A joint is the place where two bones meet, such as your ankle, wrist, or knuckles. They vary greatly in mobility and in size. Below is an overview of the anatomy of shoulders, hips, and knees, as large joints that are commonly injured and may need physical therapy, surgery, or a joint replacement.
Parts of the Joint
Joints are made up of a combination of bones, tendons, ligaments, muscles, cartilage, and synovial fluid. The configuration of these parts varies depending on the joint, and not all of them are required in every joint. Below are the definitions of some of the less-common terms.
Tendons are the bands that connect muscles to bones.
Ligaments are the bands that connect bones to other bones.
Cartilage acts as a pad between bones in your joint so that the bones don’t rub against each other and can move smoothly.
Synovial fluid is a thick fluid found in some joints that acts as a lubricant.
Types of Joints
There are several different types of joints in your body. Two of them are the hinge joint and ball-and-socket joint.
In a hinge joint, like elbows and knees, the two main bones meet end-to-end. Ligaments connect them to each other along the outside, while within the joint cartilage and synovial fluid keep the bones from rubbing against each other. Muscles, connected to each bone by tendons, are attached on two sides of the joint. One set of muscles will extend the joint, straightening your leg out or reaching out your arm, and the other set of muscles will contract the joint, bending your knee or elbow.
Hinge joints are designed to move in one direction. The knee is a special type that has slightly more motion.
In a ball-and-socket joint, the rounded end of one bone fits into a rounded cavity in the other bone. Ligaments connect the bones to each other in several places around the joint, and the cartilage and synovial fluid provide cushioning. Muscles, connected to each bone by tendons, surround the joint and allow it a greater range of motion.
Ball-and-socket joints are designed to move in almost all directions: up, down, front, back, left, right. Compare the directions you can move your shoulder to the directions you can move your knee.
Injuries to Joints
Your knee, hip, or shoulder may need to be replaced, often due to age-related conditions or other injuries.
Osteoarthritis is a form of arthritis caused by the cartilage in your joints wearing away and allowing bones to rub directly against each other. Hip replacements and knee replacements are commonly caused by osteoarthritis.
Osteoporosis is the weakening of bones, usually due to age. It is more common in women than in men. Weak bones can lead to fractures, especially if someone loses their balance and falls.
Sports injuries take several forms. You may injure your rotator cuff, the ring of muscles around your shoulder joint, by overdoing specific forms of arm exercise. You may move quickly in a sports game such as tennis or soccer and tear your ACL, or a major ligament in your knee.