A joint is any place in your body where two bones meet. Some joints allow wide, sweeping movement like swinging your arm overhead. Others barely move at all, and a few are fused so tightly they function more like a single bone. Together, your joints let you stand, sit, bend, twist, and absorb impact as you move through daily life.
How Joints Are Classified by Movement
The simplest way to think about joints is by how much they move. Every joint in your body falls into one of three categories.
- Immovable joints (synarthroses): These joints don’t move at all. Their purpose is purely structural. The joints between the flat bones of your skull are the clearest example. In infants, these skull plates are slightly separated to allow for brain growth, but by adulthood they’ve fused into rigid seams called sutures.
- Slightly movable joints (amphiarthroses): These joints offer a small amount of give, providing a balance of stability and flexibility. The joints between your vertebrae are a good example. Each individual disc allows only a tiny range of motion, but the combined flexibility of all those small movements is what lets you bend forward, arch backward, and rotate your torso.
- Freely movable joints (diarthroses): These are the joints most people picture when they hear the word. Your knees, shoulders, hips, elbows, and finger joints all fall into this group. They allow movement in one or more directions and are responsible for nearly every deliberate motion you make.
How Joints Are Classified by Structure
Joints can also be grouped by what physically holds them together. This structural view explains why different joints feel and behave so differently.
Fibrous joints are connected by dense, tough connective tissue with no gap between the bones. The skull sutures are fibrous joints, and so are the connections between your lower leg bones (the tibia and fibula). Because there’s no space or cushion between the bones, these joints are rigid or nearly so.
Cartilaginous joints are held together by cartilage, a firm but slightly flexible tissue. The discs between your vertebrae and the joint at the front of your pelvis (the pubic symphysis) are cartilaginous. The cartilage acts as a built-in shock absorber, allowing limited compression and movement.
Synovial joints are the most complex and the most mobile. They’re the ones that have an actual joint cavity, a small fluid-filled space between the bones. Your knee, shoulder, hip, elbow, wrist, and ankle are all synovial joints. Because they move so freely and bear so much load, they have several built-in support structures that simpler joints don’t need.
Inside a Synovial Joint
Synovial joints have an architecture designed to reduce friction and absorb shock during movement. The ends of each bone are covered in articular cartilage, a smooth layer of tissue that prevents bone from grinding directly against bone. This cartilage is made of hyaline cartilage, the same type found in your nose and the rings of your windpipe, and it’s remarkably slippery.
Surrounding the joint is a fibrous capsule called the articular capsule, which attaches to both bones just beyond the surfaces that move against each other. The inner lining of this capsule is the synovial membrane, which secretes synovial fluid into the joint cavity. Synovial fluid is thick and slightly viscous, similar in consistency to egg white. It serves two purposes: lubricating the joint surfaces so they glide smoothly and delivering nutrients to the cartilage, which has no blood supply of its own.
As people age, the volume of synovial fluid tends to decrease. Research comparing adults over 65 to younger individuals found that older adults had an average synovial fluid volume of about 6 milliliters compared to roughly 15 milliliters in younger people. The fluid also becomes more viscous with age. These changes contribute to the increased stiffness many people notice in their joints over time.
Ligaments and Tendons
Joints don’t hold themselves together. They rely on two types of dense, fibrous connective tissue to stay stable and functional.
Ligaments connect bone to bone. They wrap around or run alongside a joint, keeping the bones aligned and preventing them from shifting out of position. The anterior cruciate ligament (ACL) in your knee, for instance, is a key stabilizer that controls rotation and prevents your shinbone from sliding forward under your thighbone.
Tendons connect muscle to bone. When a muscle contracts, the tendon pulls on the bone, producing movement at the joint. Together, tendons and ligaments transmit mechanical forces that both stabilize the skeleton and allow the body to move. Spraining a joint means stretching or tearing a ligament. Straining a joint typically involves damage to a tendon or muscle.
Types of Synovial Joints
Not all synovial joints move in the same way. Their shape determines their range of motion.
- Hinge joints move in one plane, like a door hinge. Your elbow and knee are hinge joints, allowing flexion (bending) and extension (straightening).
- Ball-and-socket joints allow the widest range of motion. The hip and shoulder are ball-and-socket joints, where a rounded bone head fits into a cup-shaped socket, permitting movement in almost every direction.
- Pivot joints allow rotation around a single axis. The joint at the top of your neck that lets you turn your head side to side is a pivot joint.
- Saddle joints allow movement in two directions. The base of your thumb is a saddle joint, which is why your thumb can move so differently from your other fingers.
- Gliding joints allow bones to slide past each other in limited, flat movements. The small joints in your wrists and ankles are gliding joints.
- Condyloid joints allow movement in two planes without rotation. Your knuckles are condyloid joints, letting you flex, extend, and move your fingers side to side.
Common Joint Problems
Because joints are load-bearing, constantly moving structures, they’re vulnerable to both wear-and-tear damage and immune system disorders. The most widespread joint condition is osteoarthritis, which occurs when the cartilage covering the bone ends gradually breaks down. Without that smooth cushion, bones begin to rub against each other, causing pain, swelling, and stiffness. Osteoarthritis most commonly affects the knees, hips, hands, and spine, and becomes more likely with age, excess body weight, or a history of joint injury.
Rheumatoid arthritis is a different process entirely. Rather than wear and tear, it’s an autoimmune condition in which the body’s immune system attacks the synovial membrane lining the joints. This causes chronic inflammation that can eventually erode the cartilage and bone inside the joint. It affects roughly 0.5 to 1 percent of the global population and can appear at any age, though incidence has been rising slightly over recent decades.
Other common joint problems include bursitis (inflammation of the small fluid-filled sacs that cushion joints), gout (caused by uric acid crystals depositing in a joint, often the big toe), and dislocations (where bones are forced out of their normal alignment, usually by trauma). Joint infections, though less common, can occur when bacteria enter the joint space, either through the bloodstream or directly through a wound, and require prompt treatment to prevent cartilage destruction.
Keeping Joints Healthy
Joint cartilage has no blood supply. It depends entirely on synovial fluid for nutrients, and that fluid circulates best when you move. Regular, moderate physical activity like walking, swimming, or cycling keeps synovial fluid flowing across cartilage surfaces and strengthens the muscles around a joint, reducing the mechanical load the joint itself has to bear.
Maintaining a healthy body weight has an outsized effect on joint health, particularly for weight-bearing joints like the knees and hips. Every extra pound of body weight translates to roughly three to four additional pounds of force on your knees with each step. Strength training that targets the muscles supporting major joints, especially the quadriceps for knee health and the rotator cuff muscles for shoulder stability, can reduce injury risk and slow cartilage breakdown over time.