Common causes of knee pain can really be broken up into two categories: traumatic and atraumatic.
From a traumatic standpoint, you can break something in the knee, like a bone, which can cause pain from a fall, car accident, or some other type of trauma. You can also sprain a ligament or a piece of cartilage, or shear off a piece of cartilage. These are some examples of traumatic causes.
From an atraumatic standpoint, you may just wake up one day with a sore knee, which could be due to arthritis, which is really just the loss of cartilage. It can also be due to inflammation in the knee that occurs for no known reason. Additionally, from a serious standpoint, an atraumatic cause of knee pain could be something like an infection.
