Introduction to Joints
A joint is a point where two or more bones of the skeleton meet. The joints hold the skeletal framework together and aid in movement. Joints can be classified in numerous ways. Depending on the possible range of movement, they are classified as:
- Synarthrosis (no movement)
- Amphiarthrosis (little movement)
- Diarthrosis (full range of movement)
Diarthrosis, also known as synovial joints allow for full range of motion due to their structure and presence of a joint cavity filled with synovial fluid. Ball and socket joint is one type of such a synovial joint.
Ball and Socket Joint – Definition
Ball and socket joints, also known as spheroidal joints are defined as joints, which are composed of a ball-like rounded structure of one bone that moves freely within a socket-like depression in another bone.
Picture 1: Schematic representation of a ball & socket joint
Range of motion of ball & socket joints
These joints allow for a wider range of motion than any other type of joint. Thus, they need more musculature around them to ensure stability. These joints allow for movement in three different planes. The possible movements include flexion/extension, abduction/adduction, rotation, and circumduction. As a result, these joints are also classified under as multiaxial joints.
Factors limiting the movement include the depth of the socket and presence of cartilaginous labrum around the joint.
Examples
Examples of such ball and socket joints in the human body include the hip joint and the shoulder joint.
Shoulder Joint
In the shoulder joint, or more accurately the glenohumeral joint, the spherical head of the humerus (ball) fits into the depression of the glenoid fossa (socket) of the scapula bone. Although the glenoid fossa is much shallower than a proper cup-like socket, the joint is reinforced by a cartilaginous labrum around the joint. The muscles of the rotator cuff also aid in stabilizing the joint by keeping the head of the humerus in place.
Picture 2: Shoulder joint
Hip Joint
The hip joint is another example of ball & socket joint, where the head of the femur (ball) fits into a cup-like cavity known as acetabulum (socket) in the hip bone. This joint is provided extra stability by the strong musculature and ligaments surrounding it. Moreover, the acetabulum is a much deeper cavity as compared to the glenoid fossa of the shoulder joint, adding to the stability of the joint, while at the same time, limiting some movement.
Picture 3: Hip joint
