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.
Podophyllotoxin (PPT) is the active ingredient in Podofilox, which is a medical cream that is used to treat genital warts and molluscum contagiosum. It is not recommended in HPV infections without external warts. It can be applied either by a healthcare provider or the person themselves.It is a non-alkaloid toxin lignin extracted from the roots and rhizomes of Podophyllum species. A less refined form known as podophyllum resin is also available, but has greater side effects.Podophyllotoxin was first isolated in pure form in 1880 by Valerian Podwyssotzki (1818 – 28 January 1892), a Polish-Russian privatdozent at the  University of Dorpat (now:  Tartu, Estonia) and assistant at the Pharmacological Institute there.It is on the World Health Organizations List of Essential Medicines. Medical uses
Podophyllotoxin possesses a large number of medical applications, as it is able to stop replication of both cellular and viral DNA by binding necessary enzymes. It can additionally destabilize microtubules and prevent cell division. Because of these interactions it is considered an antimitotic drug, although modern medicine instead use less orally toxic derivatives when such effect is wanted.Podophyllotoxin cream is commonly prescribed as a potent topical antiviral. It is used for the treatment of HPV infections with external warts as well as molluscum contagisum infections. 0.5% PPT cream is prescribed for twice daily applications for 3 days followed by 4 days with no application, this weekly cycle is repeated for 4 weeks. It can also be prescribed as a gel, as opposed to cream.