2009 Abstract : 25

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Authors: Nugent M, McLaren A, McLemore R, Kaul H, Vernon B, Banner Good Samaritan Medical Center, Phoenix, AZ

Title: Dose Related Effect of Particulate Fillers on Elution and Compressive Strength of Antibiotic Loaded Bone Cement

Purpose: To determin the effect of volume fraction of a particulate filler on the elution performance and compressive strenght of ALBC

Methods: ALBC test cylinders 12 mm in length and 6 mm in diameter were made using ALBC formulated with Simplex P bone cement (Stryker), 1 gram of tobramycin and particulate xylitol in the following doses: 0, 1, 2, 4, 8 and 16 grams. The PMMA powder, tobramycin and xylitol were first mixed homogeneously using a commercial bone cement mixing bowl. The monomer was then added and the ALBC mixed by hand without vacuum. Three batches ALBC were made for each Xylitol dose. The ALBC was introduced into a mold in the dough phase to make the test cylinders. The ends of the test cylinders were machined flat and square.
Elution- 3 groups of 5 cylinders for each xylitol dose were eluted in 20 ml of de-ionized water. Total elutant exchange was performed on days 1, 3, 7, 15 and 30. The tobramycin concentration was measured using disc diffusion bioassay. Total recovered tobramycin was calculated.
Compressive Strength- 3 groups of 5 cylinders for each xylitol dose for 4 time periods (0, 7, 15, and 30 days) were eluted following the above protocol. Cylinders were tested to failure in axial compression at deformation rate of 2 mm /sec in a material test frame (MTS Sintech 1/S). A custom Matlab algorithm was used to calculate the compressive strength in accordance with ISO standard 5833.
Data was subjected to statistical analysis using repeated measures ANOVA with P < 0.05 defined as significant. Tukey's multiple comparison test (P < .05) was used as a post-hoc test to identify differences between groups.

Results: Elution- Tobramycin elution increased with time in days (p < .05) and grams of xylitol per batch (p < .05), following first order kinetics. For 1 gram of xylitol, 440µg was recovered by 3 days and 816µg after 30 days. For 16 grams of xylitol, 1043µg was recovered by 3 days and 2722µg after 30 days. Compressive strength- Compressive strength decreased with time in elution (p < .05) and grams of xylitol per batch (p < .05) with the majority of the change occurring in the first 7 days. For 1 gram of xylitol, compressive strength decreased by 7.4% by day 7 and 10.5% by day 30. For 16 grams of xylitol, compressive strength decreased by 27.8% by day 7 and 35.4% by day 30. For 0, 1 and 2 grams of xylitol the compressive strength stayed above the ISO standard for the full duration of the test. For 4, 8 and 16 grams, compressive strength fell below the ISO standard during the study (4 grams at 20 days, 8 grams at 6.5 days and 16 grams at 2 days).

Discuassion and Conclusion: 1) Increasing doses of particulate filler cause progressively increased antibiotic elution. 2) Increasing doses of particulate filler cause progressively decreasing compressive strength during elution, however for 0,1 and 2 grams of xylitol the loss of compressive strength was not below the ISO standard.