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Surgical Site Infection (SSI) - MS Orthopaedics 20 Marks

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DEFINITION

• CDC/NHSN 2017 guidelines provide the current standard definitions.
• Reported incidence: 2-5% of all surgical procedures; in orthopaedics, deep implant-related infection rates are 0.5-2% for joint arthroplasty.

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CLASSIFICATION (CDC/NHSN)

1. Superficial Incisional SSI

• Occurs within 30 days after surgery
• Involves only skin and subcutaneous tissue
• Criteria (at least one): purulent drainage, positive culture, pain/tenderness/swelling/erythema/heat + deliberate opening by surgeon, or diagnosis by attending surgeon

2. Deep Incisional SSI

• Occurs within 30 days (no implant) or within 1 year (with implant)
• Involves deep soft tissues: fascia and muscle layers
• Criteria (at least one): purulent drainage from deep incision (not organ/space), spontaneous or deliberate dehiscence with fever >38°C or localized pain/tenderness, abscess or other evidence of infection on direct examination or reoperation, or diagnosis by surgeon

3. Organ/Space SSI

• Involves any part of the body other than the skin incision, fascia, or muscle that was opened or manipulated during surgery (e.g., joint space, bone, intra-articular abscess)
• Particularly relevant in orthopaedics: periprosthetic joint infection (PJI), osteomyelitis

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WOUND CLASSIFICATION (NRC/CDC)

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ETIOLOGY AND MICROBIOLOGY

Common Causative Organisms in Orthopaedic Surgery

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RISK FACTORS

A. Patient-Related (Non-Modifiable / Modifiable)

B. NHSN SSI Risk Index (NNIS Score)

1. ASA score ≥ 3
2. Wound class III or IV (contaminated or dirty)
3. Operative duration > 75th percentile for that procedure

C. Procedure-Related Factors

• Wound classification
• Perioperative antibiotics (timing, selection, duration)
• Intraoperative hypothermia
• Blood loss and transfusion
• Surgical duration (risk increases 37% per additional 60 minutes; 25% per 20 minutes)
• Skin preparation method
• Hair removal technique (clipping - not shaving)
• Operating room traffic and door openings
• Surgeon technique - dead space, tissue handling

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PATHOGENESIS

• Inoculation of pathogens at time of surgery (primary) or hematogenous spread to implant (secondary, can occur years later)
• Biofilm formation on orthopaedic implants: bacteria adhere to metal/polymer surfaces, secrete extracellular polysaccharide matrix, rendering them resistant to antibiotics and host defenses
• Contamination sources: patient's own skin flora (primary), airborne particles (5,000-55,000 shed/min per person in OR), surgical team, equipment
• Airborne bacteria are predominantly gram-positive and originate almost exclusively from humans in the OR - Campbell's Operative Orthopaedics, 15e, p.963

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DIAGNOSIS

Clinical Features

• Early infection (within weeks): wound erythema, warmth, swelling, discharge, dehiscence, fever
• Late/chronic infection (months-years with implants): persistent or new-onset pain, sinus tract formation, implant loosening, failure of fracture fixation

Investigations

• WBC count - often elevated but non-specific
• ESR - sensitive but non-specific
• CRP - more reliable; typically returns to normal by 3 weeks post-op; persistently elevated = suspect infection
• D-dimer - noninferior to CRP in diagnosis of PJI (2023 data)
• Procalcitonin, IL-6 - emerging markers

• Gram stain and culture - mandatory before empiric treatment; critical in era of multidrug-resistant pathogens
• Deep wound swabs, tissue biopsy, joint aspirate
• Synovial fluid analysis: WBC >3000 cells/µL, PMN >80% suggestive of PJI

• Plain X-ray: periosteal reaction, bone resorption at bone-cement interface, osteolysis, cyst formation (usually late)
• CT scan: soft tissue involvement, sinus tracts, abscess
• MRI: best for soft tissue and marrow involvement
• Nuclear medicine:
  • Technetium-99m bone scan: sensitive but non-specific
  • Indium-111-labeled WBC scan combined with technetium scan: differentiates infection from aseptic loosening; sensitivity 64-77%, specificity 78-86%

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PREVENTION

I. Preoperative Measures

1. Screen and treat remote infection before elective orthopaedic surgery
2. MRSA decolonization: nasal swab PCR; mupirocin ointment + chlorhexidine baths if positive
3. Glycemic control: HbA1c optimization before elective procedures
4. Smoking cessation: at least 30 days preoperatively
5. Nutritional optimization: correct hypoalbuminemia
6. Skin antisepsis: chlorhexidine-alcohol preparation is superior to povidone-iodine
7. Hair removal: clip (do not shave) - shaving creates skin micro-abrasions

II. Perioperative/Intraoperative Measures

• Begin 30-60 minutes before skin incision (60-120 min for vancomycin/fluoroquinolones)
• First-generation cephalosporin (Cefazolin) is drug of choice: weight-adjusted - 1g (<60 kg), 2g (60-120 kg), 3g (>120 kg)
• Redose every 4 hours intraoperatively or when blood loss exceeds 1500 mL
• Stop within 24 hours post-operatively (not extended even with drains/catheters in situ)
• If penicillin allergy (anaphylaxis): vancomycin (15 mg/kg over ≥1 hr to prevent Red Man Syndrome) or clindamycin
• Cephalosporins are preferred over semisynthetic penicillins as they are more effective against S. epidermidis
• For hip/knee arthroplasty: cefazolin or cefuroxime; vancomycin if β-lactam allergy

• Strict aseptic technique; minimize OR personnel and door openings
• Gentle tissue handling; obliterate dead space
• Minimize operative time (<2.5 hours when possible)
• Control blood loss; use tranexamic acid
• Irrigation: 3L for Gustilo I, 6L for Gustilo II, 9L for Gustilo III open fractures
  • 0.3% Povidone-iodine irrigation - 2.3-fold reduction in SSI, recommended by CDC and WHO (avoid if articular cartilage present)
  • Do NOT add antibiotics to irrigation solution routinely (no added benefit)
• Topical vancomycin powder: reduces SSI in spine surgery; mixed evidence vs IV vancomycin

• Hand washing: 2-minute hand scrub equally effective as traditional 5-minute scrub; alcohol-based hand rubs with chlorhexidine are superior to povidone-iodine scrubs
• Laminar flow: NOT routinely recommended; horizontal laminar flow may paradoxically increase infection rates
• Ultraviolet light: effective but not recommended due to risk to personnel; useful for terminal room cleaning
• Double-glove technique; change gloves when perforation noted

• Maintain normothermia (hypothermia impairs neutrophil function and wound healing)
• Supplemental oxygen intraoperatively
• Perioperative glucose <180 mg/dL

III. Postoperative Measures

• Incisional negative pressure wound therapy (iNPWT): superior to silver-impregnated dressings in high-risk patients (e.g., total joint arthroplasty)
• Subcuticular sutures with skin adhesive preferred; avoid non-absorbable sutures
• Avoid excessive electrocautery
• Wound monitoring, early detection

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TREATMENT

SSI Treatment - Four-Step Strategy (Mulholland & Greenfield's Surgery, 7e)

1. Early empiric antimicrobial therapy - directed against likely pathogens (gram-positive coverage as default; gram-negative cover for open fractures, immunocompromised)
2. Decision on wound opening - open if fluctuance, purulent drainage, or wound dehiscence present; debride necrotic tissue
3. Pathogen identification - Gram stain + culture from wound ALWAYS before or at time of opening (mandatory in era of MDR organisms)
4. De-escalation of antimicrobial therapy once culture and sensitivity results are available

Management by SSI Type

• Wound opening, irrigation, debridement
• Dressing changes
• Oral/IV antibiotics guided by culture

• Surgical debridement, washout
• Open wound management with delayed primary closure or secondary healing
• IV antibiotics

• Debridement, repeated if needed
• External fixation if internal fixation compromised
• Antibiotic-impregnated beads/spacers (local delivery)
• Bone grafting after infection eradication

MRSA Bundle (when MRSA SSI diagnosed):

1. MRSA nasal screening (PCR) on admission/transfer/discharge
2. Contact isolation of positive patients
3. Standardized hand hygiene
4. Cultural transformation - staff and leadership engagement
5. Targeted decolonization therapy

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CURRENT CHALLENGES IN SSI

• Rising MRSA prevalence: increased from 22.5% (1986-2003) to 49.2% of all SSI isolates by 2007
• Increasingly elderly, obese, and immunocompromised patients undergoing orthopaedic surgery
• More prosthetic device implantations - biofilm-forming organisms
• Antibiotic resistance: coagulase-negative staphylococci (S. epidermidis) increasingly resistant; need for antibiogram-guided prophylaxis
• Cost burden: PJI is one of the most expensive complications in orthopaedic surgery

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SUMMARY TABLE: Key Points for Exam

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• Campbell's Operative Orthopaedics, 15th Ed 2026 (Chapter 22 - SSI Prevention)
• Mulholland & Greenfield's Surgery: Scientific Principles and Practice, 7e (Chapter 8 - SSI)
• Sabiston Textbook of Surgery, The Biological Basis of Modern Surgical Practice
• CDC/NHSN Guidelines 2017; WHO Global Guidelines for Prevention of SSI, 2017; SCIP/SIP Performance Measures