Bone marrow edema is when fluid builds up in your bone marrow. Bone marrow is soft, spongy tissue in the middle of most bones, containing blood stem cells that can become red blood cells, white blood cells, or platelets. Bone marrow edema causes swelling and pain, often due to injury, arthritis, osteoporosis, bone tumors, and bone infections. Treatment includes rest, therapy, and pain medications, with surgery needed in severe cases.