New York University

David B. Kriser Dental Center

College of Dentistry

Graduate School of Arts and Science                                                                                        Nov. 7, 1998

Department of Dental Materials Science

345 East 24th Street

New York, NY  10010

Tel.:  212-998-9637

Fax: 212-995-4085

Dr. Samuel Waknine

DRM Research Laboratories Inc.

29 Business Park Drive

Branford, CT  06045

Dear Dr. Waknine,

Please find enclosed a final report entitled “Diametral tensile strength(DTS) of a crown and bridge restorative material: Multiple cure modalities and Test house/operators.

This study of ‘DiamondCrown’(DRM Labs. Inc., Branford, CT), Crown and Bridge material, includes additional data generated for comparison of the effect of cure mode and test house operator.  I thank you for providing the Diametral tensile test data from ‘In house testing’ and from ‘University of Sydney (Dr.Tyas)’.  The comparison of data indicates that Argon laser curing provides the highest DTS in comparison to other conventional cure modalities.  All other cure modes provide DTS values that are not significantly different

Weibull regression analysis’ of test data for all test data is also included in this report.  Please contact me for any clarifications or additional information.

I thank DRM Labs for the Material and Equipment grants required for this study.

Sincerely,

T.V.V. Raghavan

T . V. Vijayaraghavan, PhD

Associate Professor

NYU Kriser Dental Center

Dental Materials Science

Diametral tensile strength(DTS) of a crown and bridge restorative material:

Multiple cure modalities and Test house/operators.

Objective      The objective of this study is to compare the Diametral Tensile Strength (DTS) data measured for ‘DiamondCrown’ crown and bridge composite material, (DRM laboratories Inc., Branford, CT, USA) for four cure modalities or devices and from three test houses/operators.

Introduction and background                  Dental restorative composite materials are blends of hard glass filler or glass ceramic particulates in a polymeric resin matrix.  They are manipulated and handled by laboratory technicians to create anatomical forms.  Due to the technique sensitivity of these materials, operator manipulation and handling differences can lead to property differences in the set form.  Mechanical testing is one way to estimate the strength property of the material and the reliability validity of the data can be estimated by the use of ‘Weibull statistical analysis’.

The test data from ‘N’ number of trials or replicates of evaluation of DTS can be analyzed using ‘Weibull statistics’ to estimate two parameters, viz: s₀, termed the characteristic or most probable or a median strength and ‘m’ the ‘Weibull modulus’ or shape parameter; which can be used to estimate the failure probability.  A high value of ‘m’ indicates a greater level of homogeneity in material character while a smaller ‘m’ value would be an indicator of heterogeneity or a wider spread in test data.  The spread in test data is often due to the differences in ‘flaw population characteristics’ from sample to sample.  Flaws such as , voids and micro-porosity in the volume, may be material  inherent and /or infiltrated or incorporated during manipulation technique.  From the value of s₀ and ‘m’ the failure stress for a failure probability of 0.001, 0.1, 0.9 and 0.999((1 - 999 per 1000).labeled as s_0.001, s_0.1, s_0.9, and s_0.999 can be estimated.

Fracture can occur at low stresses, this depending on the operator and the technique used to manipulate the material.  The values of s_0.001, s_0.1, indicate the possible performance of the material, for an operator who may be a junior operator or a first time user; while the values s_0.9, and s_0.999 reflect the performance obtainable from a master technician or an experienced user.  The value of s₀, can be considered as a performance of the material obtained by a ‘median’ operator.  The materials’ inherent limitation or attainable performance is also represented by the value of s_0.999; i.e., as good as it gets!. The correlation or validity of the data conformity to the ‘Weibull distribution function’ for a given sub group of N trials is indicated by the value of R.  The higher the value of R, the greater the confidence in the test data and the statistical confidence.  When R is >0.9 the test data has a greater validity.

Materials and Methods    DiamondCrown^TM (DRM research laboratories, Inc., Branford, CT) was used to make DTS test samples.  DTS tests were carried out as per ADA Spec. #27 at the various test houses.  Test samples were prepared using a cylindrical metallic mold of 6 mm diameter and height 3 mm.  The test arrangement is shown in figure 1.  The load at failure was measured and the DTS value was calculated as per; DTS= 2P_f /pdh, where P_f  is the load at failure and ‘d’ and ‘h’ are the diameter and thickness of the test specimen, respectively.

Curing devices, 1) an Optilux 400 ^TM VLC unit (Demetron Research Corp., Danbury, CT) (Op), 2)a Xenon plasma cure unit (DiamondPlasma ^TM (DRM Res. Lab. Inc., Branford, CT), (Xe) 3) A Halogen light booth, DiamondLite ^TMVL Halogen (DRM Res. Labs. Inc., Branford, CT), (Hl) and 4). a prototype Argon laser source, DiamondLase ^TM(DRM Res. Labs. Inc., Branford, CT), (Ar) were used.

Test houses/operators       DTS data was made available from three test houses with three different operators; 1) NYU Kriser dental center, NY, NY. USA, Oa, 2) In house (DRM Res. Labs. Inc., Branford, CT, USA) , Ob, and 3) University of Sydney, Sydney, Australia. Oc.

Results and Discussion     The results of ‘Weibull analysis of the DTS raw data obtained from four cure modes and three test houses are summarized in Table 1.  The time of cure for the various cure modes are shown in the same table.  Overall, the test data represents a high correlation to the Weibull distribution function(high R value).  The mean values are not statistically significant for operator differences at constant cure mode.  The data indicates that the highest values are obtained for Argone laser cure and the lowest for the Halogen booth cure mode.  Operator contribution are reflected in the differences in s_0.001 for a given cure mode and may also be due to the technique sensitivity of the material.  If operator errors are a minimum or when the material manipulation is technique insensitive the value of the property obtainable approaches the s_0.999. value.  Conversely, when operator skill required in handling a technique sensitive material requires more experience or low, the value of the property obtainable may be closer to s_0.001.  The Weibull plots at low and high failure probability values indicate that operator manipulation tends to have a greater effect at low failure probabilities, in the rapid rate of drop in limiting failure stress, compared to that at higher failure probability.  The differences in DTS values obtained by operators, Oa and Ob may be related to the expertise in handling of the material.  In the case of Oa no attempt was made to evaluate the soundness of the test samples for surface or subsurface defect prior to testing.

Conclusions

a)     The DTS test data evaluated conforms to Weibull statistics.

b)     The characteristic strength, s_o, is higher for Argon laser curing compared to the Optilux 400 mode of curing.  A maximum variation  of +/- 10% in s_o is noted among the various sub-groups.  All the cure modes provide a similar strength value.

c)     Test house operator Oa, obtains lower values of s_o and s_0.001  irrespective of the cure modality.  while the in house operator, Ob, obtains a higher ‘s₀’and ‘m’ value (columns 1,  and 5; Table 1).  This is attributed to a differences in expertise in handling of this material, compared to Oa.

d)     The handling and manipulation of this material by the operators tend to provide fewer samples with a lower strength in the sub group.  In other words the characteristic strength is closer in value to the attainable s_0.999, in comparison to the value of s_0.001 for all operators.

Table 1.  Results of Weibull statistical analyses of DTS data as a function cure modes and Test house/operator

Weibull Characteristic DTS as a function of Cure device and Operator

Weibull plots at high and low end of failure probability for DT stress as a function of cure mode and  operator. (Ar- Argon, Xe- Xenon, Op-Optilux 400 and Hl - Halogen light booth; curing units.  Oa, b, c - Operators a, b and c.