In the last video, we covered the bones of the spine. Today, we’ll learn about the joints, ligaments, and discs. How they hold the spine together, and how injury to those structures can result in pain, herniation, and instability. So in this video we’ll start with the why do we have vertebral joints, how are the vertebral joints structured and how do we classify them? The vertebral joints are well organized and it makes so much sense once you know fibrous joints, cartilagionous joints, synovial joints and the bony joints. So that’s our goal for today, understanding all of these joints, and also look specifically at common injuries like disk herniations as well. What’s up everyone, my name is Taim. I’m a medical doctor, and I make animated medical lectures to make different topics in medicine visually easier to understand. If you’d like a PDF version or a quiz of this presentation, you can find it on my website, along with organized video lectures to help with your studies. Alright, let’s get started. Why, How, Classification. Why do we have vertebral joints? I think this is one of the things that really makes you appreciate the structure of the spine. If the vertebrae were fused together into a single bone, we wouldn’t be able to move properly. The vertebral joints allow flexibility and movement, making it possible for the spine to bend forward in flexion, bend backward in extension, and rotate along its axis. At the same time, they maintain spinal stability and keep the vertebrae properly aligned, so the spine doesn’t collapse under its own weight. Another important role of the vertebral joints is protecting the spinal cord and the spinal nerves during movement, by keeping the vertebral canal stable and minimizing any compression or stretching. They also help absorb and distribute mechanical forces, especially when we walk, run, or lift — so that the forces don't damage the bones or nerves. And they also enable coordinated movements of the head and trunk, allowing us to perform daily activities. So how are the vertebral joints structured? If we go a little bit down. … And just, take a moment and appreciate all the ligaments you see here, they are the ones that stabilize your whole body. We see that the vertebral joints are made up of different types depending on what kind of tissue connects the bones. First, we have fibrous joints that connect bones by dense fibrous connective tissue, and they allow little to no movement. We can also see cartilaginous joints. These are joints where the bones are united by cartilage, allowing more flexibility than fibrous joints but still providing a lot of stability. And a key example here are the intervertebral discs that allow for slight movement between vertebrae and act as important shock absorbers. Now, if we look from the lower perspective, we can also see synovial joints. Synovial joints are characterized by a joint cavity filled with synovial fluid, surrounded by a capsule. They allow much more movement compared to fibrous and cartilaginous joints. And we also have bony joints, where two bones fuse together into a single bone over time. In the vertebral column, a good example is the fusion of the sacral vertebrae to form the sacrum. Once fused, these joints no longer allow movement between the original vertebrae. Each of these contribute in their own way to the overall function and stability of the spine. And this is how they’re generally structured. All the joints fall under these categories. Awesome, let’s do the fibrous joints first. Fibrous joints in the spine are classified as syndesmoses. This means that two bones are connected by a ligament or a sheet of connective tissue, without a true joint cavity. They are slightly movable joints to provide stability, what we call amphiarthroses. Here you see two bones. They are held together tightly by a flexible rope, that’s basically what fibrous joints are. If we look at the spine, you can see that they do look different. Some are very short and thin, some are short but a little bit thicker, and some are very long, spanning across the whole spine. And because of this, we can break the fibrous joints into two groups, the long ligaments and the short ligaments. The long ligaments are the ones that run along the full length of the vertebral column. Their main job is to limit hyperextension and maintain stability during movement. The short ligaments are ligaments that connect adjacent vertebrae directly, and they help control motion between individual vertebrae and provide elasticity to the spine. Let’s cover the long ligaments first. Starting with the anterior longitudinal ligament. This ligament runs along the front of the vertebral bodies from the base of the skull all the way down to the sacrum. Its main job is to prevent hyperextension of the vertebral column, which means it stops the spine from bending too far backward. At the bottom of the spine, this ligament continues as the anterior sacrococcygeal ligament, where it anchors onto the front of the sacrum and coccyx, maintaining stability of the lower end of the spine. Alright, next. If we now zoom in a little bit and remove the body of one vertebrae, we can see the posterior longitudinal ligament. Highlighted here. It runs along the posterior surfaces of the vertebral bodies, but this time inside the vertebral canal, just in front of the spinal cord. Here’s another view showing you that it runs along the whole spine. Its function is to limit hyperflexion of the spine, meaning it prevents excessive bending forward, and it also helps support the intervertebral discs. Then we have the superficial posterior sacrococcygeal ligament. If we look at the spine from this view, we’ll see this. We can find the superficial posterior sacrococcygeal ligament at the very end, attached to the back of the sacrum and coccyx. So those are the long ligaments. They are major stabilizers across the whole length of the spine. Now let’s move into the short ligaments. The first short ligament we see is the ligamenta flava. These are elastic ligaments that connect the laminae of adjacent vertebrae, and they are special because they contain a lot of elastic fibers, giving them a yellow color. Their job is to maintain posture and assist with the recoil of the spine after flexion, like a built-in spring system if it makes sense. Turning the spine a little to the side, we see the intertransverse ligaments here highlighted in orange. They run between the transverse processes, and they limit lateral flexion, helping to keep the spine stable when we bend to the sides. Alright let’s highlight some other structures. Here we see the interspinous ligaments, connecting one spinous process to the next. They help limit flexion by resisting the separation of spinous processes when you bend forward. And then, we got the supraspinous ligament, running all the way along the tips of the spinous processes, from the sacrum up to the cervical spine. This ligament also limits flexion and keeps the midline of the spine together during movement. What’s special here, is that if we look at the cervical region, the supraspinous ligament broadens and becomes the nuchal ligament. The nuchal ligament is much thicker and supports the weight of the head, while also serving as a site of muscle attachment. So those were the fibrous joints I wanted to mention. Let’s do the cartilaginous joints. Now, just something I need to mention here first. Notice I wrote synchondroses and symphyses. Cartilaginous joints in general are connections between bones that are made of cartilage, but they are divided into two types based on the kind of cartilage and movement they allow. Synchondroses are joints connected by hyaline cartilage and are usually temporary, like the ones found during growth, while symphyses are joints connected by fibrocartilage and are permanent, providing strength with a little bit of movement. In the adult vertebral column, all the cartilaginous joints are symphyses. There are no synchondroses between vertebral bodies after development is complete. So first ones are the intervertebral discs. These are classic examples of symphyses. They sit between adjacent vertebral bodies from the cervical region all the way down to the sacrum, acting as important shock absorbers. Then, as we move downward to the junction between the last lumbar vertebra and the sacrum, we find the lumbosacral symphysis. This junction needs to be especially strong because of the heavy weight-bearing demands in this region, so the fibrocartilaginous disc here is very thick and durable. At the very bottom of the spine, there is the sacrococcygeal symphysis, which connects the sacrum to the coccyx. In some adults, this joint can even ossify completely over time, but early in life, it remains a fibrocartilaginous symphysis, providing slight mobility which can be important during activities like childbirth. Let’s now zoom a little bit out. We can see the intervertebral discs distributed along the entire length of the vertebral column. I removed one vertebra here, so if we just isolate this specific region, we can see that the intervertebral disc is made up of two distinct regions. The outer layer is called the anulus fibrosus, which consists of concentric layers of fibrocartilage. This tough structure provides strength and contains the inner part. At the center is the nucleus pulposus, which is a soft, gelatinous core rich in water content. The nucleus pulposus functions as a hydraulic cushion, absorbing compressive forces and distributing them evenly across the disc. How does that look like? As you can see here, nerve roots extend from the spinal cord through openings located on either side of the vertebrae. The intervertebral discs play a really important role here. They prevent friction between the vertebral bodies and allow for smooth, controlled movement during bending, twisting, and lifting. However, sometimes the integrity of these discs can be compromised, and it can start to degenerate. Disc degeneration may occur, often due to age-related wear and tear, repetitive stress, or trauma. When degeneration weakens the anulus fibrosus, the nucleus pulposus can herniate outward, creating what we call a herniated disc. This herniated material can compress nearby spinal nerve roots against the hard vertebrae. And depending on which nerve root is compressed, patients may experience pain, numbness, tingling, or muscle weakness along the distribution of the affected nerve. For example, compression of a nerve root in the lumbar spine can cause symptoms down the leg, a condition commonly known as sciatica. Now imagine this clinical case. A 38-year-old man comes to your clinic. He tells you he was lifting a heavy box at work about two weeks ago when he felt a sharp pain that starts in his lower back and radiates down the right side of his buttock, thigh, and all the way into the lateral part of his right lower leg and foot. He also mentions some tingling and numbness in these areas, and since then, he has noticed some weakness when trying to lift his foot Based on this description, you immediately start thinking about a possible lumbar nerve root compression. To confirm the diagnosis, you order an MRI. So here we have an MRI machine. You send him in, scan happens, he goes out. And then you get an mri scan of something like this. This sagittal view shows a clear bulging of the intervertebral disc at the lower lumbar level, most prominently at the L5/S1 junction. On the axial view, you can see the herniated disc material pushing toward the right side of the patient, compressing the nerve root exiting there. This matches perfectly with his symptoms on the right leg. Putting it all together: the patient has a right-sided L5/S1 disc herniation, compressing the right S1 nerve root. This explains his pain radiating down the posterior thigh and calf, and numbness. It’s a classic presentation of a disk herniation with nerve root compression. And with that, we covered the cartilaginous joints. Next let’s do the synovial joints. Synovial joints are characterized by a joint cavity filled with synovial fluid, surrounded by a capsule, and lined by synovial membrane. They allow for a wide range of motion depending on their type and structure. In the spine, the first example of a synovial joint is in this area. One vertebra is connected with its adjacent vertebra though facet joint. It’s called this way because it is formed between the superior articular facet of one vertebra and the inferior articular facet of the vertebra above. Add a membrane, and we get the zygapophysial joints, or facet joints. These are plane-type synovial joints, allowing gliding movements between the vertebrae. Degeneration of these joints is common with aging and can lead to facet joint arthropathy, causing localized back pain and referred pain patterns that can mimic nerve root compression. If we now turn this image towards the posterior view of the head. We can find the lateral atlanto-axial joints between the first cervical vertebra, the atlas, and the second cervical vertebra, the axis. There are actually three joints here, two lateral atlanto-axial joints and one median atlanto-axial joint. The lateral atlanto-axial joints are synovial plane joints between the inferior articular facets of the atlas and the superior articular facets of the axis. These allow gliding movements, but the majority of rotation occurs thanks to the median atlanto-axial joint. Just for orientation, we can see the ligamenta flava here as well. The median atlanto-axial joint is interesting, let’s just remove the posterior parts of the vertebra here. This joint is a pivot-type synovial joint between the dens of the axis and the anterior arch of the atlas. The dens acts like a pivot, allowing the head to rotate from side to side, like shaking the head to say "no." This joint is stabilized by strong ligaments, most importantly the transverse ligament of the atlas, which holds the dens tightly against the anterior arch. Damage to this ligament, such as from trauma or diseases like rheumatoid arthritis, can cause instability and even life-threatening spinal cord compression. Another thing we can see is the atlanto-occipital joint. This is a joint between the occipital condyles and the superior articular facets of the atlas. It allows mainly flexion and extension, like when you nod your head. This joint is supported by membranes and ligaments that strengthen the joint capsule. First, we see the posterior atlantooccipital membrane, connecting the posterior margin of the foramen magnum to the posterior arch of the atlas. It helps stabilize the joint and leaves a small opening for the vertebral artery. Then, the lateral atlantooccipital ligament strengthens the capsule on each side. And if we turn over to the anterior view, we can see the anterior atlantooccipital membrane, which strengthens the front of the joint and helps limit extension. So those were the joints. Let’s move to the last category. Bony joints, or synostoses. Bony joints represent the complete fusion of bones, with no remaining joint cavity. In the vertebral column, the best examples are the synostoses of the sacrum and synostoses of the coccyx. During development, the individual sacral and coccygeal vertebrae are separated, but they later fuse into a single sacral bone and a coccygeal bone. With that, we have now finished covering all the joints of the vertebral column, fibrous, cartilaginous, synovial, and bony joints. In the next video on the skeletal system series, I’ll cover the bones of the thorax, including the ribs and sternum. Click on the next video, and I’ll see you there. If you want a handmade PDF version of this lecture, take a quiz to test your knowledge, or access an organized list of all my videos, you can find everything on my website. Thanks for watching! See you in the next one.