A joint is held together by ligaments and the joint capsule, and the joint surfaces are covered with smooth cartilage. If the foot rolls over, the knee twists or the hand breaks a fall, these structures can be overstretched or tear. The upper ankle joint is affected most often, usually the outer ligament at the ankle.
Injuries range from a sprain, in which the ligaments are only overstretched, to a torn ligament or capsule, up to a dislocation (luxation): here the joint surfaces lose contact with each other, for example in the shoulder, elbow, kneecap or finger. With great force, a piece of cartilage can also shear off, sometimes with bone attached.
Swelling, bruising and pain say little about how serious the injury is. What matters is the stability test and imaging: the X-ray rules out a fracture, while ultrasound and magnetic resonance imaging show ligaments, capsule and cartilage. If a ligament injury goes undetected, the joint can become permanently unstable and keep giving way.
Until you see a doctor, the RICE rule helps: rest, ice (never directly on the skin), compression with an elastic bandage and elevation. A dislocated joint should be seen by a doctor promptly – trying to put it back yourself can damage nerves, blood vessels and cartilage.