Knee swelling, often referred to as “water on the knee,” is a very common knee symptom—in athletes and non-athletes, young and old alike. It may be acute or chronic, and it can stem from a variety of causes. Read on to learn about some of the most common causes behind a swollen knee.
Before examining the root causes of knee swelling, it is important to note that knee swelling can actually occur in two places. The swelling can be an accumulation of fluid within the knee joint itself; or it can be an accumulation of fluid in the surrounding soft tissue, as in the case of prepatellar bursitis where fluid accumulates above the kneecap. This article will focus on the root causes behind fluid accumulation within the knee joint itself.
Acute
Acute swelling occurs relatively quickly—typically within 24–48 hours of a traumatic injury. Blood accumulation tends to occur more rapidly, though the swelling could also be an accumulation of non-bloody fluid (such as synovial fluid.)
ACL tear
An ACL tear is one of the most common causes of blood accumulation in the knee, and you’ll typically see swelling occur within minutes following the tear.
Fracture
Fracture of the knee bone and cartilage is another common cause of blood accumulation. As with an ACL tear, this type of fracture will allow blood to enter the joint, resulting in rapid swelling.
Meniscus tear
Meniscus tears involve damage to one of the two menisci (a type of cartilage) in your knee joint and can lead to an accumulation of joint fluid that occurs over a number of hours or even days.
Ligament sprain
Ligament sprains are similar to meniscus tears in that they can also cause an accumulation of joint fluid over a number of hours or days. There are four major ligaments that contribute to the stability of the knee, plus several smaller ligaments surrounding the knee, so ligament sprains are more likely to happen than you might think.
Chronic
There is also such thing as chronic swelling of the knee. This is usually due to chronic conditions like arthritis.
Arthritis
Arthritis is perhaps the most common cause of chronic knee swelling. Knee arthritis in particular causes the body to produce extra fluid in the knees, resulting in chronic swelling. Often with knee arthritis the amount of fluid in the knee will fluctuate over time, and one knee will appear larger than the other.