Anatomy: What is the difference between locked joints and closed-packed joints?

"Locked joint" is not an anatomical term, but a common term. In some cases (e.g. knee and elbow), when we "lock" the joint (full extension), it is in fact in its close-packed position for that joint (true close-packed for elbow is full extension and supination). But we don't usually talk about a "locked shoulder" because the bones of that joint are never as congruent and close-packed as those in the knee and elbow (which are hinge joints with little separating bone from bone, at the joint). In the case of the knee and elbow, a slight bend will take the joint out of a locked/close-packed position. This is why we always suggest to "microbend" the knee and elbow in many postures. Once bent, the adjacent muscles contract to support the joint, rather than the "support" coming from bone resting on bone.

However, we want to clarify one point. It is not necessarily unsafe simply to put a joint in its close-packed position.

The knee or elbow in a close-packed position is not unsafe, on its own, it is when you put body weight over the close-packed joint so that the weight is going through the joint that problems may arise. 

Further, it's more complicated yet, as a locked knee with weight on it might also be fine if the person doing it (for example, a trained athlete, gymnast, or dancer) is supporting the weight with strong, contracted muscles adjacent to the joint. The danger arises when:

1. The joint is close-packed/locked
2. AND there is weight-bearing by the locked joint
3. AND there is insufficient adjacent muscular contraction/support