Blood in the Knee (Hemarthrosis): Causes, Symptoms, Diagnosis, and Outlook

What causes blood in the knee?

Blood in the knee, medically termed hemarthrosis, is a significant symptom indicating bleeding within the joint capsule. It is most commonly caused by acute traumatic injuries to the knee's internal structures, but can also stem from underlying medical conditions or medication use.

Understanding Hemarthrosis: Blood in the Joint

The knee joint is a complex structure enclosed by a synovial capsule, which contains synovial fluid that lubricates and nourishes the cartilage. When blood accumulates within this normally clear fluid, it indicates a breach of a blood vessel inside or around the joint. This accumulation, known as hemarthrosis, can lead to severe pain, swelling, and stiffness, and if left unaddressed, can cause significant long-term damage to the joint's delicate cartilage due to the irritating and destructive properties of blood products.

Common Traumatic Causes of Knee Hemarthrosis

Acute injuries are by far the most frequent cause of blood in the knee, particularly those involving structures with a rich blood supply.

• Anterior Cruciate Ligament (ACL) Tear: The ACL is highly vascularized, and a tear, especially a complete rupture, almost invariably leads to significant hemarthrosis. This is a very common sports injury.
• Posterior Cruciate Ligament (PCL) Tear: While less common than ACL tears, PCL injuries can also result in bleeding into the joint.
• Meniscal Tears (Peripheral): The menisci are C-shaped cartilage pads that act as shock absorbers. Only the outer third (red zone) has a blood supply. Tears in this vascularized region can cause hemarthrosis, whereas tears in the avascular inner two-thirds (white zone) typically do not.
• Patellar Dislocation or Fracture: Direct trauma to the kneecap or its dislocation can damage surrounding tissues and cause bleeding into the joint. Fractures of the patella or other bones around the knee (e.g., tibial plateau, femoral condyles) often lead to hemarthrosis, sometimes with fat globules visible in the aspirated blood, indicating bone marrow involvement.
• Osteochondral Fractures: These involve damage to both the cartilage and the underlying bone. Such injuries, often resulting from direct impact or twisting forces, can cause bleeding due to the disruption of bone tissue.
• Capsular or Synovial Tears: The joint capsule and synovial lining are vascular structures. A tear due to severe trauma can lead to direct bleeding into the joint space.

Non-Traumatic (Medical) Causes of Knee Hemarthrosis

While less common, certain medical conditions or medications can predispose an individual to spontaneous or minor-trauma-induced hemarthrosis.

• Bleeding Disorders (Coagulopathies): Conditions like Hemophilia (A or B), Von Willebrand disease, or other inherited or acquired clotting factor deficiencies significantly impair the body's ability to form clots, leading to spontaneous bleeding into joints (target joints) or severe bleeding after minor trauma.
• Anticoagulant Medication Use: Individuals taking blood thinners such as warfarin, direct oral anticoagulants (DOACs), or even high doses of NSAIDs (which have antiplatelet effects) are at an increased risk of hemarthrosis, even from minor injuries that would not typically cause significant bleeding.
• Synovial Hemangioma: A rare, benign vascular tumor within the joint's synovial lining that can spontaneously bleed.
• Pigmented Villonodular Synovitis (PVNS): A rare, benign but locally aggressive condition characterized by the overgrowth of the synovial membrane. This overgrown tissue is often iron-laden and prone to bleeding.
• Gout and Pseudogout: While primarily inflammatory arthropathies, severe flares can sometimes be associated with micro-bleeding into the joint, though frank hemarthrosis is rare.
• Charcot Arthropathy (Neuropathic Joint): Severe joint degeneration due to nerve damage (e.g., from diabetes, syphilis). The loss of protective sensation and repetitive microtrauma can lead to joint destruction and sometimes bleeding.
• Infection (Septic Arthritis): In some severe cases of joint infection, the inflammation can be so intense that it causes damage to small blood vessels, leading to a hemorrhagic effusion, though pus is the primary component.

Recognizing the Symptoms of Knee Hemarthrosis

The presence of blood in the knee typically manifests with a distinct set of symptoms that often appear rapidly after an injury or, less commonly, spontaneously.

• Acute Onset Swelling: The knee joint rapidly swells, often within minutes to hours after the injury, as blood fills the joint space. This swelling can be significant.
• Pain: Often severe and throbbing, exacerbated by movement or weight-bearing. The blood itself is an irritant to the synovial lining.
• Stiffness and Reduced Range of Motion: The increased fluid pressure and irritation within the joint severely limit the ability to bend or straighten the knee.
• Warmth to the Touch: The joint may feel noticeably warmer than the surrounding skin due to inflammation.
• Bruising: While not always immediately apparent externally, bruising may develop around the knee as blood extravasates into the surrounding soft tissues.
• Difficulty Bearing Weight: The pain and instability caused by the effusion often make it challenging or impossible to put weight on the affected leg.

Diagnosis and Medical Intervention

Accurate diagnosis of hemarthrosis and its underlying cause is crucial for effective treatment and preventing long-term complications.

• Clinical Examination: A healthcare professional will assess the knee for swelling, tenderness, warmth, and evaluate the range of motion and stability.
• Imaging Studies:
  • X-rays: Primarily used to rule out fractures.
  • Magnetic Resonance Imaging (MRI): The gold standard for visualizing soft tissue injuries, such as ACL tears, meniscal tears, and osteochondral lesions. It can also identify synovial abnormalities.
  • Ultrasound: Can quickly confirm the presence of an effusion and sometimes identify its nature (e.g., simple fluid vs. complex fluid with blood clots).
• Arthrocentesis (Joint Aspiration): This involves drawing fluid from the knee joint with a needle. It is both diagnostic and therapeutic.
  • Diagnostic: The presence of frank blood confirms hemarthrosis. Analysis of the fluid can also reveal fat globules (suggesting a fracture), or be sent for cell count, crystal analysis (for gout/pseudogout), or culture (for infection).
  • Therapeutic: Removing the blood reduces pressure, pain, and irritation within the joint, potentially minimizing long-term cartilage damage.
• Blood Tests: If a bleeding disorder is suspected, blood tests to assess clotting factors or platelet function may be ordered. If anticoagulant medication is involved, blood tests to check medication levels may be performed.

Implications for Athletes and Active Individuals

For athletes and active individuals, hemarthrosis is a serious sign that demands immediate medical attention. Prompt and accurate diagnosis of the underlying injury is critical. Ignoring or delaying treatment can lead to:

• Cartilage Damage: Blood in the joint is detrimental to articular cartilage, increasing the risk of premature degradation and early-onset osteoarthritis.
• Arthrofibrosis: The inflammatory response to blood can lead to excessive scar tissue formation within the joint, causing chronic stiffness and loss of motion.
• Chronic Pain and Instability: Untreated underlying injuries can result in persistent pain, weakness, and instability, severely impacting athletic performance and daily function.
• Prolonged Rehabilitation: Complex injuries causing hemarthrosis often require extensive rehabilitation, sometimes including surgery, leading to significant time away from sport or activity.

Conclusion: When to Seek Medical Attention

Blood in the knee is never a normal finding and should always be evaluated by a healthcare professional. If you experience acute, unexplained swelling, severe pain, stiffness, or difficulty moving your knee, especially after an injury, seek immediate medical attention. Early diagnosis and appropriate treatment are essential to address the underlying cause, manage symptoms, and minimize the risk of long-term complications, ensuring the best possible outcome for joint health and function.