Clinical History
An 11 year old female came to our institution with complaints of intermittent pain in her left knee in the last two years. The physical examination and laboratory tests were unremarkable.
Imaging Findings
An 11 year-old female, with complaints of intermittent pain in her left knee in the last two years was referred to our institution. The physical examination and laboratory tests were unremarkable.
The initial radiograph was performed when the complaints of knee pain first started. The frontal radiograph depicts a subtle ovoid, osteolytic lesion involving the metaphysis, the physis, and the epiphysis of the medial proximal left tibia (Figures 1a-1b). The lesion increased in size over a period of nineteen months (Figure 1c-1d), it acquired sclerotic borders, a narrow zone of transition, and at least two smaller internal lytic areas became apparent.
MRI of the left knee was them performed (see Figure 2). In correspondence with the lesion described, a medullary cavity is shown in the metaphysis, the physis, and the epiphysis of the proximal left tibia. The medullary cavity content has low signal in T1-weighted images (WI), has high signal in T2-WI fat suppressed, and enhances after gadolinium contrast, which is consistent with a infectious/inflammatory nature. It communicates with the cortical surface through sinus tracts and contains a small hypointense area on all sequences suggestive of sequestrum. The nearby bone marrow exhibits low signal in T1-WI, high signal in T2-WI fat suppressed, and enhances after gadolinium contrast, denoting also the inflammatory process. Additionally, similar signal changes are appreciated in the periosseous soft-tissues and a small juxtacortical abscess (see arrow) adjacent to the antero-medial tibial surface is visible. These findings are highly suggestive of chronic osteomyelitis complicated with a periosseous abscess.
Discussion
Osteomyelitis is a multifaceted disease that comprehends three distinctive clinical and radiological entities: acute, subacute, and chronic osteomyelitis. This spectrum reflects among others: duration, patient’s age, site of infection, host susceptibility and inadequate therapy, the latter being important harbingers of subacute and chronic infection. The 6th week of evolution is the temporal frame used to define the chronic form.
Three routes of infection are possible: haematogenous, by contiguity, or direct implantation through a penetrating wound or an infected foreign body.
Osteomyelitis usually affects a single bone, being polyostotic in 7% of cases. Sites of predilection are the fast-growing and large metaphyses around the knee, wrist, and proximal humerus. Flat bones, vertebrae, and calcaneus are affected in 25% of cases.
The regional vascular anatomy around the physis differs according to age and elucidates the different haematogenous spread in infants, toddlers, and adults. Transphyseal blood supply during infancy originates the combined metaphyseal, epiphyseal, and joint involvement. Sites adjacent to apophyseal cartilage (metaphyseal equivalents) exhibit a similar vascular pattern and have therefore the same pattern of spread.
The clinical and laboratory signs are often elusive. It is often clinically silent and a causative organism is not identified in 25% of cases, particularly in the chronic form. Chronic osteomyelitis may be indolent for a long time and then reactivate, due to continuous low grade infection.
Imaging approach aims a timely and accurate diagnosis to prevent sequelae. All techniques should be considered and advantage taken of theirs specificities, in a tailored strategy for a given patient and a given institution.
Radiographs are usually the first step. The earliest sign is deep soft tissue swelling; bone destruction and periosteal reaction being visible 10-21 days after onset. Comparative views of the contralateral limb better appreciate subtle early findings.
Bone scintigraphy has moderate specificity and may help to localize osteomyelitis and distinguish it from cellulitis.
MR and CT provide complementary and detailed evaluation of osseous and soft tissue changes. Coronal or sagittal imaging is useful for biopsy and debridement planning, and for physis’ and epiphysis’ assessment. MRI is considered the gold standard for osteomyelitis evaluation (sensitivity: 88-100% and specificity: 75-100%). The penumbra sign is useful for subacute osteomyelitis detection, with a sensitivity of 75% and specificity of 90%. It is identified on unenhanced T1-WI as a thin and slightly hyperintense peripheral zone on Brodie’s abscess, probably due to granulation tissue.
Ultrasound application for soft tissues evaluation and abscess drainage guidance has encouraging reports. Ultrasound availability and low cost make it an attractive alternative for centres with limited access to cross-sectional imaging.
Acute osteomyelitis has often an aggressive appearance and can be mistaken for trauma, eosinophilic granuloma, leukemia or other aggressive neoplasms. The differential diagnosis of subacute and chronic osteomyelitis includes: chondroblastoma, eosinophilic granuloma, granulomatous infections, osteoid osteoma, or Ewing’s sarcoma.
Possible complications comprise septic arthritis, pathologic fracture, growth disturbances, and rarely squamous cell carcinoma decades after chronic osteomyelitis with longstanding sinus tract drainage. Early diagnosis and prolonged antibiotherapy are vital to eradicate osteomyelitis and prevent sequelae.
Differential Diagnosis List
