Osteonecrosis can be caused by disease or by severe trauma, such as a fracture or dislocation, that affects the blood supply to the bone. Osteonecrosis can also occur without trauma or disease. This is called idiopathic -- meaning it occurs without any known cause. The following are possible causes:
https://medlineplus.gov/ency/article/007260.htm
Osteonecrosis is bone death caused by poor blood supply. It is most common in the hip and shoulder, but can affect other large joints such as the knee, elbow, wrist and ankle. Osteonecrosis occurs when part of the bone does not get blood and dies. After a while, the bone can collapse.
https://medlineplus.gov/ency/article/007260.htm
In general, one or both knees may be swollen due to. increased joint fluid. blood in the joint. enlargement of the bones near the knee joint. swelling near the knee joint (without an actual joint problem)--for example, bursitis can cause swelling directly on top of the kneecap. fluid behind the knee, called a Baker's cyst.
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The most common signs of a DVT are a noticeable swelling of the leg, a warmth or a redness in the area surrounding the knee (the calf or the thigh area), and leg pain that seems to worsen when standing or walking. A physician may conduct a variety of tests if a DVT is suspected in the knee area.
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Causes of Blood Clots on the Knee. According to the National Library of Medicine a blood clot may form in the knee area for a variety of reasons. Age can be a factor, and the chance of developing a blood clot can increase two-fold in each decade for people over the age of 40.
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Behind knee pain is discomfort or soreness behind the knee joint. It may occur with or without movement and can be severe enough to limit movement. People may describe behind knee pain as discomfort, inflammation, increased warmth or burning, soreness, stiffness, or pain. Behind knee pain is a common sign of a Baker’s cyst, which is a buildup of synovial fluid behind the knee.
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Black and blue marks are often associated with bruises. A bruise, or “contusion,” appears on the skin due to trauma. Examples of trauma are a cut or a blow to the area. The injury causes tiny blood vessels called capillaries to burst. The blood gets trapped below the skin’s surface, which causes a bruise. Bruises can occur at any age. Some bruises appear with very little pain, and you might not notice them.
http://www.healthline.com/health/bruise
There is a difficulty in diagnosing small blood clots, especially below the knee. Most will go undetected because they do not cause any obvious swelling or pain. It is not until a DVT occurs above the knee that symptoms will appear, and then it becomes crucial to begin treatment for the clotting.
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Other causes of blood clots on the knee include: prolonged bed rest or long periods of sitting and lying down, leg paralysis any major surgery under general anesthesia (especially involving hips or legs), and trauma to the knee area from a blow or fall.
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Find an Orthopedic Specialist Near You. However, behind knee pain may be a symptom of deep vein thrombosis (blood clot in the leg), which is a serious and life-threatening condition. The blood clot can break loose and cause a pulmonary embolism in the lung, a heart attack, or even stroke. Deep vein thrombosis has similar symptoms to a Baker’s cyst.
http://www.healthgrades.com/symptoms/behind-knee-pain
what can cause blood on the knee
Prolonged bed rest or long periods of sitting and lying down, leg paralysis any major surgery under general anesthesia especially involving hips or legs, and trauma to the knee area from a blow or fall can cause blood on the knee. Prolonged bed rest or long periods of sitting and lying down, leg paralysis any major surgery under general anesthesia, and trauma to the knee area from a blow or fall can cause blood on the knee.