay so it is you might actually think that you will just insert the needle over here but no that doesn't happen because this is quite a vascular area you don't want to damage the delicate vasculature which is present here and cause bleeding instead what we do is we basically use a straight needle and the needle basically comes out through the celery sulcus like this okay so as the needle comes out from the celery sulcus traveling through the Scara and out of the Scara the distance between the limbus and the emergence of the suture or the needle when using a straight needle is about 2 to 2.5 mm from the mid limber line or the posterior surgical line okay so that is one very important thing that you must remember and if the suture is being tied U or if the I is being fixated using the P planner route the P plan situated at a distance of about 3 mm from the posterior surgical limbus right now over here there's quite some disparity some books say that it is 1 to 1.5 however I found that in the article and what I have learned at my Institute it was 2 to 2.5 mm so do comment in the comment section and tell me what do you follow at your institutes okay the distance of the C sulkus from the posterior surgical limbus right so basically to summarize if you take an Inc at the anterior limble border you will be basically entering through uh at the end of the Bowman's membrane through the desmids into the anterior chamber if you take an incision at the mid limble line you will be entering again the AC but here you have the termination of the desmet's membrane chances of desm stripping are more in these cases the angle structures obviously located in the white limble zone that means behind the mid limble line or behind the Blue Zone and if you make an insert uh an incision at the posterior limit of the scerotic scatter you will insert your needle or insert your incision or blade basically at the scar spur or near thecar spur and in this case you will basically be entering your supraciliary space and not the anterior chamber now if you want to carry out the iris uh repair Iris dialysis repair or idro dialysis repair that is done at a distance of about 1.5 mm from the mid limble line if you want to enter this space which is the cery sulkus using a straight needle the distance would be 2 to 2.5 5 mm and if you carrying out a parts plan of fixation of the I that will be about 3 mm from the mid limble line okay so that was all for today I hope it was useful thank you and have a nice day if you liked our content make sure to subscribe to our YouTube channel also you can actually join our YouTube memberships and I often get a question as to how do I join the membership house so let me 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