Purple or bluish discoloration on the skin around the knee can range from a simple, temporary issue to a sign of a serious medical condition. Understanding the underlying reasons for this color change is the first step toward determining whether the symptom requires self-care or immediate professional attention. This information helps distinguish between a common bruise and a more urgent circulatory or infectious problem that needs prompt evaluation.
Understanding Skin Discoloration
Skin takes on a purple or bluish hue due to two primary biological reasons: a lack of oxygen in the blood or the presence of blood outside of the vessels. Healthy skin is reddish due to oxygen-rich hemoglobin within the capillaries. When blood loses oxygen, the hemoglobin shifts to a darker, bluish-purple color, a phenomenon known as cyanosis.
Localized cyanosis can cause the knee to turn purple when blood flow is temporarily slowed, causing the tissue to extract more oxygen than usual. Alternatively, a deeper purple, black, or red-purple discoloration, known as bruising or a hematoma, occurs when damaged blood vessels leak blood into surrounding tissue layers. The breakdown of hemoglobin in the tissue causes this visible color change.
Common Causes of Localized Purple Knees
The most frequent cause of a purple knee is a simple bruise, or contusion, resulting from minor trauma like a fall or bumping into furniture. This trauma breaks small blood vessels, leading to a localized collection of blood under the skin that appears dark purple or black shortly after impact. Bruising is usually accompanied by tenderness and changes colors over several days as the body reabsorbs the blood.
Another common, temporary cause is positional cyanosis, which arises from restricted blood flow due to pressure or an awkward position. Sitting cross-legged or kneeling for an extended period can temporarily compress blood vessels, slowing circulation and causing a mild, reversible bluish tint. This discoloration resolves quickly once the position is changed and is not usually accompanied by intense pain or swelling. Chronic inflammation from conditions like arthritis can also cause minor localized color changes due to increased blood flow, sometimes appearing reddish-purple.
Systemic Conditions and Urgent Symptoms
Purple discoloration paired with other significant symptoms can signal a serious underlying systemic or vascular condition. Deep Vein Thrombosis (DVT) occurs when a blood clot forms in a deep vein, which can happen in the popliteal vein behind the knee. DVT symptoms often include throbbing pain, significant swelling usually affecting only one leg, warmth to the touch, and reddish or purple skin discoloration around the affected area.
A severe joint infection, known as septic arthritis, can also cause the skin around the knee to change color. This condition involves bacteria or other germs entering the joint space, leading to rapid and intense inflammation. Symptoms progress quickly and include severe pain, significant swelling, warmth, and a high temperature or fever. The skin may appear red, darkened, or purplish due to the intense inflammatory response.
Circulatory issues that limit blood flow to the lower extremities, such as Peripheral Artery Disease (PAD) or Chronic Venous Insufficiency (CVI), can also manifest with purple or bluish skin changes. PAD involves narrowed arteries and may cause the limb to feel cold and look pale or purplish, particularly when the leg is elevated. CVI involves faulty valves in the veins, leading to blood pooling, which causes skin discoloration, often a reddish-brown or purple hue, typically starting around the ankles and moving up the leg.
When to Seek Medical Attention
If the purple discoloration is an isolated bruise resulting from a known injury, self-care measures like rest, ice, compression, and elevation (RICE) are appropriate. However, a bruise that does not fade or is accompanied by increasing pain after several days should be evaluated by a healthcare professional.
Immediate medical evaluation is necessary if the purple discoloration is accompanied by signs of a serious underlying condition. Seek emergency care if the knee discoloration is sudden and paired with severe pain, rapid swelling, warmth, or difficulty bearing weight. Prompt attention is also required if the discoloration is accompanied by a high fever, chills, or if you experience chest pain or shortness of breath, as these may signal a pulmonary embolism, a complication of DVT.