Effect of Radio-Opacifier Content on an MTA-like Root Filling Material

Objectives: Investigate the effect of varying radio-opacifier content (bismuth oxide-Bi₂O₃) on the mechanical stability and relative porosity and hence the potential longevity of a Portland cement based root filling material (similar to Mineral Trioxide Aggregate-MTA). Methods: An MTA-like model system (grey Portland cement/Bi₂O₃/gypsum mixture) was established which enabled the inert radio-opacifying Bi₂O₃ content to be varied. Wet compressive strengths of set cylindrical samples (n≥7, 12x6 mm) made from cement pastes at a powder to liquid ratio of 3.3 g/ml were measured for every compositional variation and the commercial MTA control. Strut densities were determined by helium pycnometry; together with apparent dry densities the relative porosities of the samples were calculated. Results: The wet compressive strength of the set cement deteriorated significantly (p<0.01) from 82±11 MPa to 40±11 MPa when Bi₂O₃ content was increased from 0 to 10wt% while relative porosity increased slightly from 15% to 17%. Gradually increasing Bi₂O₃ content from 10wt% to 40wt% decreased strength to 29±5MPa accompanied by a pronounced increase in relative porosity to 31%. Strength and porosity at 20wt% Bi₂O₃ content (analogous to commercial MTA) was 40±3MPa and 24% for the model system, compared to 33±1MPa and 31% for the commercial MTA control. Conclusion: The results clearly indicate that the necessary addition of radio-opacifying Bi₂O₃ to an MTA-like root filling material deteriorates mechanical stability by increasing porosity and introducing flaws into the set cement matrix. As a weaker and more porous cement structure will degrade faster in an aqueous environment the radio-opacifier content should be kept to the necessary minimum for dental application.