Locking symptoms can occur in the knee if loose fragments of cartilage or bone get caught in the mechanism of the joint.
The classical presentation of a ‘locked knee’ occurs following a period of kneeling or crouching followed by an inability to straighten the knee. Usually this is due to a tear in the shock absorbing meniscal cartilage flipping into the joint. Often the knee can be unlocked by shaking or wiggling the leg until the loose fragment moves back into place.
Occasionally, catching and locking symptoms arise if a damaged and roughened area develops in the smooth articular cartilage on the joint surface. This is particularly common if arthritis develops behind the patella.