Locking of the knee joint
This is verbal shorthand used in orthopaedics to describe the inability to fully extend the knee for an appreciable period of time (more than a few minutes). This "passive flexion deformity" is brought on by a mechanical obstacle which makes the knee stop short of full extension, usually at about 10-15 degrees of flexion. The knee will flex reasonably normally but cannot be straightened out. The cause may be a bucket-handle tear of the meniscus, or a bulky flap that has dislocated forwards in the joint; a loose body or an ACL stump may also be to blame.
Patella catching (false locking)
Many patients consider their knee locked when a momentary "sticking" of the knee causes the inability to flex or extend. This is usually relieved as soon as weight is transferred to the other side. Patellofemoral chondral damage is the most likely cause of "catching".
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