Cutibacterium acnes Shoulder Periprosthetic Infection

• Gram-positive anaerobic bacillus (formerly Propionibacterium acnes)
• Slow-growing: requires 14-21 days of anaerobic culture
• Biofilm-forming: adheres to implant surfaces, resistant to host immunity
• Low virulence: minimal local inflammation, no systemic response typically
• Skin commensal: deep hair follicles and sebaceous glands of upper trunk

• Male predominance: males affected more than females (greater pilosebaceous density)
• Most common shoulder PJI organism: overtakes Staphylococcus aureus and coagulase-negative staphylococci
• Increasing recognition: improved culture techniques and longer incubation have revealed true prevalence
• Risk factors: male sex, prior shoulder surgery, haematoma, prolonged operative time

Unique anatomy: The shoulder region has the highest density of pilosebaceous units harboring C. acnes on the human body. Standard chlorhexidine or povidone-iodine skin preparation does not fully eradicate organisms from deep follicles. Intra-operative contamination rates of 15-30 percent have been reported from tissue cultures taken during primary shoulder arthroplasty.

Protected state: Within biofilm, C. acnes exists in a metabolically quiescent state that is resistant to cell-wall active antibiotics. Minimum biofilm eradication concentration (MBEC) can be 100-1000 times greater than planktonic minimum inhibitory concentration (MIC). This is why antibiotics alone rarely cure established implant-related C. acnes infection.

• Pain: Persistent or worsening pain after shoulder arthroplasty (most common presenting symptom)
• Timing: May present months to years after index surgery
• Stiffness: Progressive loss of range of motion
• No wound problems: Typically no erythema, warmth, or drainage
• No systemic features: Afebrile, no rigors, no malaise

• Inspect: Well-healed wounds, no erythema or sinus
• Palpate: May have mild tenderness around implant, often unremarkable
• Range of motion: Decreased, often with pain at end ranges
• Instability: Component loosening may cause subtle instability
• Neurovascular: Usually intact, no specific deficit

• C. acnes is the most common organism in shoulder PJI worldwide, with prevalence estimates of 30-60 percent across North American, European, and Australasian series
• Male predominance is consistent across all geographic populations studied
• Intra-operative contamination rates of 15-30 percent during primary shoulder arthroplasty are reported in studies from multiple continents
• Recognition increasing globally as culture techniques (prolonged incubation, sonication) become standard

• Europe: Strong trend toward one-stage exchange for C. acnes PJI, supported by multicentre data from France, Germany, and Scandinavia
• North America: Two-stage exchange remains more commonly performed, though one-stage is gaining acceptance
• Australasia: Individualised approach based on organism, bone stock, and surgeon preference
• Universal principle: Prolonged anaerobic culture (14-21 days) and multiple tissue samples are mandatory regardless of region

Current evidence suggests comparable eradication rates for one-stage and two-stage revision in C. acnes PJI. One-stage avoids a second surgery and may offer faster functional recovery. Two-stage remains traditional for cases with extensive bone loss, uncertain microbiology, or soft tissue compromise. No high-quality RCT exists to definitively guide the choice.

Debate continues about whether a single positive C. acnes culture (of 5 or more samples) represents true infection or contamination. Most authorities now treat it as significant in the appropriate clinical context (male, shoulder, indolent pain), but the threshold for intervention varies between centres.

Optimal antibiotic duration is not established by RCT evidence. Protocols range from 4 weeks to greater than 12 weeks of combined IV and oral therapy. Some centres use prolonged oral suppression for 6-12 months in high-risk patients. The role of rifampicin combination therapy for C. acnes biofilm is extrapolated from Staphylococcus data.

The MSIS criteria were developed for hip and knee PJI and have poor sensitivity for shoulder infections. Proposed shoulder-specific thresholds (lower synovial WCC, lower neutrophil percentage) have not been universally validated. An international consensus on shoulder-specific PJI diagnostic criteria is still evolving.