Transcript for:
Understanding Synovial Joints and Osteoarthritis

you'd think after recording all these videos for youtube that teaching by video live from the lab to students would be um easy or satisfying it's not it's still a pain oh well uh students are back we're back in the lab we're doing things here's another video today we're going to talk about how did the patella get around there we're going to talk about synovial joints these are the joints in the body that give a wide range of movement and i'm going to talk about the general structure and parts of a synovial joint i'm not going to talk about the various different shapes and classifications and that sort of thing this is very much how you make a synovial joint i probably will talk far too much about articular cartilage because i used to be an articular cartilage biologist before this job in fact during part of this job as well and we'll talk a little bit about osteoarthritis because that is the main disease of the synovial joint [Music] yes i know rheumatoid arthritis is a big thing as well but indulge me osteoarthritis used to be my my thing um okay so a synovial joint is a joint that allows [Applause] a good range of movement or a lot of movement so some joints in the body think about the sutures of the skull allow no movement and some joints allow a little movement and some joints allow a great deal of movement so the synovial joint has a number of structures within it which make that possible it's a it's a joint that's mobile but of course it has to be held together because it's also mobile there are a number of challenges in here and essentially we have the ends of bones here we're looking at at the knee so we have the femur and the tibia and the bones themselves tend to be shaped in such a way so that they articulate in such a way so that the the knee for example is a hinge joint it does rotate a little bit but generally it doesn't rotate it doesn't make any other significant movements it's it's a hinge joint now the bone is covered by articular cartilage the um hole joint is surrounded by synovial capsule we've got some cling film for this and that creates a potential space and inside that potential space we have synovial fluid and all those things combined give us a low friction joint which in some cases is very good with standing compressive loads that's it in a nutshell did i do a nutshell for once that's unusual okay so if we look at the knee joint and forget the patella now what we can see here really is probably the bone and the cartilage has been stripped away certainly that's what we see when we look at a skeleton because the bone survives and the cartilage doesn't so this looks nice and shiny but articular cartilage then is it's like hyaline cartilage blue tape this is going to be this is going to be articular cartilage the articular cartilage then covers the articulating surfaces of the bone which i've chosen bare tape now you know maybe actually once i should actually rehearse something that i do instead of making it up on the fly but who's got time for that okay so i'm just going to do one surface something like that but so the the articulating surface of the bone and you can see that it actually curves all the way around so it's not just the bit you stand on but it's the whole bit that you move anyway articular cartilage covers the articulating surface of the bone and articular cartilage is made up of largely type 2 collagen like hyaline cartilage there are other collagens involved like type 9 and type 11 collagens that help manage the fibril diameter of type 2 collagen and also link those collagen fibers together so what you get is kind of like a mesh weave bag of collagen in your extracellular matrix in your cartilage and then also inside that articular cartilage we see proteoglycans particularly agrican with lots of glycosaminoglycans now what the proteoglycans do is they pull in water and hold on to water there's lots of hyaluronan in there and what you've then got is you've got a mesh bag holding in all those proteoglycans holding on to all that water so you've got a material that is then very good withstanding compressive loads and it does deform a little bit and you also produce this very smooth friction free articulating surface so the joint can move smoothly and easily inside the articular cartilage there are cells maintaining that extracellular matrix all of those collagen proteins and proteoglycans and what have you and those cells are chondrocytes and there aren't very many chondrocytes they're spaced out inside the extracellular matrix they um they sense the their surroundings and they react to changes in their surroundings so for example you know the chondrocytes will manage the extracellular matrix they'll they'll make the collagen they'll make the protein glycols and what have you to make sure that the articular cartilage is is healthy and there's a balance between the rate at which the articular cartilage breaks down and the rate at which is is repaired and normally that's kept in balance and you have good nice thick healthy articular cartilage and in fact the chondrocytes are very good at responding to load so there's an old wives tale that you know runners will wear their knees out this isn't true there's lots of evidence lots of studies that show that in fact runners have very healthy articular cartilage so the cells respond to the repetitive loading and they make thicker cartilage they make better cartilage to respond to that just like your bone responds to load just like your muscles respond to load the trick comes when there's an imbalance in the joint and then you get very very high loads in one area where you wouldn't normally expect very high loads and that causes problems here's a cool fact articular cartilage has no nerves in it and it has no blood vessels in it which means that pain from articular cartilage degeneration must be caused by another component in the joint and it means that the nutrients must get to the articular cartilage through another root and the metabolites must be taken away similarly i wonder how it turns out articular cartilage has a progenitor cell population in its surface so when you lose the surface of your articular cartilage you lose those progenitor cells that can make more chondrocytes so that's an issue anyway articular cartilage right the synovial capsule surrounds the joint and completely separates it from everything else so the synovial capsule it's actually made up of two layers it's got an outer fibrous layer so it's a it's a fairly tough capsule and it's continuous with the periosteum covering the bone and it has an inner serous layer and that in a serious layer is largely responsible for creating synovial fluid so when you look at diagrams in textbooks you tend to see like this is all pulled apart and there's a gap between the two bones and what have you and there's a big big blown up synovial cavity that's not what it actually looks like there's no gap between the articular cartilage of the tibia and the articular cartilage of the femur of course those two are pressed together and the whole thing is wrapped in this and cased in this synovial capsule so then the potential space that's inside there is filled with synovial fluid so the space in the joint it's kind it's more of a potential space than a real space there's only a little bit of synovial fluid in there and this is input so the synovial fluid is a lubricant and you need the synovial capsule then to keep all that in place otherwise your snow your fluid is just going to leak out it's not going to work do you see but maybe maybe it's the synovial capsule and the nerves there that cause some pain anyway um the snowball capsule is sometimes thickened in places forming ligaments helping to hold the joint together and keeping it stable in whichever way it needs to be stable in some places it's thinner so synovial fluid then so inside here if you were to pierce through the synovial capsule some of your fluid would leak out and it would be very um very very oily like it's a very good lubricant it's very very slippery and i know this because i've done this a lot i've taken um bovine joints apart to get the cartilage and culture the cells and and do various research and i've got i've got a very nasty scar on on that finger where um i slipped when i was dissecting because the um the synovial fluid once you've pierced the synovial capsule it is really it makes everything really really slippery it's a lubricant it's viscous it's got hyaluronan in it and lubricin it's the synovial fluid inside the synovial capsule which is responsible for carrying nutrients to the articular cartilage and carrying metabolites away from the articular cartilage the other thing to remember here is that the chondrocytes of the articular cartilage they're not very active they're not very busy so they don't have a great metabolic requirement but that's the synovial fluid apparently synovial fluid is also a non-newtonian fluid you know like custard right the rate at which you load it affects its mechanical properties if you load custard slowly it's it's fluidy if you load custard hard it becomes hard if you load it fast it becomes hard right um rheologists will explain it much better than i did but so so i think the synovial fluid can also um be involved in resisting compressive loads but those are the main important components of a synovial joint the articular cartilage the snowville capsule and the synovial fluid they all work together with the specially shaped joint surface to give you a freely mobile smoothly articulating joint which you kind of take for granted until they don't work anymore and that's the challenge here isn't it is creating a joint that's freely mobile may be restricted in one or two ways but also strong enough and stable enough to stay in place if you're making a fixed joint like a suture and a skull that's pretty easy but with synovial joints it's very difficult if you think about the glenohumeral joint and the wide range of movement there because that joint is actually very shallow it then becomes a real challenge for the body to hold this heavy upper limb into the glenohumeral joint as you move around so it's a it has it's a joint with a wide range of movement particularly when you add the the shoulder girdle into it but it's also quite a weak joint whereas the pelvis the hip joint has a kind of a reduced range of motion but it's a very strong joint and the knee is somewhere in between the two the knee has to take all of our body weight um and so the synovial capsule running around it is strengthened by ligaments but the knee also has it has ligaments running from bone to bone outside it it has ligaments running from bone to bone inside it the cruciate ligaments and then it also has all the muscles think about all of these muscles that cross the knee joint they have their big muscles with big tendons because they have to raise our entire body weight but also the muscles themselves are very important in stabilizing the the knee joint so if you have a knee injury it's really important to consider these muscles and are they balanced and are they strong and what have you right so muscles and ligaments work together to stabilize the knee joint or to stabilize any joints and hold it together and the other thing we see in the knee is we see these miniski now a meniscus is a like it's a fibrocartilaginous it's not like perfect hyaline cartilage it's a bit more fibrous than that um and this is kind of like a semi-circular disc that is because we've got these articulating surfaces here and this disc is um kind of thin in the middle and thicker outwards and the wedge then helps stabilize the joint even more so you need all these things to work together to have a a good stable joint and what we tend to see is that it's not wear and tear that causes osteoarthritis it's when a joint is not square anymore when it's not loaded normally and osteoarthritis is a degenerative disease of the articular cartilage within the joint and it has a hereditary aspect there's a genetic component to it and it's also associated with with with people who you know work moving with manual work and that sort of thing um kneeling down spending a lot of time on your knees is bad it is likely to cause osteoarthritis the degeneration of the articular cartilage so it's not running the wears out the articular cartilage it's when you've had that injury if it had that tweak or imbalance and you start loading the cartilage inappropriately and one bit has a high load then you're likely to damage the cartilage but what we see in osteoarthritis is i said there was a balance between the degradation rate of the articular cartilage and and it's repair its restoration it's when that gets out of balance and there are a whole bunch of pathological things that happen here but the rate at which the cartilage is destroyed is greater than the rate at which the cartilage is repaired and we see some changes in in the cartilage as well so over time the articular cartilage gets worn away and it may get worn away all the way to the bone for decades well we've been saying for a long time or in 10 years time there'll be a there'll be a method of fixing this because there are so many potential treatments out there but it turns out it's a really hard biomechanical problem to solve and still i think the best way of fixing it is to actually replace the whole joint with a prosthetic joint which is what the orthopedic surgeons do my dad's had his hip done my dad's got osteoarthritis so i'm likely to develop osteoarthritis as well because of the hereditary component and it's not just in the big joints it often starts in the thumb and the fingers and other joints around the body and so what's the currently recommended treatment for osteoarthritis it is keep using the joint not my recommendation this is this is the british this is the nhs the nice guidelines this is the recommendation keep using the joint and manage the pain because if you keep using the joint um then it seems to well the cartilage seems to last for longer um of course when you're the thing about these synovial joints is that when they stop working they they cause a whole bunch of other problems because they limit people's activity and when you limit someone's activity not only does it have a knock-on effect to their working life but also to their to their weight because people usually put on weight obesity then causes exacerbation of osteoarthritis and with obesity of course we have all these other health problems as well so with osteoarthritis it's really important to try and help that joint last as long as possible and also to stay active for the rest of your body but that's it the synovial joint is is those structures osteoarthritis is a disease of the articular cartilage i did didn't i talked too much about um articular cartilage and osteoarthritis that's still pretty good for me um anyway right okay so uh synovial joint which incidentally is also known as a diarthrodial joint is made up of articular cartilage synovial capsule and synovial fluid and it gives a a freely moving low friction um joint all right okay so when you're looking at any joint in the body right lots of these things consider the shape of the joint and the synovial joint how it how it's structured around it how it might be injured and that sort of thing all right good see you next time [Music]