What is the recommended approach for diagnosing osteomyelitis in the distal phalanx of a finger?

Diagnosis of Osteomyelitis in the Distal Phalanx of the Finger

For diagnosing osteomyelitis in the distal phalanx of a finger, begin with plain radiographs and clinical examination including probe-to-bone testing if an open wound is present, followed by MRI if the diagnosis remains uncertain or for surgical planning.

Initial Clinical Assessment

Suspect osteomyelitis when:

• An open wound or ulcer overlies the distal phalanx and fails to heal after 6 weeks of appropriate care 1
• The finger appears swollen, erythematous, and lacks normal contours ("sausage digit") 1
• There is visible bone or a draining sinus tract 1
• The wound area exceeds 2 cm² 1

Perform probe-to-bone (PTB) testing if an open wound is present by gently inserting a sterile blunt metal probe through the wound 1:

• A positive test (palpable hard, gritty bone) substantially increases the likelihood of osteomyelitis with a positive likelihood ratio of 7.2 in high-prevalence settings (>60% pretest probability) 1, 2
• A negative test in low-risk patients (≤20% prevalence) essentially rules out osteomyelitis with a negative likelihood ratio of 0.48 1, 2
• The test accuracy is highly dependent on clinician experience and the pretest probability of disease 1

Laboratory Testing

Order inflammatory markers to support the diagnosis 1:

• Erythrocyte sedimentation rate (ESR) >70 mm/h increases the likelihood of osteomyelitis (positive LR of 11) 1
• ESR <70 mm/h reduces the likelihood (negative LR of 0.34) 1
• C-reactive protein, procalcitonin, and white blood cell count are less useful; normal white blood cell count does not exclude osteomyelitis 1, 2
• Combining laboratory findings with clinical assessment improves diagnostic accuracy 1

Imaging Algorithm

Step 1: Plain Radiographs (Initial Study)

Obtain plain radiographs of the affected finger in all cases 1:

• Look for cortical erosion, periosteal reaction, mixed lucency and sclerosis, or bone destruction 1, 2
• Sensitivity is only 54% and specificity 68%, with positive LR of 2.3 and negative LR of 0.63 1
• Radiographic changes typically require 2-3 weeks to develop, so early osteomyelitis may show normal films 1

If initial radiographs are negative but suspicion persists:

• Treat the soft tissue infection for 2 weeks, then repeat radiographs 2-4 weeks later to look for interval changes 1

If radiographs show classic osteomyelitis changes:

• Proceed with treatment after obtaining bone culture specimens 1

Step 2: MRI (Preferred Advanced Imaging)

Order MRI when 1:

• Plain radiographs are equivocal or negative but clinical suspicion remains high
• Surgical planning requires precise anatomic definition
• Deep soft tissue abscess is suspected

MRI characteristics of osteomyelitis 1, 2:

• Low focal signal intensity on T1-weighted images
• High focal signal on T2-weighted images
• High bone marrow signal on STIR sequences
• Sensitivity ~90-97%, specificity ~85-93%, with positive LR of 3.8 and negative LR of 0.14 1, 2
• Normal marrow signal reliably excludes infection with 100% negative predictive value 2

Step 3: Nuclear Medicine (Alternative if MRI Unavailable)

Consider leukocyte or antigranulocyte scan combined with bone scan if MRI is contraindicated or unavailable 1. However, this is a second-line option with lower diagnostic accuracy 1.

Definitive Diagnosis: Bone Biopsy and Culture

The gold standard for diagnosis is bone culture plus histology showing inflammatory cells and osteonecrosis 1:

Obtain bone biopsy when 1:

• The diagnosis remains uncertain after imaging
• Antibiotic-resistant organisms are suspected
• The patient fails to respond to empiric therapy
• You need to justify prolonged antibiotic therapy or major surgery

Biopsy technique 1:

• Percutaneous biopsy should be performed under fluoroscopic or CT guidance, traversing uninvolved skin if possible
• Use bone-cutting needles (Jamshidi or Ostycut types)
• Obtain 2-3 specimens: send one for culture and another for histology 1
• Operative biopsy during debridement is preferable to percutaneous needle biopsy 2

Common Pitfalls to Avoid

Do not rely on soft tissue or wound cultures to guide antibiotic therapy for osteomyelitis, as they do not accurately reflect bone culture results 2, 3:

Do not assume normal inflammatory markers exclude osteomyelitis, particularly normal white blood cell count 1, 2:

Do not order MRI as a first-line test in straightforward cases where plain radiographs show classic changes 1:

Recognize that the PTB test accuracy depends heavily on pretest probability - it is most useful in intermediate-probability cases, not when osteomyelitis is already clinically obvious or highly unlikely 1:

Be aware that distal phalanx osteomyelitis in fingers can occur from contiguous spread following trauma, infected wounds, or severe skin infections with scratching 4: