Differentiating Occult Propionibacterium acnes Infection From Aseptic “Biologic” Interference Screw Hydrolysis After Anterior Cruciate Ligament Reconstruction

Historically, arthroscopic anterior cruciate ligament (ACL) reconstruction has a low rate of infection, with a reported prevalence of 0.14% to 5.7%. While uncommon, these infections generally occur early and are characterized by either cellulitis at the graft insertion site and/or septic arthritis. Subacute infections are less well characterized. Although bioabsorbable interference screws have the potential advantages of osseointegration, limited postoperative imaging artifact, and decreased hardware removal rates, the potential for acting as the nidus for a sterile abscess, which can both mimic and/or set the stage for infection, exists. Typically, distinguishing a sterile abscess from infection is clinically apparent. However, we describe 2 patients with subacute postsurgical medial tibial pain in which this distinction was not evident. Application of a novel culture protocol taken from the prosthetic joint infection literature identified infection by a low-virulence organism and directed therapeutic treatment.