Microbial contamination assessment of cryostored autogenous cranial bone flaps: should bone biopsies or swabs be performed?

Autogenous cranioplasty infection requiring bone flap removal is under-recognised as a major complication causing significant morbidity. Microbial contamination of stored bone flaps may be a significant contributing factor. Current infection control practices and storage procedures vary. It is not known whether 'superficial' swabs or bone cultures provide a more accurate assessment. Twenty-five skull flaps that were cryo-stored for more than 6 months were studied. Two swab samples (superficial and deep) and a bone biopsy sample were taken from each skull flap sample and cultured. Half blood agar and half chocolate agar plates were inoculated with the swabs for anaerobic and aerobic cultures respectively. The bone biopsy samples were cultured in brain-heart broth and subcultured similar to the swabs for 5 days. Incidence of microbial contamination was 20 % in the bone flaps studied. One swab culture and five bone biopsy cultures were positive for bacterial growth, all of which contained Propionibacterium acnes (p = 0.014). Positive cultures were from bone flaps stored less than 18 months, whereas no growth was obtained from bone flaps that were stored longer (p = 0.014). Bone biopsy culture is a more sensitive technique of assessing microbial contamination of cryo-stored autogenous bone flaps than swab cultures. The clinical implications of in vitro demonstration of microbial contamination require further study.