What are the causes of fluid collection in the hip?

Causes of Fluid Collection in the Hip

Fluid collections in the hip are most commonly caused by infection (septic arthritis or periprosthetic joint infection), inflammatory conditions, trauma, or post-surgical complications. 1

Infectious Causes

Septic Arthritis

• Characterized by:
  • Joint effusion with synovial thickening and enhancement 1
  • Lamellated appearance of inflammatory synovitis on MRI 1
  • Soft tissue edema and bone marrow edema 1
  • Periosteal reaction 1
• Diagnostic features:
  • Intraarticular effusion with extraarticular extension has 100% specificity for infection 1
  • Ultrasound can detect even minor fluid collections of 1-2 ml 2
  • Echofree effusions typically represent transient synovitis or fresh hemorrhagic effusions 2
  • Non-echofree effusions suggest clotted hemorrhagic collections or septic arthritis 2

Periprosthetic Joint Infection (PJI)

• Key findings:
  • Soft tissue edema (sensitivity 86.7%, specificity >73.3%) 1
  • Enlarged lymph nodes (sensitivity 80%, specificity 86.7%) 1
  • Abnormalities at both acetabular and femoral components 1
  • Irregular soft tissue mass, bone destruction, and fistulas 1
  • Fluid collections that may be serous, purulent, or haematic 3

Non-Infectious Causes

Post-Surgical Collections

• Common in the immediate postoperative period (55% of amputations) 4
• Typically decrease in size and frequency over time 4
• Not necessarily indicative of infection without other clinical signs 4
• May involve skin/subcutaneous tissues (76%), deep soft tissues (91%), or the implant (57%) 3

Trauma-Related Collections

• Subperiosteal fluid collections following trauma 1
• Hematomas (appear as echofree or non-echofree collections depending on clotting) 2

Inflammatory/Degenerative Conditions

• Femoroacetabular impingement (FAI) syndrome can present with fluid collections 5
• Aseptic loosening of prosthetic components 1
• Metal-related disease and foreign body reactions 1
• Pseudotumors 1

Diagnostic Approach

Imaging Modalities

1. Ultrasound:

   • First-line for detecting joint effusion and fluid collections 1
   • Can detect even small fluid collections of 1-2 ml 2
   • Limited by metal artifact and inability to detect bone abnormalities 1

2. MRI:

   • Highly reproducible for detection, localization, quantification, and characterization of fluid collections 3
   • Can differentiate phlegmon from abscess with IV contrast 1
   • Metal reduction sequences have mitigated limitations from implants 1

3. CT:

   • Helpful for identifying necrotic bone fragments or sequestra in chronic osteomyelitis 1
   • Can show effusions and soft tissue fluid collections 1
   • Addition of IV contrast aids in detection of synovial thickening or abscesses 1

Joint Aspiration

• Gold standard for diagnosis of septic arthritis 1
• Can be performed using fluoroscopic, ultrasound, or CT guidance 1
• Modified techniques for patients with prosthetic hips have high success rates (97.8%) 6

Clinical Pearls and Pitfalls

• Important caveat: A negative ultrasound will exclude a fluid collection, while the depiction of an echofree effusion will virtually rule out septic arthritis 2

• Pitfall to avoid: Fluid collections are common after surgery and not necessarily indicative of infection without other clinical signs such as erythema or drainage 4

• Warning sign: The presence of objective clinical signs at the extremity (erythema and/or drainage) strongly correlates with infection 4

• Surgical consideration: Fluid collection bags used during surgery can become contaminated, especially in procedures lasting longer than 90 minutes 7

• Diagnostic challenge: Bone marrow signal abnormalities may persist for months following injury or surgery, complicating MRI interpretation 1