Antibiotic-impregnated polymethylmethacrylate (PMMA) bone cement has been used successfully to treat infected joint arthroplasties, and some surgeons advocate using antibiotic-impregnated PMMA bone cement prophylactically for all joint replacement surgeries. There is, however, concern that the added antibiotic reduces the mechanical properties of the cement. The general consensus is that the quasi-static mechanical properties of PMMA bone cement are reduced by the addition of antibiotic, but there is reason to believe that the fracture toughness of PMMA bone cement may increase with the addition of antibiotic. The objective of this study is to demonstrate the possible toughening effect of the addition of antibiotic to PMMA bone cement.

The Mode I critical stress intensity factor or fracture toughness of Simplex® P bone cement with and without the addition of the antibiotic Tobramycin was determined experimentally using the procedures of ASTM Standard D5045. Cast rectangular compact tensile specimens were cyclically loaded to grow an initial fatigue crack to the recommended length. The specimen was then ramp-loaded to failure while the crack opening displacement was monitored. From the load-crack opening displacement data, the measured geometry of the test specimen and the fatigue crack, the Mode I fracture toughness was calculated. The fracture surfaces of representative specimens were examined by scanning electron microscopy to identify toughening mechanisms.

The fracture toughness of Simplex® P (1.22 ± 0.22 MPa√m, n = 9) bone cement was statistically significantly increased (p<0.05) by the manual addition of Tobramycin (1.45 ± 0.08 MPa√m, n = 9) as it would be done at the time of surgery. The increase in fracture toughness of Simplex® P bone cement due to the addition of Tobramycin by the cement manufacturer (1.27 ± 0.12 MPa√m, n = 9) did not reach statistical significance. Fracture surfaces of Simplex® P bone cement with added antibiotic were more convoluted with greater plastic deformation than those of the cement without antibiotic, and did not reveal the biphasic, PMMA bead-newly polymerized matrix structure seen on the plain cement surfaces.

From the results of this study, it was concluded that the addition of antibiotic to PMMA bone cement can increase the fracture toughness of the cement by increased crazing and microcrack formation in the newly polymerized matrix. The magnitude of this increase in fracture toughness depends on the method of addition of the antibiotic to the PMMA bone cement.