This is a list of some of the commonest knee problems that may occur in the absence of any injury or overuse:
Problems relating to kneecap alignment
If the kneecap does not track smoothly a patient may develop pain in the front of the knee - so-called 'anterior knee pain' - often on going down stairs or on squatting and often accompanied by alarming crunchy noises. Noises may precede any pain and are a warning symptom.
The pain and noises in the early stages do not necessarily mean the there is joint damage - but that the mal-alignment (i.e. bad alignment) and mal-tracking of the bone over the knee joint on bending and straightening is causing excessive strain on the bones and soft tissues.
The telling symptom is that the pain and noises are related to going down hills or stairs, and that there is no problem with going up hill or up stairs.
Another developmental problem which can manifest without significant injury is the plica - a fold of soft tissue which may be present normally inside the knee, but which may on occasion become nipped (like biting the lining of your cheek) and it becomes swollen and repetitively traumatised, leading to 'catching' pain and occasional 'giving way'.
Fat pad syndrome
An anatomically large fat pad may get nipped between the bones of the knee, causing intermittent pain. Later inflammation and scarring of the fat pad may lead to the pain becoming more chronic.
Pain experienced in the knee may, surprisingly, have nothing whatever to do with the knee, and may be 'referred' from a problem elsewhere - for example the hip or the spine.
This is a condition where a wedge of joint cartilage and underlying bone dies (osteonecrosis) due to failure of its blood supply. It is painful. Eventually the starved piece breaks loose and may fall into the joint as a 'loose body'.