Antimicrobial Mechanisms and Preparation of Antibiotic-impregnated Cement-coated Locking Plates in the Treatment of Infected Non-unions

Abstract

Background: Antibiotic-impregnated cement-coated plates (ACPs) have been used successfully for temporary internal fixation between stages in the two-stage treatment of infected non-unions. We describe our approach of using an ACP in the staged treatment of a methicillin-resistant Staphylococcus aureus (MRSA)-infected distal femoral non-union below a total hip prosthesis. In addition, we present the results of an in vitro experiment to provide an in-depth insight into the capacity of ACPs in (i) treating residual biofilm and (ii) preventing bacterial recolonisation.

Materials and methods: In the first stage, we used a titanium LISS plate coated with hand-mixed PALACOS with vancomycin (PAL-V) for temporary internal fixation combined with commercially prepared COPAL with gentamicin and vancomycin (COP-GV) to fill the segmental defect. In the second stage, the non-union was treated with double-plate fixation and bone grafting.A Kirby-Bauer agar disc diffusion assay was performed to determine the antimicrobial activity of both ACPs and a drug-release assay to measure antibiotic release over time. A biofilm killing assay was also carried out to determine if the antibiotic released was able to reduce or eradicate biofilm of the patient's MRSA strain.

Results: At one-year follow-up, there was complete bone-bridging across the previous non-union. The patient was pain-free and ambulatory without need for further surgery. Both ACPs with COP-GV and PAL-V exerted an antimicrobial effect against the MRSA strain with peak concentrations of antibiotic released within the first 24 hours. Concentrations released from COP-GV in the first 24 hours in vitro caused a 7.7-fold log reduction of colony-forming units (CFU) in the biofilm. At day 50, both COP-GV and PAL-V still released concentrations of antibiotic above the respective minimal inhibitory concentrations (MIC), likely contributing to the positive clinical outcome.

Conclusion: The use of an ACP provides stability and infection control in the clinical scenario of an infected non-union. This is confirmed in vitro where the release of antibiotics from ACPs is characterised by an early burst followed by a prolonged sustained release above the MIC until 50 days. The burst release from COP-GV reduces CFU in the biofilm and prevents early recolonisation through synergistic activity of the released vancomycin and gentamicin.

Clinical significance: An antibiotic-impregnated cement-coated plate is a useful addition to the surgeon's armamentarium to provide temporary internal fixation without the disadvantages of external fixation and contribute to infection control in an infected non-union.

How to cite this article: Wagner RK, Guarch-Pérez C, van Dam AP, et al. Antimicrobial Mechanisms and Preparation of Antibiotic-impregnated Cement-coated Locking Plates in the Treatment of Infected Non-unions. Strategies Trauma Limb Reconstr 2023;18(2):73-81.

Keywords: Antibiotic; Cement; In vitro; Infection; Non-union; Osteosynthesis; Plate.

Figs 1A and B

(A) Anteroposterior radiograph of the right femur with the external fixator in situ at the first presentation to our institution; (B) Coronal CT-scan image taken 2 years after the second stage. There is bone bridging across the non-union site

Figs 2A to C

Intraoperative images of the first stage. (A) The holes left by the external fixator and previous internal fixation are clearly visible; (B) The plate is covered with cement, and the screw holes are ‘protected’ with a locking drill sleeve to prevent obstruction of the holes; (C) The antibiotic-impregnated cement-coated locking plate in situ

Figs 3A to C

Case I: Intraoperative images of the second stage. (A) Before removal of the antibiotic plate; (B) After removal of the plate and opening of the medullary cavities. One of the cortical bone grafts is shown in the hand of the surgeon; (C) After placement of the stainless-steel VA distal femur LCP and demineralised bone matrix around the non-union

Figs 4A to E

(A) Graphical example of the set-up. A silicone mould is placed on the bottom of each compartment to prevent leaking of excess cement through the screw holes of the plate segments; (B) Custom-made aluminum mould with two coated plate segments in situ as an example. On the bottom of each compartment a screw is placed to press out the plate segment after coating; (C) Pressure is applied on the mould with a glass plate to drive out excess cement; (D) Antibiotic-impregnated cement-coated plate segments; (E) Visual example of the zones of inhibition for the plate segments coated with PALACOS LV + V cement or COPAL G+V cement. Pictures are taken from the bottom side of the agar plate. The coated plate segments were transferred daily to fresh agar plates seeded with the MRSA strain isolated from our patient. Results for the first 5 days of the experiment are shown

Figs 5A to C

(A) Schematical presentation of the zone of inhibition measurement; (B) Average of the zones of inhibition (in mm) on agar plates seeded with the MRSA clinical strain around the plate segments with either PALACOS LV + V cement or COPAL G+V cement. The plate segments were transferred to fresh inoculated plates each day for 28 consecutive days (n = 5 per antibiotic formulation); (C) Schematical presentation of the drug release assay. Eluates were collected and stored at −20ºC for further analysis. The amount of gentamicin was measured with the O-phthalaldehyde reagent, and the amount of released vancomycin was quantified with quartz cuvettes by a UV–Vis spectrometer based on UV absorption at 280 nm

Figs 5D to I

(D) Amount of vancomycin released (in µg) PALACOS LV + V; (E) Cumulative release (in w/w% of the original drug loading) over time up to 50 days from the PALACOS LV + V (n = 3); (F) Amount of vancomycin released (in µg) COPAL G + V; (G) Cumulative release (in w/w% of the original drug loading) over time up to 50 days from the COPAL G + V cement (n = 3); (H) Amount of gentamicin released (in µg); (I) Cumulative release (in w/w% of the original drug loading) over time up to 50 days from the COPAL G+V cement (n = 3)

Fig. 6

Results from the checkboard biofilm-killing assay of gentamicin sulphate (512, 256, 128, 64, 32, 16, 8 and 4 µg/mL) and vancomycin (2048, 1024, 512, 256, 128, 64 and 32 µg/mL) against the MRSA strain. The data show the logarithmic reduction on CFU counts compared to the non-treated well in a colour pattern from a logarithmic reduction of 9–0