Metal instrumentation (rods and screws) is used to stabilise the spine after trauma, malignancy or deformity. Approx 3% become infected often necessitating removal of metal. At surgery tissue samples and metal are removed for culture, but many clinical laboratories are not equipped to process metal or use simple culture methods. The causative bacteria exist as biofilms on the metal and they are often anaerobic and slow-growing, so conventional culture methods often fail to detect them. Also, they are common contaminants leading to diagnostic uncertainty. We have established a laboratory protocol to overcome these problems. Removed metalwork was sonicated and the sonicate centrifuged and the supernatant discarded. Quantitative aerobic and anaerobic culture of the resuspended pellet for 14 days and microscopy were carried out. Metalwork from 11 suspected infected cases was culture-positive (median 2857, 60–5000cfu/mL). Microscopy revealed an infection due to Candida albicans that would not have been detected otherwise. Bacteria were isolated from 8 of 10 non-infected cases (median 15, 0–35 cfu/mL). Conventionally processed samples failed to grow in 4 infected cases. (cfu/mL infected vs noninfected cases p=0.0093). Micro-organisms on spinal metalwork grow as biofilms and they require sonication to dislodge them. The causative bacteria are slow-growing and
Background. Deep infection rates of 1 - 2% following primary hip and knee arthroplasty are mainly due to endogenous contamination of the surgical site from bacteria within the patient's own skin. However surgical skin preparation removes only bacteria from the surface of the skin, leaving viable bacteria in the deeper layers of the skin within hair follicles and sweat and sebaceous glands. The aim of our study was to test the hypothesis that surface skin swabs taken after skin preparation with alcoholic povidone iodine would not grow bacteria, whereas full thickness biopsies taken from the line of surgical incision would grow bacteria. Methods. Under LREC approval, informed consent was obtained from 22 patients undergoing primary hip (n=9) or knee (n=13) arthroplasty. All patients received intravenous antibiotic prophylaxis at the time of induction of anaesthesia. After surgical skin preparation with alcoholic povidone iodine, a surface skin swab and full thickness skin biopsy, using an 8mm x 4 mm elliptical punch, were taken. The swab culture was incubated aerobically and anaerobically at 37°C. The skin biopsy was cut aseptically in half. One half was crushed using artery forceps, placed in 5mL anaerobe basal broth and incubated anaerobically at 37°C. The other half of the skin biopsy was frozen in isopentane and gram – stained after sectioning. Results. Seven of 20 surface swabs were positive for bacteria (2 S. epidermidis, 3
We have designed a prospective study to evaluate
the usefulness of prolonged incubation of cultures from sonicated
orthopaedic implants. During the study period 124 implants from
113 patients were processed (22 osteosynthetic implants, 46 hip
prostheses, 54 knee prostheses, and two shoulder prostheses). Of
these, 70 patients had clinical infection; 32 had received antibiotics
at least seven days before removal of the implant. A total of 54 patients
had sonicated samples that produced positive cultures (including
four patients without infection). All of them were positive in the
first seven days of incubation. No differences were found regarding
previous antibiotic treatment when analysing colony counts or days
of incubation in the case of a positive result. In our experience, extending
incubation of the samples to 14 days does not add more positive
results for sonicated orthopaedic implants (hip and knee prosthesis
and osteosynthesis implants) compared with a conventional seven-day incubation
period. Cite this article:
