Rare presentation of Brodie's abscess in the acromion process and the value of the penumbra sign

Abstract

A 17-year-old male patient presented with a swelling on his right shoulder 1 week after a fall. MRI revealed a superficial fluid collection of the acromion and trapezius muscle, with slight enhancement of the wall. The swelling burst open spontaneously after failed conservative therapy and was treated with surgical drainage and antibiotics. Tissue cultures showed a Staphylococcus aureus 1 year later, he presented with local red discolouration and a recurrent painful swelling that fluctuated at palpation. Radiographs showed a lytic lesion of the acromion process. MRI showed a sinus in the lytic cavity and a penumbra sign, which helped to identify this extremely rare occurrence of Brodie's abscess in the acromion process that was breaking through to the subcutaneous fat. The abscess was treated successfully with surgical drainage and intravenous antibiotics. In retrospect, the first MRI showed a small cortical indentation, with slight oedema of the acromion process.

Figure 1

Axial MRI of the right shoulder made at the time of the initial presentation. Axial T1 shows a dorsocranial fluid collection (star). Adjacent, in retrospect, there is some oedema in the acromion process, and a cortical indentation in the lateral acromion (arrow) in the location where the sinus was later discovered.

Figure 2

Photograph of the right shoulder taken at the time of the second presentation during surgical treatment. The anterior view shows the swelling on the shoulder (white arrow).

Figure 3

Radiograph of the right shoulder taken during the second presentation. The endorotated anterior-posterior radiograph shows an oval lucency with slight surrounding sclerosis in the acromion (arrow). Cranially from the acromioclavicular joint and clavicle, a soft tissue mass (no fat) is shown (star).

Figure 4

Axial MRI of the right shoulder made at the second presentation T1-weighted image at the level of the AC joint shows the abscess (star) in the acromion and small cloaca laterally (arrow). The abscess is outlined by a higher signal (isointense to muscle) curvilinear rim (called penumbra sign, representing granulation tissue). Low signal (oedema, and on this image posteriorly cortical bone) surrounding the abscess.

Figure 5

Coronal MRI of the right shoulder made at the time of the second presentation. (A) Coronal T2 Spectral Presaturation with Inversion Recovery sequence depicts an oval, slightly lobulated fluid collection in the acromion, with little surrounding oedema. A subtle lower signal rim demonstrates the penumbra (arrow). Cranially of the acromion and clavicle it shows a large collection in the subcutaneous fat (star). (B) Coronal T1 SPIR after intravenous gadolinium administration demonstrates both the collections with low-to-intermediate signal, surrounded by a high signal enhancing rim, representing Brodie's abscess with extension of the abscess superficially in the subcutaneous fat. SPIR, spectral presaturation with inversion recovery.

Figure 6

Photographs made during surgical treatment after the second presentation. (A) The dorsal view shows the incision over the posterior aspect of the acromion (arrow). Purulent fluid is draining from the incision (star). (B) The sinus was discovered dorsocranial on the acromion process (arrow). (C) The sinus was enlarged with a drill (arrow). (D) A gentamicin sponge was placed in the abscess cavity.

Figure 7

Radiograph of the right shoulder taken 2 months postoperatively. The endorotated AP radiograph shows partial filling-in of the lucency peripherally and normalised soft tissue cranial of the AC joint area.

Figure 8

Radiograph of the right shoulder taken 1 year postoperatively. The exoratated AP radiograph shows a remaining slight cortical irregularity at the craniolateral side of the acromion.