Effect of manual vancomycin loading on the bending strength and flexural modulus of bone cement

Objectives: Manual addition of antibiotics when prefabricated antibiotic-loaded bone cement is not available is possible but should be performed with caution due to the effects of the antibiotic on the mechanical properties of polymethyl methacrylate (PMMA). This study investigates differences in bending strength and modulus and morphology of bone cement resulting from two variations in manual vancomycin loading. Methods: Three groups of bone cement – control without antibiotic, group with coarsely mixed 1 g vancomycin, and group with finely mixed 1 g vancomycin/40 g bone cement were tested for bending strength and bending modulus according to ISO5833:2002. The broken surfaces of the tested samples were analyzed using scanning electron microscopy. Results: Both methods of manual vancomycin loading caused a significant decrease in bending strength but not in bending modulus. The bending strength of the group with finely mixed vancomycin was significantly above the ISO5833:2002 threshold of 50 MPa. The bending strength of the group with coarsely mixed vancomycin did not differ significantly from the minimum set by the standard, with 4 of 9 specimens failing to meet the minimum requirement per ISO standards. The bending modulus exceeded 1800 MPa, with no significant differences among groups. The addition of vancomycin increased microporosity and the incidence of microcracks in the cement samples. Conclusion: Manual loading of finely mixed vancomycin causes an acceptable decrease in the tested mechanical properties of bone cement and can be considered for use in arthroplasty procedures. Additional testing of compressive strength and fatigue could provide a sufficient mechanical profile of these formulations and a better understanding of whether manually loaded properties of PMMA with vancomycin are mechanically safe for implant fixation.