Joints are points in your skeleton where separate bones come together to allow movement. If you did not have joints, you would not be able to walk, move your arms, or otherwise perform functions. Because joints just work, we do not usually think about them. They are just there, and we expect them to perform. However, issues can develop in your joints, which can hamper your freedom of mobility.
There are several types of joints. Immovable joints (synarthrosis) do not allow motion. It seems odd that these should be considered joints, but they are places on the body, such as where your skull bones have joined together. Skull bones are separate at birth. They slide over each other so babies can go through the birth canal. Thereafter, they start fusing together with fibrous tissue making the skull one rigid bone group. These joints are called sutures. This process can take until age 20 to complete. The skull is importantly formed to protect the brain from injury from the outside world. We do not think much about these joints unless they fuse improperly, creating a space too small for the developing brain or fusing and altering the head's shape. Correction of this is usually treated with a helmet to remold the skull. Immovable joints are also found where teeth fit into their sockets. They are held in place by the periodontal ligament.
Slightly moveable joints (amphiarthrosis) are bones that are just as their name indicates. These joints only allow a small bit of motion. An example is the individual bones of the spine or the spine vertebrae, where there is some flexibility between each bone that helps with trunk movement and allows adjustments for balance. Cartilage keeps these bones from moving too far. These joints may be fused due to a genetic issue or maybe fused surgically to provide stability to the spinal column after an injury or disease process. Too much movement from the spine vertebrae can lead to spinal cord injury (or additional injury). Too little movement can overwork the spine vertebrae above or below the fusion making the spinal bones unstable over time. This can occur after a spinal fusion but typically takes years if it even occurs. Newer, slightly flexible spinal stabilization rods have reduced this complication immensely. There are also slightly moveable joints on the ends of the tibia and fibula joints which provide little movement but enhance stability and weight bearing. The pubic synthesis is also a fibrocartilage joint between both sides of the pelvis that helps manage upper body weight and allows slight separation of the pelvis for childbirth.
Freely moveable joints (diarthrosis) are those we think the most about. These provide movement to most of the body. The ends of these bones are enclosed in a capsule that contains fluid, so they slide easily with movement. The types of freely moving joints and their locations are provided in this chart:
| Type of Freely Moving Joint |
Joint Form |
Example of Body Location |
| Ball and socket joint |
A round head bone fitting into a cup-shaped bone |
Both shoulders and hips |
| Saddle joint |
A concave (turned inward bone) fits into a convex bone (turned outward). The concave bone looks like the shape of a saddle. The convex bone looks like the shape of a rider. |
Thumbs, shoulders, and inner ear |
| Hinge joint |
Allows flexion and extension only |
Elbows, fingers, knees, ankles, toes |
| Condyloid joint |
Convex and concave bones create an elliptical formation |
Wrists, joints at the junction of the palm of the hand and fingers, joints at the junction of the toes to the foot |
| Pivot joint |
Allows rotation movement |
Neck, wrists, and elbows |
| Gliding (plane) joint |
Surfaces are flat, which allows the bones to slide over each other |
Hands, skull before fusion of bones |
There are medical conditions that can affect your joints. These include arthritis or joint inflammation which includes rheumatoid arthritis, gout, or degenerative arthritis (osteoarthritis). Bursitis is inflammation of the fluid that pads bones. Infection can occur within the joint. Tendonitis is inflammation, irritation, and swelling of a tendon or tendons attached to a joint.
Paralysis can affect joints in a variety of ways. The muscles and tendons around a joint can become lax or weak, which will not then hold the joint in place. This can lead to joint displacement. If the muscles are weak, handling may create a movement that is not within the purpose of a particular joint’s flexibility. For example, this can occur by moving a body part in a rotation when the joint should only be moved in flexion and extension. Another issue is tone or spasticity, where the muscles pull the joint past its functionality. Muscle imbalances can pull joints out of alignment. An example is scoliosis of the spine, where muscle imbalance pulls the spine out of its usual position.
Joint issues due to paralysis can be avoided and reduced. Gentle handling of a joint during movement and exercise is critical to the health of your joints. Moving limbs carefully is essential. One of the most common issues with joint handling is the knee ‘backslapping’ or being stretched beyond its extension forward due to rough handling. This can happen to any joint, like pulling sideways on the leg in positioning or tugging the shoulder in moving the upper body. Knowing your joints' extension and flexion limitations is essential in preventing injury.