joint
Meaning of joint: Several tissues share the work at every joint, so pain can come from bone, cartilage, ligament or lining without the others failing.
Definition of joint
A joint is the place where two or more bones meet, held together by ligaments and, in most movable joints, cushioned by cartilage that lets the bone ends glide with almost no friction. Joints range from the immovable seams between skull bones to the shoulder, which moves in every direction. Structure follows the movement required.
What types of joint are there?
| Type | What holds it together | Movement | Example |
|---|---|---|---|
| Fibrous | Dense fibrous tissue directly between the bones | None, or almost none | The seams of the skull |
| Cartilaginous | Cartilage between the bone ends | Slight | Between vertebrae, and the front of the pelvis |
| Synovial | A capsule enclosing lubricating fluid | Free, in directions set by the shape | Knee, hip, elbow, shoulder |
Synovial joints are divided again by shape, and shape dictates the movement available. A hinge such as the elbow bends in one plane. A ball and socket such as the hip or shoulder rotates in all of them. A pivot lets one bone turn against another, as when the head turns. Trading stability for range is the recurring theme: the shallow socket of the shoulder, formed partly by the shoulder blade, allows enormous range and dislocates far more readily than the deep socket of the hip.
How does a synovial joint work?
Each bone end is covered by a smooth layer of cartilage that has no blood supply or nerves of its own and draws its nutrition from the fluid around it. A tough fibrous capsule encloses the whole joint, and its inner lining produces synovial fluid, a viscous liquid that lubricates the surfaces and feeds that cartilage. Ligaments running across or within the capsule restrict movement to the intended directions. Some joints add a wedge of fibrous cartilage between the surfaces to improve fit and spread load, as the knee does. Small fluid-filled sacs called bursae sit where tendons pass over bone, reducing friction outside the joint itself.
Where does joint pain come from?
Several tissues are involved and each fails in its own way. Cartilage wears thin, leaving bone surfaces poorly cushioned, which is the process in osteoarthritis. The lining can become inflamed and thickened, which is the process in rheumatoid arthritis and produces swelling, warmth and prolonged morning stiffness. Ligaments tear under sudden force. Crystals can deposit within the fluid and provoke intense inflammation, as in gout. Infection can settle in the joint space. Pain may also arise outside the joint entirely, from a tendon, a bursa, or referred from elsewhere, which is why an examination distinguishes pain on active movement from pain when the joint is moved passively.
Used in a sentence
Physical examination focused on the affected joint to assess swelling, warmth, and range of motion.
Good to know
Joint pain arising from the joint itself is distinguished from pain referred from a nearby muscle, tendon or bursa, since evaluation and treatment differ depending on which structure is actually involved.