8 April 2021

Combining image modalities can enhance diagnosis

Clinical osteomyelitis

HISTORY

A 67-year-old man had cut his second toe four weeks prior to presenting with a hot, red, swollen but painless toe. He had a long history of non-insulin dependent type 2 diabetes. On examination, there was an erythematous and swollen distal phalanx with ulceration and purulent discharge and cellulitis.

The initial clinical concern was that of peripheral neuropathy and underlying possible osteomyelitis.

IMAGE FINDINGS

Figure A: X-ray of the foot showing probable loss of cortex and bony destruction of the distal phalanx of the second toe (arrow)

First-line imaging included radiographs of the foot, which revealed an apparent lucency of the cortex of the distal phalanx with surrounding soft tissue thickening. A subsequent CT confirmed the presence of bony destruction of the distal tuft of the second toe, but no sequestrum (devitalised fragment of bone) or periosteal reaction.

Figure B: Sagittal CT showing destruction of the cortex distal 2nd phalanx (arrow)
Figure C: Sagittal T2 fat-saturated image showing oedema in the soft tissues and bone of the distal phalanx (red arrows) and slight oedema in the intermediate phalanx (blue arrow). Note that fluid is white and there is paucity of fluid in the distal interphalangeal joint indicating lack of effusion or septic arthritis 

Both the X-rays and CT are sensitive at assessing bony destruction which can be seen in late osteomyelitis, but not so much at detecting septic arthritis, abscess formation or early osteomyelitis.

An MRI of this region demonstrated an intensely enhancing marrow in the distal phalanx, with mild enhancement of the intermediate phalanx, but no effusion or periosteal lifting. 

Figure D: T1 weighted sequence showing dark signal in the marrow of the distal 2nd phalanx indicating osteomyelitis. Marrow in the other toes is bright due to the presence of normal fatty marrow

Also, low signal in the marrow on the T1 sequence within the distal phalanx is confirmatory of osteomyelitis, but this was not present in the intermediate phalanx. 

Figure E,F: Sagittal and coronal post contrast images show a non-enhancing sinus tract with surrounding cellulitis, and enhancing osteomyelitis only in the distal phalanx (blue arrow)

In addition, there is diffuse cellulitis in the soft tissues with a tract of non-enhancing tissue representing an infected sinus tracking from the skin to the bone of the distal phalanx.

DIAGNOSIS

Therefore, the diagnosis was a diabetic infected ulcer with a sinus tract extending into the distal phalanx, causing distal phalanx osteomyelitis, but no septic arthritis. There is reactive oedema without osteomyelitis in the intermediate phalanx. Confidence in this diagnosis was the result of the combination of imaging modalities, in particular the CT and MRI.

MANAGEMENT

Management was amputation of the distal phalanx with resection including the distal interphalangeal joint. The diagnosis, including the absence of septic arthritis in the joint, was confirmed at the time of surgery.

Dr Sebastian Fung is a musculoskeletal radiologist who undertook an MRI imaging fellowship in Hospital for Special Surgery in New York. He now works in Sydney at St Vincent’s Private Hospital and Mater Hospital