Joints, Cartilage, Ligaments, and Tendons

A joint is a place where two or more bones meet, cartilage cushions and smooths the bone surfaces, ligaments connect bone to bone, and tendons connect muscle to bone. In brief: fixed joints, such as the skull sutures, do not move; hinge joints, such as the elbow, move in one plane; ball-and-socket joints, such as the hip and shoulder, move in many directions. Synovial joints contain a lubricating fluid.

Joints make movement possible and also provide stability, and the soft tissues around them determine how well they work. This guide describes the types of joint, the structure of a synovial joint, the roles of cartilage, ligaments and tendons, compares the joint types in a table, works through an example of the knee and corrects common mistakes.

What is a joint?

A joint, or articulation, is any junction between two bones. Joints are classified by structure and by how much movement they allow. The right type depends on the task: some places need strong protection and fixed bones, while others need a wide range of motion.

What are the types of joint?

Fixed (immovable) joints

Fixed joints allow no movement. The bones are joined by fibrous tissue or fused. Examples are the sutures between the bones of the skull, which protect the brain, and the joints that hold the teeth in their sockets.

Slightly movable joints

These joints allow a small amount of movement. The bones are joined by cartilage. Examples are the joints between the vertebrae, which are separated by discs of cartilage, and the joints between the ribs and the sternum.

Freely movable (synovial) joints

These joints allow considerable movement, and they are the most common type in the limbs. They are classified by the shape of the surfaces and the movement allowed.

• Hinge joints move in one plane, like a door: bending and straightening. The elbow, knee and finger joints are hinge joints.
• Ball-and-socket joints have a rounded end of one bone fitting into a cup of another. They allow movement in all directions and rotation. The shoulder and hip are examples. The shoulder has the largest range of motion but is less stable and dislocates more easily than the hip.
• Pivot joints allow rotation around an axis. The joint between the first two vertebrae of the neck lets you turn your head from side to side.
• Gliding joints allow sliding between flat surfaces, as between the small bones of the wrist.

What is the structure of a synovial joint?

A synovial joint has several parts that work together.

• Articular cartilage: a smooth, slippery layer covering the ends of the bones. It reduces friction and absorbs shocks.
• Joint capsule: a tough fibrous sleeve that surrounds the joint and holds the bones together.
• Synovial membrane: lines the capsule and produces synovial fluid.
• Synovial fluid: a thick, clear liquid that lubricates the joint and supplies nutrients to the cartilage.
• Ligaments: strong bands that stabilize the joint.
• Bursae and menisci: small fluid-filled sacs and pads of cartilage (as in the knee) that reduce friction and cushion the joint.

What do cartilage, ligaments and tendons do?

Cartilage

Cartilage is a firm but flexible connective tissue made of cells in a matrix of collagen and other molecules. It has no blood vessels, so it repairs itself slowly after damage. Types include the glassy articular cartilage on bone ends, the fibrous cartilage of vertebral discs and the elastic cartilage of the outer ear. A baby’s skeleton contains much cartilage, which gradually turns into bone.

Ligaments

Ligaments are bands of dense connective tissue rich in collagen fibers. They connect bone to bone, hold joints together and limit movements that could injure the joint. They have a limited blood supply, so they heal slowly. A sprain is a stretch or tear of a ligament.

Tendons

Tendons are cords of dense connective tissue that join muscle to bone. They transmit the force of a muscle contraction to the bone, which makes the bone move. They are very strong but do not stretch much. The Achilles tendon, which joins the calf muscle to the heel bone, is the thickest tendon in the body. A strain is a stretch or tear of a muscle or tendon.

Joint types compared

Worked example: the knee joint

The knee is the largest joint in the body, and it joins the femur with the tibia. Analyze its parts by asking what each does.

1. Type: mainly a hinge joint, allowing bending and straightening, with a small amount of rotation when bent.
2. Articular cartilage covers the ends of the femur and tibia and the back of the patella, to reduce friction.
3. Menisci: two C-shaped pads of fibrous cartilage between the femur and tibia act as shock absorbers and improve the fit of the bones.
4. Ligaments: the anterior and posterior cruciate ligaments cross inside the joint and prevent the tibia from sliding forward or backward. The medial and lateral collateral ligaments on each side stop sideways movement.
5. Tendons: the quadriceps tendon connects the thigh muscles to the patella, and the patellar tendon connects the patella to the tibia, so contraction of the thigh muscle straightens the leg.
6. Synovial fluid in the capsule lubricates all these surfaces.

A twisting injury in sport can tear a cruciate ligament or a meniscus. Because cartilage and ligaments heal slowly, treatment may include rest, physical therapy or surgery. The example shows that the movement of a joint depends on the combination of bone shape, cartilage, ligaments and muscles with their tendons.

How were joints studied?

The study of joints and movement was part of the early anatomy of Vesalius in the sixteenth century. The Italian physiologist Giovanni Borelli applied mechanics to the body in the seventeenth century, treating bones as levers. In the twentieth century, artificial joints were developed. Sir John Charnley introduced a successful hip replacement in the 1960s, using a metal ball and a plastic cup fixed with bone cement, and joint replacement surgery is now common.

What problems affect joints?

Osteoarthritis is the gradual wearing away of the articular cartilage, often in older people or after injury, and it causes pain and stiffness. Rheumatoid arthritis is an autoimmune condition in which the immune system attacks the synovial membrane. Dislocation occurs when the bone ends are forced out of position. Regular moderate exercise, a healthy body weight and good technique in sports help to keep joints healthy.

Which mistakes are common?

Mistake 1: Mixing up ligaments and tendons

Ligaments join bone to bone. Tendons join muscle to bone.

Mistake 2: Saying that cartilage is bone

Cartilage is a softer, more flexible tissue with no blood supply, and it forms the smooth surface of bones.

Mistake 3: Thinking that all joints move

Fixed joints, such as the skull sutures, allow no movement, and this is useful for protection.

Mistake 4: Saying that synovial fluid is blood

It is a lubricating fluid made by the synovial membrane. It is not blood.