A Bone to Pick: Diagnosing Osteomyelitic Foot Infections

Last updated on September 4th, 2024 at 02:02 pm

Osteomyelitis is a diagnostic term used to describe the destruction of bone via inflammatory processes, often associated with pathogenic infection.  Osteomyelitis exists on a wide chronicity spectrum, with variable treatment and prognosis.  Acute osteomyelitis is generally used for symptoms < 2 weeks duration; chronic osteomyelitis is generally reserved for patients with symptom duration > 6 weeks.  Of note, there are 3 main etiological categories of osteomyelitis (listed in order of descending frequency): contiguous spread, vascular insufficiency, and hematogenous spread.  Contiguous spread is used to describe osteomyelitis wherein the causative pathogen is translocated directly from the external world to the affected bone: trauma, recent surgery, chronic skin breakdown (eg. pressure ulcers).  Vascular insufficiency describes cases where the inflammatory bone destruction occurs secondary to poor blood flow, most notably in patients with poorly controlled or long-standing diabetes or peripheral arterial disease.  Osteomyelitis caused by vascular insufficiency occurs most commonly in the distal extremities (particularly in the foot, where concomitant neuropathy and skin breakdown are common).  Hematogenous spread occurs when bacteremia leads to seeding of the affected bone and occurs most commonly in pediatric populations.

Clinical gestalt plays an important role here, with at least moderately impressive positive and negative likelihood ratios in patients with diabetic foot wounds according to one meta-analysis.^​1​ Unfortunately none of the vital signs (alone or in combination) are sensitive enough for clinical practice in an emergency setting.^​2​  Labs have variable test characteristics as well.  WBC count understandably has increasing sensitivity as the cutoff is increased, but unfortunately loses specificity and thus should not be used on its own for diagnostic determination.^​2–5​ Definitionally, one would reasonably expect inflammatory markers to have diagnostic value in osteomyelitis; unsurprisingly, CRP has a higher sensitivity than ESR for acute osteomyelitis but neither are sensitive or specific enough across the spectrum of chronicity.^​2​  When paired with physical exam findings (specifically ulcer depth > 3 mm + EITHER of ESR > 60 or CP > 3.2) however, the sensitivity increases to 100%.^​2​  The most sensitive singular physical exam finding for osteomyelitis of any chronicity is a positive probe-to-bone (PTB) test, which involves the clinician physically probing to bone with a sterile instrument (I recommend a sterile metal probe if insensate and a sterile q-tip if sensate).^​6​ This essentially proves an easily accessible route for contiguous spread.  Note: many bacteria commonly found in patients with osteomyelitis secondary to contiguous spread form thin biofilms which are easily spread away but will prevent the ability to see the bone, necessitating the use of the probe in all open areas before confidently calling the PTB testing negative.

Plain films (xrays) have variable sensitivities across the chronicity spectrum of osteomyelitis, with no radiographic changes consistently demonstrated until day 10-21 of disease.^​7,8​ Bone scans alone are fairly sensitive (81%) but have poor specificity so will often require follow up imaging/testing for confiramation.^​7​ FDG (Fluorodeoxyglucose) in combination with PET/CT and radionucleotide-labeled monoclonal antibody (MoAb) bone scan are both expensive but highly sensitive and specific and can be considered, but are unlikely to be available to in the emergency department setting.^​9,10​  CT, with or without contrast, is neither sensitive or specific by itself but still remains an important imaging modality in these cases for assessing for soft tissue air or other findings concerning for necrotizing infection which would require immediate surgical management.  MRI is highly sensitive for osteomyelitis, with a sensitivity of 90% rivaled only by PTB testing described above.^​7​
To summarize I offer the clinical decision pathway shown below, which takes into account all of the different diagnostic modalities described above.^​11​ Note that this pathway has been internally validated but has yet to be externally validated and should thusly be used with caution and subjected to your own clinical gestalt.

Cite this post: Taylor Blackwell, MD, Arman Hussain, MD. “A Bone to Pick: Diagnosing Osteomyelitic Foot Infections”. GW EM Blog. July 26,2023. Available at: https://gwemblog.com/foot-osteomyelitis.

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