What is the differential diagnosis between septic arthritis and osteomyelitis?

From the Guidelines

The differential diagnosis between septic arthritis and osteomyelitis is crucial, as septic arthritis is a medical emergency requiring immediate intervention to prevent joint destruction, whereas osteomyelitis may require more extensive surgical intervention and longer antibiotic courses. Septic arthritis and osteomyelitis are distinct but related infections of the musculoskeletal system. Septic arthritis is an infection of the joint space and synovial fluid, while osteomyelitis is an infection of the bone itself. The diagnosis of septic arthritis requires joint aspiration with synovial fluid analysis showing elevated white blood cell count, positive Gram stain, or culture, as recommended by the Infectious Diseases Society of America 1. Osteomyelitis diagnosis often requires blood cultures, bone biopsy, and imaging studies like MRI or bone scan, with MRI with contrast being the modality of choice in the evaluation of osteomyelitis 1. Some key points to consider in the differential diagnosis include:

• The typical presentation of septic arthritis, which includes acute onset of joint pain, swelling, warmth, limited range of motion, and fever, usually affecting a single large joint like the knee or hip.
• The presentation of osteomyelitis, which often includes deep bone pain, localized tenderness, and sometimes fever, though it can also develop acutely.
• The use of the Kocher criteria, which include fever, erythrocyte sedimentation rate, white blood cell count, and an inability to bear weight on the affected side, to distinguish septic arthritis from transient synovitis 1.
• The importance of clinical suspicion of septic arthritis, as diagnosis is made by arthrocentesis, and the need for prompt treatment to prevent joint destruction. Treatment for both conditions includes antibiotics, with empiric therapy typically starting with vancomycin or cefazolin for Gram-positive coverage, plus a third-generation cephalosporin or fluoroquinolone for Gram-negative coverage, as well as joint drainage for septic arthritis and surgical debridement for osteomyelitis 1. The distinction between septic arthritis and osteomyelitis is critical, as it guides the management and treatment of these conditions, with septic arthritis requiring immediate intervention and osteomyelitis often requiring more extensive surgical intervention and longer antibiotic courses. In terms of management, the most recent and highest quality study recommends that typical management of uncomplicated osteomyelitis in the pediatric population consists initially of intravenous antibiotic therapy followed by a prolonged course of outpatient antibiotics, either oral or intravenous, while typical management of septic arthritis consists of antibiotics, arthrotomy, irrigation, and debridement 1. Overall, the differential diagnosis between septic arthritis and osteomyelitis requires careful consideration of the clinical presentation, laboratory results, and imaging studies, as well as prompt treatment to prevent long-term morbidity and mortality.

From the Research

Differential Diagnosis

The differential diagnosis between septic arthritis and osteomyelitis can be challenging, as both conditions present with similar symptoms such as joint pain, swelling, and fever. However, there are some key differences:

• Septic arthritis typically presents with acute onset of monoarticular joint pain, erythema, heat, and immobility, whereas osteomyelitis may present with more chronic symptoms such as bone pain and swelling 2, 3.
• Synovial fluid analysis is crucial in diagnosing septic arthritis, as it can help distinguish between septic arthritis and other conditions such as crystal arthropathy 3, 4.
• The presence of a high synovial fluid white blood cell count (usually greater than 50,000 per mm3) is indicative of septic arthritis, whereas a lower count may be seen in osteomyelitis 3.

Diagnostic Criteria

The diagnosis of septic arthritis is typically made based on a combination of clinical presentation, laboratory results, and synovial fluid analysis. The following criteria are commonly used:

• Acute onset of monoarticular joint pain, swelling, and fever
• Synovial fluid white blood cell count greater than 50,000 per mm3
• Presence of bacteria on Gram stain or culture of synovial fluid
• Elevated erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) levels 2, 3.

Microbiological Differences

The microbiological profile of septic arthritis and osteomyelitis may also differ:

• Septic arthritis is most commonly caused by Staphylococcus aureus, followed by other bacteria such as Streptococcus and Gram-negative organisms 2, 5.
• Osteomyelitis may be caused by a wider range of microorganisms, including bacteria, fungi, and mycobacteria.
• The microbiological profile of septic arthritis may vary depending on the joint affected, with MRSA being more common in shoulder infections 5.

Treatment Differences

The treatment of septic arthritis and osteomyelitis also differs:

• Septic arthritis typically requires prompt antibiotic therapy and drainage of the infected joint, either through arthrocentesis or surgical debridement 2, 3.
• Osteomyelitis may require longer-term antibiotic therapy and possibly surgical debridement of the affected bone.
• The choice of antibiotic therapy may depend on the suspected or confirmed microbiological cause of the infection, with vancomycin being a common choice for MRSA infections 3, 5.