Mixing method affects elution and strength of high-dose ALBC: a pilot study

Abstract

Background: High-dose antimicrobial-loaded bone cement (ALBC) is used to treat orthopaedic infections. High-dose ALBC is not commercially available and requires surgeon directed formulation, and there are several different methods used to mix high-dose ALBC.

Questions/purposes: We asked whether the mixing method affected antimicrobial elution and mechanical properties of high-dose ALBC.

Methods: ALBC was formulated with Simplex P bone cement and 10 g of vancomycin per batch using one of three mixing methods: (1) hand-stirred using a standard bowl and spatula, (2) bowl-mixed using a mechanical mixing bowl, and (3) dough-phase mixing where the vancomycin was left in chunks (1-5 mm) and folded into the cement during the dough phase after adding the monomer. We eluted 45 standardized test cylinders (15 per mixing technique) for 30 days under infinite sink conditions. We tested 135 (45 per mixing method) similarly eluted cylinders in axial compression to failure.

Results: Dough-phase mixing lead to greater antimicrobial delivery, but lower compressive strength than the hand-stirred or bowl-mixed methods. Dough-phase cement released 18,570 lg of vancomycin versus 11,731 for hand-stirred and 7700 μg for bowl mixed. Compressive strength for dough-phase mixing after 30 days of elution was 36 MPa, while both hand-stirred and bowl mixed cements were 56 MPa.

Conclusions: Performance of high-dose ALBC was affected by mixing method. Dough-phase mixing led to greater antimicrobial delivery, but caused greater loss in compressive strength.

Fig. 1

The diagram shows the experimental design for mixing method as a determinate for antimicrobial release and compressive strength of high-dose ALBC.

Fig. 2A–B

The photographs show (A) ground vancomycin powder prior to mixing with PMMA powder, and (B) vancomycin chunks prior to addition to cement. This figure was provided to document the qualitative differences in antimicrobial preparation prior to their use in the different techniques.

Fig. 3

The graph shows the cumulative elution of high-dose cement formulated with 10 g of vancomycin. Error bars show standard deviation. White bars indicate the elution from the hand-stirred ALBC. Black bars indicate the elution from the bowl-mixed ALBC. Grey bars indicate the elution of the dough-phase mixed ALBC. Dough-phase mixed ALBC delivered more vancomycin than the other two mixing methods.

Fig. 4

The graph shows compressive strength for high-dose ALBC formulated with 10 g of vancomycin per batch of Simplex^® P cement. White bars indicate the strength of the hand-stirred ALBC. Black bars indicate the strength of the bowl-mixed ALBC. Grey bars indicate the strength of the doughphase mixed ALBC. Dough phase mixing produced ALBC that was weaker than the other two mixing methods. Compressive strength decreased over time in elution for all three mixing methods. Error bars indicate standard deviation.