What is the most common organism causing septic complications after total knee replacement?

Most Common Organisms in Post-Total Knee Replacement Sepsis

Staphylococcus aureus and coagulase-negative Staphylococcus species (including Staphylococcus epidermidis) are the most common organisms causing infection after total knee replacement. 1, 2

Primary Causative Organisms

Staphylococcal Species Dominate

• Staphylococcus aureus and coagulase-negative staphylococci together account for the majority of post-TKA infections, as consistently reported across multiple guidelines from the American College of Radiology 1, 2
• Among coagulase-negative staphylococci, Staphylococcus epidermidis is the most frequently isolated species, representing approximately 65.5% of coagulase-negative staphylococcal infections 3

Distribution Between Organism Types

• Research data from infected joint replacements shows Staphylococcus aureus accounts for approximately 36% of infections while coagulase-negative staphylococci account for 35% 4
• Both organism types combined represent the overwhelming majority of periprosthetic knee infections 5

Critical Antibiotic Resistance Patterns

Methicillin Resistance is Alarmingly High

• Coagulase-negative staphylococci demonstrate 63-72% methicillin resistance, which is substantially higher than Staphylococcus aureus 3, 4
• Staphylococcus aureus shows 20% methicillin resistance in revision arthroplasty infections 4
• Methicillin-resistant Staphylococcus aureus (MRSA) accounts for approximately 46% of positive cultures in both hip and knee revisions 5

Multi-Drug Resistance is Increasing

• At least 32% of coagulase-negative staphylococci are resistant to both methicillin and gentamicin, compared to only 4% of Staphylococcus aureus 4
• The prevalence of multi-drug resistant organisms has increased dramatically since the mid-1990s, with essentially no multi-drug resistant isolates before 1996 but most isolates showing resistance patterns afterward 5

Clinical Implications for Treatment

Poor Treatment Outcomes with Coagulase-Negative Staphylococci

• Only 47% of coagulase-negative staphylococcal infections achieve successful treatment at one year, representing significantly worse outcomes than other organisms 3
• This poor success rate reflects the organisms' ability to form biofilms and decreased antibiotic efficacy 3
• Surgical debridement with implant retention shows a 60% failure rate for coagulase-negative staphylococcal infections 3

Timing and Presentation Patterns

• Infection is responsible for 37.6% of early revisions and 21.9% of revisions performed more than 2 years after the initial operation 1
• Chronic infections frequently present with pain alone, without fever, erythema, or warmth 2
• Night pain or pain at rest characteristically indicates infection rather than mechanical loosening 2

Diagnostic Approach

Obtain Cultures Before Antibiotics

• Withhold antibiotics for at least 2 weeks prior to joint aspiration when clinically feasible to avoid false-negative cultures 2, 6
• Obtain synovial fluid for aerobic and anaerobic bacterial cultures, as both Staphylococcus aureus and coagulase-negative staphylococci can be isolated 2
• Very small numbers of Staphylococcus aureus are sufficient to initiate infection 7