Signs and symptoms typically include:

• Swelling. The skin around your kneecap can puff up noticeably, especially when you compare the affected knee to the normal one.
• Stiffness. When your knee joint contains excess fluid, you might not be able to bend or straighten your leg completely.
• Pain. Depending on the cause of the fluid buildup, the knee might be very painful — to the point that it's difficult or impossible to bear weight on it.

Many types of problems, ranging from traumatic injuries to diseases and other conditions, can cause a swollen knee.

Injuries

Damage to any part of your knee can cause excess joint fluid to accumulate. Injuries that can cause fluid buildup in and around the knee joint include:

• Torn ligament, particularly the anterior cruciate ligament (ACL)
• Cartilage (meniscus) tear
• Irritation from overuse
• Broken bones

Diseases and conditions

Underlying diseases and conditions that can produce fluid buildup in and around the knee joint include:

• Osteoarthritis
• Rheumatoid arthritis
• Infection
• Gout
• Pseudogout
• Bursitis
• Cysts
• Tumors

• Age. Your likelihood of developing a swollen knee related to arthritis increases as you age.
• Sports. People who participate in sports that involve twisting the knee, such as basketball, are more likely to experience the types of knee injuries that cause swelling.
• Obesity. Excess weight puts added stress on the knee joint, contributing to the tissue and joint overload and knee degeneration that can lead to a swollen knee. Obesity increases your risk of osteoarthritis, one of the more frequent causes of knee swelling.

Complications of a swollen knee can include:

• Muscle loss. Fluid in the knee can harm the working of your muscles and cause thigh muscles to weaken and atrophy.
• Fluid-filled sac (Baker's cyst). The buildup of fluid in your knee can lead to the formation of a Baker's cyst in the back of your knee. A swollen Baker's cyst can be painful, but usually improves with icing and compression. If the swelling is severe, you might need to have fluid removed (cyst aspiration).