Last Updated: October 21, References. There are 22 references cited in this article, which can be found at the bottom of the page. This article has been viewed 14, times. Knee bruises can occur because of a fall, a sports injury, a car accident, or even something as simple as hitting your knee against the edge of a table. All three types of bruises are treated the same way, but bone bruises will take much longer to heal several months than a subcutaneous bruise about 2 weeks.