By Janet Rae-Dupree, Pat DuPree

Arthrology, which stems from the ancient Greek word arthros (meaning “jointed”), is the study of those structures that hold bones together, allowing them to move to varying degrees — or fixing them in place — depending on the design and function of the joint. The term articulation, or joint, applies to any union of bones, whether it moves freely or not at all.

Inside some joints, such as knees and elbows, are fluid-filled sacs called bursae that help reduce friction between tendons and bones; inflammation in these sacs is called bursitis. Syndesmosis joints are stabilized by connective tissue called ligaments, or interosseus membranes, that range from bundles of collagenous fibers that restrict movement and hold a joint in place to elastic fibers that can repeatedly stretch and return to their original shapes. Examples are

  • The shoulder joint, or corticohumeral ligament that extends from the coracoid process of the scapula to the greater tubercle of the humerus

  • The pubofemoral ligament that extends from the pubic bone to the femur

  • The knee joint (oblique popliteal ligament), where the tendon of the semimembranous muscle expands to cross the posterior of the knee joint

The three types of joints are as follows:

  • Fibrous: Fibrous tissue rigidly joins the bones in a form of articulation called synarthrosis, which is characterized by no movement at all. The sutures of the skull are fibrous joints. Any slight movement in a joint depends on the length of the fibers uniting the bones.

  • Cartilaginous: This type of joint is found in two forms:

    • Synchondrosis articulation involves hyaline (rigid) cartilage that allows no movement. Once bone growth ends, the joint becomes ossified and immobile. The most common example is the epiphyseal plate of the long bone. Other examples are the joint between the ribs, costal cartilage, and sternum.

    • Symphysis joints occur where fibrocartilage fuses bones in such a way that pressure can cause slight movement, called amphiarthrosis. Examples include the intervertebral discs and the symphysis pubis.

  • Synovial: Also known as diarthrosis, or freely moving, joints, this type of articulation involves a synovial cavity, which contains articular fluid secreted from the synovial membrane to lubricate the opposing surfaces of bone covered by smooth articulating cartilage.

    The synovial membrane is covered by a fibrous joint capsule layer that’s continuous with the periosteum of the bone. Ligaments surrounding the joint strengthen the capsule and hold the bones in place, preventing dislocation. In some synovial joints, such as the knee, fibrocartilage pads called menisci (singular: meniscus) develop in the cavity, dividing it into two parts. In the knees, these menisci stabilize the joint and act as shock absorbers.

There are six classifications of moveable, or synovial, joints:

  • Gliding: Curved or flat surfaces slide against one another, such as between the carpal bones in the wrist or between the tarsal bones in the ankle.

  • Hinge: A convex surface joints with a concave surface, allowing right-angle motions in one plane, such as elbows, knees, and joints between the finger bones.

  • Pivot (or rotary): One bone pivots or rotates around a stationary bone, such as the atlas rotating around the odontoid process of the axis at the top of the vertebral column.

  • Condyloid: The oval head of one bone fits into a shallow depression in another, to allow for five movements: flexion, extension, abduction, adduction, and circumduction. Examples are the carpal-metacarpal joint at the wrist and the tarsal-metatarsal joint at the ankle.

  • Saddle: Each of the adjoining bones is shaped like a saddle (the technical term is reciprocally concavo-convex). The saddle joints resemble condyloid joints but have even greater freedom of movement. An example is the carpometacarpal joint of the thumb.

  • Ball-and-socket: The round head of one bone fits into a cuplike cavity in the other bone, allowing movement in many directions so long as the bones are neither pulled apart nor forced together, such as the shoulder joint between the humerus and scapula and the hip joints between the femur and the os coxa.

The following are the types of joint movement:

  • Flexion: A decrease in the angle between two bones

  • Extension: An increase in the angle between two bones

  • Abduction: Movement away from the midline of the body

  • Adduction: Movement toward the midline of the body

  • Rotation: Turning around an axis

  • Pronation: Downward or palm downward

  • Supination: Upward or palm upward

  • Eversion: Turning of the sole of the foot outward

  • Inversion: Turning of the sole of the foot inward

  • Circumduction: Forming a cone with the arm or leg