Engineering definition
For engineers, joints are designed and engineered structures that have a specific purpose. Two common types of joints are hinge joints and ball joints. Hinge joints restrict motion to rotation within a plane, and the center of rotation lies on the axis of the pin. Ball joints allow rotation around the pivot point at the geometric center of the ball. Lower tolerances may allow a joint to have out of plane movement, but this is generally not an intentional part of the design.
Medical definition
For clinicians, the term has a similar meaning to that defined by engineers. Common anatomical joints include the knee joint, the hip joint, and the spine. These joints allow the body to move. Bones form the rigid structural members, and ligaments connect adjacent bones to form a joint. Cartilage over the tip of the bone provides a cushion, and some joints like the knee have additional tissue within the joint (meniscus). Although at first glance, the knee joint may appear to be a simple hinge joint, its movement is much more complex. As the knee bends, the femur rolls and slides on the tibial plateau. This causes the contact point to move, the contact area to change, and the center of rotation to change during knee flexion. The movement of the hip joint more closely represents an engineering ball and socket. However, the anatomical geometry deviates more from a perfect sphere to that of a machined engineering component. The spine is not a single joint, but rather a series of bending and sliding joints that overall allow movement. In healthy people, each segment moves a portion of the overall movement. The center of rotation does not lie within a specific joint and changes location with movement. As a result, spinal movement can be quite complex.
Confusion
Engineers tend to oversimplify the knee joint by assuming it is a simple hinge joint. External braces used to support the joint must hinge around the axis of rotation to perform well and avoid causing binding. This can be a difficult task, because the location of this center of rotation moves with flexion and extension. Similar problems arise when integrating mechanical systems with other body joints.
