A synovial joint, also known as a diarthrosis, is the most common and most movable type of joint in the body of a mammal. As with most other joints, synovial joints achieve movement at the point of contact of the articulating bones. Structural and functional differences distinguish synovial joints from cartilaginous joints (synchondroses and symphyses) and fibrous joints (sutures, gomphoses, and syndesmoses). The main structural differences between synovial and fibrous joints are the existence of capsules surrounding the articulating surfaces of a synovial joint and the presence of lubricating synovial fluid within those capsules (synovial cavities).
Several movements may be performed by synovial joints. Abduction is the movement away from the mid-line of the body. Adduction is the movement toward the middle line of the body (Figure 1). Extension is the straightening of limbs (increase in angle) at a joint. Flexion is bending the limbs (reduction of angle) at a joint (Figure 2). Rotation is a circular movement around a fixed point.
There are six types of synovial joints. Some are relatively immobile, but are more stable. Others have multiple degrees of freedom, but at the expense of greater risk of injury. The six types of joints include:
- Gliding joints, which only allow sliding movement
- Hinge joints, which allow flexion and extension in one plane
- Pivot joints, which allow bone rotation about another
- Condyloid joints, which allow flexion, extension, abduction, and adduction movements
- Saddle joints, which permit the same movement as condyloid joints (and condylar joints and saddle joints combine to form compound joints)
- Ball and socket joints, which allow all movements except gliding