Abstract
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 P acnes is anaerobic and requires prolonged incubation. S epidermidis and P acnes are common contaminants and quantitative culture helps to distinguish pathogens from contaminants, removing the diagnostic uncertainty that conventional methods give. Microscopy of the sonicate can reveal micro-organisms that fail to grow on culture. We recommend that sonication of metalwork, prolonged anaerobic incubation and quantitative culture be adopted to improve diagnostic clarity for spinal instrumentation infections.
