The Joints

Joints are where bones come together in the human body. The adult human body usually has 206 bones, and except for the hyoid bone in the neck, each bone is connected to at least one other bone. Many joints let bones move smoothly against each other, while other joints are built for support and very little movement.

Joint stability and movement are closely linked. Joints that are very stable usually allow little or no mobility. Joints that allow the most movement are the least stable.

This...

Joint mobility is the basis of the functional classification of joints. Joints are grouped by how much movement they allow between nearby bones. They are classified as synarthrosis, amphiarthrosis, or diarthrosis.

A synarthrosis is an immobile or nearly immobile joint. These joints form a strong union between bones. That strength is important where the bones help protect internal organs. Examples include sutures between the bones of the skull, which protect the brain, and the manubriosternal...

Body joints, or articulations, are grouped by how the bones are connected and how much they move. The main structural features are the tissue between the bones and whether a fluid-filled joint cavity is present. These differences divide joints into three structural classes.

Fibrous joints connect adjacent bones with fibrous connective tissue. They do not have a synovial cavity. The bones are joined by dense irregular connective tissue rich in collagen fibers, so movement is minimal to none.

Fibrous joints connect bones with fibrous connective tissue. They give the body stability and allow little to no movement between the bones that meet there.

There are three kinds of fibrous joints: sutures, syndesmoses, and gomphoses. A suture joins the bones of the skull, except for the mandible. The strong fibrous tissue at these joints helps protect the brain and shape the face. In adults, the skull bones lie close together, and fibrous tissue fills the narrow space between them. Many...

Cartilaginous joints are bone connections made with cartilage, a tough but flexible connective tissue. They do not have a joint cavity like synovial joints. Instead, the bones are linked by either hyaline cartilage or fibrocartilage.

One type of cartilaginous joint is the synchondrosis, which means bones are joined by hyaline cartilage. A synchondrosis can be temporary or permanent. The temporary form is the epiphyseal, or growth, plate in a growing long bone. This plate is the growing hyaline...

Synovial joints are the body’s most mobile joints. They have a joint cavity, which is a fluid-filled space where the bones meet. Unlike fibrous or cartilaginous joints, the bone surfaces in a synovial joint are not directly joined by fibrous connective tissue or cartilage.

This structure lets the bones move smoothly against each other. The walls of the joint cavity are made by the articular capsule, a fibrous connective tissue structure that attaches to each bone just outside the articulating...

The vertebral column has synovial joints that help control how the spine moves. Adjacent vertebrae connect not only through the intervertebral discs, but also through joints between the superior and inferior articular processes. These joints are called zygapophysial joints, or facet joints, and they are plane joints that allow only limited motion.

The direction of the articular processes changes in different regions of the spine. This change affects which movements are possible in each area.

The knee joint is the most complicated joint in the body. It includes three articulations: two tibiofemoral joints and one patellofemoral joint. As a synovial joint, it has a thin articular capsule that partly surrounds the joint cavity.

Several ligaments, muscles, and cartilaginous structures help the knee move and stay stable. Seven ligaments support the knee joint. The patellar ligament, which is also attached to the quadriceps femoris muscle group, is the main ligament that stabilizes the...

The ankle joint is a hinge joint that links the foot and lower leg. It is formed by the talocrural joint, which brings together the talus bone of the foot with the distal ends of the tibia and fibula in the leg. The talus has a square-shaped top surface with three areas of articulation.

The upper part of the talus meets the inferior tibia and carries body weight between the leg and foot. The medial malleolus of the tibia and the lateral malleolus of the fibula hold the talus in place on the...

Limb synovial joints form during embryonic development as the bones of the arms and legs begin to take shape. The process starts with mesenchyme, the embryonic tissue that gives rise to bones, cartilage, and connective tissues. Mesenchymal stem cells first become chondrocytes, which make hyaline cartilage. This cartilage later becomes the model for each bone and then turns into bone through endochondral ossification.

The limbs first appear as small limb buds on the sides of the embryo near the...