By your description, prepatellar bursitis is the most likely cause of your father’s symptoms.

Bursae are sac-like cavities located in many areas of the body. Most importantly they lie next to large joints such as the hips, knees, shoulders and elbows. They provide a smooth gliding surface just outside the joint. This helps the muscles and other surrounding structures move easily when the joint is moved.

However, sometimes a bursa becomes inflamed (a condition called bursitis). It usually causes pain with pressure or use. Repetitive motion, injury and diabetes are risk factors for this sort of bursitis. But it is a common condition that may develop for no identifiable reason.

The most common forms of bursitis cause no swelling. But if the region over the kneecap (also called the patella) becomes swollen, it likely signals prepatellar bursitis. Most often this condition is caused by infection, crystal deposits (such as gout) or blood.

Of these, infection is the most important to recognize. Even though the absence of prominent pain would be unusual.

Hemorrhagic (or bloody) prepatellar bursitis is the most common cause of swelling in that area. It usually develops because of trauma, even if the injury is minor. People who have a bleeding tendency (including those taking blood thinners) have a higher risk for this problem. If there is no pain, prepatellar bursitis due to trauma does not require treatment.

It might not be bursitis. For example, swelling around the knee could develop because of arthritis or a cartilage injury. Another common cause of a “spongy” area near the knee is fatty deposits, including a lipoma (a benign fatty tumor).

It’s important for your father to be examined by his doctor. He or she may remove a small amount of fluid from the swollen bursa. It can be analyzed for signs of infection, inflammation, crystals or blood.

Making sure there is no infection is the top priority. If there is an infection, your father would need antibiotics. Otherwise, the most common options include observation without drug therapy, acetaminophen (Tylenol), or an anti-inflammatory drug such as ibuprofen or naproxen. A cortisone injection is occasionally recommended and may be quite helpful.