How should I evaluate and manage a 2‑week history of left knee swelling, erythema, and pain in a construction worker who suspects a splinter in the knee?

Evaluation and Management of Suspected Knee Foreign Body with Infection

In a construction worker with 2 weeks of knee swelling, erythema, and suspected splinter, you must immediately obtain knee radiographs to detect radiodense foreign bodies and exclude fracture, followed by urgent joint aspiration to rule out septic arthritis, and if radiographs are negative but clinical suspicion remains high, proceed with ultrasound or MRI to detect radiolucent foreign bodies like wood splinters.

Initial Imaging Evaluation

Radiographs are mandatory as the first-line imaging study for any patient with knee swelling and suspected foreign body 1. The radiographs should include:

• Anteroposterior, lateral, and patellofemoral views to evaluate for radiodense foreign bodies (metal, graphite, stone), fractures, and soft-tissue gas 1
• Radiographs detect most metal and glass fragments, though small glass pieces may be missed 1
• Soft-tissue gas on radiographs is a critical finding that may signal necrotizing fasciitis requiring emergent surgical intervention 1

Ruling Out Septic Arthritis

Joint aspiration should be performed immediately given the 2-week duration of symptoms with swelling and erythema 1, 2. This is critical because:

• Septic arthritis is a rapidly progressive, debilitating process with significant morbidity and mortality 1
• Pain, erythema, swelling, and diminished range of motion are classic presenting features 1
• A positive culture from joint aspirate is the reference standard for diagnosis, though negative cultures do not exclude infection, especially if antibiotics were already started 1
• Joint aspiration can be performed by radiology or the clinical service 1

Laboratory Testing

Before or concurrent with aspiration, obtain:

• C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) as initial screening tests 1
• CRP has 73-91% sensitivity and 81-86% specificity for prosthetic joint infection (cutoff 13.5 mg/L), and similar utility for native joint infection 1
• Peripheral leukocyte counts are often not elevated in joint infections, limiting their utility 1
• ESR is abnormal in infection but also in uninfected inflammatory conditions 1

Advanced Imaging for Foreign Body Detection

If radiographs are negative but clinical suspicion for a retained foreign body remains high, ultrasound is the optimal next imaging modality 1:

• Ultrasound excels at detecting radiolucent foreign bodies including wood and plastic splinters 1
• US allows precise foreign body localization for removal 1
• US also detects soft-tissue abscesses, joint effusions, and tenosynovitis 1
• US is particularly valuable in detecting subperiosteal abscesses if osteomyelitis is developing 1

When to Use MRI

MRI should be obtained if osteomyelitis is suspected based on prolonged symptoms (2 weeks), persistent pain, or if initial management fails 1:

• MRI is the modality of choice for suspected bone infection, with 100% negative predictive value for excluding osteomyelitis 1
• MRI shows decreased T1-weighted bone marrow signal with increased signal on fluid-sensitive sequences in osteomyelitis 1
• MRI is superior for evaluating the extent of soft-tissue involvement and detecting abscesses 1
• However, CT is superior to MRI for detecting wood foreign bodies and should be considered if US is inconclusive 1

Management Approach

If Foreign Body is Identified

Reactive materials like wood, thorns, and vegetative matter should be removed immediately before inflammation or infection worsens 3:

• Superficial horizontal splinters can be removed by incising skin over the splinter's long axis and lifting it out with forceps 3
• Deeper splinters near important structures (nerves, tendons, vessels) should be referred for surgical removal 3
• For penetrating knee injuries with joint involvement, arthroscopic evaluation with irrigation and foreign body removal is recommended 4

Antibiotic Management

Based on construction worker penetrating knee injury data 4:

• Initiate IV first-generation cephalosporin immediately after cultures are obtained 4
• Continue IV antibiotics for 12-48 hours, then transition to oral antibiotics for 10 days total 4
• Ensure tetanus prophylaxis is current 4
• If septic arthritis is confirmed, prolonged IV antibiotics (typically 2-4 weeks) are required 2

Critical Pitfalls to Avoid

• Do not delay joint aspiration in a patient with 2 weeks of symptoms—this duration significantly increases infection risk 1, 2
• Do not rely solely on radiographs to exclude foreign bodies; wood splinters are radiolucent and require US or CT 1
• Do not miss necrotizing soft tissue infection—while rare, subacute presentations exist and can be mistaken for simple cellulitis 5
• Do not assume normal white blood cell count excludes infection—peripheral leukocytosis is often absent in joint infections 1
• Retained vegetative foreign bodies trigger granulomatous reactions and subsequent infection if not removed 1