What skin changes are associated with early osteomyelitis?

Skin Changes in Early Osteomyelitis

In early osteomyelitis, you may see only mild soft tissue swelling and erythema overlying the affected bone, or the skin may appear entirely normal, as radiographic and visible bone changes typically lag behind the infection by 7-10 days. 1

Key Clinical Skin Findings to Look For

Acute Presentation

• Erythema and swelling over the affected area, though these may be subtle or absent in early disease 1
• Soft tissue edema that may be the only visible sign before bone destruction occurs 1, 2
• "Sausage toe" appearance (erythematous and indurated digit) is highly suggestive when present, particularly in diabetic foot osteomyelitis 1
• Ulceration overlying a bony prominence, especially if the ulcer is large (>2 cm²) or fails to heal after 6 weeks of appropriate care 1

Important Clinical Caveat

The absence of skin changes does NOT exclude osteomyelitis. Bone infection can occur without overlying local signs of inflammation, particularly in early disease. 1 This is a critical pitfall—clinicians often miss early osteomyelitis because they expect to see obvious skin changes that may not yet be present.

Specific Skin Findings That Increase Likelihood of Underlying Osteomyelitis

High-Risk Skin Presentations

• Visible or palpable bone through a wound (positive likelihood ratio 9.2) 1
• Ulcer area >2 cm² (positive likelihood ratio 7.2) 1
• Non-healing ulcer despite adequate off-loading and wound care for ≥6 weeks 1
• Ulcer located over bony prominence (metatarsal heads, calcaneus, malleoli) 1

Additional Skin/Soft Tissue Signs

• Sinus tracts or fistulae draining from the skin surface to underlying bone 2
• Effacement of normal fat planes visible on examination 2
• Localized warmth over the affected area 1

Why Skin Changes Are Often Minimal or Absent Early

The fundamental problem is timing: bone destruction requires >30% of osseous matrix to be destroyed before it becomes visible on plain radiographs, and this typically takes 7-10 days from symptom onset. 1, 2 During this early window, skin changes may be limited to mild swelling or may be completely normal despite active bone infection. 1, 2

Diagnostic Approach When Skin Changes Are Subtle

When you suspect osteomyelitis but skin findings are minimal:

• Perform probe-to-bone test if an ulcer is present—a positive test (feeling hard, gritty bone with a sterile blunt probe) has a positive likelihood ratio of 7.2 in high-risk patients 1
• Obtain inflammatory markers (ESR >60 mm/hour or CRP >3.2 mg/dL support the diagnosis) 3
• Order plain radiographs first to exclude alternative diagnoses, but recognize they will be normal in early disease 1, 3, 2
• Proceed to MRI if clinical suspicion remains high despite normal radiographs—MRI has 97% sensitivity and 100% negative predictive value 3, 4

Special Population: Diabetic Foot

In diabetic patients, skin changes may be even more subtle due to neuropathy masking pain and inflammation. Look specifically for:

• Swollen foot with history of ulceration 1
• Erythematous, indurated toe ("sausage toe") 1
• Chronic non-healing ulcer (>6 weeks) despite appropriate care 1, 3
• Exposed bone in the ulcer base 1

The probe-to-bone test is particularly valuable in this population and should be performed routinely when evaluating diabetic foot ulcers. 1, 3