Hemarthrosis is blood inside the knee or bleeding into the knee joint space. The swelling and fluid inside the knee joint is usually relieved with an aspiration. During the aspiration, the physician will insert the needle on the lateral side of the knee, just above the upper border of the patella. The needle enters below the patella into the suprapatellar bursa which is continuous with the joint cavity. This aspiration technique is different than how physicians perform injections. For a knee injection, the needle is inserted at the lower border of the patella on either side of the patellar tendon at the soft spot.
The color of the fluid aspirated—not bloody effusion—is probably due to synovial irritation caused by chronic processes such as gout, pseudogout, arthritis, rheumatoid arthritis, or degenerative meniscus. A degenerated meniscus may be associated with swelling and fluid collection; usually not bloody. The peripheral portion of the meniscus is vascular (about 3-5 mm). The blood supply of the meniscus originates from the medial and lateral genicular arteries. Although a degenerative meniscus effusion is not bloody, a traumatic tear of the meniscus may cause bleeding inside the knee joint.
A bloody effusion could be trauma related or non-trauma related. For example, hemarthrosis can be caused by trauma or injury to the structures of the knee such as the ACL, PCL, or meniscus. Hemarthrosis can also occur due to tibial plateau fractures, chondral fractures, patellar dislocations, or a meniscal tear. Non-traumatic conditions that can cause hemarthrosis include: PVNS, sickle cell anemia, hemophilia, anticoagulation, or hemorrhage following total knee replacement.
Hemoarthrosis from trauma or injury indicates a significant knee injury such as ACL (75-80%) or a meniscal tear. If aspirations of the knee after trauma shows hemarthrosis, early evaluation of the injury may be necessary to define the extent of damage. The physician may get an MRI early.
Absence of hemarthrosis does not mean a less severe ligament injury—the blood may escape without distending the capsule. A severe injury may cause minimal or severe joint effusion. More than 20cc of fluid may affect the quadriceps function and prevent full extension of the knee. A hematoma should be evacuated. The bloody aspirate should be examined for fat to rule out a fracture. The aspirate may vary in color depending on the severity of the injury and the duration of the symptoms. Fat is less dense than blood and fat floats on the surface, whereas blood is heavier and stays on the bottom.
The presence of a fat/fluid level is diagnostic of a fracture even if a fracture is not seen on an x-ray (occult). Fat/fluid level is usually seen with tibial plateau, chondral, and patellar fractures. The cross table lateral view of the knee shows it well. When a fat/fluid level is seen, look for intra-articular fractures. Lipohemarthrosis is only seen on horizontal x-ray beams with the beam parallel to the floor. It occurs in 40% of all fractures inside the joint.