Background

Calcium sulfate and phosphate have a long clinical history of use as bone-void fillers (BVF) with established biocompatibility and resorption profiles. It has been widely reported that the addition of ‘impurity’ elements such as Silicon, Strontium and Zinc to calcium phosphate is advantageous, resulting in an improved bone healing response.

Daptomycin has a unique mechanism of action against Gram-positive bacteria. Daptomycin is only bactericidal in the presence of calcium ions. [1]

Kanellakopoulou et al [2] investigated elution of daptomycin from calcium sulfate. The results indicated above MIC elution concentrations out to 28 days. Experience reports that the ability for calcium sulfate to set hard when combined with daptomycin can be problematic.[3] This study aimed to investigate the combination of daptomycin with a synthetic recrystallised form of calcium sulfate and investigate zone of inhibition (ZOI) testing against susceptible organisms.

6mm hemispherical beads, were prepared using a commercially available calcium sulfate hemihydrate powder (CSH) – CaSO4 ·1/2H2O. [4] In order to combine daptomycin [5] with the CSH and enable it to set hard, 7mls of saline solution was added to 20g CSH powder and mixed for 80 seconds to initiate the setting reaction. Then 1g of daptomycin powder was added and mixed for a further 30 seconds. The resultant paste was applied to a bead mat and allowed to set.

Tryptone soya agar plates were seeded with 0.2ml of a 10e6 – 10e8 cfu/ml suspension of the relevant organism. The plates were incubated at 33 °C ± 2 °C for 30 minutes. The plates were then removed from the incubator and the beads placed on the surface. The plates were then incubated at 33 °C ± 2 °C for 24 hours before examination for the absence of growth as seen by a clear zone around the test sample.

Triplicate samples were tested against Staphylococcus epidermidis, Staphylococcus aureus, MRSA, VRE Enterococcus faecium and Propionibacterium acnes.

Repeat tests were carried out for beads that had been stored at 37 °C for 21 days to simulate in-vivo conditions.

Setting times for the CSH/daptomycin beads were approximately 20 minutes. ZOIs indicating efficacy were seen for all samples both ‘fresh’ and ‘incubated’ with MRSA and Propionibacterium acnes having the largest ZOIs at 31–33mm.

A mixing protocol was established to enable set beads to be formed with daptomycin loaded calcium sulfate. As assessed by ZOI testing, the eluted antibiotic maintained efficacy against susceptible pathogens. Results obtained in-vitro may not be indicative of in-vivo performance.

Published infection rates following wounds sustained in combat vary dramatically. We reviewed UK military extremity trauma, sustained in Afghanistan over a two year period, to evaluate early infection rates and causative organisms. Data on wound site, time to onset of infection, organisms detected and method of presentation were collected.

351 patients had full datasets for clinical wound surveillance and microbiological data. 58 (16.5%) patients were diagnosed with wound infections. Median time to diagnosis was 17 days (range = 749, Interquartile range =31.75 days). Limb infection was detected in 53 (15.1%) patients. Infection was statistically significantly more likely to be incurred in the lower extremity (p=0.0220). Multiple organisms were identified in 34 (64.2%) of the 53 patients with a limb infection. Fungi were significantly more common in early presenters (<30 days after injury) (p=0.0024). Staphylococcus aureus was significantly more likely in late presentation (p=0.002). Infection was more likely in those injured by an improvised explosive device (p=0.0019).

The overall infection rates recorded are low when compared to historical data. Organisms isolated from infected wounds are frequently multiple. The microbial spectrum and the number of organisms present on diagnosis change with time from wounding.