Rheological studies of dental materials: prosthetic materials and dental cements

Introduction

The handling, working and setting times, adhesion, and stability of dental materials are dependant on their rheological properties. The values of these and their measurement are, in turn, affected by the temperature, humidity, the type of rheometer and the geometry of the measuring device[1]

Rheological parameters commonly in use in dental material science are viscosity, loss and storage modulus, the loss tangent, known as tan delta (tan δ), in shear stress and elastic modulus in elongational flow. Measurements of creep strain and creep recovery are recorded for creep.
The values of these can be modified for any particular material by the particle size distribution, powder:liquid ratio, particle shape, density, volume fraction and the use of additives[2,3]. These latter can be particular (e.g. superplasticisers) or liquid (surfactants). Also involved in these controlling parameters are particle deformability and particle/particle interaction (i.e. attraction or repulsion) [4]

Rheology is defined as the study of the deformation and flow of matter[5]
In rheological terms, materials are either Newtonian, where viscosity is not affected by shear stress, or non-Newtonian, where stress produces shear thinning (pseudoplastic) or shear thickening (dilatant) behaviour[2,5].
The terms psuedoplastic and dilatant, used in earlier studies, may be misleading and are no longer used. Most materials used in dentistry are non-Newtonian. Rheological behaviour is often classified into established models e.g Bingham, Crosse, which can be used to predict future behaviour of the material in differing conditions[1]. These models are used by theoretical rheologists, the building industry, food technologists and pharmaceutical industries[4]. They are used infrequently in dental studies.

Material category

The studies included in this review are of prosthetics materials and also various categories of dental cements. This review does not include endodontic materials, adhesives used in restorative dentistry, fluoride gels, fissure sealants, dental waxes or ceramics. These subjects form the basis of separate reviews.


Viscometer / rheometer type

The geometry and type of the measuring device is of great importance as these will have an effect on the measurement results[1]. The apparatus used in these studies range from two glass plates[6,-9], penetrometer[10-14], reciprocating rheometer[10,15-20,7], oscillating plates[21-44,11], capillary extrusion[6,45-51] , rotational spindle[52,53], ram and piston[14], cone and plate[54-74,24,47,48,27,43], parallel plates[71-73,75-80,17, 35,44], concentric cylinder[ 80,81,50,72], compression rheometer[6,82], cup and bob[81,], displacement rheometer[83-85,30], Instron Universal testing machine[72], creep measurement[83,86] and sinusoidal vibration[87,88]. Some studies use a shark fin method[89, 62], a laser/magnet apparatus[90,91], squeeze film technique[92] and an indenter apparatus[93]. Other methods described are a flow point test[94] , flexural vibration[95] , an amalgam condenser [96],a modified ADA test[97], a Gilmore needle with a Zwick1440 mechanical testing machine [98] and custom made capsule extrusion [99]

Temperature and humidity

The values of rheological measurements are affected by both temperature and humidity. In the prosthetic studies, ambient or working temperature varies from 10ºC to 30ºC. Mouth temperature varies between 32ºC and 37ºC and one early study of compo impression material was tested at temperatures of 45, 50 and 60º C. In most of the studies humidity is not stated but in three studies humidity was controlled at 50% RH [10,55,27] and in one humidity was controlled also at 15%[27].
The dental cements were tested for ambient or working temperatures of 5, 8, 18 (62º F), 22, 23, 25 and 26ºC. Mouth and higher temperatures quoted varied from 29ºC and 60 º C. Again humidity is usually not stated. In four studies [34,37,42,69], humidity was controlled at 50%. Four studies were controlled at 100% humidity [66-68, 86]. One study was controlled between 40 – 60% [96] while another was controlled at 90% [98]


Study design and statistical analysis

Very few studies adhere to the concept of hypothesis testing and in many studies it would appear that the sample size was one or not stated. However sample sizes of between 2 and 10 are reported. Statistical analysis was carried out in 49 of a total of 94 studies reviewed. Of these 43 showed some statistically significant results. However only 20 studies reported use of an experimental control, although some stated that the apparatus had been calibrated before the start of experiment.


Main findings of each material type

Impression materials

Rheometers quoted as suitable for measuring impression materials are reciprocating, capillary extrusion, oscillating, extrusion from disposable syringes and micro-fourier squeeze film. It is stated that the ideal setting property of an impression material is a long working time followed by a sharp set at mouth temperature. It has been found that the viscosity during setting is highly dependant on the temperature. Manufacturers’ working times have been rendered inaccurate by some of these studies and clinically relevant shear rates for syringe extrusion have been suggested. It is generally considered that tan delta is a suitable measure for setting characteristics of dental impression materials.

Acrylics and resins

Increased viscosity and therefore reduced doughing time is produced by increase in small particles and aesthetic fibres. Shear thickening and thixotropy have been demonstrated and light cured resins have higher viscosity than rubber based materials. Viscosity increases with time, temperature and rotational speed of the apparatus. There may be an induction time before a certain increase in viscosity is reached and this increase is an exponential function of time.


Soft linings/tissue conditioners

Significant difference in rheology of these materials has been found. Gelation can be controlled by presence of ethyl alcohol and polymer molecular weight. Particle size and molar volume also have effect. High values of tan delta and storage modulus improve masticatory function and in a study on dietary effects, corn oil and heptane produced a rapid reduced compliance.

Adhesives

The viscosity of denture adhesives is in the order of 1000000 poise and is affected by the rate of dilution of saliva.


Dental cements

Unset dental composites exhibit a yield stress and on setting are pseudoplastic (i.e. exhibit shear thinning behaviour) and shear dependant (i.e. non-Newtonian). However one study found that composites were dilatant (i.e. shear thickening) while setting. Polymerization rate of composites depends on constituent particle size. There is much discrepancy in the flow of different brands of composites and this affects their handling properties. As would be expected, flowable composites have greater flow but less mechanical strength and are unsuitable for stress bearing areas. Increased filler content increases viscosity and reduces creep strain.
Glass ionomers show power law behaviour and their viscosity may be associated with polymer cross-linking. Mixing glass ionomer orthodontic cements on a chilled slab increases their working time with no effect on the sharpness of set at mouth temperature. The effect of +-tartaric acid varies with concentration: at medium concentration it prolongs working time and sharpens set. Flow properties can be manipulated by cement formulation but also by the geometry of capsule design.
The viscosity of luting cements is shear rate dependant and dual cure setting greatly reduces the working time. The variation in yield stress of temporary pastes has an effect on their handling properties.
Viscosity of cements is affected more by temperature and geometry than shear rate. It is also affected by particle size distribution. Reduced viscosity and increased strength can be achieved by interparticle fillers.

Discussion

This review demonstrates that there has been extensive rheological studies of prosthetic materials and dental cements but there has been great variation in the aim of the studies, the geometry of the apparatus used and the experimental temperatures. With a few notable exceptions, the importance of humidity has not be known or considered. In some cases, this may be because it was felt that within the confined space of certain devices, humidity was not an important factor. The measurement of relative humidity as a percentage of the total humidity possible is however dependant on barometric pressure and height above sea level.
The actual measurement of humidity would be more accurate although the use of the term absolute humidity is no longer recommended.

Study design

For an experiment to be reproducible, the duration of these rheological tests should be stated, as this could vary across a particular range of shear and might give different results. In some studies, different shear rates are used for different materials, which means that the results for the different materials cannot be accurately compared.
Sometimes more than one rheometer or experimental geometry has been used because of the wide range of viscosity of the experimental materials or lengthy setting times. Most modern stress controlled rheometers have a very wide range, so this should no longer a problem. The amount of material required for each test varies from one device to another and can be a factor in choice of rheometer.
Some studies tested different materials at different temperatures, again rendering the results not accurately comparable.
In many of the studies, not all of the results of the reported experiments have been given, only some being quoted as representative. This leads to incompleteness of the data and reduced measurement quality.

Rheological studies are laboratory based and not clinical. In many of these studies, a clinical application or explanation is suggested. There is often a tendency for this to be subjective and not proven.

Table 3 Rheological studies of dental materials 1966-2008


Conclusion

Rheology is a very useful tool for the study of dental materials and has given much information about the intrinsic nature of materials, their properties and behaviour. Many of these excellent studies could be improved by reducing the number of variables, ensuring an adequate sample size followed by suitable statistical analysis and completeness of data reporting.


References

1. Barnes HA (2000). 'A Handbook of Elementary Rheology.' (University of Wales Institute of Non-Newtonian Fluid Mechanics: Aberystwyth, Wales.)
2. Barnes HA, Hutton JF, and Walters K (2001). 'Introduction to Rheology.' (Elsevier: Amsterdam.)
3. Bentz DP, Jensen OM, Haecker CJ, and Garboczi EJ (1999). Effects of cement particle size distribution on performance properties of Portland cement -based materials. Cement and Concrete Research 29, 1663-1671.
4. Barnes HA (2002). 'Viscosity.' (University of Wales, Institute of Non-Newtonian Fluid Mechanics, Aberystwyth: Aberystwyth,Wales.)
5. Walters K (1975). 'Rheometry.' (Chapman and Hall: London.)
6. Braden, M. (1967). Rheology of dental composition (impression compound). Journal of Dental Research 46, 620-622.
7. Graham BS, Jones DW, Sutow EJ. . Clinical implications of resilient denture lining material research. Part II: Gelation and flow properties of tissue conditioners.
J Prosthet Dent. 1991 Mar;65(3):413-8.
8 . Jacobsen PH, Whiting R, Richardson PC. Viscosity of setting anterior restorative materials. Br Dent J. 1977 Dec 20;143(12):393-6.
9. Moon, P. C., Tabassian, M. S., and Culbreath, T. E. (2002). Flow characteristics and film thickness of flowable resin composites. Operative Dentistry 27, 248-253.
10. Wilson HJ (1966). Elastomeric impression materials 1 The setting material. British Dental Journal 121, 277-283.
11. Mutlu G, Huggett R, Harrison A. Determination of doughing and manipulation times of acrylic resin denture base material and a proposal for a specification for a packing plasticity test. J Dent. 1992 Oct;20(5):311-7.
12. Katsikas NG, Huggett R, Harrison A, Vowles RW. The effect of esthetic fibers on the flow properties of an acrylic resin denture base material.
Dent Mater. 1994 Jan;10(1):2-5.
13. Jepson NJ, McGill JT, McCabe JF. Influence of dietary simulating solvents on the viscoelasticity of temporary soft lining materials. J Prosthet Dent. 2000 Jan;83(1):25-31.
14. Lorton, L., Moore, B. K., Swartz, M. L., and Phillips, R. W. (1980). Rheology of luting cements. Journal of Dental Research 59, 1486-1492.
15. Mansfield MA, Wilson HJ. Elastomeric impression materials. A comparison of methods for determining working and setting times. Br Dent J. 1972 Feb 1;132(3):106-10.
16. McCabe JF, Spence D, Wilson HJ. Doughing time of heat-cured dental acrylic resins and its dependence on polymer particle size distribution. J Oral Rehabil. 1975 Apr;2(2):199-207.
17. McCabe JF, Bowman AJ. The rheological properties of dental impression materials. Br Dent J. 1981 Sep 15;151(6):179-83.
18. Jones DW, Sutow EJ, Graham BS. Gel strength and rate of gelation of soft polymers. Dent Mater. 1991 Apr;7(2):138-44.
19. Inoue K, Song YX, Fujii K, Kadokawa A, Kanie T. Consistency of alginate impression materials and their evaluation. Oral Rehabil. 1999 Mar;26(3):203-7.
20. Inoue K, Song YX, Kamiunten O, Oku J, Terao T, Fujii K. Effect of mixing method on rheological properties of alginate impression materials. J Oral Rehabil. 2002 Jul;29(7):615-9.
21. Inoue K, Wilson HJ. Viscoelastic properties of elastomeric impression materials. I. A method of measuring shear modulus and rigidity during setting. J Oral Rehabil. 1978 Jan;5(1):89-94. PubMed PMID: 272446.
22. Inoue K, Wilson HJ. Viscoelastic properties of elastomeric impression materials. II: variation of rheological properties with time, temperature and mixing proportions. J Oral Rehabil. 1978 Jul;5(3):261-7. PubMed PMID: 279652.
23. Inoue K, Wilson HJ. Viscoelastic properties of elastomeric impression materials. III. The elastic recovery after removal of strains applied at the setting time. J Oral Rehabil. 1978 Oct;5(4):323-7. PubMed PMID: 280633.
24. McCabe JF, Carrick TE. Rheological properties of elastomers during setting. J Dent Res. 1989 Aug;68(8):1218-22. PubMed PMID: 2632608.
25. Mutlu G, Huggett R, Harrison A, Goodwin JW, Hughes RW. Rheology of acrylic denture-base polymers. Dent Mater. 1990 Oct;6(4):288-93.
26. Murata H, Iwanaga H, Shigeto N, Hamada T. Initial flow of tissue conditioners--influence of composition and structure on gelation. J Oral Rehabil. 1993 Mar;20(2):177-87.
27. Takahashi H, Finger WJ. Effects of the setting stage on the accuracy of double-mix impressions made with addition-curing silicone. J Prosthet Dent. 1994 Jul;72(1):78-84.
28. Mutlu G, Huggett R, Harrison A. Factors that affect the rheologic properties of acrylic resin denture base materials. J Prosthet Dent. 1994 Feb;71(2):186-91.
29. Iwanaga H, Murakami S, Murata H, Shigeto N, Hamada T. Factors influencing gelation time of tissue conditioners. J Oral Rehabil. 1995 Mar;22(3):225-32.
30. Murata H, Hamada T, Djulaeha E, Nikawa H. Rheology of tissue conditioners. J Prosthet Dent. 1998 Feb;79(2):188-99
31. Murata H, Haberham RC, Hamada T, Taguchi N. Setting and stress relaxation behavior of resilient denture liners. J Prosthet Dent. 1998 Dec;80(6):714-22.
32. Murata H, Hamada T, Harshini, Toki K, Nikawa H. Effect of addition of ethyl alcohol on gelation and viscoelasticity of tissue conditioners. J Oral Rehabil. 2001 Jan;28(1):48-54.
33. Berg JC, Johnson GH, Lepe X, Adán-Plaza S. Temperature effects on the rheological properties of current polyether and polysiloxane impression materials during setting. J Prosthet Dent. 2003 Aug;90(2):150-61
34. Plant C.G., Jones IH, and Wilson HJ (1972). Setting characteristics of lining and cementing materials. British Dental Journal 133, 21-24.
35. Cook WD, Brockhurst P. The oscillating rheometer--what does it measure? J Dent Res. 1980 May;59(5):795-9.
36. Pearson GJ, Atkinson AS. Effects of temperature change on the working and setting characteristics of water-based dental cements. Dent Mater. 1987 Oct;3(5):275-9.
37. Fricker J, Hirota K, Tamiya Y. The effects of temperature on the setting of
glass ionomer (polyalkenoate) cements. Aust Dent J. 1991 Jun;36(3):240-2.
38. Shortall AC, Baylis RL, Fisher SE, Harrington E. Operating variables
affecting the working time of a dual-cure composite luting cement. Eur J Prosthodont Restor Dent. 1993 Jun;1(4):185-8.
39. Rubbi E, Baffa O, Vinha D, Thomé LH. Rheological studies of photopolymerized dental composites. Dent Mater. 1993 Nov;9(6):361-4.
40. Ogawa T, Tanaka M, Matsuya S, Aizawa S, Koyano K. Setting characteristics of five autopolymerizing resins measured by an oscillating rheometer. J Prosthet Dent. 2001 Feb;85(2):170-6.
41. Jiang P, See H, Swain MV, Phan-Thien Using oscillatory squeezing flow to measure the viscoelastic properties of dental composite resin cements during curing Rheologica acta 2003, vol. 42, no1-2, pp. 118-122
42. Fleming, G. J. and Zala, D. M. (2003). An assessment of encapsulated versus hand-mixed glass ionomer restoratives. Operative Dentistry 28, 168-177.
43. Lee IB, Son HH, Um CM. Rheologic properties of flowable, conventional hybrid, and condensable composite resins. Dent Mater. 2003 Jun;19(4):298-307.
44. Lee IB, Cho BH, Son HH, Um CM. Rheological characterization of composites using a vertical oscillation rheometer. Dent Mater. 2007 Apr;23(4):425-32. Epub 2006 Mar 29.
45. Combe, E. C. and Moser, J. B. (1976). An apparatus for measuring the rheological properties of dental materials. Journal of Dental Research 55, 223-228.
46. Combe, E. C. and Moser, J. B. (1978). The rheological characteristics of elastomeric impression materials. Journal of Dental Research 57, 221-226.
47. Eyre D, van Noort R, Ellis B. The rheology of silicone rubber impression materials. J Dent. 1989 Aug;17(4):171-6.
48. Wanis TM, Combe EC, Grant AA. Measurement of the viscosity of irreversible hydrocolloids. J Oral Rehabil. 1993 Jul;20(4):379-84.
49. Watts, D. C., Combe, E. C., and Greener, E. H. (1980). Capillary rheology of two composite resin systems. Journal of Oral Rehabilitation 7, 475-480.
50. Watts, D. C., Combe, E. C., and Greener, E. H. (1981). The rheological properties of polyelectrolyte cements II. Glass ionomers. Journal of Oral Rehabilitation. 8, 61-67.
51. Combe, E. C., Watts, D. C., and Greener, E. H. (1981). The rheological properties of polyelectrolyte cements. I. polycarboxylates. Journal of Oral Rehabilitation 8, 55-60.
52. Vermilyea, S. G., Powers J.M. and Koran A. (1978). The rheologic properties of fluid denture-base resins. Journal of Dental Research 57(2):227-231
53. Vermilyea, S., Powers, J. M., and Craig, R. G. (1977). Rotational viscometry of a zinc phosphate and a zinc polyacrylate cement. Journal of Dental Research 56, 762-767.
54. Herfort, T. W., Gerberich, W. W., Macosko, C. W., and Goodkind, R. J. (1977). Viscosity of elastomeric impression materials. J.Prosthet.Dent. 38, 396-404.
55. Cook WD Rheological studies of the polymerization of elastomeric impression materials. II. viscosity measurements. J Biomed Mater Res. 1982 Jul;16(4):331-44.
56. Tulachka GJ, Moser JB. Evaluation of the viscoelastic behavior of a light-cured denture resin. J Prosthet Dent. 1989 Jun;61(6):695-9.
57. Tavas, M. A. and Watts, D. C. (1989). The use of a cone and plate viscometer for determination of flow properties of unfilled resins and etching gels. Journal of Oral Rehabilitation 16, 185-192.
58. Pang IC, Chai J. The effect of a shear load on the viscosities of 10 vinyl polysiloxane impression materials. J Prosthet Dent. 1994 Aug;72(2):177-82.
59. McCabe, J. F. and Arikawa, H. (1998). Rheological properties of elastomeric impression materials before and during setting. J.Dent.Res. 77, 1874-1880.
60. Martinez JE, Combe EC, Pesun IJ. Rheological properties of vinyl polysiloxane impression pastes. Dent Mater. 2001 Nov;17(6):471-6.
61. Murata H, Chimori H, Hamada T, McCabe JF. Viscoelasticity of dental tissue conditioners during the sol-gel transition. J Dent Res. 2005 Apr;84(4):376-81.
62. German MJ, Carrick TE, McCabe JF. Surface detail reproduction of elastomeric impression materials related to rheological properties. Dent Mater. 2008 Jul;24(7):951-6. Epub 2007 Dec 27.
63. Braden, M. (1977). Rheology of composite filling material pastes. Journal of Dental Research 56, 627-630.
64. Ferracane, J. L. and Greener, E. H. (1981). Rheology of acrylic bone cements. Biomaterials Medical Devices and Artificial Organs 9, 213-224.
65. Ferracane, J. L., Moser, J. B., and Greener, E. H. (1981). Rheology of composite restoratives. Journal of Dental Research 60, 1678-1685.
66. Cook, W. D. (1983). Dental polyelectrolyte cements. III. Effect of additives on their rheology. Biomaterials 4, 85-88.
67. Cook, W. D. (1983). Dental polyelectrolyte cements: II: effect of powder/liquid ratio on their rheology. Biomaterials 4, 21-24.
68. Cook, W. D. (1984). Rheological studies of the setting of water-based cements. Australian Dental Journal 29, 44-49.
69. Hill, R. G. and Wilson, A. D. (1988). A rheological study of the role of additives on the setting of glass-ionomer cements. Journal of Dental Research 67, 1446-1450.
70. Silikas N, Watts DC. Rheology of urethane dimethacrylate and diluent formulations. Dent Mater. 1999 Jul;15(4):257-61.
71. Schulze KA, Zaman AA, Söderholm KJ. Effect of filler fraction on strength, viscosity and porosity of experimental compomer materials. J Dent. 2003 Aug;31(6):373-82.
72. Lee JH, Um CM, Lee IB. Rheological properties of resin composites according to variations in monomer and filler composition. Dent Mater. 2006 Jun;22(6):515-26. Epub 2005 Sep 19.
73. Beun S, Bailly C, Dabin A, Vreven J, Devaux J, Leloup G. Rheological properties of experimental Bis-GMA/TEGDMA flowable resin composites with various macrofiller/microfiller ratio. Dent Mater. 2009 Feb;25(2):198-205. Epub 2008 Jul 14.
74. Chadwick RG, McCabe JF, Carrick TE. Rheological properties of veneer trial pastes relevant to clinical success. Br Dent J. 2008 Mar 22;204(6):E11. Epub 2008 Feb 15
75. Ellis B, Al-Nakash S, Lamb DJ. The composition and rheology of denture adhesives. J Dent. 1980 Jun;8(2):109-18.
76. Shigeto N, Yamada Y, Iwanaga H, Subianto A, Hamada T. Setting properties of alginate impression materials in dynamic viscoelasticity. J Oral Rehabil. 1997 Oct;24(10):761-5.
77. Love BJ , Piguet Ruinet F , Teyssandier F Chemorheology of photopolymerizable acrylates using a modified Boltzmann sigmoidal model. Journal of Polymer Science Part B: Polymer Physics Volume 46 Issue 21, Pages 2319 – 2325
78, Ellakwa A, Cho N, Lee IB. The effect of resin matrix composition on the polymerization shrinkage and rheological properties of experimental dental composites. Dent Mater. 2007 Oct;23(10):1229-35. Epub 2006 Dec 19.
79. Lee I, Chang J, Ferracane J. Slumping resistance and viscoelasticity prior to setting of dental composites. Dent Mater. 2008 Dec;24(12):1586-93. Epub 2008 Apr 22
80. Cook WD. Rheological studies of the polymerization of elastomeric impression materials. III. Dynamic stress relaxation modulus. J Biomed Mater Res. 1982 Jul;16(4):345-57.
81. Stannard JG. Comparative flow characteristics of selected dental materials with emphasis on thixotropic behavior. Dent Mater. 1988 Apr;4(2):94-7.
82. Jamani KD, Harrington E, Wilson HJ. Consistency, working time and setting time of elastomeric impression materials. J Oral Rehabil. 1989 Jul;16(4):353-66.
83. Abuasi, H. A., McCabe, J. F., Carrick, T. E., and Wassell, R. W. (1993). Displacement Rheometer: a method of measuring working time and setting time of elastic impression materials. Journal of Dentistry. 21, 360-366.
84. Murata H, McCabe JF, Jepson NJ, Hamada T. The determination of working time and gelation time of temporary soft lining materials. Dent Mater. 1997 May;13(3):186-91.
85. Baroudi K, Silikas N, Watts DC. Time-dependent visco-elastic creep and recovery of flowable composites. Eur J Oral Sci. 2007 Dec;115(6):517-21.
86. el Hejazi AA, Watts DC. Creep and visco-elastic recovery of cured and secondary-cured composites and resin-modified glass-ionomers. Dent Mater. 1999 Mar;15(2):138-43
87. Murata H.1; Taguchi N.; Hamada T.; McCabe J.F.
Dynamic viscoelastic properties and the age changes of long-term soft denture liners
Biomaterials, Volume 21, Number 14, July 2000 , pp. 1421-1427(7)
88. Murata H, Taguchi N, Hamada T, Kawamura M, McCabe JF. Dynamic viscoelasticity of soft liners and masticatory function. J Dent Res. 2002 Feb;81(2):123-8.
89. Perry RD, Goldberg JA, Benchimol J, Orfanidis J. Applicable research in practice: understanding the hydrophilic and flow property measurements of impression materials. Compend Contin Educ Dent. 2006 Oct;27(10):582-6.
90. Helvatjoglu-Antoniades M, Papadogiannis Y, Lakes RS, Dionysopoulos P, Papadogiannis D. Dynamic and static elastic moduli of packable and flowable composite resins and their development after initial photo curing. Dent Mater. 2006 May;22(5):450-9. Epub 2005 Aug 11
91. Papadogiannis DY, Lakes RS, Papadogiannis Y, Palaghias G, Helvatjoglu-Antoniades M. The effect of temperature on the viscoelastic properties of nano-hybrid composites. Dent Mater. 2008 Feb;24(2):257-66. Epub 2007 Jul 20.
92. King S, See H, Thomas G, Swain M. Determining the complex modulus of alginate irreversible hydrocolloid dental material. Dent Mater. 2008 Nov;24(11):1545-8. Epub 2008 May 7
93. Tsukada G, Tanaka T, Kajihara T, Torii M, Inoue K. Film thickness and fluidity of various luting cements determined using a trial indentation meter. Dent Mater. 2006 Feb;22(2):183-8. Epub 2005 Jul 21
94. Mohsen NM, Craig RG. Effect of silanation of fillers on their dispersability by monomer systems. J Oral Rehabil. 1995 Mar;22(3):183-9.
95. Labella R, Lambrechts P, Van Meerbeek B, Vanherle G. Polymerization shrinkage and elasticity of flowable composites and filled adhesives. Dent Mater. 1999 Mar;15(2):128-37.
96. Judge, R. B. and Wilson, P. R. (1999). The effects of oscillating forces upon the flow of dental cements. Journal of Oral Rehabilitation 26, 892-899.
97. Attar N, Tam LE, McComb D. Flow, strength, stiffness and radiopacity of flowable resin composites. J Can Dent Assoc. 2003 Sep;69(8):516-21
98. Gbureck U, Spatz K, Thull R, Barralet JE. Rheological enhancement of mechanically activated alpha-tricalcium phosphate cements. J Biomed Mater Res B Appl Biomater. 2005 Apr;73(1):1-6
99. Crowley CM, Doyle J, Towler MR, Hill RG, Hampshire S. The influence of capsule geometry and cement formulation on the apparent viscosity of dental cements. J Dent. 2006 Sep;34(8):566-73. Epub 2006 Mar 29. Erratum in: J Dent. 2007 Jan;35(1):93.