Retentive Strengths of Two Different Phosphate Cements under Different Mixing Conditions

Objectives: The cooled mixing of phosphate cements is recommended to improve the physical properties of the hardened cement. However, it is difficult to put this into daily practice. An automated cooling device (dentalCooly/RP-Dentalelectronic) might solve this problem. The objective of the study was therefore to determine the retentive strength (RS) of 2 phosphate cements (Fixodont/DeTrey Dentsply, CoolCem P/Pharmamed) after mixing on the dentalCooly compared to a standard mixing procedure on a frozen glass slab. Methods: Cylindrical dies (6° angle of convergence) where machined of brass in heights of 1-10 mm in 1 mm steps. Crowns (Phantom-Metall/Degussa) where fabricated on the dies and sandblasted with 250 µm Al2O3. The luting cements where stored at 8°±1°C prior to use and mixed according to the manufacturer’s instructions either on a frozen glass slab (8°±1°C) or on the dentalCooly. The crowns (N=10 per material and die) where seated with a constant pressure of 10 N for 10 minutes. RS was determined after 1h and after two days of thermocycling (1000x5-55°C), respectively in a Zwick 1454 universal testing device (crosshead speed 10 mm/min.). Results: The mean RS-values ranged from 3.1 to 5.0 MPa. Statistical difference between the two different mixing procedures could not be detected for Fixodont (ANOVA, n.s. p>0.05), however there was a significant difference for CoolCem P (slab: 3.7 MPa vs. dentalCooly: 4.0 MPa; ANOVA, p<0.05). Overall CoolCem P showed significantly higher RS-values than Fixodont (ANOVA, p<0.01). After thermocycling RS-values increased for CoolCem P (ANOVA, p<0.01). Conclusions: Within the limits of the study it can be concluded, that the use of dentalCooly leads to a retention quality of the cement, which is comparable to a quality obtained from a well established mixing procedure on a frozen glass slab. In addition, the influence of the cement used is of major importance.