A joint is defined as the juncture where two or more bones come together for the purpose of movement or for stability. Contraction of muscles crossing the joint causes it to move.
Normal joint function is defined as a joint’s ability to move throughout its range of motion, bear weight and perform work.
Doctors and physical therapists are trained in anatomy and physiology to know how a normal joint should look and act. They know the structure and function of the bones, muscles, ligaments, tendons, nerves, and blood supply for the joint. All of this can be shown in diagrams and in dissecting cadavers. But beyond that, they have to know how a normal joint works. That includes how far it should be able to flex, extend or rotate. The ability to bear weight or to resist pressure opposing the action of the joint are other aspects of joint function.
When you go to the doctor with a complaint about a bone, muscle or joint, she will perform a musculoskeletal examination that includes assessing joint function. Joint function can be affected by injury or inflammation, so she will look for redness, bruising, and how it compares with the opposite joint that may be uninjured.
Are they the same size and shape?
The doctor will have the patient move the joint through its usual range of motion - flexing, extending, rotating. She will also manipulate the joint to move it. In both cases, she looks for restrictions on the range of motion and whether there is any pain during the movement.
She places a hand on the joint to feel for crepitus (crackling) showing a degradation of the joint. She will perform some specific function tests for suspected injuries. She sees how well the patient can resist movement of the joint.