A joint is the spot where two or more bones come together. Most joints allow for mobility, although several are fixed in their position. Joints are stabilized by ligaments and cartilage. Joints are classified as fibrous, cartilaginous, or synovial based on their structures.

  • Fibrous joints: The bones of a fibrous joint are connected by fibrous tissue. They range from being immovable (like joints between the bones of the skull) to being slightly moveable (joints between the tibia and fibula in the legs).

  • Cartilaginous joints: The joint surfaces in cartilaginous joints are covered with hyaline cartilage and have fibrocartilaginous discs between them. Like the fibrous joints, the cartilaginous joints can be immoveable or slightly moveable.

  • Synovial joints: Synovial joints allow for the most movement. A typical synovial joint includes bones covered in hyaline cartilage and a joint cavity lined with a synovial membrane and filled with synovial fluid. A durable fibrous joint capsule surrounds the joint. Some synovial joints also have fibrocartilaginous discs between the bones.

    The shapes of the bones determine what types of movements can occur in synovial joints. They’re categorized and described by these shapes:

    • Plane joints, such as the acromioclavicular joint of the shoulder, have flat articular surfaces that allow for a sliding motion.

    • Hinge joints allow flexion and extension of joints like the elbow and the knee.

    • Pivot joints include a bone shaped like a pivot and a ring made of bone or ligament. Pivot joints, such as the atlantoaxial joint of the cervical spine, have rotational movements.

    • Condyloid joints have an oval surface on one bone that articulates with an oval-shaped depression in another bone. This configuration allows for flexion, extension, abduction, adduction, and circumduction.

    • Ball-and-socket joints involve a ball-shaped head that fits into a bony socket. The shoulder and hip are ball-and-socket joints. This type of joint allows for free movement in several directions, including flexion, extension, abduction, adduction, medial and lateral rotation, and circumduction.

    • Saddle joints have the appearance of a saddle and allow for flexion, extension, abduction, adduction, and circumduction. The carpometacarpal joint at the base of the thumb is an example of a saddle joint.

Cartilage is a form of connective tissue made up of cells and fibers in a flexible matrix. It’s found in joints as well as in your nose and ears. You’ll run into three types of cartilage:

  • Hyaline cartilage: This durable type covers most of the bone surfaces in synovial joints. It’s also found in the nasal septum, rings of the trachea, and costal cartilages of the ribs, and it forms the epiphyseal plates of growing bones.

  • Fibrocartilage: This cartilage has a larger number of collagen fibers and less matrix. It’s found in the discs in joint spaces including the temporomandibular joint, knee joint, and joints between the bodies of the vertebrae.

  • Elastic cartilage: This type contains elastic fibers in the matrix, so it’s more flexible than either hyaline or fibrocartilage. Your ear has elastic cartilage, and your epiglottis is also formed from elastic cartilage.

Arthritis is an umbrella term for more than 100 diseases that affect the joints in people of all ages. Two noteworthy types of arthritis include osteoarthritis and rheumatoid arthritis:

  • Osteoarthritis is the most common type of arthritis. It’s more likely to occur after the age of 60, generally due to wear and tear on the joints that normally happens during the aging process. The damage occurs when the joint cartilage wears thin and eventually breaks down. Without the cushioning effect of cartilage, the joint just doesn’t function like it should. Symptoms of osteoarthritis include joint pain and stiffness. Osteoarthritis is more likely to occur in joints that have been previously injured and in people who are obese. It can affect any joint but usually shows up in the hands, wrists, neck, back, knees, and hips. Osteoarthritis can’t be cured, but it can be treated with nonsteroidal anti-inflammatory medication, exercise, and sometimes surgery.

  • Rheumatoid arthritis is an inflammatory chronic joint condition that can occur much earlier than osteoarthritis (as early as childhood in some cases). It’s an autoimmune disease, but the actual cause isn’t always known. Patients with rheumatoid arthritis suffer from joint pain and stiffness along with fatigue and flu-like symptoms. Without treatment, rheumatoid arthritis can do a great deal of damage to the joints. Rheumatoid arthritis can’t be cured either, but medications such as nonsteroidal anti-inflammatory drugs, steroid medications, and immunosuppressants may slow down progression of the disease.