Did you know that there are between 250-350 joints in the human body? The reason why this is not an exact number is because the number of joints can vary with age, the number of sesamoids can vary, and the definition of joints can either be the point at which two bones connect or the point where bones connect for the purpose of movement.
Joints in the human body are defined by their movement (or absence of movement). There are three types of joints:
- Synarthroses (immovable joints), also known as fixed or fibrous joints, are defined as two or more bones in close proximity that have no movement. An example of immovable joints are the plates of the skull.
- Amphiarthrosis (slightly movable joints), also known as cartilaginous joints, are defined as two or more bones held together so tightly that movement is limited. An example of cartilaginous joints are the vertebrae.
- Diarthroses (freely movable), also known as synovial joints, are defined as containing synovial fluid that allows for all parts of the joint to move against each other. Synovial joints are the most common joint in the body and are the type of joint that most people are familiar with.
When it comes to joint problems and injuries, synovial joints are affected far more than fixed or cartilaginous joints. For this reason, we will be looking closely at the different types of synovial joints, where they are found in the body, and what problems or injuries commonly affect them. There are six different types of synovial joints, including:
Ball and Socket Joints
Ball and socket joints are composed of one bone with a rounded head that fits into the cup of another bone. Since the rounded head can move freely within the cup, or socket, this allows for movement in all directions. Common ball and socket joints include the hips and shoulders. Osteoarthritis is a common problem seen in the hip joints that occurs as the lubricating cartilage of the joint degenerates. In the shoulder, it is common to develop rotator cuff tendinitis, which is a swelling of the tendons that support the rotator cuff.
Hinge Joints open and close in a single direction, much like a door. Some examples of hinge joints include fingers, toes, ankles, elbows, and knees. Like ball and socket joints, hinge joints can be affected by osteoarthritis as the joint degenerates. Additionally, hinge joints are also prone to dislocation and tissue damage from sports injuries.
Condyloid joints, also known as ellipsoid joints, are composed of an egg-shaped bone known as a condyle that fits into a similarly shaped cavity. Although it sounds similar to a ball and socket joint, condyloid joints only allow for forward-backward and side to side movement and do not allow rotation. An example of condyloid joints is the wrist. Condyloid joints are also found in the hands and allow for the movement of fingers. Sprains, carpal tunnel syndrome, and arthritis are common problems that can affect the condyloid joints and surrounding structures.
Also known as rotary joints or trochoid joints, pivot joints are composed of one bone that swivels within the ring of a second bone allowing for rotation. The joint between the ulna and radius, as well as the joint between the first and second vertebrae are pivot joints. Pivot joints can dislocate, fracture, become sprained or strained, and be affected by wear and tear problems, such as arthritis.
Also known as the plane joint, gliding joints are composed of two smooth surfaces that slide over one another to produce limited movement. They are primarily found in the ankles, wrist, and spine. Like the other joints in the body, plane joints can be affected by arthritis.
Saddle joints are composed of one bone that is concave, resembling a saddle, and another bone that is convex, resembling a rider. They are found in the thumb, shoulder, and inner ear, and allow for a variety of movement in multiple directions. Saddle joints are especially prone to osteoarthritis.