My knee joint keeps locking after injury,very painful. it creaks, had an x-ray after paramedic popped back into place, but found nothing? why?

X-ray shows bones: The X-ray will predominantly show the presence or lack of any injury to the bone. You could have a normal X-ray, but still have a sprain injury, or more serious injuries such as to tendons, ligaments, or to a meniscus. Some of these more serious injuries will heal with time, but some of them require surgery. Locking is often a symptom of an injured meniscus.