Osteomyelitis: Clinical sciences

Osteomyelitis refers to an infection of the bone, which is typically caused by bacteria, such as Staphylococcus aureus.

Osteomyelitis develops by one of three routes; direct inoculation of the bone, like from an open fracture; contiguous infection, like from an infected foot ulcer overlying the bone; or hematogenous spread, like from bacteremia due to endocarditis. Moreover, acute osteomyelitis develops within days to weeks of infection, whereas chronic osteomyelitis is characterized by long standing infection over months or even years.

If your patient presents with chief concerns suggesting osteomyelitis, you should first perform an ABCDE assessment to determine if the patient is unstable or stable. If unstable, stabilize the airway, breathing, and circulation, obtain IV access, and start IV fluids. Next, put your patient on continuous vital sign monitoring, including blood pressure, heart rate, and oxygen saturation. Finally, if needed, provide supplemental oxygen, and don’t forget to start broad-spectrum antibiotics.

Okay, now let’s go back to the ABCDE assessment and take a look at stable individuals. In this case, first, obtain a focused history and physical examination. Next, order labs, including CBC, CRP, and ESR.

Patients typically report fever, as well as pain, redness, and swelling at the site of the infection. Additionally, the physical exam usually reveals erythema, warmth, and purulent drainage, as well as tenderness to palpation over the affected bone area. Finally, labs usually show leukocytosis and elevated CRP and ESR. With these findings, you should suspect osteomyelitis.

Your next step is to order imaging, such as an X-ray or MRI. An MRI is the best imaging study to diagnose osteomyelitis, so make sure to order one if the X-ray results are normal but there's high clinical suspicion. Additionally, a bone biopsy can help you reveal the histopathologic changes specific to osteomyelitis, and bone cultures, or deep tissue cultures, can reveal the causative pathogen. Together, histopathological examination and microbiological examination of bone are the gold standard when it comes for diagnosing osteomyelitis!

Alright, first let’s take a look at acute osteomyelitis! In this case, your patient will usually report symptoms lasting for up to 6 weeks. The X-ray can be normal or may show overlying soft tissue swelling with cortical bone destruction and an underlying lucent bony lesion;

MRI typically reveals bone marrow edema and overlying periosteal and subcutaneous edema. Finally, the bone biopsy will reveal neutrophil-rich infiltrate and thrombosis of small blood vessels. With these findings, you can diagnose acute osteomyelitis!

On the other hand, we may have chronic osteomyelitis. These patients will report symptoms lasting for a time period that’s longer than 6 weeks. In these individuals, the X-ray typically demonstrates a lucent lesion with a surrounding poorly defined sclerotic border and overlying periosteal thickening;

MRI shows a bony abscess with a thickened enhancing rim, as well as surrounding bone marrow and soft tissue edema. This type of abscess is also known as a Brodie abscess. Finally, the bone biopsy reveals lymphocyte- and plasma cell-rich inflammatory infiltrate, bone marrow fibrosis, along with bone necrosis and new bone formation. With these findings, diagnose chronic osteomyelitis!

Now, regardless of acute versus chronic, once you diagnose osteomyelitis, begin empiric antibiotics that cover the most likely pathogens, and that have good bone penetration. Since the most common causative pathogen of osteomyelitis is Staphylococcus aureus, initiate the treatment with antistaphylococcal penicillins or first-generation cephalosporins. However, if you suspect methicillin-resistant Staphylococcus aureus, your choice should be vancomycin!

Additional antibiotic coverage might be needed in patients with certain risk factors! For example, if your patient has a history of hemoglobinopathies like Sickle Cell Disease, they’re at higher risk for osteomyelitis from Salmonella.