Title

A Novel Approach for Incorporating Benzoyl Peroxide in Two-Solution Bone Cement

Date of Award

5-2013

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Biomedical and Chemical Engineering

Advisor(s)

Julie M. Hasenwinkel

Subject Categories

Biomedical Engineering and Bioengineering

Abstract

Commercially available bone cements come pre-packaged and separated into powder and liquid components before they are combined during a vacuum mixing process. The powder portion consists of poly (methyl methacrylate) (PMMA), the initiator benzoyl peroxide (BPO) and typically a radiopacifier, such as barium sulfate (BaSO4) or zirconium dioxide (ZrO2). The liquid constituents include methyl methacrylate (MMA) monomer and the activator N, N-dimethyl-p-toluidine (DMPT). Two-solution bone cements (TSBC) vary from commercially available cements in that they incorporate the powder and liquid components together within a closed two-cartridge system. In TSBC's the polymer and initiator are thoroughly mixed within the liquid monomer to allow for complete dissolution. The two-cartridge system divides the BPO and DMPT into distinct containers to prevent the curing of the cement before application.

Concerns arise with the shelf life of two-solution bone cement due to the potential for spontaneous free radical polymerization as benzoyl peroxide interacts with the monomer during cement storage. Decomposition of benzoyl peroxide creates free radicals that can interact with the available methyl methacrylate molecules. Currently TSBC's require refrigerated storage to prevent polymerization that may be caused by heat or light exposure. The limited shelf life and storage condition of two-solution cements complicates the commercialization of the cement. As a solution to pre-polymerization concerns, we have removed the initiator from the cement solution and developed a solvent casting method to deposit BPO within the static mixing nozzle that is used for cement delivery. This technique will allow for the benzoyl peroxide to integrate with the two-solution bone cement as it is mixed for use.

A solution containing PMMA, MMA, and BPO was developed to create a thin film coating within the mixing nozzle as a method for introducing the initiator into the cement. The major goals of this thesis were to determine the effect of variable PMMA and BPO concentrations within the nozzle coatings on the thermal characteristics, flexural mechanical properties, and residual monomer content of the resulting cements. Altering the polymer and initiator concentrations within the nozzle affects the entrapment of BPO in the coating and the release of benzoyl peroxide during cement mixing.

Generally, exotherm testing resulted in a reduction in the maximum temperature released during cement curing, while the setting times were extended when compared to commercial and standard two-solution bone cements. Monomer conversion and setting times measured through differential scanning calorimetry testing showed low initial conversions and extended setting times respectively. This was due to the limited amount of benzoyl peroxide available at the initial point of injection. The cement requires a period of time to dissolve the polymer coating within the nozzle in order to access the available BPO. Mechanical testing resulted in comparable flexural properties that have previously been recorded in commercial and formerly tested two-solution bone cements. As the exotherm testing showed a reduction in maximum temperature, residual monomer experiments showed elevated levels of excess methyl methacrylate. There was a surplus of excess MMA found within the cement samples generated from the initial cement passage. The low maximum exotherm temperatures, extended setting times, and poorly polymerized mechanical samples were significantly influenced by the high residual monomer values that were achieved.

The solvent casting technique has shown to be a practical method for creating a BPO containing PMMA thin film within the static mixing nozzle that allows for integration into two-solution bone cement. Due to several undesired cement properties, further development of the coating system is necessary to improve the thermal and mechanical properties while reducing the residual monomer content within the resulting cements. Although the method is feasible, the kinetics and BPO release mechanism of the film must be investigated to improve the consistency of benzoyl peroxide liberated throughout cement injection.

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