It is hard to appreciate something if you don't understand it. Understanding your joints and how they function helps you not only appreciate them but also understand how best to care for them. There are different basic kinds of joints in your body. All of the mobile joints (e.g. knees, hips, shoulders, elbows, fingers) have the same basic structure.
An outer tough joint capsule surrounds the joint. The fibrous tissue in the capsule provides strength and stability to the joint. Inside the capsule the articulating bones come together.
The inner part of the capsule is lined with a sheet of synovium, which is made up of synovial cells. The synovial cells secrete the joint fluid called synovial fluid. Synovial fluid and water are the chief constituents of joint fluid. Synovial fluid is straw-colored and thick. The thickness of the fluid is due largely to a component of the fluid called hyaluronic acid. You may have heard of this substance before. One common treatment for osteoarthritis is hyaluronic acid injections. The purpose of these injections is chiefly to replace the joint's natural fluid, which decreases in osteoarthritis. The injections "pave the pot-holes" that are left in the joint as the fluid gets eroded. But that topic is for another blog. Let's get back to the anatomy of the joint.
Inside the joint is an amazing substance called cartilage. I like to say that in the world of joints, cartilage is a true superhero. Up to eight times more slippery than ice and with the ability to soak up and push out water as easily as a sponge, cartilage really is perfectly designed for joints. The qualities of cartilage allow it to have dual functions: (1) Cartilage provides ideal shock-absorbing capacity for the joint. (2) Cartilage lubricates the joint and allows for the joint's seamless motion.
Cartilage is so perfect for joints that, though we have put a man on the moon and created an artificial heart, we haven't yet been able to make a product as ideal as the body's own healthy cartilage. Cartilage stores the joint's fluid. When you are sitting, the knee's joint fluid rests within the cartilage. When you get up from sitting (and load the knee joint), the fluid is squeezed out of the cartilage, lubricating and cushioning the joint. When sitting back down, the knee's joint fluid moves back into the cartilage.
Cartilage is also filled with negatively charged chondroitin molecules. This gives the cartilage an important added cushioning effect. When cartilage is loaded with weight, the chondroitin molecules are pushed together but, just as magnets oppose each other as they get closer and closer together, these negatively charged chondroitin molecules also repel each other with stronger and stronger force as they get closer together.