Chapter 9: the joints. In today's lecture, we're going to cover rheumatoid arthritis and why this disease can strike people at a young age. We'll talk about slipped discs and how they don't actually slip, and what their link is to lower back pain. We'll talk about labral tears and why this can increase the risk of dislocations to that join.t. And we'll talk about the unhappy triad-- I do not cover this in the historical fashion, I am going off of the most recent scientific information that I have read, so it may be different from your textbook. So to do all of this, we will classify joints. You know we're anatomists, we love to make lists of things. We'll make a list of the different types of motions, and then I'm going to cover a few of the specific joints. I'm going to focus us on just the ones that are most frequently injured and therefore in need of care. And then we'll talk about some of the major types of arthritis, a disease that affects joints. Joints can be classified based off of both of their structure and their function. Luckily for us, there's a significant amount of overlap between these two lists of three things. Fibrous joints are also synarthrosis. Most of our cartilaginous joints are amphiartheroses. And our synovial joints are diarthroses. Most of the joints in the axial region of the body are synovial joints, whereas most of the joints in my skull and thorax are either synarthrosis or amphiarthroses. Let's start with a little bit of terminology. A joint is where two or more articulations are located, and an articulation is where two bones connect. Frequently, a joint and an articulation are the same thing. It really should not be a big deal throughout the rest of the lecture. The structure of these joints will determine how movable and how strong they are. One way to classify joints is based off of what's in between them. A fibrous joint contains dense connective tissue between the bones. There's not enough of it to really determine whether it's dense regular or dense irregular, it's a lot of collagen. Cartilaginous joints contain some form of cartilage that connects two bones together. And synovial joints are connected by a fluid-filled pocket called a synovial cavity. We can also classify joints based off of what they do. Synarhtoses are joints that don't move. The sutures of the skull are an excellent example of joints that do not move. The slightly movable joints are called amphiarthroses, and the vertebral column is an excellent example of a number of amphiarthoses: each vertebra does not move much relative to its neighbor, but it moves a little bit, and because there's a lot of them this can add up. The diarthroses, on the other hand, are freely movable joints. These are all synovial joints, and there's going to be a number of different types of diarthroses. Oh but we're not done with lists yet! There's a number of different types of Synarthrotic joints. You will see a lot of sutures in the skull, but our teeth get their own special joint name called a Gomphosis. And i have a number of synchondroses, and this is where there is some dense cartilage anchoring to bones rigidly together. The amphiarthroses are slightly movable joints. They come in two basic flavors: those where the bones are connected by ligaments, and those where the bones are connected by fibrocartilage. Lastly, the diarthroses are the freely movable joints. These are where two long bones connect to one another via a capsule full of synovial fluid. Muscles can move one bone relative to the other bone in these types of joints. In your basic synovial joint, two bones will come together-- but we've left a little bit of articular cartilage we discussed that in Chapter six. A synovial membrane encapsulate s' the space between the two bones, and this is full of a little bit of synovial fluid which can act as a lubricant. Synovial fluid is a non-newtonian fluid, which means that when it's under low pressure it's a liquid, but if you put it under significant stress it gelatinizes and absorbs shock. There are a number of accessory structures which may or may not be inside of a synovial joint. In addition to the articular cartilages, some synovial joints have a meniscus of fibrocartilage that helps to provide extra cushioning. Some joints will have fat pads which are good at absorbing shock. Joints will have a number of ligaments that help to anchor one bone to the next bone and hold them in place. Joints will also have tendons-- more dense regular connective tissue, only this time it's anchored to muscles, and these can also help hold the joint in place. Lastly, some joints will have bursae-- or small pockets of synovial fluid that are found elsewhere besides between the two bones. These help to provide cushioning wherever ligaments and tendons would rub on a bone. For all of these structures that a joint has, it tends to increase the stability of that joint but reduce its range of motion. Conversely, joints that have a wide range of motion tend to not be very stable and are prone to dislocation. A dislocation is where the head of one bone pops out of the socket of the next. A partial dislocation would be called a subluxation. Hopefully you know what joint this is here, you should if you've done the bones lab. You recognize the letter U here for the ulna. So that's how we classify joints, we can do it based on what they look like, or what they're made of. Next up, let's talk about range of motion. And now we're focusing on the synovial joints, these are the only ones that have different ranges of motion. So we'll have another list of things for you to learn! Joints that can produce gliding motion are ones where bones slide past one another. Joints where there is angular motion are those where the angle of a joint changes as two bones move relative to one another. Joints that can circumduct do a similar motion, only across a central axis. Of the angular motions, the two most common are flexion and extension. Flexing your biceps, for instance, reduces the angle of your elbow joint, whereas extending that joint increases the angle. a few joints can hyper-extend. The head can flex forwards, extend back to its original position, and continue to hyper-extend backwards. Two more are abduction and adduction. There are different ways of remembering these, I like to remember abduction is what you do when you're abducted by aliens: your arms go away from the midline. Adduction means moving bones back towards the midline. Again, circumduction would be tracing a circle, whereas rotation is rotating around a central axis. These pictures can be a little bit confusing, so try it out on yourself. The elbow joint gets its own two types of motion called a supination and pronation. When you are holding a bowl of soup, you are supinating. If you were to pour that soup on the floor that would require pronation. In class I would demonstrate to you the difference between supination and pronation versus rotation. Move your hand, try rotating it, then try making your hand move in the same way only using your elbow joint rather than your shoulder joint. If you're using your elbow, then you are supinating and pronating it to cause your hand to do what looks like rotate. Just for fun, when the radius and ulna supinate and pronate, the two bones must criss-cross one another. Apparently Leonardo da Vinci wondered whether that led to a reduction in the size of the forearm when this happened. It turns out that the two bones also shift in position, so that the forearm remains the same size despite the fact that the two bones now form an X (which should be shorter than two bones parallel to one another). There are a number of other movements that the skeletal system is capable of doing. I'm not going to cover these as they involve joints that I'm not going to be talking about. Next up, we can classify synovial joints based off of the shape of the bones and how they come together. Here is a small list of those names that you should learn. I am NOT going to go through these, take a look at this picture, read the names, and look at the shapes of how two bones can come together to form something that would move one way or other way. The more that a joint can move the less stable that it is, and more prone to dislocation. On the other hand, the less that a joint can move it's typically more stable and less prone to injury. So that wraps up the basics of all of the joints. We talked about different ways to categorize the joints. and then we said of the synovial joints we could further categorize those based on their shape. Let's move to discuss some specific joints. First up are the intervertebral joints. The vertebrae articulate with one another at two positions: the bodies of each vertebrae articulate with one another in a cartilaginous joint, and the superior articular process of one vertebra articulates with the inferior articular process of the superior vertebra. Did you get that? If not go, back hit rewind, listen to it again. There's also a significant number of intervertebral ligaments that anchor one part of each vertebra to another. These ligaments contain a high amount of elastic tissue, which means as we bend the elasticity of these ligaments tends to bring us back into an upright position. I don't have any of the ligaments on models in the lab, but I do have models that show the intervertebral discs. These regions of fibrocartilage actually have two portions to them. There is an outer tougher annulus fibrosus, and an inner gooier nucleus pulposus. This organization is very useful, it's both tough, but also capable of adjusting for motion. A slipped disc occurs when the nucleus pulposus either bulges or ruptures through the annulus fibrosus. There are a number of different names for specific injuries to a vertebral disc, but all of them are considered slipped discs. I'm not going to go into any more detail on the names than that a slipped disc can cause pain if the nucleus pulposus bulges or ruptures and pushes on a nerve or the spinal column. Otherwise, it may not cause pain. What's important is that a slipped disc doesn't actually slip out of place, it stays put. It's just the inner gooier portion tends to move outwards-- either bulging or rupturing. If this puts pressure on a nerve or the spinal column, that can cause a number of symptoms, including pain or partial paralysis. On the other hand, it may not push on anything, and it would be asymptomatic. Here's an x-ray that's been contrast-enhanced on the left side to show where a slipped disc is impinging upon the spinal cord. This would cause significant symptoms in this patient, including but not limited to pain and paralysis. It turns out that most people who have lower back pain do not have any slipped discs, and most people that have slipped discs do not have any lower back pain or other symptoms. Nevertheless, lower back pain is a very very complicated condition to treat, therefore if somebody does have low back pain and they do have a slipped disc, that is an easy target for treatment. We will talk in much greater detail about pain in Bi232. It's a very complex subject. Their vertebral column can move quite a lot. Each joint between the vertebrae does not move much, we considered them amphiarthroses (or the slightly movable joints), but there's a whole lot of them. There are seven cervical, 12 thoracic and five lumbar vertebrae with joints in between all of those, and when you add up that much small range of motion, it leads to a fairly wide range of motion, including flexion, extension, hyperextension, lateral flexion, and rotation. Next up is the shoulder joint, or the more anatomically correct name is the glenohumeral joint. "Gleno" of course stands for the glenoid cavity of the scapula, where it connects to the humerus. This joint has the widest range of motion of any of our joints, and for that reason it's the least stable. It's a ball and socket joint, which makes it a diarthroses (or a freely movable joint). Think back to the glenoid cavity of the scapula. You may recall that it is fairly flat, it is not shaped much like a cavity at all. It would be very difficult to carry an orange around if it was on a plate, it would be a lot easier to carry that orange if it was in a bowl. So to make the glenoid cavity shaped more like a bowl, we've got a lip of fibrocartilage that runs around the edge. This deepens the socket, and it's known as the glenoid labrum. It's possible to do damage to this fibrocartilage, in which case it might get replaced with scar tissue, and scar tissue doesn't move around as much. So somebody who's had a labral tear is going to be at an increased risk for dislocating this joint. There's also a number of ligaments that help to stabilize this joint, as well as a number of bursa. I don't have these on models so I'm not gonna be testing on these in lab. Here's a little bit of application about what we've learned. When we do surgeries on joints, we often need to provide pain medication afterwards. However, we don't need that pain medication traveling to the entire body, especially not up to the brain where it could induce addiction. It was noticed in knee surgeries that if you modified an insulin pump to deliver an anesthetic just to the knee after the surgery, this provided adequate pain relief without the risk of addiction. Once a procedure has been okayed for one surgery, it becomes okay for all surgeries. And surgeons began to use this in shoulder surgeries. The problem is this pain medication is toxic to chondrocytes, and the cartilage in people's shoulders was destroyed after these surgeries unintentionally. This led to a condition called osteoarthritis, which we will be covering in a moment. Why was this procedure okay for the knees, and not for the shoulders? Well, that's a big question, I don't really know the answer. It's possible that the cartilage in the knees is a lot thicker and so was more resistant to a small amount of damage from this anesthetic, whereas the thinner cartilages in the shoulder were more susceptible to damage. Nevertheless, it's always important to really know whether something is safe before doing it. We should not jump to conclusions, and just assume that because it's okay in one situation that it'll be okay in another. The human body is just not that simple! It has ways of making your life miserable. Next up, when the ligaments or tendons of the shoulder become damaged, the dense regular connective tissue often gets replaced with scar tissue. Scar tissue is very similar to dense regular connective tissue-- it is composed of collagen fibers running in parallel. However, in scar tissue these collagen fibers are cross-linked to one another. This increases their strength, but decreases the range of motion. So when people have injuries to the dense regular connective tissue of the shoulder, this can lead to adhesive capsulitis, or frozen shoulder. It reduces the range of motion of the shoulder. Next up is the hip joint (or the coxal joint). This is another ball and socket joint, one that is more stable than the shoulder. The head of the femur fits into the acetabulum of the hips. There are a large number of ligaments that completely surround this joint, which provide for added stability but reduces the range of motion. Let's move on to the knee joint, which suffers more injuries than the hip joint. This is a hinge joint, meaning it has a lower range of motion than the hip, and yet it is more susceptible to injury. So that's something we're going to need to understand. There are a number of articulations in this joint: the femur and the tibia connect, but the patella is also here, so we could discuss the femur- patellar articulation, or the patellar- tibial articulation. Let's remove the patella for a moment, and take a look at some of the internal structures. There are a pair of fibrocartilage rings called the lateral and the medial meniscus. These provide extra strength to this joint, which is important because the knees are under a lot of stress. They not only have to hold all of our body weight, they need to hold any weight that the arms are carrying at the time. It's possible to lock the knees, to jam the femur into these menisci, to hold this joint stable without using as much muscle force. This is not a good idea to do long term, we'll understand why that is next quarter when we talk about the circulatory system. The word meniscus means Crescent. These rings of fibrocartilage are partial rings. There are a few other small meniscusses throughout the body, the only one that we will discuss are those in the temporomandibular joint up in the jaw. There are also a number of ligaments which help to stabilize this joint. These are fairly skinny ligaments, and do not completely surround the joint cavity. Interior are the posterior cruciate ligament, and the anterior cruciate ligament. These two ligaments criss-cross. Next up are the lateral collateral ligament, and the medial collateral ligament. I knew that the first one was the lateral collateral ligament because it was anchored to the fibula. You may also call it the fibular collateral ligament, and the one over here the tibial collateral ligament. Just don't put both words. In there there's also a number of other ligaments ,which I'm not drawing here because they are not represented on the models in the lab, and therefore are not on the lab exams. There's also this big tendon right here, the quadriceps tendon. Sometimes books will break this up into two structures, and call it the quadriceps tendon from upwards down to the patella, and then down below they call it the patellar ligament. They call it a ligament assuming that it anchors bone to bone. I would suggest calling the entire structure of the quadriceps tendon, because that's what it is, and sure enough there is a bone that lives inside of it called the patella, but there's no beginning middle or end to the structure, it's all one thing. The three most commonly injured structures in the knee are the anterior cruciate ligament, the medial collateral ligament, and the lateral meniscus. It's this last one that your textbook may or may not agree with, depending on how old that it is. The "unhappy triad" technically does not include the lateral meniscus, and that's a historical term. But based off of more modern research, these are the three structures that most frequently get injured in the knee. If you're in my face-to-face class, I'll have a demonstration in lab as to why it's these three and not any of the others. It's not that the others don't get injured, it's just if you count which ones do get injured, these are the top three. Next up is the temporomandibular joint. This joint also has a meniscus. It's under a lot of stress because our jaws can produce a huge amount of force. When this joint gets damaged, chewing and talking can become difficult. This could be due to any number of factors, but stress is often an underlying characteristic. Some people are said to be double-jointed. A more technical term would be hypermobility of the joints. They have the same number of joints, they don't have double the number of joints. It's just that their joints are a lot more mobile than the average person's joints. This extra mobility increases the risk of both stress and damage to these joints, which can lead to osteoarthritis later in life. This can be caused by mutations to collagen, which makes the ligaments and tendons a little weaker and more prone to stretching. It can also be completely structural, and have to do with where the tendons are attached on the bones. If a tendon attaches close to the joint, that joint will be more mobile than a tendon that attaches further away. If we look at these two joints here, the person on the right has a tendon which attaches more distally. To hyper-extend this joint would require a lot more stretching of that tendon, something that that tendon may not be capable of without rupturing. The person on the left, however, may be able to stretch that tendon to hyper-extend this joint. It requires less stretching to their tendons to increase the range of motion to a point of hyper-extension. Our last topic for today is Arthritis. There's a number of different types of arthritis, I would like to cover the three most common. Arthritis is any damage to our joints. Osteoarthritis is caused by gradual wear and tear over time. It typically affects people over the age of 60 (but sometimes we'll strike at a younger age), and this will lead to destruction of the articular cartilages, which allows bones to rub on bones as the joint moves, which will trigger inflammation and pain, and damage to the skeletal system. Next up, rheumatoid arthritis can strike at a much younger age, often in somebody's 20s. A young person could go to sleep healthy one day, and wake up with rheumatoid arthritis the next day. It's thought that an infection or possibly an allergy can trigger this disease, but this disease is caused by the immune system, and is characterized by inflammation of the synovial cavity. Synovial fluid, to be useful, needs to be thin. Extra thickness in a lubricant does not lead to extra lubrication, in fact, it leads to the exact opposite. For a lubricant to be optimal it needs to be a very thin layer of that lubricant. Thicker layers provide less lubrication. So inflammation of the synovial fluid actually reduces the amount of lubrication that can occur, and this can lead to damage over time to the articular cartilage and bone tissue. Lastly is gouty arthritis. This is caused by any number of metabolic disorders, where crystals of some sort of waste product show up in the synovial fluid, and crystals do not act as a good lubricants. And this can do damage to the bone and cartilage tissue here. Rheumatoid arthritis can be diagnosed with any number of blood tests. We test the blood because this disease is caused by the immune system attacking synovial proteins. One marker might be C reactive peptide (CRP), which is a generic marker for inflammation. A more specific test would be for anti-ccp, or rheumatoid factor. These are antibodies that can attack things found in the synovial cavity. I think anti-ccp is interesting. For those who've taken biochemistry, you may not remember citrulline being one of the amino acids that you learned about. It is an amino acid, just not one that's used in human proteins. It turns out that if the human body accidentally uses citrulline in the proteins found in a synovial cavity, our immune system does not recognize those proteins, and thinks that they're foreign, and starts to attack them. This leads to inflammation known as rheumatoid arthritis. It's important to get the symptoms of rheumatoid arthritis under control as soon as possible. Prolonged inflammation is not good for any tissue, and can lead to damage. If we damage the articular cartilages they do not grow back, in which case rheumatoid arthritis will have turned into osteoarthritis, which is irreversible. To treat rheumatoid arthritis, we can either try and block inflammation with the use of anti-inflammatory drugs like aspirin and ibuprofen, or we can use drugs that target the immune system more directly, like methotrexate and cyclosporine. The risk of inhibiting the immune system is that this will increase the risk of infections. So that's arthritis. Make sure that you don't get osteo-arthritis and osteo-porosis confused. They sound similar, but osteoporosis is a disease of bone tissue, whereas osteoarthritis is a disease of joints (more than two bones). And that should wrap up all of the specific joints that we've talked about, and this will be the end of chapter 9.