A. Elashmawy, B. Merte, and A. J. Medak
Chief Complaint: Left knee pain and swelling
History of Present Illness
A 63-year-old male with a past medical history of hypertension and diabetes presented with five days of progressive left knee swelling, redness, and discomfort. Symptoms were localized to the midline area inferior to the patella. The patient described worsening swelling, warmth, and tenderness to palpation in the area. He denied fever, recent trauma, or prior similar episodes. The patient had recently been prescribed trimethoprim-sulfamethoxazole, but he had not started taking the medication as of yet.
Vitals BP 134/79 | HR 84 | Temp 98.9°F (37.2°C) | RR 17 | SpO₂ 97% on RA
Physical Exam
Gen: Patient alert and in no distress, well-appearing.
Skin: Skin warm. Capillary refill less than 2 seconds.
Extremities: Neurovascularly intact in all four extremities. Examination of the left lower extremity revealed an approximately 6 cm × 6 cm area of fluctuance, erythema, warmth, and tenderness several centimeters inferior to the patella in the midline. Inferior to this area, there was diffuse mild swelling with faint erythema and warmth. Compartments were soft and nontender. Left knee noted to have minimal pain with active ROM. ROM was full.
Laboratory Studies:
- WBC: 12.4 ×10³/mm³ (elevated)
- ESR: 27 mm/hr (minimally elevated)
- CRP: 3.76 mg/dL (elevated)
Additional labs, including CMP and coagulation studies, were otherwise unremarkable.
ED Course
- Treatments and interventions: Intravenous vancomycin and ceftriaxone were administered. Ultrasound-guided aspiration of the infrapatellar bursa was performed, and fluid was sent for culture. Analgesia was provided.
- Consultations: Orthopedic surgery was consulted, and aspiration and intravenous antibiotics were recommended. The patient was admitted for continued management.
- Clinical course: The patient remained hemodynamically stable with persistent localized tenderness but no progression of swelling. Compartments remained soft without evidence of systemic toxicity or evolving septic arthritis.
- Disposition: Admitted to the Medicine service for intravenous antibiotics and continued management of suspected septic infrapatellar bursitis.
Discussion
Infrapatellar bursitis is an inflammatory condition involving the superficial or deep infrapatellar bursae located adjacent to the patellar tendon. These bursae function to reduce friction between the patellar tendon and surrounding soft tissues during knee movement. Inflammation may occur due to repetitive trauma, direct pressure, infection, or underlying inflammatory disease. Patients typically present with localized swelling, erythema, warmth, and tenderness over the anterior knee, which can mimic other soft tissue processes, including cellulitis, abscess, or septic arthritis.
Point-of-care ultrasound (POCUS) has become an increasingly valuable diagnostic tool in the emergency department for evaluating superficial musculoskeletal complaints. Ultrasound allows clinicians to rapidly distinguish between bursal fluid collections, joint effusions, abscesses, and cellulitis, which may have overlapping clinical presentations. In cases of infrapatellar bursitis, ultrasound commonly demonstrates a well-circumscribed fluid collection superficial to the patellar tendon, sometimes with surrounding soft-tissue edema. Ultrasound may also facilitate procedural guidance for diagnostic aspiration, particularly when septic bursitis is suspected.
In this case, bedside ultrasound demonstrated a localized fluid collection within the infrapatellar bursa, helping to narrow the differential diagnosis and guide further evaluation. Subsequent CT imaging confirmed an organized fluid collection without evidence of deeper infection, and ultrasound-guided aspiration was performed for diagnostic evaluation.
Bottom line: Point-of-care ultrasound is a rapid, effective method for evaluating anterior knee swelling in the emergency department. Early ultrasound evaluation can help identify bursal pathology, differentiate soft-tissue infections from other etiologies, and guide procedural management, thereby improving diagnostic accuracy and facilitating timely treatment.
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