Identifying Types of Joints

The sternoclavicular and trapeziometacarpal (thumb) joints are unique in that the involved bones have both convex and concave portions that give their saddle shape.

There are only two ball and socket joints in the human body. They are the hip and the shoulder joints (there are two in the hip and one in each shoulder). Of the two areas where there are ball and socket joints, the shoulder is the weakest. More shoulder dislocations occur than any other joint. The greater the range of movement, generally the weaker the joint is. Unlike the hip which has a deeper depression for the femur to fit into, the shoulder joint has a relatively shallow depression for the humerus to fit into. This coupled with its weaker tendons makes it the weaker of the two ball and socket joints.

The shoulder is a ball and socket joint, which allows a wide range of movement. The elbow is a hinge joint, which only allows movement in one plane. The thumb is a saddle joint, which allows movements in a variety of directions. Finally, the knuckles are a condyloid joint which is a rounded bone moving within a shallow "bowl" shape of another bone.

A syndesmosis is a type of immovable fibrous joint that is made up of collagen. This same interosseous membrane also exists between the radius and ulna. Sutures and gomphoses are other examples of fibrous joints.

Intra-articular discs are structures made of fibrocartilage that increase joint congruency, and improve shock absorption. They can be found in the AC joint, the SC joint, the TMJ, and several other joints. Similarly, the acetabular labrum in the coxafemoral joint is comprised of fibrocartilage, and improves joint congruency. However, it is not an intra-articular disc.

Ginglymus joints are also known as hinge joints, which only permit flexion/extension movements. The humeroulnar joint of the elbow is the best example of this. Cotyloid and ball and socket joints are synonymous, and permit motion in all three planes. Pivot joints and lateral ginglymus are synonymous, and permite transverse plane movement.

Joints allowing a great range of motion require lubrication. This lubrication is provided by sinovial fluid, which is present in sinovial joints. The fluid is encased inside a membrane to provide this lubrication, allowing a greater degree of motion.

Fibrous joints and suture joints are significantly less mobile, while cartilaginous joints are somewhat less mobile. Synarthrosis is a term used to describe any type of joint that does not allow for motion.

Fibrous joints, also known as synarthroses, are joined by fibrous tissue, have no joint cavity, and allow little movement. An example of this is a suture, which is found between the flat bones of the skull. Cartilaginous joints are joined by cartilage, have no joint cavity, and allow little or no movement. An example of this is the epiphyseal plate. Synovial joints, in contrast, do contain a joint cavity and allow a great deal of free movement. An example of this is the hip joint.

Where the first rib meets the manubrium (top of sternum), the joint is cartilaginous and called a synchondrosis. Syndesmoses are fibrous joints of connective tissue, and syostoses are areas of bone fusion.

A diarthrosis is another name for a synovial joint. Synovial joints (diarthroses) allow more movment than synarthroses. The tibiofemoal joint (the knee), the humero-ulnar joint (one of the elbow joints), and the glenohumeral joint ("the shoulder") are all synovial joints. Although the proximal tibiofibular joint is a diarthrosis, the distal tibiofibular joint is not; it is a type of synarthrosis.