US - Pre-patellar bursitis: Ultrasound of the Month

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A diabetic patient in his 60s presents to the Emergency Department 1 week after falling from a standing height onto his left knee. Over the past 4 days he noted swelling and redness and he had a visit to this ED 2 days ago and was placed on cephalexin and sulfa/trimethoprim. Since then he notes worsening pain and spreading redness since that time. He denies fevers, chest pain and shortness of breath and has been able to bear weight on the knee, although with pain.

On exam his left knee has circumferential anterior erythema and swelling with associated tenderness. Range of motion of the knee is intact but limited by pain. The erythema extends distally to the mid-tibia and without proximal streaking. He is distally neurovascularly intact.

The patient is hemodynamically stable and afebrile, and lab studies show a WBC count of 17, stable renal function, CRP of 260.

A bedside ultrasound was obtained of the patient's bilateral patellar tendons in the longitudinal plane:

Left (affected) patellar tendon

Right (unaffected) patellar tendon

+ What is the diagnosis?

Septic Bursitis

Bursa are synovial membrane lined sacs that help facilitate frictionless sliding of tissue layers. Although there are over 140 in the human body, the olecranon bursa and patellar bursa are the predominantly affected and will be the focus of this review.

Looking at all cases of bursitis, the majority of cases are non-septic traumatic bursitis. These are commonly associated with local and repetitive trauma (athletes, etc).

Bursitis is a common diagnosis in the emergency department, however even with a clinical diagnosis, the possibility of an infectious cause of bursitis is often clouded by concurrent presence of overlying cellulitis or pain with range of motion concerning for septic joint and is an important consideration before performing arthrocentesis through overlying septic bursitis. Immune status is also important to consider as 50% of septic bursa are in immunocompromised patients (1).

Studies evaluating exam and diagnostics strategies for septic bursitis note that systemic evidence of infection (fever > 37.7, other SIRS) in the setting of bursitis is enough to presume causative septic bursitis. If more subtle, guidelines recommend aspiration, which can safely be performed in the ED, with or without ultrasound guidance. WBC > 3000 in the aspirate is concerning for septic bursa.

Initial antibiotics should include skin flora, as staph species are most common infectious agents, with direct invasion being the most common cause (2).

Ultrasound is a quick way to differentiate between a joint effusion and other pathology of the knee (3). Ultrasound is better than physical exam for determining the presence of knee effusion and has a sensitivity of 81% and specificity of 100% when using MRI as the gold standard (4,5,6). In the knee, the typical location to evaluate for a joint effusion with ultrasound is to evaluate the suprapatellar bursa. This is a superior recess of the knee joint and communicates with the knee joint. The probe is placed above the patella. In the longitudinal plane, the quadriceps tendon, femur and patella are all visible. Fluid within the suprapatellar bursa appears as an anechoic area between the quadriceps tendon and the femur. Fluid can be milked into this space using hands, an inferiorly placed ace bandage or asking to the patient to flex the quadriceps muscle to facilitate detection of a small effusion (7,8).

A normal knee ultrasound with the probe superior to the patella in a longitudinal plane

Same Knee, now with an effusion!

In this case, fluid was see overlying the patella and the superior aspect of the patellar tendon, thus confirming the diagnosis of bursitis.

Modified from cc work by openstax at https://upload.wikimedia.org/wikipedia/commons/a/a2/908_Bursa.jpg

In this case there was diagnostic uncertainty of failure of outpatient antibiotics vs deep infection of the bursa or joint, and ultrasound was used to engage consultants that were otherwise reticent to tap through overlying cellulitis. Ultrasound easily identified fluid in the prepatellar bursa and not within the joint, thus alleviating a great deal of diagnostic uncertainty and ultimately providing the patient with faster, targeted care. This case stands as an example of POCUS's utility in rapid bedside diagnostic and therapeutic changes.

In follow up for this patient - he had a local aspiration performed with 10cc of puruluent fluid isolated, he was admitted on IV vancomycin for MRSA coverage and recovered well without need for further intervention.

 
 

Case and Post by Ryan LaFollette, MD

Peer Review by Lori Stolz, MD RDMS