There are many possible answers, but you can break it down to some simple groups:
- Traumatic injuries from fractures, meniscus injuries, and ligament tears always lead to swelling because of blood in the knee. These require an evaluation.
- Infections are rare and are associated often with fevers and redness. Luckily this is not seen often, but it can be severe.
- The most common reason, though, is due to "overuse" of the joint, and I'll discuss this one in detail.
Sometimes the swelling goes away with the standard RICE protocol (Rest, Ice, Compression, Elevation). If this isn't helpful, then over-the-counter medications like anti-inflammatories may help. This class of drugs is associated with some side effects, so be sure to take as directed and contact your doctor prior to taking them if you have any cardiac, kidney, or gastrointestinal problems.
A lot of patients ask if they can "play through" the swelling. My response is this: if swelling is occurring, then something is abnormal in the knee. If the reason is overuse, then changing your running patterns or trying cross-training may help. If pain and swelling are worsening, then you need to see your primary care physician (PCP) or orthopedic surgeon to get a good knee exam.
Surgery is always the last option when dealing with chronic conditions of the knee, but you should always seek some assistance if the simple treatments don't help.