Blood in the Joint

When a major injury occurs, the joint swells. Part of the swelling is good, and part is bad. Let’s look at what can be done to optimize the outcome.


Joint swelling after an injury is usually due to blood vessels being broken as tissue tears. These tears leak blood into the joint, which already holds about a milliliter of synovial (joint) fluid. Blood contains both healing factors and inflammatory agents, the latter mainly white blood cells. The swollen joint becomes inflamed, heated by the rush of blood into the joint spaces, and additionally swollen with the extracellular fluid that surrounds cells in normal tissue. This chicken soup of bioactive factors sends siren calls to the stem cells located throughout the body. These stem cells are then activated to form progenitor cells that mobilize to the joint, fulfilling a range of key functions to stimulate the healing response.  

So: What are the fluids that we want to have in the joint and which ones do we not want there?

As with most of biology, there are positive and negative aspects to each response. The white blood cells are primarily responsible for the inflammatory response and produce pain and more swelling. On the positive side, however, white cells are scavengers. They consume any foreign body or bacteria, clearing our injured tissues of some of the detritus produced when collagen tissue is ruptured.

The red blood cells carry oxygen to the damaged and healing tissue. Oxygen is required for most tissues to repair themselves. Sometimes, as with hyperbaric chambers, additional oxygen is provided to accelerate the healing. Red blood cells, however, have a toxic effect on articular cartilage, which is normally avascular (i.e., lacking blood vessels). Repeated exposure to these cells (hemarthrosis) becomes toxic to articular cartilage, leading to arthritic joints. People suffering from hemophilia, for example, have frequent episodes of bleeding into their joints, due to their deficient clotting mechanism, and unfortunately, frequently get joint arthritis. 

Still smaller particles, called platelets, are packed with granules that contain potent growth factors. The platelets release their growth factor packets at the site of collagen injury. These factors stimulate the healing response, much as anabolic steroids stimulate muscle building. Other bioactive factors in the platelet granules are chemotactic—meaning they send out signals that recruit other cell types, such as macrophages, which eat the torn tissues and damaged cells. They induce the first type of macrophage to become Type II macrophages, which are directly involved in tissue repair. Several of these bioactive factors are the most potent stem cell activation and recruitment compounds ever discovered. Since everyone has billions of stem cells living on their blood vessels, these factors do a great job of ramping up the healing response.

Now the question becomes: When you injure your joint and the joint swells, should the doctors draw off the blood and fluid? The answer is sometimes yes—if it makes you feel better and lessens the pain from the swelling. But your doctor should then spin the aspirate down in a centrifuge, removing the white and red blood cells and concentrating the platelets. These platelets, with their multiple growth factor packets, can then be reinjected into the joint.

The next question is: Should all injured joints be injected with bioactive factors to accelerate healing? The answer is coming, as our research in this field continues. My intuition is yes: We will not let injured joints become arthritic if we can change the course of the injury from devastating to just inconvenient with a rapid return to normal. If we aim to reach age 100 playing the sports we love, we can’t afford to let sports injuries become fatal for the joints.

Medically authored by
Kevin R. Stone, MD
Orthopaedic surgeon, clinician, scientist, inventor, and founder of multiple companies. Dr. Stone was trained at Harvard University in internal medicine and orthopaedic surgery and at Stanford University in general surgery.