Diabetic Foot Infections

Wound Examination:

• Size: measure length, width, depth in centimeters
• Location: pressure points (metatarsal heads, heel) high risk
• Depth: superficial vs deep (probe-to-bone test)
• Base: granulation tissue (healthy) vs necrotic tissue
• Drainage: serous (uninfected) vs purulent (infected)
• Odor: foul smell suggests anaerobic infection

Probe-to-Bone Test:

• Use sterile metal probe or cotton swab
• Gently insert into ulcer and probe to base
• Positive: hard, gritty bone felt at base
• Sensitivity 87%, Specificity 83%, PPV 89% for osteomyelitis
• Most accurate in ulcers greater than 2cm² and depth greater than 3mm

Surrounding Tissue Assessment:

• Measure erythema extent from wound edge
• Palpate for induration, fluctuance (abscess)
• Check for lymphangitic streaking
• Assess for crepitus (gas in soft tissues)
• Document any exposed bone, tendon, or joint

The probe-to-bone test should be performed in all diabetic foot ulcers greater than 2cm² or depth greater than 3mm. A positive test in the appropriate clinical context is highly predictive of osteomyelitis and may be sufficient to initiate treatment without confirmatory imaging.

Perfusion Examination:

• Inspect for skin changes: hairless, shiny, atrophic
• Palpate dorsalis pedis and posterior tibial pulses (grade 0-2)
• Capillary refill time (normal less than 2 seconds)
• Dependent rubor and elevation pallor (Buerger test)
• Assess for foot temperature asymmetry

Ankle-Brachial Index (ABI):

• Normal: 0.9-1.3
• PAD: less than 0.9
• Non-compressible vessels: greater than 1.4 (calcified, unreliable)
• If ABI greater than 1.4, obtain toe-brachial index (TBI)
• TBI less than 0.7 indicates significant ischemia

Arterial assessment is critical because infection will not resolve without adequate perfusion. If ischemia present (ABI less than 0.7, absent pulses, or TcPO2 less than 30mmHg), vascular surgery consultation for revascularization is essential before definitive treatment.

Sensory Testing:

• 10g Semmes-Weinstein monofilament test (standard screening)
• Test 10 sites on plantar foot including metatarsal heads
• Unable to feel at 4 or more sites indicates loss of protective sensation
• Vibration testing with 128Hz tuning fork at hallux IPJ
• Pinprick sensation (sharp vs dull discrimination)

Motor Examination:

• Intrinsic muscle wasting (claw toes, hollowing of foot)
• Foot deformities: claw toes, hammer toes, Charcot arthropathy
• Muscle strength testing: ankle dorsiflexion, plantarflexion, toe flexion/extension
• Gait assessment if ambulatory

Autonomic Testing:

• Skin examination for anhidrosis (dry, cracked skin)
• Assess for callus formation at pressure points
• Fissures in dry skin as portals of entry

Severe neuropathy explains why patients often present late with advanced infection. The lack of pain means they continue weight-bearing on infected ulcer, driving bacteria deeper into soft tissues and bone.

Vital Signs:

• Temperature: fever suggests systemic infection but may be absent in diabetes
• Heart rate: tachycardia greater than 90 one SIRS criterion
• Blood pressure: hypotension indicates septic shock
• Respiratory rate: tachypnea greater than 20 suggests SIRS
• Oxygen saturation: hypoxia concerning for severe sepsis

General Examination:

• Mental status: confusion suggests severe sepsis in elderly
• Hydration status: assess for dehydration from sepsis
• Other ulcers: check entire foot and contralateral foot
• Lymphadenopathy: palpate inguinal nodes

Systemic Complications:

• Diabetic ketoacidosis (DKA) can be precipitated by infection
• Check for Kussmaul breathing, fruity breath odor
• Acute kidney injury from sepsis or contrast (if angiography planned)
• Cardiac events (MI) can occur with sepsis stress

Up to 30% of diabetic foot infections present without fever despite severe infection. Absence of systemic signs does NOT exclude severe infection. Rely on local wound severity and SIRS criteria rather than patient symptoms.

Mild Infection (Outpatient Oral):

First-Line Options:

• Cephalexin 500mg QID, OR
• Amoxicillin-clavulanate 875/125mg BID, OR
• Clindamycin 300mg TID (if penicillin allergic)

Duration: 1-2 weeks for soft tissue infection only

Coverage: S. aureus and Streptococcus spp (primary pathogens in acute mild infection)

Moderate Infection (Inpatient IV):

Recommended Regimen:

• Vancomycin 15-20mg/kg IV q8-12h (target trough 15-20 for serious infection), PLUS
• Piperacillin-tazobactam 4.5g IV q6h or 3.375g q4h

Alternative (if penicillin allergic):

• Vancomycin PLUS ciprofloxacin 400mg IV q12h PLUS metronidazole 500mg IV q8h

Duration: 2-3 weeks for soft tissue, 4-6 weeks if osteomyelitis with surgical debridement

Coverage: MRSA, Gram-negatives including Pseudomonas, anaerobes

Severe Infection (ICU, Broad-Spectrum):

Recommended Regimen:

• Vancomycin 15-20mg/kg IV q8-12h (or linezolid 600mg IV q12h), PLUS
• Meropenem 1g IV q8h or imipenem-cilastatin 500mg IV q6h, PLUS
• Consider adding metronidazole 500mg IV q8h if extensive necrosis

Duration: Minimum 4-6 weeks, often longer depending on clinical response

Empiric therapy must be started immediately after obtaining cultures in moderate-severe infection. Do NOT wait for culture results to initiate antibiotics.

Antibiotic Adjustment Based on Culture Results:

Once deep tissue culture results return (typically 48-72 hours for preliminary, 5-7 days for final), narrow spectrum based on susceptibilities:

MSSA (Methicillin-Sensitive S. aureus):

• Cefazolin 2g IV q8h (preferred), OR
• Nafcillin or oxacillin 2g IV q4h, OR
• Cephalexin 500mg PO QID (if transitioning to oral)

MRSA (Methicillin-Resistant S. aureus):

• Vancomycin IV (continue, monitor troughs), OR
• Linezolid 600mg PO/IV q12h (excellent bone penetration, oral bioavailability), OR
• Daptomycin 6-8mg/kg IV q24h (not for osteomyelitis, inactivated by surfactant)

Streptococcus spp:

• Penicillin G 2-4 million units IV q4-6h, OR
• Ceftriaxone 2g IV q24h, OR
• Amoxicillin 875mg PO TID

Enterococcus spp:

• Ampicillin 2g IV q4-6h (if susceptible), OR
• Vancomycin (if VRE, use linezolid or daptomycin)

Pseudomonas aeruginosa:

• Cefepime 2g IV q8-12h, OR
• Piperacillin-tazobactam 4.5g IV q6h, OR
• Ciprofloxacin 400mg IV q12h or 750mg PO BID (excellent bone penetration)

Anaerobes (Bacteroides, Peptostreptococcus):

• Metronidazole 500mg IV/PO q8h, OR
• Clindamycin 600mg IV q8h (if susceptible), OR
• Beta-lactam/beta-lactamase inhibitor combinations

The goal is to de-escalate from broad empiric coverage to narrow targeted therapy based on culture results, minimizing antibiotic resistance and side effects.

Treatment Duration by Infection Type:

Transition from IV to Oral:

• Criteria: Afebrile for 48 hours, improving local signs, tolerating oral intake, reliable patient
• Use antibiotics with high oral bioavailability: fluoroquinolones (ciprofloxacin, levofloxacin), linezolid, clindamycin, TMP-SMX
• Continue total duration based on infection type

Monitoring Parameters:

• Clinical: daily wound assessment for erythema, drainage, wound size
• Laboratory: CRP weekly (should decrease by 50% in 2 weeks if responding), ESR monthly
• Imaging: repeat radiographs at 4-6 weeks (MRI not useful for monitoring, changes lag by months)
• Drug levels: vancomycin troughs, aminoglycoside peaks/troughs if used

Failure to improve after 2 weeks of appropriate antibiotics suggests inadequate source control (undrained abscess, residual necrotic tissue, persistent ischemia) and requires re-evaluation with imaging and possible surgical re-exploration.

Goals of Surgical Debridement:

1. Remove all necrotic and infected tissue
2. Obtain deep tissue and bone cultures
3. Reduce bacterial burden
4. Convert chronic wound to acute healing wound
5. Assess extent of infection

Surgical Technique:

Preparation:

• Patient supine, thigh tourniquet available (use controversial, may limit bleeding assessment)
• Ensure adequate IV access for resuscitation
• Broad-spectrum antibiotics already on board
• Prepare for possible amputation if extensive involvement

Debridement:

• Excise all necrotic skin and subcutaneous tissue
• Remove callus around ulcer edges (keratinocytes impair healing)
• Open all deep spaces and compartments
• Probe for tracking sinuses and abscess pockets
• Resect obviously infected bone (visual inspection: dark, mushy, non-bleeding)
• Send multiple deep tissue specimens for culture (3-5 samples)
• Copious irrigation with normal saline (3-6 liters)

Principles:

• Debride to bleeding viable tissue (the 4 C's: Color pink/red, Consistency firm, Contraction with stimulus, Circulation bleeding)
• Be aggressive but preserve functional structures if possible
• Leave wound open for delayed closure or healing by secondary intention
• Plan for serial debridements every 48-72 hours if needed

The concept of damage control debridement applies in severe infections: remove obviously infected tissue at first operation, reassess at 48-72 hours, and perform definitive reconstruction only after infection controlled.

Surgical Options for Diabetic Foot Osteomyelitis:

1. Bone Resection with Preservation of Function:

• Indications: localized bone infection, viable surrounding soft tissue, adequate perfusion
• Technique: excise infected bone segment en bloc with margin of healthy bleeding bone
• Examples: partial phalangeal resection, metatarsal head resection, calcanectomy
• Advantages: preserves foot length and function
• Success rate: 80-85% if complete resection achieved

2. Ray Amputation:

• Indications: osteomyelitis of toe and metatarsal, adequate soft tissue for closure
• Technique: resect entire ray (toe and metatarsal) en bloc
• Central rays (2nd, 3rd, 4th) better tolerated than border rays
• Advantages: removes all infected tissue, allows primary closure
• Rehabilitation: usually ambulatory in normal shoe with filler

3. Transmetatarsal Amputation (TMA):

• Indications: forefoot infection involving multiple rays, adequate heel pad perfusion
• Technique: long plantar flap, short dorsal flap, bevel metatarsals
• Preserve at least 50% of first metatarsal length if possible
• Advantages: maintains functional ambulation, fits in modified shoe
• Success rate: 70-80% healing with proper patient selection (adequate perfusion)

4. Partial Calcanectomy:

• Indications: calcaneal osteomyelitis (often from pressure ulcer), adequate soft tissue coverage
• Technique: excise infected portion of calcaneus, preserve insertion of Achilles if possible
• Can remove 40-50% of calcaneus and maintain some weight-bearing
• Requires prolonged off-loading during healing

5. Major Amputation (Below-Knee or Above-Knee):

• Indications: extensive infection uncontrolled by lesser procedures, life-threatening sepsis, non-salvageable limb
• Patient selection: balance limb salvage attempts vs mortality risk in severe sepsis
• Below-knee preferred (better functional outcome) if adequate gastrocnemius perfusion
• Above-knee if below-knee fails or severe PAD

The decision between bone resection vs amputation level depends on extent of infection, perfusion status, soft tissue availability for coverage, patient mobility goals, and overall medical status.

Vacuum-Assisted Closure (VAC) in Diabetic Foot Wounds:

Indications:

• Large wounds after debridement (greater than 10cm²)
• Exposed bone, tendon, or joint (after infection controlled)
• Wounds with heavy exudate
• Bridge to delayed primary closure or skin grafting

Contraindications:

• Active untreated infection with necrotic tissue
• Exposed vascular structures (arteries)
• Malignancy in wound bed
• Untreated osteomyelitis

Technique:

• Apply after initial debridement and infection control
• Black foam (polyurethane) for most wounds
• White foam (polyvinyl alcohol) if less granulation needed
• Continuous pressure at -125mmHg (standard)
• Change dressing every 48-72 hours

Benefits:

• Reduces edema and wound size
• Promotes granulation tissue formation
• Removes exudate and bacteria
• Prepares wound bed for closure
• May reduce time to healing

Evidence:

• Several RCTs show faster wound healing with VAC vs standard dressings
• Particularly beneficial in post-surgical wounds greater than 10cm²
• Does NOT replace adequate debridement or antibiotic therapy

VAC therapy is an adjunct to surgical debridement and antibiotics, not a replacement. It should only be applied after infection is controlled and necrotic tissue removed.

Postoperative Management Principles

Antibiotic Completion

• Complete planned antibiotic course based on infection type
• Soft tissue: 1-2 weeks
• Osteomyelitis with resection: 4-6 weeks
• Monitor CRP weekly (should decrease by 50% in 2 weeks if responding)

Advanced Postoperative Considerations

Glycemic Optimization

Inpatient Goals:

• Target glucose 140-180 mg/dL during acute illness
• Avoid hypoglycemia (impairs wound healing)
• Transition to outpatient regimen when stable

Serial Debridement Protocol

For severe infections requiring staged approach:

• First debridement: Remove obviously necrotic tissue, obtain cultures
• 48-72 hour reassessment: Evaluate tissue viability, further debridement
• Definitive procedure: When infection controlled, granulation tissue forming

Vascular Re-Assessment

If healing delayed despite adequate treatment:

• Repeat ABI/TBI
• Consider CT angiography or MRA
• Vascular surgery consultation for revascularization
• Healing unlikely without adequate perfusion (TcPO2 greater than 30mmHg)

Transition to Outpatient

Criteria for discharge:

• Afebrile for 48 hours
• Wound improving with controlled drainage
• Tolerating oral intake and antibiotics
• Reliable patient with appropriate support
• Follow-up arranged within 1 week

Key Evidence Summary

Surgical vs Medical Management of Osteomyelitis:

• Level I evidence supports surgical resection for forefoot osteomyelitis with 80-90% remission rates vs 60-70% with antibiotics alone
• Antibiotics-only acceptable in patients with high surgical risk or minimal bone involvement
• IWGDF 2019 guidelines recommend 6 weeks antibiotics if surgery leaves residual infection, 3-5 days if complete bone resection achieved

Probe-to-Bone Test Validation:

• Grayson 1995 landmark study: PPV 89% in high-prevalence population
• Lavery 2007 validation: Sensitivity 87%, Specificity 83%
• Test most useful in ulcers greater than 2cm² with visible bone or depth greater than 3mm

Multidisciplinary Team Approach:

• Cochrane review 2020: MDT care reduces amputation rates by 40-50%
• Teams should include: vascular surgery, infectious diseases, podiatry, endocrinology, wound care, orthotist
• Weekly case conferences improve outcomes

Critical Evidence Review

Antibiotic Duration Studies:

• SIDESTEP trial (Tan 2023): 3 weeks vs 6 weeks for post-surgical osteomyelitis showed non-inferior outcomes with shorter duration if adequate debridement achieved
• DFI antibiotic duration meta-analysis: 4-6 weeks for residual bone, 1-2 weeks for soft tissue only

Negative Pressure Wound Therapy:

• NPWT meta-analysis (Liu 2017): 50% faster healing, 25% reduction in amputation
• Most benefit in moderate-to-large wounds post-debridement
• Apply 75-125 mmHg continuous or intermittent suction

Revascularization Impact:

• BASIL-2 trial implications: Angiosome-directed revascularization improves wound healing
• Revascularization before major debridement in patients with critical limb ischemia
• Ankle pressure greater than 70 mmHg or toe pressure greater than 50 mmHg needed for healing

Biomarkers for Osteomyelitis:

• ESR greater than 70 mm/hr: sensitivity 78%, specificity 84% for osteomyelitis
• CRP for monitoring treatment response (greater than 50% reduction by week 2 predicts success)
• Procalcitonin may help distinguish bone vs soft tissue infection