What is the best course of treatment for a patient presenting with effusion in the knee joint patella?

Treatment of Knee Joint Effusion

Begin with plain radiographs (anteroposterior, lateral at 25-30° flexion, and tangential patellar view) as the mandatory first step, followed by clinical assessment to determine whether aspiration, advanced imaging, or conservative management is appropriate. 1, 2

Initial Diagnostic Workup

Mandatory First Step: Plain Radiographs

• Obtain at minimum two views (AP and lateral), with a third tangential patellar view strongly recommended to exclude fracture, loose bodies, prosthetic complications, or chronic joint pathology before proceeding with any treatment. 1, 2, 3
• The lateral view must be taken at 25-30 degrees of knee flexion and can demonstrate lipohemarthrosis (indicating intra-articular fracture) when obtained as a cross-table lateral with horizontal beam. 1, 3
• Never skip radiographs even when effusion is clinically obvious, as approximately 20% of patients inappropriately receive advanced imaging without baseline radiographs, and painless effusions can harbor serious pathology including sleeve fractures or septic arthritis in immunocompromised patients. 2, 3

Clinical Assessment to Guide Next Steps

• Determine whether the effusion is traumatic or atraumatic, as this fundamentally changes the diagnostic approach. 4, 5
• Assess for infection indicators including fever, systemic symptoms, recent bacteremia, or inability to bear weight, which suggest septic arthritis requiring urgent intervention. 4
• Apply Ottawa Knee Rules: obtain radiographs if age >55 years, isolated patellar tenderness, fibular head tenderness, inability to flex to 90°, or inability to bear weight for 4 steps. 1, 3

Treatment Algorithm Based on Radiographic Findings

If Radiographs Show Fracture or Bony Abnormality

• Order MRI without IV contrast to evaluate for sleeve avulsion fracture, quadriceps tendon integrity, cartilage injury, and loose bodies that may require surgical intervention. 1, 2
• MRI has 96% sensitivity and 97% specificity for detecting meniscal tears and is superior to CT for evaluating bone marrow lesions and ligamentous injuries. 1

If Radiographs Are Normal: Traumatic Effusion

• Conservative management with close observation is appropriate if the patient can bear weight fully and perform straight leg raise without difficulty. 2
• Order MRI without contrast at 5-7 days if symptoms persist, mechanical symptoms develop (locking, catching), inability to bear weight continues, or joint instability is present. 1, 2, 4
• In adolescents with traumatic knee effusion, MRI should be obtained early due to high rates of patellar dislocation (36% in ages 10-14,28% in ages 15-18) and ACL tears (22% in younger, 40% in older adolescents) that are missed on physical examination. 6

If Radiographs Are Normal: Atraumatic Effusion

• Perform diagnostic arthrocentesis to establish diagnosis and rule out infection or crystal disease, particularly when etiology remains unclear after initial assessment. 2, 4
• Obtain ESR and CRP, as the combination provides the best sensitivity and specificity for suspected prosthetic joint infection. 4
• Ultrasound may confirm effusion and guide aspiration, with quadriceps contraction technique superior to static scanning (identifying effusions in 54.2% of knees not seen on static scanning). 1, 7

Critical Management Principles

When to Aspirate

• Aspiration is indicated when infection or crystal disease is suspected, when there is atraumatic effusion of unknown etiology, or when surgery is not immediately planned. 2, 4
• Obtain blood cultures if fever is present or there is acute symptom onset. 4
• The combination of self-noticed swelling and positive ballottement test has a likelihood ratio of 3.6 for detecting effusion on MRI. 8

When to Use MRI

• MRI without contrast is the appropriate next imaging study for suspected internal derangement, significant joint effusion with inability to fully bear weight after 5-7 days, mechanical symptoms suggesting meniscal injury, or joint instability suggesting ligamentous injury. 1, 4
• MRI can diagnose bone marrow contusions that predict associated soft-tissue injuries and is more sensitive than radiographs for detecting anterolateral ligament injuries associated with ACL tears. 1
• Knee effusion >10 mm on lateral radiograph in patients <40 years old with acute knee injury should prompt consideration for MRI to decrease delayed diagnosis and improve outcomes. 1

When Conservative Management Is Appropriate

• Close observation without aspiration or advanced imaging is appropriate for benign-appearing effusion if the patient can bear weight fully, perform straight leg raise without difficulty, and has no mechanical symptoms. 2
• Reassess at 5-7 days for development of mechanical symptoms, inability to bear weight, or progression of swelling. 2

Critical Pitfalls to Avoid

Never Inject Corticosteroids Prematurely

• Corticosteroid injection should be avoided until infection is definitively excluded, as injecting infected joints or supporting structures can cause catastrophic outcomes including tendon rupture and septic spread. 2
• Even in painless swelling, infection must be ruled out as septic arthritis in immunocompromised patients can present with minimal pain. 2

Do Not Delay Aspiration in Suspected Infection

• Septic arthritis is an orthopedic emergency requiring urgent intervention, and delaying aspiration can lead to irreversible joint destruction. 4
• Do not routinely use bone scans, PET, or MRI for initial diagnosis of prosthetic joint infection, as these should not replace the standard workup of radiographs, ESR/CRP, and arthrocentesis. 4

Do Not Assume Painless Effusion Is Benign

• Approximately 20% of patients with effusion have sleeve avulsions, bipartite patella variants, or other serious pathology that presents with minimal pain. 2
• Quantitative measurement of suprapatellar effusion by ultrasound correlates significantly with pain, symptoms, and quality of life in knee osteoarthritis patients. 9