What causes hematoma or hemorrhagic knee swelling?

Causes of Hematoma or Hemorrhagic Knee Swelling

Hemorrhagic knee swelling results from intra-articular bleeding (hemarthrosis) or intramuscular hematoma, with the most common causes being trauma with ligamentous injury, anticoagulation therapy, vascular lesions, or spontaneous bleeding in elderly patients with osteoarthritis.

Traumatic Causes

Acute knee trauma is the most frequent cause of hemarthrosis, particularly in younger patients:

• Anterior cruciate ligament (ACL) injury is the leading traumatic cause, accounting for isolated ACL tears in approximately 40% of acute hemarthrosis cases, with an additional 23% involving combined ACL and meniscal injuries 1
• Meniscal tears alone cause hemarthrosis in approximately 11% of cases 1
• Chondral surface ruptures account for roughly 14% of traumatic hemarthrosis 1
• Patellar dislocation with lateral retinaculum disruption represents another mechanism of acute bleeding 1
• Osteochondral injuries including osteochondritis dissecans can produce significant intra-articular bleeding 1

Anticoagulation-Related Bleeding

Patients on oral anticoagulants face substantial risk of extremity bleeding complications:

• Intramuscular and intra-articular bleeding in anticoagulated patients presents with joint pain, swelling, and decreased range of motion 2
• The American College of Cardiology classifies extremity bleeds as potentially critical, with consequences including compartment syndrome, paralysis, limb loss, or irreversible joint damage 2, 3
• Hemoglobin drops ≥2 g/dL associated with hematomas indicate major bleeding events with significantly increased mortality risk 2
• Both vitamin K antagonists (VKAs) and direct oral anticoagulants (DOACs) can cause spontaneous hemarthrosis or intramuscular hematomas 4, 3

Spontaneous Hemorrhage in Elderly Patients

Atraumatic spontaneous hemarthrosis occurs predominantly in elderly patients with underlying osteoarthritis:

• Peripheral arteries supplying the posterior horn of the lateral meniscus are the identified bleeding source in spontaneous knee hemarthrosis 5
• This condition typically presents with acute posterolateral knee pain, swelling, and severely limited weight-bearing in patients with pre-existing lateral compartment osteoarthritis 5
• Complex meniscal tearing with full-thickness chondral loss creates the anatomic substrate for arterial bleeding 5

Vascular Malformations

Rare vascular lesions can cause recurrent spontaneous hemarthrosis:

• Juxta-articular hemangiomas or hemangiohamartomas produce repeated spontaneous bleeding episodes that may require multiple arthroscopic examinations before diagnosis 6
• These lesions are typically identified only after angiography when other causes have been excluded 6
• Meniscal hematomas represent an unusual entity where bleeding occurs within the meniscal substance itself 7

High-Energy Trauma Considerations

Major trauma mechanisms warrant evaluation for associated injuries:

• High-energy injuries to the lower extremity, particularly involving the knee, may be associated with compartment syndrome requiring urgent intervention 2, 8
• Pelvic fractures with knee trauma should raise concern for massive hemorrhage, as hemorrhage is the leading cause of death in major pelvic fractures 2, 8

Diagnostic Approach

When evaluating hemorrhagic knee swelling, systematically assess the following:

• Trauma history and mechanism of injury - high-energy mechanisms increase risk of ligamentous disruption and associated injuries 1
• Anticoagulation status - check PT/INR for VKAs or specific assays for DOACs 2, 3
• Age and osteoarthritis history - elderly patients with lateral compartment osteoarthritis are at risk for spontaneous hemarthrosis 5
• Pattern of presentation - recurrent spontaneous episodes suggest vascular malformation requiring angiography 6
• Associated symptoms - pain, pallor, paresthesia, weakness, or diminished pulse indicate potential compartment syndrome requiring urgent surgical consultation 2, 3

Critical Pitfalls to Avoid

• Do not dismiss normal platelet counts in patients with malignancy, as a decreasing trend from elevated levels may be the only sign of disseminated intravascular coagulation contributing to bleeding 2
• Avoid using PT/INR alone to assess bleeding risk in patients on direct factor Xa inhibitors, as these tests have important limitations 4
• Do not delay evaluation of recurrent spontaneous hemarthrosis, as vascular malformations require angiographic diagnosis and definitive excision 6
• In anticoagulated patients with intramuscular hematomas, maintain high suspicion for compartment syndrome even with seemingly minor trauma 2, 3