What are the next steps for managing left knee pain and effusion one week post-injury with worsening symptoms, including evident effusion lateral and below the joint, warmth, and tender patella grind?

Management of Worsening Post-Traumatic Knee Effusion with Inflammatory Signs

This patient requires urgent imaging with knee radiographs as the immediate next step, followed by MRI if radiographs are negative, to rule out fracture and assess for internal derangement including meniscal tears, ligamentous injury, and bone contusions. The presence of warmth, worsening effusion, and tender patella grind one week post-injury raises concern for significant internal derangement or occult fracture that requires definitive diagnosis before treatment 1.

Immediate Diagnostic Workup

Initial Imaging: Radiographs

• Obtain anteroposterior and lateral knee radiographs immediately as the first imaging study, even one week post-injury with worsening symptoms 1.
• The lateral view is critical for assessing joint effusion and should demonstrate the patella in profile with 25-30 degrees of knee flexion 1.
• A cross-table lateral view with horizontal beam can visualize lipohemarthrosis, which indicates intra-articular fracture 1.
• Consider adding a patellofemoral view given the tender patella grind, as this can detect patellar fractures or dislocation that may be missed on standard views 1, 2.
• The "sliver sign" (linear or curvilinear intra-articular ossific density with effusion) on radiographs is highly specific for recent lateral patellar dislocation and should be actively sought 2.

Advanced Imaging: MRI if Radiographs Negative

• If radiographs show no fracture but symptoms persist, MRI is the next appropriate study 1.
• MRI has 96% sensitivity and 97% specificity for meniscal tears and provides superior evaluation of ligamentous injuries, bone marrow contusions, and occult fractures 1.
• Knee effusion >10 mm on lateral radiograph in patients <40 years should prompt strong consideration for MRI, as this can decrease delayed diagnosis and improve outcomes 1.
• MRI can diagnose bone marrow contusions that predict associated soft-tissue injuries and future osteoarthritis development 1.

Therapeutic Interventions

Arthrocentesis Considerations

• Aspiration provides only temporary symptomatic relief (lasting approximately one week) due to early re-accumulation and does not improve long-term outcomes 3.
• Aspiration is most beneficial for establishing diagnosis in effusions of unknown etiology rather than for therapeutic benefit 3.
• In post-traumatic effusions specifically, aspiration showed temporary improvement but no difference in clinical outcomes at follow-up 3.
• Perform aspiration with caution in traumatic effusions; it should be reserved primarily for diagnostic purposes or immediate clinical relief when diagnosis is uncertain 3.

Intra-Articular Corticosteroid Injection

• If acute inflammatory exacerbation with effusion is confirmed and fracture is excluded, intra-articular corticosteroid injection is indicated 1.
• Corticosteroids demonstrate strong short-term efficacy (effect size 1.27) for pain relief over 7 days in knee effusions 1.
• The benefit is relatively short-lived, typically lasting 1-2 weeks with no significant difference from placebo at 24 weeks 1.
• Evidence regarding predictors of response is mixed: one study found better outcomes in patients with effusion, while another found no clinical predictors 1.

Pharmacologic Management

• NSAIDs (oral or topical) should be initiated for inflammatory symptoms, particularly given the warmth and effusion 1.
• NSAIDs demonstrate superior efficacy compared to acetaminophen (effect size 0.32-0.45) with median effect size of 0.49 versus placebo 1.
• Topical NSAIDs (such as diclofenac) are effective alternatives for those unable to tolerate oral NSAIDs, with effect size of 0.91 versus placebo 1.

Critical Pitfalls to Avoid

Red Flags Requiring Urgent Evaluation

• The lateral location of effusion "below the joint" is atypical and may represent extra-articular fluid collection, abscess, or sympathetic effusion from adjacent pathology 4.
• Warmth with worsening symptoms one week post-injury raises concern for septic arthritis, which must be definitively excluded 4.
• Sympathetic joint effusion can occur from concurrent pathology in close anatomical proximity (infection, DVT, intramuscular collection) in 40% of cases 4.

Diagnostic Errors to Avoid

• Do not miss patellar dislocation: the sliver sign is often misinterpreted (48% described vaguely, 22% interpreted incorrectly, 22% missed entirely) 2.
• Standard trauma knee series does not include patellar view, which should be added especially in young patients with effusion 2.
• Fat pad separation sign on lateral radiograph is the most accurate indicator of effusion (88% accuracy for diagnosis, 90% for exclusion), detecting volumes as small as 1-2 ml 5.

Treatment Sequencing

• Never perform therapeutic aspiration or injection before excluding fracture with imaging 1.
• Do not rely on aspiration for sustained therapeutic benefit in post-traumatic effusions; focus on identifying and treating the underlying pathology 3.
• If corticosteroid injection is performed, counsel patients that benefit is short-term and does not address structural pathology 1.