Most knee bruises are mild and heal on their own, but some can be severe. Steps that can support healing include rest, ice, compression, and elevation (RICE). However, some bruises may need medical attention.

This article explores how bruises form, how to treat them, and how to tell if a bruise could be a sign of a more severe injury, such as a fracture.

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A bruise or contusion forms when small blood vessels under the skin break, but the skin remains intact. Blood leaks out of these broken capillaries into the surrounding soft tissues, which causes discoloration.

There are three types of bruises:

  • subcutaneous, which occur just under the skin’s surface
  • intramuscular, occurring within the underlying muscle
  • bone bruises, which are an injury to the bone

A bone bruise is typically the most painful type of bruise. Doctors can only detect these using magnetic resonance imaging, as they are not easily visible on X-rays.

A bruise on the knee may occur because of damage to the skin, tendons, ligaments, or the bone of the knee cap. For example, a knee injury may cause a tear in the anterior cruciate ligament, in addition to bruising.

A bruise on the knee may appear black, blue, or purple on lighter skin, or dark purple or brown on darker skin. Bruises may change color as they heal, turning yellow or green.

A knee bruise may also trigger symptoms such as:

  • pain and tenderness
  • swelling
  • stiffness
  • weakness
  • a lump, or hematoma, caused by blood pooling in the area

If there is a bone bruise, the knee may be particularly stiff, swollen, and take longer to heal. However, with no other symptoms, most bruises are not serious and typically heal on their own.

Bruises to the knee generally occur from direct and blows to the body involving something blunt. This may occur due to a fall or when a person bangs their knee into something hard.

Bruising can also develop alongside other types of injury, such as fractures, broken bones, torn tendons, or dislocated joints.

Older adults are more likely to bruise than younger adults. Additionally, people may bruise more easily than others.

Bruises without any injury

Sometimes, people develop bruises and forget how they occurred. However, some bruise-like markings on the skin that do not result from an injury may be due to blood spots, also known as purpura.

Purpura is not the same as bruising, although they look similar. This symptom can occur due to a range of conditions, including:

  • medications that thin the blood, such as aspirin or warfarin
  • malnutrition and vitamin deficiencies
  • inflammatory conditions, such as lupus
  • inflamed blood vessels, or vasculitis
  • a bleeding or clotting disorder, such as hemophilia
  • liver disease, such a cirrhosis
  • infections that cause toxins to build up in the blood, such as sepsis
  • some types of cancer, such as leukemia

If a person notices red or purple patches under the skin that they cannot explain, it is best to speak with a doctor.

According to the American Academy of Orthopedic Surgeons, rest, ice, compression, and elevation (RICE) may help reduce symptoms and aid healing.

  • Rest: rest the knee joint
  • Ice: cool the area with a compress or ice pack wrapped in a towel for 20 minutes at a time, several times per day
  • Compression: lightly wrap the knee in a soft bandage
  • Elevation: Raise the knee above the level of the heart, if possible, to prevent blood from pooling

Do not apply ice directly to the skin, or try to drain the bruise of blood using a needle. If there is a large lump that does not decrease in size after several days, a doctor may need to drain it.

A bruised knee will usually heal within around 2–4 weeks. A bone bruise may take as long as 1–2 months or longer if the bruise is large.

Sometimes, a direct blow to the knee causes a sprain or a kneecap fracture. It may be difficult to tell the difference between these injuries.

A kneecap or patella fracture can cause:

  • pain on and around the kneecap
  • pain when moving the knee in any direction
  • swelling
  • difficulty extending the leg
  • a deformed appearance due to broken pieces of bone

If a person has a sprain or kneecap fracture, severe pain, bruising, or swelling will set in within 30 minutes of the injury. A doctor can determine if someone has a fracture with an X-ray.

Treatment for a fractured kneecap may involve surgery to repair the broken bones or replacing all or part of the kneecap.

A person should speak with a doctor if:

  • a knee bruise does not improve within 2–4 weeks
  • they develop a large hematoma on the knee that does not decrease in size after a few days
  • their knee shows signs of a fracture or more serious injury, such as severe pain or swelling
  • they develop red or purple or discolored patches that are not due to an injury
  • the knee shows signs of infection, such as swelling, pus, and warmth

A person should seek emergency medical care if they experience extremely painful swelling in the muscles in the foot, leg, buttocks, or arms.

This may be the result of compartment syndrome, a rare but serious condition that requires immediate treatment.

A bruised knee is typically a minor injury that heals on its own in a few weeks. A bone bruise may cause more pain and take longer to heal.

The RICE approach may help relieve symptoms and promote healing in knee injuries. However, if there is severe pain and swelling, or the bruise is not due to an injury, a person should speak with a doctor.