The onus is on the physician to distinguish a septic joint from all other diagnoses when faced with a patient with an acutely swollen and painful joint. Septic bursitis, in particular, can mimic this emergent condition. Factors such as patient habitus or decreased range of motion because of pain may prevent a thorough examination. Bedside ultrasound provides a quick method to differentiate rapidly between bursitis and effusion and to save an unnecessary procedure.
Multiple joints can be affected by effusions, but the knee and elbow are the most likely to be affected by bursitis. Bursitis will appear very similar from joint to joint, appearing as superficial, well-defined, anechoic or hypoechoic (black or dark gray) collections consistent with an enlarged and fluid-filled bursa. (Image 1.) Signs of soft tissue edema, like thickening or cobblestoning, may be noted on ultrasound when there is a significant surrounding inflammatory component. It would appear adjacent to the enlarged bursa.
A high-frequency transducer should be used for maximum resolution when evaluating the knee. Identifying landmarks can be difficult in some patients, and placing the transducer midline directly over the patella can be helpful to orient the examiner. (Image 2.) The patella is seen as a hyperechoic, slightly curved, superficial structure. Usually, prepatellar bursitis will be quickly noted on this view superficial to the patella. The transducer can then be moved slightly medially or laterally until the patella is seen to disappear, and it can be used to look “under” the patella. Joint effusions can be seen as anechoic or hypoechoic collections in this area. If no obvious effusion is seen, the transducer should be advanced slightly proximal to evaluate the suprapatellar recess of the joint, a frequent hiding spot for fluid. (Image 3.)
The elbow can be evaluated from the anterior or posterior aspect. Extension may be more comfortable than flexion, so evaluation from the posterior aspect may be easier for the examiner. (Image 4.) The transducer should be placed in a transverse orientation just proximal to the olecranon to evaluate the olecranon fossa, which appears as a U-shaped recess. (Image 5.) Joint effusions will be seen to fill this U, while bursitis will be seen superficial to this area. (Image 1.)
Next month, we will look at the final installment of the basics of soft tissue ultrasound by evaluating the hands and feet.
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