Imaging for Osteomyelitis of Coccyx

Initial Diagnostic Algorithm

The American College of Radiology recommends plain radiographs as the first imaging test due to their wide availability, low cost, and ability to provide anatomic evaluation 2, 3
Radiographs help evaluate for radiodense foreign bodies, soft tissue gas, and exclude alternative diagnoses such as fractures or tumors 3, 4
Early acute osteomyelitis may show only normal findings or mild soft tissue swelling on radiographs, limiting their sensitivity 2
Radiographic findings of established osteomyelitis include erosions, periosteal reaction, bone sclerosis, and mixed lucency 2, 3
MRI is the modality of choice for suspected bone infection due to its superior sensitivity for bone marrow abnormalities and excellent soft tissue contrast and anatomic detail 3, 5
MRI has a reported 100% negative predictive value for excluding osteomyelitis; a normal marrow signal reliably excludes infection 3

CT scanning depicts cortical bone well and can detect periosteal reaction, bone destruction, and sequestra, as well as soft tissue infections, abscesses, and sinus tracts 3, 5
Nuclear medicine examinations are most appropriate when MRI is contraindicated, infection is suspected to be multifocal, or infection is associated with orthopedic hardware or chronic bone alterations 3, 5, 6
Three-phase bone scan is highly sensitive but lacks specificity, and can become positive as early as 1-2 days after symptom onset 5
Combined approaches, such as bone scan with labeled leukocyte scan, enhance specificity 5, 1

Relying solely on radiographs to rule out infection, as early osteomyelitis may have normal radiographic appearance 2, 1
Misinterpreting post-traumatic changes as infection on MRI 1
Failing to obtain contrast-enhanced MRI when evaluating soft tissue involvement 1
Not considering multifocal involvement, which is more common in children but can occur in adults 6
Overlooking the need for tissue sampling in cases where imaging findings are inconclusive 7, 8

Contrast-enhanced MRI is particularly important in this region to differentiate infection from other conditions such as pilonidal disease or perirectal abscess 1
When interpreting imaging, consider that the coccyx may be affected by pressure-related changes in bedridden patients, which can mimic infection 3, 4