Detection of bacteria in healthy spinal bone during pedicle screw placement: colonization versus evidence for future low-grade infection.

Abstract

Symptomatic implant failure presents a burdensome complication for patients undergoing spinal instrumentation. The origin of screw loosening remains a question of debate; mechanical load, patient-related factors, and the hypothesis of low-grade infections with biofilm production can contribute to the need for revision surgery. Scientific evidence lacks a prospective investigation of contributing factors. Therefore, the aim of this study was to evaluate the causes of implant loosening, including microbiological analysis of the vertebral bone before instrumentation, using prospectively collected data. The authors prospectively enrolled patients undergoing spinal instrumentation for degenerative or trauma indications at their tertiary care center between April 2020 and January 2022. Bone meal obtained during the drilling process for pedicle screw placement was microbiologically analyzed for each pilot hole. Clinical prognostic risk factors, including comorbidities, surgical data, and postoperative complications, were assessed. Patients were screened for symptomatic screw loosening over a 2-year period following surgery. In total, 867 pilot holes in 146 patients (79 female, median age 72.5 years) were analyzed. Bacterial presence was detected in 65.4% of the patients (significant amount in 7.3%, sporadic in 21.8%, in enrichment medium in 54.2%, and in a single probe in 20.7%). The most prevalent bacteria identified was Cutibacterium acnes (31.5%), followed by Staphylococcus epidermidis (23.3%), Staphylococcus hominis (14.4%), Staphylococcus capitis (8.2%, significantly more present in spinal revision cases 17.6% vs 3.6%), and Staphylococcus aureus (0.7%). After a 2-year follow-up period, screw loosening was detected in 12.3% of patients. Previous surgery at the same level, an open surgical approach, antiplatelet therapy, and previous steroid injection to the spine were identified as potential risk factors. The presence of Staphylococcus species was significantly associated with future screw loosening, while the presence of C. acnes had no prognostic value. In a large patient cohort, no significant association between C. acnes and screw loosening was found. Meanwhile, low-virulent Staphylococcus species, open surgery, previous surgery, antiplatelet therapy, or infiltration were associated with a higher rate of screw loosening. These findings emphasize the multifactorial nature of implant failure, with several contributing risk factors identified, including revision surgery and open approaches, particularly when combined.