Authors: AC McLaren MD, SG McLaren BS, CL Nelson MD, DL Wassell
Title: Phenolphthalein Used to Assess Permeability of Antibiotic Laden PMMA- A Pilot Study
Addresses: UAMS, Dept of Orthopaedic Surgery, 4301 W Markham, Little Rock AR 72211
Purpose: To determine the feasibility of assessing permeability of antibiotic loaded PMMA by adding phenolphthalein to the antibiotic laden PMMA and using an elution fluid with pH > 10.0.
Methods: Antibiotic laden PMMA was prepared in the ratio of 1 batch of Palacos PMMA (40gm powder), 4 gm of vancomycin, 3.6 gm of tobramycin and 6 gm of Ancef. Three additional porosities of PMMA were made by adding crystalline glycine in the ratios of 0.45 gm, 7.0 gm, and 28.0 gm per batch of PMMA. One half gram of phenolphthalein was homogeneously mixed to each mixture before adding the monomer. Seven millimeter beads were made using a silicone mold. Elution fluid was prepared by mixing NaOH with filtered water to produce pH>10. Five beads of each mixture were placed in 10 ml of eluant at room temperature. The eluant was exchanged every 24 hours. Two beads were taken for analysis from each group on days 3, 7, and 14. The beads were dried, split along the equator, and examined for red coloration. Depth of the coloration was measured and correlated with time and mixture.
Results: The elution fluid caused a dramatic red color change immediately upon contact with the eluant. A distinct zone of red colorization could be seen upon splitting the beads. The depth of the red zone increased with time and with the amount of antibiotic or glycine powder that had been added to the PMMA.
Discussion: The release rate of antibiotic from PMMA is important clinically. Porosity/permeability of the PMMA is an important factor in determining the release rate. The time and expense of antibiotic assays to study the effect of changes in porosity are considerable. If the depth of penetration of fluid into the PMMA could be assessed with a simple visual technique, the factors affecting the permeability could be studied without the expense of the antibiotics and the antibiotic assays. Phenolphthalein/basic eluant caused a clearly visible colorization to the depth of penetration of the eluant into the PMMA. This technique is extremely inexpensive, immediate, and reproducible. Glycine has been shown to increase the elution of antibiotics from PMMA and is shown here to increase the penetration of fluid into PMMA. It is feasible to use this methodology to study the variables that effect permeability of PMMA.
Significance: The cost of antibiotic assays needed to study multiple combinations and permutations of antibiotic laden PMMA mixtures can be dramatically reduced by first determining the mixture that will produce the optimal fluid penetration. The permeability of PMMA can now be studied in relation to such factors as temperature, amount of added materials, and mixing techniques, etc., without the expense of antibiotic assays for the preliminary studies. These relationships now need to be documented.