Why Is One Knee Swollen but Not the Other?
The human body is an extraordinary design of balance, where each side typically mirrors the other in structure and function. So when you notice that one knee appears swollen while the other looks normal, it can be startling and concerning. Swelling in just one knee—known as unilateral knee swelling—is a signal from the body that something may be wrong in that specific joint. While swelling in both knees at once often points to systemic conditions like rheumatoid arthritis or heart failure, a single swollen knee almost always indicates a local problem. Understanding the potential causes requires careful observation and medical insight, distinguishing between minor overuse and more serious structural issues.
One of the most common reasons for sudden swelling in one knee is trauma or repeated mechanical stress. The knee is one of the body’s most complex joints, supporting the full weight of the torso while allowing bending, rotation, and pivoting. When injury occurs, the body responds with inflammation, sending fluid and white blood cells into the joint to create a protective cushion. This reaction often presents as visible swelling. Sometimes the injury is obvious—a sharp pop while playing sports or tripping on a curb—but other times it develops gradually and silently over time.
Structural problems inside the joint are frequent causes of swelling. Tears in ligaments like the Anterior Cruciate Ligament (ACL) or Medial Collateral Ligament (MCL) can trigger immediate fluid buildup within the knee, a condition called hemarthrosis. Meniscus tears, which damage the C-shaped cartilage that cushions the joint, may cause swelling to develop slowly over hours or even days. In older adults, even minor falls can result in bone bruises or micro-fractures that produce prolonged swelling. If the knee feels like it is locking or giving way, it often points to a structural tear.
Soft tissues surrounding the joint can also lead to one-sided swelling. Bursitis, for example, is an inflammation of the small fluid-filled sacs (bursae) that help reduce friction between tissues. The prepatellar bursa, located over the kneecap, is particularly susceptible. Often called “housemaid’s knee,” prepatellar bursitis arises from repeated kneeling or direct impacts. Unlike deep joint injuries, bursitis swelling forms a soft, fluid-filled bump on top of the kneecap. While it can be painful and restrict movement, it is technically outside the joint, creating a visible asymmetry.
Another source of localized swelling is a Baker’s cyst, or popliteal cyst. This happens when excess synovial fluid produced by the knee—often due to osteoarthritis or a small cartilage injury—collects in a pocket at the back of the knee. The front of the knee may appear normal, but the back can feel tight and swollen, making full extension or bending difficult. If the cyst ruptures, fluid can flow down into the calf, mimicking the symptoms of a potentially dangerous blood clot.
More serious, non-mechanical causes of one-sided swelling also exist. A red, hot, and swollen knee may indicate an infection, known as septic arthritis, which requires urgent medical attention. Infections can enter the joint through a cut or via the bloodstream. Such cases often include fever and an inability to bear weight. Crystalline arthropathies like gout or pseudogout can also target a single joint, rapidly causing inflammation from uric acid or calcium pyrophosphate crystals. Although it may appear similar to a traumatic injury, it is a metabolic event affecting only one joint.
Vascular issues must also be considered. Swelling around the knee could signal a Deep Vein Thrombosis (DVT), a dangerous blood clot in the leg. DVT can cause significant swelling below and around the knee and may be accompanied by calf pain, heaviness, or discoloration. Because clots can travel to the lungs, this is a medical emergency.
Managing unilateral swelling begins with rest, ice, compression, and elevation (R.I.C.E.). Elevating the knee above heart level helps the lymphatic system drain excess fluid. However, home care has limits. If swelling follows significant trauma, the joint is visibly deformed, weight-bearing is impossible, or there are systemic symptoms such as fever, professional medical attention is critical.
In a world where physical demands often exceed natural limits, a swollen knee is a signal to slow down and care for your body. Whether caused by prolonged kneeling, athletic injury, or structural damage, swelling demonstrates the body’s capacity to protect and repair itself. Observing the swelling’s location, texture, and onset, and consulting healthcare professionals, helps ensure mobility and symmetry are restored. The knee may be a single joint, but it is a vital hinge, supporting the freedom of movement essential to daily life.
