Prepatellar bursitis is caused by either a single instance of acute trauma to the knee, or repeated minor trauma to the knee. The trauma can cause extravasation of nearby fluids into the bursa, which stimulates an inflammatory response.[2] This response occurs in two phases: The vascular phase, in which the blood flow to the surrounding area increases, and the cellular phase, in which leukocytes migrate from the blood to the affected area.[2]:p.