Investigation of the replacement of bismuth oxide by zirconium oxide in mineral trioxide aggregate (MTA) and characterization and evaluation of the radiopacity and physical properties of varying replacements of zirconium oxide mixed at either water–powder or water–cement proportions of 0.3. The suitable filler loading of zirconium oxide for Portland cement in a MTA system for use as a root-end filling material was thus determined.
Portland cement replaced by zirconium oxide in varying amounts ranging from 0% to 50% in increments of 10 was mixed with water either at a water/powder (WP) proportion or at a water/cement (WC) proportion of 0.3. Portland cement and ProRoot MTA were used as controls. The materials’ microstructures were investigated using optical light microscopy. The radiopacity, strength, setting time, water uptake, solubility, sorption and porosity of the specimens were also evaluated. The optimum formulation was selected using the digital logic method.
Portland cement replaced with 30% zirconium oxide mixed at a water/cement proportion of 0.3 resulted to have the optimum combination of properties. This material exhibited radiopacity, compressive strength, setting time, water uptake, solubility and sorption comparable to ProRoot MTA. Both microscopy and the evaluation of porosity from the solubility and sorption experiments indicated a degree of porosity consisting mainly of capillary pores and entrapped air voids.
A filler loading of 30% zirconium oxide to Portland cement mixed at a water to cement proportion of 0.3 resulted in a material with comparable properties to mineral trioxide aggregate.
Mineral trioxide aggregate is used primarily as a root-end filling material in endodontic surgery . It is composed primarily of Portland cement with bismuth oxide added as radiopacifier. Bismuth oxide was added in 1:4 proportions to the cement so the material could easily be distinguished from the surrounding anatomical structures on a radiograph . The radiopacity of Portland cement has been reported to be 0.86–2.02 mm aluminum (Al) , values lower than the 3 mm aluminum recommended by the International Standards for dental root canal sealing materials (ISO 6876 Section 7.8: 2002) .
The addition of bismuth oxide renders the MTA sufficiently radiopaque . MTA has been reported to have radiopacity values ranging from 6 to 8 mm Al thickness . The addition of various proportions of bismuth oxide was reported not to affect the compressive strength of the material . Contrarily, other studies claim that increasing percentages of bismuth oxide adversely affect the material strength . Dilution of MTA with an excess of water has been reported to reduce the radiopacity of the material . Bismuth oxide in MTA does not act as inert filler. It becomes incorporated in the calcium silicate hydrate structure replacing the silicon and is also leached from the cement in increasing amounts . Bismuth oxide can exert some toxicity and negatively affect the growth and proliferation of human dental pulp cells and can also induce cell death .
Alternative radiopacifiers have thus been suggested for use with Portland cement. These alternative radiopacifiers are materials with a high relative molecular mass that can be used to replace the bismuth oxide in MTA. Addition of gold powder, silver/tin alloy , barium sulfate , iodoform , zirconium oxide , zinc oxide , lead oxide, bismuth subnitrate, bismuth carbonate and calcium tungstate results in a radiopacity value of more than 3 mm Al. The effects of the various additions of the different radiopacifiers on the hydration and the physical properties of the resultant material have not been completely investigated. Addition of gold powder, silver/tin alloy and barium sulfate has been shown to result in a material with similar properties to mineral trioxide aggregate . In addition the alternative radiopacifiers did not affect the hydration mechanisms of the cement. The gold powder was completely inert and was not leached out in solution . This, accompanied by the improvement in physical properties of the cement with 25% replacement with gold powder resulted in a prospective root-end filling material replacing MTA. The main disadvantages with using gold to replace the bismuth oxide in MTA include the cost of the gold and also the resultant color of the cement imparted by the gold.
Zirconium oxide was initially introduced as a biomaterial for use in hip or other joint implants in orthopedic surgery due to its hardness, high density and good wear resistance. In dentistry yttrium-stabilized zirconium oxide is usually used to replace the metal framework in crowns and bridges. Zirconium oxide is also used as radiopacifier with glass ionomer cements . Zirconium oxide added to Portland cement in a 1:4 ratio, resulted in adequate radiopacity .
The aim of this study was to investigate the replacement of bismuth oxide by zirconium oxide in mineral trioxide aggregate (MTA) and to characterize and evaluate the radiopacity and physical properties of varying replacements of zirconium oxide mixed at either water–powder or water–cement proportions of 0.3 thus determining the suitable filler loading of zirconium oxide for Portland cement in a MTA-like system for use as a root-end filling material.
Materials and methods
The materials used in this study included white Portland cement (PC; CEM 1, 52.5N; LaFarge Cement, Birmingham, UK) and zirconium oxide (ZrO 2 ; Sigma–Aldrich, Buchs, Switzerland). ProRoot MTA (Dentsply Tulsa Dental, Johnson City, TN, USA) lot number: 08003394 was used as control. The Portland cement was replaced by zirconium oxide in varying amounts ranging from 0% to 50% in increments of 10. The Portland cement/zirconium oxide mixtures were mixed with water either at a water/powder (WP) proportion or at a water/cement (WC) proportion of 0.3. The mix proportions are shown in Table 1 . ProRoot MTA was mixed at a water to cement proportion of 0.3 as recommended by the manufacturer .
|Sample name||PC weight %||ZrO 2 weight %||Water/powder ratio||Water/cement ratio|