There are six basic types of synovial joints. Anatomical joints may consist of a combination of two or more joint types. Some synovial joints are relatively immobile but stable. Others have multiple degrees of freedom, but at the expense of greater risk of injury. The types of the synovial joints are based on their shapes and can be classified as plane, hinge, pivot, condyloid, saddle, and ball-and-socket. The following descriptions are in ascending order of mobility:
- The articulating surfaces of the plane joint are usually flat to allow slipping and gliding properties. Examples include the carpals of the wrist and the acromioclavicular joint.
- A hinge joint (ginglymus) is formed when the cylindrical end of a bone fits into a trough-shaped surface of another bone, like that of an elbow joint (between the humerus and the ulna). These joints act as a hinge, allowing flexion and extension in just one plane.
- In a pivot joint, the rounded end of the bone fits into a sleeve or ring of bone. The atlanto-axial joint, proximal radioulnar joint, and distal radioulnar joint are examples of pivot joints.
- The condyloid joint occurs where an egg-shaped surface of a bone fits into a concavity in another bone. Examples include the wrist joint (radiocarpal joint) and the temporomandibular joint. Some classifications make a distinction between condyloid and ellipsoid joints, but both allow flexion, extension, abduction, adduction, and circumduction movements.
- The surface of a saddle joint has both convex and concave areas which resemble a saddle and permit the same movements as the condyloid joints. The carpometacarpal or trapeziometacarpal joint of the thumb (between the metacarpal and carpal, the trapezium) and the sternoclavicular joint are examples of saddle joints.
- A ball-and-socket joint occurs where one bone ends in a spherical head and the other bone has a round socket. This joint creates the ball-and-socket movement found in such places as the hip and shoulder (glenohumeral). This type of joint allows for all movements except gliding.
The knee joint is an example of a compound joint/modified hinge joint where different types of joints combine. In this example, the condyles of the femur join with condyles of tibia and the saddle joint, where the lower end of the femur joins with the patella.
Plane Joint
The left shoulder and acromioclavicular joints, and the proper ligaments of the scapula.
Ball and Socket Joint
Hip joint: the ball of the femur head fits in the socket of the acetabulum of the pelvis.
Saddle Joint
Sternoclavicular articulation. Anterior view.
Collection of Joint Movements (Example): Wave your arm (hinge joint) and hand (condyloid joint) while nodding your head (pivot joint), and giving a thumbs up (saddle joint). Wind up for the pitch (ball-and- socket joint), raise your knee (compound joint/modified hinge joint), and throw that ball. Strike out! Stand there and enjoy the applause (plane joint at clavicle).