360  Flexion and extension of the knee may be accompanied by crepitus, the audible grating of bones, ligaments, or particles within the excess synovial fluid.: p. 20

Causes
In human anatomy, a bursa is a small pouch filled with synovial fluid. Its purpose is to reduce friction between adjacent structures. The prepatellar bursa is one of several bursae of the knee joint, and is located between the patella and the skin. Prepatellar bursitis is an inflammation of this bursa. Bursae are readily inflamed when irritated, as their walls are very thin.: p. 22  Along with the pes anserine bursa, the prepatellar bursa is one of the most common bursae to cause knee pain when inflamed.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. 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.: p. 22  Other possible causes include gout, sarcoidosis, CREST syndrome,: p. 359  diabetes mellitus, alcohol use disorder, uremia, and chronic obstructive pulmonary disease.: p. 22  Some cases are idiopathic, though these may be caused by trauma that the patient does not remember.: pp. 607–8 The prepatellar bursa and the olecranon bursa are the two bursae that are most likely to become infected, or septic.
Septic bursitis typically occurs when the trauma to the knee causes an abrasion, though it is also possible for the infection to be caused by bacteria traveling through the blood from a pre-existing infection site. In approximately 80% of septic cases, the infection is caused by Staphylococcus aureus; other common infections are Streptococcus, Mycobacterium, and Brucella.: p. 359  It is highly unusual for septic bursitis to be caused by anaerobes, fungi, or Gram-negative bacteria.: p. 608  In very rare cases, the infection can be caused by tuberculosis. Diagnosis
There are several types of inflammation that can cause knee pain, including sprains, bursitis, and injuries to the meniscus. A diagnosis of prepatellar bursitis can be made based on a physical examination and the presence of risk factors in the persons medical history; swelling and tenderness at the front of the knee, combined with a profession that requires frequent kneeling, suggest prepatellar bursitis. Swelling of multiple joints along with restricted range of motion may indicate arthritis instead.: p. 608 A physical examination and medical history are generally not enough to distinguish between infectious and non-infectious bursitis; aspiration of the bursal fluid is often required for this, along with a cell culture and Gram stain of the aspirated fluid.: p. 360  Septic prepatellar bursitis may be diagnosed if the fluid is found to have a neutrophil count above 1500 per microliter,: p. 608  a threshold significantly lower than that of septic arthritis (50,000 cells per microliter).: p. 360  A tuberculosis infection can be confirmed using a radiograph of the knee and urinalysis.