title: The influence of clinically induced variability on the distribution of compressive fracture strengths of a hand-mixed zinc phosphate dental cement.
abstract: OBJECTIVE
  Conventional approaches to comparing dental cements use standard property tests under manufacturers' specified conditions. Zinc phosphate cements are supplied in powder/liquid form and manipulation frequently involves mixing the components by eye so a range of mixing ratios will inevitably occur in practice. Unfortunately, the physical, chemical, biological and mechanical properties of cements are known to be dependent on the mixing ratio.
  
  
  METHODS
  Forty dental nurses prepared a series of three cement samples to a luting consistency they considered acceptable for use in practice.
  
  
  RESULTS
  It was found that each dental nurse produced consistent cement mixes, although, the mixing ratios varied from 1.7 to 3.2 g/ml between nurses. The mean compressive strength, standard deviation and associated Weibull Moduli (m) of the cements were determined as a function of this mixing ratio range and showed considerable variation ranging from 33.5 +/- 3.2 MPa (m = 11.0) at 1.7 g/ml to 71.4 +/- 8.4 MPa (m = 8.6) at 2.6 g/ml and 42.5 +/- 10.0 MPa (m = 5.0) at 3.2 g/ml.
  
  
  CONCLUSIONS
  An analytical approach was adopted which facilitated an integrated analysis of the mixing ratio variability with the strength data. It was found for the test group of dental nurses that 25% of cement mixes produced would have achieved strengths below 40 MPa whilst strengths below the standard value were produced in at least 70% of mixes. These results indicate that a simple analysis of the properties of cements manipulated under optimum conditions, provides little information on the material characteristics obtained in practice because of clinically induced variability.
authors.affiliation: Biomaterials Unit, School of Dentistry, University of Birmingham, UK.
authors.forename: G J
authors.initials: GJ
authors.lastname: Fleming
authors.forename: P M
authors.initials: PM
authors.lastname: Marquis
authors.forename: A C
authors.initials: AC
authors.lastname: Shortall
mesh_terms: D000704:Analysis of Variance; D019245:Compressive Strength; D016018:Least-Squares Analysis; D008422:Materials Testing; D016019:Survival Analysis; D013674:Technology, Dental; D015036:Zinc Phosphate Cement / Q000737:chemistry*
chemical_list: D015036:Zinc Phosphate Cement