Repetitive microtrauma and friction between the skin and patella results in inflammation of the pre-patellar bursa present in this location. Bursae are fluid filled synovial pouches that cushion adjacent surfaces such as skin, tendons and bone. Chronic inflammation causes the bursa to become distended and a swelling develops anterior to the knee joint which is largerly painful. The condition, also known as the Housemaid's Knee, is more common in occupations which require working on the knees such as housemaids, floor workers and rug installers. (Image source: https://aneskey.com/prepatellar-bursitis/ )