Ulcer depth, probe-to-bone and osteomyelitis

01Purpose and principlesWhat the assessment is for and the core concepts behind it.

Diabetic foot osteomyelitis usually arises by contiguous spread from a chronic ulcer. Depth, duration, location over a bony prominence, visible bone and a swollen 'sausage' toe increase prior probability. The diagnosis matters because prolonged antibiotic therapy, bone resection or both may be required, yet overdiagnosis exposes people to unnecessary treatment and loss of tissue. No isolated bedside or laboratory result is reliable enough to settle every case.

Use sequential probability. Start with careful wound examination, gentle sterile metal probe-to-bone testing by a trained clinician, plain radiographs and inflammatory markers when the examination is equivocal. If the combination leaves important uncertainty, MRI provides detailed marrow and soft-tissue anatomy. Nuclear imaging can be an alternative when MRI is unavailable or unsuitable. Bone culture, ideally with histology where locally available, gives the strongest pathogen evidence, but sampling is invasive and must be timed around clinical urgency and antibiotics.

Key points

Deep infection with sepsis, abscess, necrosis, gangrene or significant ischaemia needs immediate acute referral and surgical or vascular assessment; do not wait for MRI.
A negative probe-to-bone test, normal inflammatory markers or normal early radiograph does not exclude osteomyelitis when a wound is deep, chronic or clinically suspicious.
Begin with wound depth, probe-to-bone, plain radiographs and ESR or CRP; use MRI when doubt persists and the result will change management.
Probe-to-bone is most useful when interpreted against pre-test probability: a positive result supports disease in a high-risk ulcer, while a negative result helps more in a low-risk ulcer.
MRI is sensitive and maps soft tissue, but marrow oedema from Charcot arthropathy, trauma or surgery reduces specificity.
When pathogen identification is important, obtain bone aseptically through intact skin or at surgery; superficial wound flora may not match bone organisms.
Document ulcer length, width, depth, sinus tracts, undermining and exposed structures, and assess perfusion because inadequate blood flow changes both healing and surgical choices.
Antibiotic duration follows residual bone: 2–5 days after complete resection; consider up to 3 weeks after minor amputation or bone resection with a positive margin; use 6 weeks when bone is not resected or dead bone remains. These IWGDF/IDSA branches have low-certainty support and require specialist review.

02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.

High-risk wound geometry

A deep, wide or long-standing ulcer over a bony prominence, recurrent ulceration at one site, undermining, a sinus or exposed bone increases concern. Record actual dimensions and structures rather than using vague labels such as shallow or deep.

Probe-to-bone finding

After cleansing and debridement, advance a sterile blunt metal probe gently through the ulcer. A hard gritty endpoint suggests bone. Technique, ulcer location and pre-test probability affect performance; pain is not required because neuropathy is common.

Local bone phenotype

Visible bone, an erythematous swollen sausage toe, persistent drainage or failure to heal despite offloading and perfusion care supports osteomyelitis. Acute inflammatory signs may be absent in chronic disease or severe ischaemia.

Complicated deep infectionRed flag

Systemic illness, deep fluctuance, necrosis, gas, extensive tissue tracking or gangrene indicates an urgent surgical problem. Imaging should define extent only when it does not delay drainage, debridement or revascularisation.

Important mimics

Acute Charcot arthropathy, fracture, recent surgery, reactive marrow oedema, gout and neuropathic deformity can imitate clinical or MRI changes. Relate abnormalities to the ulcer tract and compare current with prior imaging.

Red flags requiring action

Systemic toxicity, haemodynamic instability, rapidly spreading inflammation or organ dysfunction requires immediate sepsis management and surgical assessment.
Fluctuance, purulent tracking, crepitus, bullae, gas on imaging or disproportionate pain suggests a deep collection or necrotising process needing urgent source control.
Gangrene, a cool foot, absent Doppler signals or severe perfusion deficit means infection and ischaemia must be managed together urgently.
Exposed bone, a sausage toe, a deep chronic wound over bone or recurrent breakdown at the same site markedly raises pre-test probability of osteomyelitis.

03Method and interpretationA systematic approach to the test and its findings.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Probe-to-bone with wound depth assessmentFirst step
Why
Adjust pre-test probability rapidly and identify ulcers that communicate with deeper structures.
Interpretation and limitations
A hard gritty endpoint is positive. It supports osteomyelitis most strongly in a clinically high-risk wound; a negative test is reassuring mainly when baseline risk is low. Technique-dependent results cannot independently include or exclude disease.
02
Plain foot radiographs
Why
Detect cortical erosion, periosteal reaction, demineralisation, sequestrum, gas, foreign body and deformity and establish a baseline.
Interpretation and limitations
Characteristic bone destruction supports osteomyelitis, but changes may lag infection by weeks. A normal early film does not exclude disease; repeat in 2–3 weeks or obtain MRI if suspicion persists.
03
ESR, CRP and full blood count
Why
Support probability and monitor systemic impact when clinical assessment is uncertain.
Interpretation and limitations
A substantially raised ESR increases concern but thresholds are imperfect; CRP may track soft-tissue inflammation more quickly. Normal markers do not exclude chronic or local osteomyelitis, particularly with ischaemia.
04
Magnetic resonance imaging
Why
Confirm or refute suspected osteomyelitis after inconclusive initial assessment and map marrow, joint and soft-tissue extent.
Interpretation and limitations
Contiguous marrow abnormality from an ulcer and associated cortical disruption support infection. Charcot change, trauma and recent surgery cause reactive oedema and lower specificity, so expert musculoskeletal interpretation is valuable.
05
Aseptic bone culture and histology
Why
Identify bone pathogens and provide tissue confirmation when results will alter antibiotic or surgical management.
Interpretation and limitations
Obtain percutaneously through intact skin or intraoperatively to avoid contamination. Prior antibiotics can reduce culture yield; histology may remain informative. A superficial or even deep soft-tissue isolate may not represent bone.

04Clinical next stepsHow the result changes management or prompts escalation.

01Diagnostic sequenceStable suspected osteomyelitisFirst stepA deep, chronic or otherwise suspicious ulcer is present without an immediate need for drainage or sepsis intervention.

1Document wound geometry, perfusion and infection grade; perform trained probe-to-bone testing, plain radiographs and ESR or CRP as the initial combined assessment.
2If osteomyelitis remains uncertain and the answer will change care, obtain MRI; consider PET, labelled leukocyte scintigraphy or SPECT when MRI is unsuitable or unavailable.
3Discuss aseptic bone sampling when organism identification or diagnostic certainty will change prolonged therapy or surgery, and coordinate timing with any antibiotics.
4Integrate anatomy, perfusion, viable tissue, patient preferences and microbiology into a multidisciplinary plan, then define response and recurrence monitoring.

02Preferred branchDeep infection needing source controlPreferredSepsis, abscess, necrotising features, gangrene, compartment infection or severe ischaemia accompanies suspected bone infection.

1Refer immediately and stabilise the patient; obtain urgent surgical and vascular opinions and do not allow MRI scheduling to delay required source control.
2Collect blood, tissue and bone specimens when feasible before or at surgery, but start empiric antibiotics promptly when delay would be unsafe.
3Debride non-viable tissue, drain collections and coordinate revascularisation; revise antibiotics to cultures, histology, residual bone and clinical response.

03Treatment selectionMedical versus surgical strategyOsteomyelitis is sufficiently established and the team must decide whether antibiotics alone are reasonable.

1Consider bone resection plus systemic antibiotics when necrotic bone, exposed bone, spreading soft-tissue infection, poor mechanics or uncertain source control makes surgery advantageous.
2Antibiotics without surgery may be considered for selected forefoot disease without immediate drainage need, without PAD and without exposed bone, under specialist follow-up.
3Match treatment to residual bone: 2–5 days after complete resection; consider up to 3 weeks after minor amputation or resection with positive margin culture or histology; use 6 weeks without bone resection or when dead bone remains. Treat these low-certainty branches as specialist-reviewed targets, then monitor remission for at least six months.

05Procedure and medicine safetyRelevant preparation, treatment and contraindications.

Eradicates residual bone infection after diagnostic sampling and any required source-control procedure.

Culture-directed systemic antibiotic

Select agent, route, dose and frequency from the organism, susceptibility, oral bioavailability and renal or hepatic function. After complete bone resection treat 2–5 days; consider up to 3 weeks after minor amputation or bone resection with positive margin culture or histology; use 6 weeks when there is no bone resection or dead bone remains. These low-certainty durations require specialist review rather than an inflexible stop rule.

There is no single universal drug regimen. Check allergy, pregnancy, interactions, perfusion and prior antibiotics; use oral therapy for prolonged treatment when bioavailability and susceptibility permit, and stop or revise for toxicity, resistance or diagnostic reclassification.

06Risks, monitoring and follow-upComplications, safety checks and further assessment.

Measure wound area and depth, drainage, surrounding inflammation and exposed structures and assess whether offloading and perfusion are adequate at each review.
Follow renal and hepatic function, blood count and drug-specific toxicity during prolonged antibiotics; reconcile interactions and adherence at every transition of care.
Use clinical progress rather than serial MRI alone: marrow abnormalities can persist after improvement, while recurrent drainage or new breakdown requires reassessment.
After antibiotics end, monitor for remission for at least six months, including sustained epithelialisation, recurrent ulceration, inflammation and need for further imaging or sampling.
Reassess immediately for systemic illness, spreading infection, new necrosis, severe pain, vascular deterioration or failure to improve despite apparently active therapy.

07Special situationsVariants, exceptions and circumstances that change the usual approach.

Pre-test probability matters

The same negative probe-to-bone result is more reassuring in a recent shallow ulcer than in a chronic deep wound over a metatarsal head. Test characteristics cannot be interpreted without the starting probability.

Radiographs reward comparison

Early films may be normal, but serial radiographs can reveal progressive cortical destruction. Prior images also help distinguish longstanding Charcot deformity from a new focal change contiguous with an ulcer.

MRI maps more than bone

MRI can show an abscess, sinus tract, tendon sheath or joint involvement that changes surgery. Its main limitation is specificity where neuropathic, postoperative or traumatic marrow oedema is present.

Bone and wound may disagree

Organisms recovered from even a properly collected deep tissue specimen may differ from aseptic bone culture. This matters when committing to weeks of targeted therapy or when previous antibiotics have selected resistant flora.

Perfusion changes feasibility

Poor arterial inflow limits antibiotic delivery and wound healing and may make either conservative surgery or medical treatment fail. Vascular assessment belongs inside the osteomyelitis plan, not after it.

08Common pitfallsFrequent interpretation and management errors.

01
Declaring osteomyelitis absent after one negative probe-to-bone test ignores wound risk and the test's dependence on technique and prevalence.
02
Treating MRI marrow oedema as automatically infectious overdiagnoses osteomyelitis in Charcot arthropathy, trauma and postoperative feet.
03
Using a superficial swab to choose six weeks of antibiotics mistakes colonising wound flora for bone microbiology.
04
Pursuing diagnostic perfection while a patient has sepsis, an abscess or gangrene delays the source control that determines survival and limb outcome.
05
Forgetting offloading and perfusion during antibiotic therapy allows mechanical damage and ischaemia to perpetuate a wound even if bacterial burden falls.

Find your next topic