A bruised knee can be painful and because this condition is typically the result of some type of stress or injury, can occur with or without further knee injury. Sometimes referred to as a patellar contusion, a bruised knee is the result of blood vessels beneath the surface of the skin being damaged and leaking blood. The signs of a bruised knee may range from discoloration on the surface of the skin to swelling and tenderness to the touch.

Normally, the bruising that occurs because of minor bumps or impact trauma is called subcutaneous bruising and is marked by mild to moderate discoloration of the area and possibly mild pain or tenderness upon touch. This type of bruising is usually acute, or temporary, and should heal itself and disappear within a few days. A bruised knee resulting from minor impact may hurt briefly, but is normally not cause for alarm.

Periosteal and intramuscular bruising is typically more painful than a subcutaneous bruise and is the result of a more severe impact or trauma, such as might occur with a sports injury. Periosteal bruising is a bruise that occurs to the bone, while intramuscular bruising is bruising of the muscles. A bruised knee can result in one or both types of bruising, and while the tell-tale discoloration of a bruising may be present, pain, swelling, discomfort when moving and touching the knee is also a sign of injury.

Sometimes a bruised knee is a secondary symptom of a more serious injury. Bruising can occur during dislocation and even fracture of the patella, or kneecap. An injury this serious would normally be evident by marked pain, discomfort and swelling. Treatment for a knee injury should be sought and stress should not be placed on the injured knee until a doctor has diagnosed the injury and provided instructions for treatment and recovery.

It is possible for other forms of stress besides impact or injury to cause a bruised knee. Stress can be placed on the knees by excess weight and previous knee injuries are susceptible to bruising, especially during the healing process. Regardless of the cause of bruising, the signs are usually present in the form of pain, tenderness, swelling and discoloration. Ice is an acceptable form of treatment for reducing swelling and temporarily relieving pain. Avoid taking aspirin or anti-inflammatory drugs unless prescribed by a doctor as these can increase blood flow. Knee injuries that cause pain with movement or immobility should be evaluated by a physician as soon as possible.