Thought to be cellulitis or an ankle sprain, but it was pus inside the bone: Brodie abscess
A patient in her 70s came in with a red, swollen top of the foot that first looked like cellulitis. A patient in his 20s with long-lasting ankle pain looked like a common sprain. Both turned out to have Brodie abscess, a pocket of pus inside the bone. In both, a single X-ray changed the diagnosis.
What is a Brodie abscess?
A Brodie abscess is subacute osteomyelitis in which pus collects locally inside the bone. It was first described in 1832 by the British surgeon Benjamin Brodie and bears his name. The body cannot fully clear the bacteria and walls them off in hard bone, so a rim of whitened, hardened bone forms around the pocket of pus.
This disease is easy to miss because it does not look like an acute infection. A systematic review that pooled 407 reported cases up to 2019 found the following.
| Item | Findings of the systematic review |
|---|---|
| Age and sex | Many young patients (median age 17), with men about twice as common as women |
| Time to diagnosis | A median of 12 weeks after symptoms began |
| Symptoms | Pain in 98%, swelling in 53%. 84% had no fever |
| Blood tests | Inflammatory markers were raised in fewer than half |
| Causative organism | Staphylococcus aureus was the most common at 67% |
| Treatment | Surgery in 94% and prolonged antibiotics in 77% |
| Recurrence | 15.6% recurred and needed further treatment |
With no fever and blood tests near normal, it is easy to write it off as a sprain, growing pains or muscle pain, and the diagnosis is delayed.
Case 1. A patient in her 70s who looked like cellulitis
A 78-year-old woman came in with a red, swollen left foot. My first impression was cellulitis, an infection of the skin and subcutaneous tissue. She had previously had plate fixation for fractures of the tibia and fibula above the ankle and a knee replacement on the same side.
The X-ray showed a sclerotic lesion, whitened bone, on the inner side of the tibia, so I added a CT scan.
The CT confirmed inflammation and a pocket of pus inside the bone, with pus that had broken through the periosteum surrounding the bone and spread outside. That is why the skin was red and swollen.
Surgery went as follows. I removed the plate on the outer fibula and, through that space, placed antibiotic cement beads mixed with vancomycin inside the tibia. Antibiotic beads release a high concentration of antibiotic at the site of infection for a long time.
I did not remove the plate on the inner tibia. Thinking only of treating the infection, it is best to remove all the metal, but removing this plate as well carried a large risk of the weakened tibia collapsing. International consensus defines infection around a plate as a separate entity, fracture-related infection, and the heart of treatment is balancing control of the infection against keeping the bone strong enough to bear load. After this operation I left myself a note: “in chronic osteomyelitis, watch out for fractures in advance.”
Afterward the patient complained of pain in the opposite knee, and a lumbar MRI showed pockets of pus around it too, so she was transferred to a tertiary hospital to continue treatment. In an older patient with a history of other surgery, an infection can spread to other sites, so new pain should not be taken lightly.
Additional note A typical Brodie abscess occurs in young people with no surgical history. A patient who is older and has a plate, like this one, is unusual, and the picture overlaps with chronic infection at a previous fracture surgery site. It looks the same in that it forms a pocket of pus with a rim inside the bone.
Case 2. A patient in his 20s who looked like an ankle sprain
A patient in his 20s came in with ankle pain that had gone on for a long time. He had no clear memory of a fall, yet the pain would not settle.
The X-ray showed an osteolytic lesion, dissolved bone, in the lower tibia, surrounded by a rim of whitened, sclerotic bone. This is the classic appearance of a Brodie abscess.
On T2-weighted MRI the inside of the lesion was bright, suggesting fluid, that is, pus, and on CT the lesion was seen dissolving the outer cortical bone and leaking out. Taking the findings together, I strongly suspected a Brodie abscess.
MRI has one more finding that helps tell it apart: on T1-weighted images, the “penumbra sign,” a thin band slightly brighter than its surroundings along the inner wall of the abscess. In a study of 32 patients with subacute osteomyelitis it was seen in 75%, and it is known as a finding that points toward infection rather than tumor.
It can be confused with a tumor
On imaging, a Brodie abscess can look like some bone tumors, so telling them apart is essential.
| Condition | Usual site and age | What helps tell them apart |
|---|---|---|
| Brodie abscess | Ends of long bones (metaphysis), children and young adults | A well-defined pocket of pus with a white rim; the MRI penumbra sign |
| Osteoid osteoma | Long bones, ages 10–20 | Pain that worsens at night and responds well to anti-inflammatory painkillers, a small central nidus and thick sclerosis |
| Chondroblastoma | Very end of long bones (epiphysis), teenagers | A well-defined lesion at the bone end near the growth plate |
| Ewing sarcoma | Shaft side of long bones, children and adolescents | Poorly defined spreading bone destruction, onion-skin periosteal reaction, possibly with fever and weight loss |
For that reason, treatment includes a biopsy to confirm it is not a tumor. If pus is found in the operating room, a culture is sent, and the lesion is cleaned out by curettage along with aggressive antibiotic treatment. In the systematic review as well, most patients received both surgery and antibiotics and results were generally good, but about 15% recurred and needed more treatment.
In closing
- A Brodie abscess often has no fever and normal blood tests, so persistent localized pain may be almost the only signal.
- If a young person has pain in the lower tibia or ankle lasting more than a few weeks, it is worth checking with a single X-ray.
- If a site of previous plate surgery turns red and swollen, it may be a bone infection rather than a simple skin infection.
- It can look like a bone tumor, so treatment includes a biopsy and a culture.
This article is general medical information. The same symptom can have many causes, so if pain or swelling lasts, please have it checked.
References
- van der Naald N, Smeeing DPJ, Houwert RM, Hietbrink F, Govaert GAM, van der Velde D. Brodie's abscess: a systematic review of reported cases. J Bone Jt Infect. 2019;4(1):33–39.
- Grey AC, Davies AM, Mangham DC, Grimer RJ, Ritchie DA. The ‘penumbra sign’ on T1-weighted MR imaging in subacute osteomyelitis: frequency, cause and significance. Clin Radiol. 1998;53(8):587–592.
- Metsemakers WJ, Morgenstern M, McNally MA, et al. Fracture-related infection: a consensus on definition from an international expert group. Injury. 2018;49(3):505–510.