You have protein in your synovial fluid drained from the knee because the synovial membrane is inflammed. Plasma proteins from the blood diffuse into the knee joint in response to inflammation. This inflammation could be from trauma or it could be from osteoarthritis or rheumatoid arthritis. Some intraarticular cells release these proteins at an advanced rate when traumatized. The microvascular permeability of the articulation is increased.
Normally, the lymphatic system takes up excess proteins and returns them to the venous circulation. When the joint is inflammed, the lymphatic system can't uptake the protein fast enough, so you have a high concentration of these plasma proteins in the synovial fluid. Your level of 5.2 is about 4 times the normal.