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56. Radiology Case 11
Keywords
OsteomyelitisOsteoblastsBone lossDrain pustulousHematogenous spreadIndications for the Exam
This patient, a 33-year-old female, with a past medical history of intravenous drug abuse presents with fever, left lower leg pain, and drainage from an ulcer on her lower left leg.
Radiologic Findings
Anteroposterior (AP) radiograph of left tibia and fibula – diffuse soft tissue swelling with cutaneous ulceration of the distal lateral leg and periosteal reaction of distal tibia and fibula.
Diagnosis
Osteomyelitis
Learning Points
Priming Questions
What is the normal evolution of imaging findings on radiographs in osteomyelitis?
What are common pathogens for various patient populations?
How is osteomyelitis different in the adult versus pediatric patient?
Introduction/Background
Osteomyelitis is an infection of the bone typically from a bacterial source. Acute cases progress over days to weeks which can evolve to a relapsing, chronic form after months to years. A majority of cases occur in children and the elderly, although it can be seen at any age, with a 3:1 male predominance [2]. Specific risk factors include diabetes, intravenous drug abuse, trauma or surgery, and sickle cell anemia.
Pathophysiology/Mechanism
There are two main origins for developing osteomyelitis: hematogenous spread and direct inoculation. Microorganisms such as bacteria embed into the bone and initiate an inflammatory response that leads to osteoclast activation and subsequent bone destruction. Osteoblasts counteract the bone loss with intense bone production which gives the periosteal reaction demonstrated in the above case. As the inflammatory response evolves, intraosseous vascular pathways are destroyed resulting in ischemia and osseous necrosis. Necrotic fragments then can be sequestered and serve as an infection nidus for chronic episodes of infection that are difficult to target medically. Sinus tracts can develop from the infected medullary cavity beyond the periosteum and drain pustulous material through the defect.
The most frequently encountered pathogen in all cases is Staphylococcus aureus [1, 2]. Particular pathogens have a higher propensity in specific populations, such as Streptococci spp and anaerobic bacteria in patients with diabetes, gram-negative species in IV drug abusers, and Salmonella spp in patients with sickle cell anemia [1].
Children are susceptible to osteomyelitis via the hematogenous route due to abundant vascularity with slow flow at the metaphyses of long bones, especially involving rapidly growing joints such as the knee [3, 5].
Making the Diagnosis
Many radiographic findings in osteomyelitis are in fact secondary signs that are insensitive and typically appear days to weeks after infection has actually started to affect the bone. The earliest radiographic signs of developing infection are soft tissue edema and loss of the fat planes adjacent to the bone, which evolve over the period of days [5]. Apparent radiographic changes to the bone itself occur 10 days to weeks later and include cortical blurring or loss, trabeculae lysis, and periosteal reaction. [4, 5] Clinical history and physical exam findings such as cutaneous ulceration and recurring cellulitis and/or osteomyelitis episodes should warrant closer inspection for subtle imaging findings.
The gold standard for diagnosis of osteomyelitis is bone biopsy but this is of course invasive. The other available imaging modality is Magnetic Resonance Imaging (MRI), which can detect early and subtle changes to the bone marrow that cannot be detected on radiographs [6].
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