- Differentiate among the six categories of joints based on shape and structure
Types of Synovial Joints
Synovial joints are further classified into six different categories on the basis of the shape and structure of the joint. The shape of the joint affects the type of movement permitted by the joint. These joints can be described as planar, hinge, pivot, condyloid, saddle, or ball-and-socket joints.
Planar joints have bones with articulating surfaces that are flat or slightly curved. These joints allow for gliding movements; therefore, the joints are sometimes referred to as gliding joints. The range of motion is limited and does not involve rotation. Planar joints are found in the carpal bones in the hand and the tarsal bones of the foot, as well as between vertebrae.
In hinge joints, the slightly-rounded end of one bone fits into the slightly-hollow end of the other bone. In this way, one bone moves while the other remains stationary, similar to the hinge of a door. The elbow is an example of a hinge joint. The knee is sometimes classified as a modified hinge joint.
Pivot joints consist of the rounded end of one bone fitting into a ring formed by the other bone. This structure allows rotational movement, as the rounded bone moves around its own axis. An example of a pivot joint is the joint of the first and second vertebrae of the neck that allows the head to move back and forth. The joint of the wrist that allows the palm of the hand to be turned up and down is also a pivot joint.
Condyloid joints consist of an oval-shaped end of one bone fitting into a similarly oval-shaped hollow of another bone. This is also sometimes called an ellipsoidal joint. This type of joint allows angular movement along two axes, as seen in the joints of the wrist and fingers, which can move both side to side and up and down.
Each bone in a saddle joint resembles a saddle, with concave and convex portions that fit together. Saddle joints allow angular movements similar to condyloid joints, but with a greater range of motion. An example of a saddle joint is the thumb joint, which can move back and forth and up and down; it can move more freely than the wrist or fingers.
Ball-and-socket joints possess a rounded, ball-like end of one bone fitting into a cup-like socket of another bone. This organization allows the greatest range of motion, as all movement types are possible in all directions. Examples of ball-and-socket joints are the shoulder and hip joints.
The Role of Rheumatologists
Rheumatologists are medical doctors who specialize in the diagnosis and treatment of disorders of the joints, muscles, and bones. They diagnose and treat diseases such as arthritis, musculoskeletal disorders, osteoporosis, and autoimmune diseases such as ankylosing spondylitis and rheumatoid arthritis.
Rheumatoid arthritis (RA) is an inflammatory disorder that primarily affects the synovial joints of the hands, feet, and cervical spine. Affected joints become swollen, stiff, and painful. Although it is known that RA is an autoimmune disease in which the body’s immune system mistakenly attacks healthy tissue, the cause of RA remains unknown. Immune cells from the blood enter joints and the synovium, causing cartilage breakdown, swelling, and inflammation of the joint lining. Breakdown of cartilage results in bones rubbing against each other, causing pain. RA is more common in women than men; the age of onset is usually 40–50 years of age.
Rheumatologists diagnose RA on the basis of symptoms (joint inflammation and pain), X-ray and MRI imaging, and blood tests. Arthrography, a type of medical imaging of joints, uses a contrast agent, such as a dye, that is opaque to X-rays. This allows the soft tissue structures of joints, such as cartilage, tendons, and ligaments, to be visualized. An arthrogram differs from a regular X-ray by showing the surface of soft tissues lining the joint in addition to joint bones. An arthrogram allows early degenerative changes in joint cartilage to be detected before bones become affected.
There is currently no cure for RA; however, rheumatologists have a number of treatment options available. Early stages can be treated by resting the affected joints, using a cane or joint splints, to minimize inflammation. When inflammation has decreased, exercise can be used to strengthen the muscles that surround the joint in order to maintain joint flexibility. If joint damage is more extensive, medications can be used to relieve pain and decrease inflammation. Anti-inflammatory drugs such as aspirin, topical pain relievers, and corticosteroid injections may be used. Surgery may be required in cases in which joint damage is severe.
- Planar joints have bones with articulating surfaces that are flat or slightly curved, allowing for limited movement; pivot joints consist of the rounded end of one bone fitting into a ring formed by the other bone to allow rotational movement.
- Hinge joints act like the hinge of a door; the slightly-rounded end of one bone fits into the slightly-hollow end of the other bone; one bone remains stationary.
- Condyloid joints consist of an oval-shaped end of one bone fitting into a similarly oval-shaped hollow of another bone to allow angular movement along two axes.
- Saddle joints include concave and convex portions that fit together and allow angular movement; ball-and-socket joints include a rounded, ball-like end of one bone fitting into a cup-like socket of another bone which allows the greatest range of motion.
- Rheumatologists diagnose and treat joint disorders, which include rheumatoid arthritis and osteoporosis.
- Immune cells enter joints and the synovium, causing cartilage breakdown, swelling, and inflammation of the joint lining, which breaks down cartilage, resulting in bones rubbing against each other, causing pain.
- condyloid joint: consists of an oval-shaped end of one bone fitting into a similarly oval-shaped hollow of another bone
- ball-and-socket joint: consists of a rounded, ball-like end of one bone fitting into a cup-like socket of another bone, allowing the first segment to move around an indefinite number of axes which have one common center
- rheumatoid arthritis: chronic, progressive disease in which the immune system attacks the joints; characterized by pain, inflammation and swelling of the joints, stiffness, weakness, loss of mobility, and deformity