Paper of Week: Implant Sonication versus Tissue Culture for the Diagnosis of Spinal Implant Infection

Bayard C. Carlson, MD, Jeremy T. Hines, MD, William A. Robinson, MD, Arjun S. Sebastian, MD, Kerryl E. Greenwood-Quaintance, MS, Robin Patel, MD, and Paul M. Huddleston, MD

SPINE Volume 45, Number 9, pp E525–E532
DOI: 10.1097/BRS.0000000000003311

Summary by Ali Omari, MD

Spinal implant infection (SII) is not an uncommon complication of spinal surgery and often results in significant morbidity and cost.^{1, 2} Adequate treatment requires timely and accurate diagnosis, which can often be challenging. Bacteria responsible for SII often create biofilms, rendering standard peri-implant tissue cultures ineffective in diagnosing an infection.³ As a result, vortex-sonication, a method designed to break up biofilms and dislodge bacteria from implants, has been proven to be an effective method in diagnosing infection.^4,5 This study aimed to compare vortex-sonication of removed spinal implants to peri-implant tissue culture in the diagnosis of a SII. The study also sought to determine an optimal threshold of sonicate fluid culture positivity for SII diagnosis.

Between June 2007 to February 2016 at one institution, 152 patients undergoing revision or resection of spinal implants who had spinal implants sent for sonicate fluid culture as well as two or more peri-implant tissue cultures collected were included. A diagnosis of a SII was determined if one or more of the following were present: acute inflammation on histopathologic examination of tissue, intraoperative purulence surrounding the implant, sinus tract communicating with the implant, more than or equal to two positive peri-implant tissue cultures and positive sonicate fluid cultures of the same organism, or isolation of Staphylococcus aureus.

With a threshold of greater than or equal to 100 colony forming units (CFU)/10 mL, 46 patients were classified with a SII. Sensitivities of peri-implant tissue and sonicate fluid culture were 65.2% and 79.6%; specificities were 88.7% and 93.4%, respectively. With ≥ 50 CFU/10 mL threshold: 50 patients had a SII. Sensitivities of peri-implant tissue and sonicate fluid culture were 68.0% and 76.0%; specificities were 92.2% for both methods. With ≥ 20 CFU/10mL threshold: 52 patients had a SII. Sensitivities of peri-implant tissue and sonicate fluid culture were 69.2% and 82.7%; specificities were 94.0% and 92.0%, respectively.

Similar to previous studies in other orthopaedic subspecialties, vortex-sonication is as sensitive and specific as peri-implant tissue culture in the diagnosis of a SII.^6,7 Lowering the threshold for a positive sonicate culture did not drastically decrease the specificity, resulting in the authors recommending a threshold of ≥ 20 CFU/10 mL as a positive result. Limitations of this study include being retrospective in design, not having a standard criterion for SII diagnosis available, using peri-implant tissue and sonicate fluid as one of the criteria, not commenting on all diagnostic criteria for each patient, and not obtaining a uniform number of peri-implant tissue cultures per patient.

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