On their own, bones are a remarkable piece of biological machinery. But the true miracle just might be the way a skeleton that’s stronger than concrete still allows the supple movements of a ballet dance.
Joints are the means by which one bone glides, rotates, or bends around another. They ensure that your bones fit snugly together while allowing them an impressive range of movement. Different joints allow different types of movement—for example, joint limitations explain why you can’t spin your hand around your wrist or fold your head back until your ears rest on your shoulder blades. The image here shows four different joints, from the relatively simple folding joints at the knees and elbows to the more versatile ball-and-socket joints in your shoulders and hips.
Joints are a surprisingly complex part of your skeleton. The bones in a joint are padded with cartilage, braced with ligaments, and lubricated with a special fluid. Here’s how each part works:
• Cartilage. This tough, flexible tissue pads the end of the two bones that meet in a joint. Cartilage helps cushion and protect the joint, absorbing shocks and preventing friction. (Cartilage also supports other structures in your body—for example, it creates the shape of your ears and the tip of your nose.)
• Ligaments. Ligaments are sturdy bands of elastic tissue that connect one bone to another. Your body often uses them to support a joint— for example, four major ligaments hold your knee in place and connect it to your leg bones.
• Synovial fluid. This thick, stringy fluid fills many of your joints. It reduces friction between pieces of cartilage and other tissue in the joint, which helps reduce wear and tear.
Incidentally, “double-jointed” people don’t have extra joints, just abnormally expanded joint movement for some other reason. They may have misaligned joints, for example, or abnormally shaped bones, or weaker ligaments and tendons. Alternatively, they may be less sensitive to the signals that indicate when the bones of a joint are out of place. Whatever the case, their condition puts them at higher risk of bone injury and arthritis.
Synovial fluid is probably also responsible for the exquisitely annoying sound of knuckle cracking. The thinking is that over time, bubbles of gas accumulate in the synovial fluid of your joints. When you pull your finger ever-so-slightly out of place, the knuckle’s bones separate, and the tiny bubbles combine and burst in a distinctive pop. In theory, this practice could eventually cause loss of grip strength or joint pain, but so far no study has found knuckle cracking to cause anything more dangerous than a seriously annoyed spouse.
Source of Information : Oreilly - Your Body Missing Manual