Abstract
Rheology is studied extensively in the food and pharmaceutical industries, in construction, oil and manufacturing industries. It can lead to better understanding of the flow of endodontic materials and help in the development of new materials. Having considered the main points of rheological theory and some of their practical applications, the aim of this paper is to review the existing literature on the rheology of endodontic materials, with emphasis on geometry, temperature and humidity. Study design and statistical analysis will also be considered. The effect of rheology on the ideal properties of endodontic materials will be discussed followed by a review of specific rheological studies of endodontic materials. The main clinical and statistical findings will be reported. Some topics for further research are suggested as well as some rheology-related questions which still need to be addressed.
Introduction
Rheology as a subject was first introduced by Professor Eugene Bingham of Lafayette College, Indiana, USA. It is defined as the study of the flow and deformation of matter(1). The American Society of Rheology was founded in 1929 and the British Society of Rheology, founded in 1940, has members now whose scientific background is in mathematics, physics, engineering and physical chemistry(1). As well as purely scientific study, there are many practical applications of rheology in the fields of food technology, construction industry, pharmaceuticals, paints and printing, polymer processing, haemodynamics, Formula 1 motor racing lubricants and manufacturing engineering, to name just a few(1). The field is open now for advances in endodontic rheology.
In a previous article, an overview was given of the fundamental concepts and theory of rheology. In this article, the rheological properties of endodontic sealers will be discussed, with reference to relevant literature. Studies of the rheology of endodontic materials will be reviewed. Finally some suggestions are made for further studies.
Rheology and the properties of endodontic sealers
The required properties of an endodontic sealer, which might be considered ideal, are listed in Table 1. Several of them are related to rheology.
Table 1 Ideal Properties
1. Suitable viscosity
The sealer should have a viscosity which will enable it to flow to the apex without being extruded. If it is too viscous, it may not flow as far as the apex and if the viscosity is very low it may seep through the apex. The sealer should also flow into lateral canals and into dentinal tubules. The sealer viscosity, combined with pressure applied during insertion, will need to be great enough to overcome the resistance of residual material in lateral canals and the resistance of the interdentinal fluid. The optimum viscosity is as yet unknown.
2. Adhesion of the material to root dentine.
It has been shown that the debonding of soft polymer adhesive systems is related to their viscoelastic properties, in particular to the values of tan delta(2). Also known as the loss tangent , this is defined as the ratio of the loss modulus (liquid-like behaviour) to the storage modulus( solid-like behaviour) of viscolesatic behaviour as measured in oscillatory shear flow. These moduli are affected by the number of molecular entanglements or cross–linkings of polymeric material and by entanglement molecular weight(3). Further study on the effect of entanglement molecular weight on the values of tan delta may help to determine an optimum formulation for adhesive materials(2).
A study of a dentine adhesive system used in a standard peeling test found that bond fracture was related to the Young’s modulus (i.e. the ratio of extensional stress and extensional strain rate) of the test membrane(4).
Steady flow will ensure that the material flows continuously along the surface of the root dentine rather than intermittently and will give closer adaptation to the canal wall(5).
3. Adhesion to gutta percha or other core material
Adhesion to gutta percha or other core material will also depend on
rheology. Of some importance may be the smoothness of the surface of the gutta percha point, but this has not been tested as yet.
The effect of heat and length of time of heating on the adhesion of polysulphone material to glass fibres and steel wire has been studied. Maximum adhesion was found when there was still some plasticity rather than rididity of the interfacial layer, and this was related to the number of cross-linkings present. Above a certain point increased cross-linking produced more rigidity at the interface and reduced adhesion(6).
4. Particle size distribution
The size distribution of particles suspensed in the sealing material is an important factor in determining its rheological properties. As well as the viscosity, the maximum particle size relative to the size of the apical foramen , the size of lateral canals and the size and number of dentinal tubules per unit area will determine whether material will flow into these areas. Addition of silica fume of different particle sizes improved the rheological properties and wettability of certain adhesive systems. The viscosity and contact angle increase with increasing particle surface area. The addition of fumed silica also increased the peel strength of some adhesive joints(7).
Varying the powder:liquid ratio of sealers alters the volume fraction of the mixed material. It has been shown that increasing the powder:liquid ratio of ZnO/eugenol sealers decreases the flow (8,9). Concrete rheological literature also supports this finding(10).
11. Bacteriostatic or bacteriocidal
One study(11) suggested that antimicrobial activity may be related to flow rate of sealers while a later study showed that flow does not affect the bacteriocidal effect of sealers(12).
13. Minimum setting contraction or expansion
Shrinkage stress of light-cured dental resins has been found to be related to their viscosity and glass transition temperature(13).
14 Clinically satisfactory working and setting times
Rheology will affect the working and setting times of materials. and oscillating rheometry has for many years been the method to determine working and setting times(14). With modern stress controlled rheometers, analysis of loss and storage modulus may give a more accurate measure of working times. Various parameters can be used to determine setting time(15).
Studies of endodontic materials
While rheological studies of other dental and commercial materials are extensive, the literature on the rheology of endodontic materials is fairly limited (Table 2).
Table 2 Rheological Studies of Endodontic Materials 1970-2008
Various geometries are used and there is variation in temperatures, humidity and the time of experimental procedure. Often the stress or shear rate of the applied force to produce the flow of material is not specified or not applicable and in many of the studies viscosity is not calculated or measured (16,17,18,19).
One early study compared the flow of 10 sealers under vacuum in a capillary pipette(20). This geometry was chosen to simulate an ultrafine canal. Variation in flow was demonstrated for the sealers, which may be related to particle size. However one material (Klorperka) showed erratic behaviour probably due to the evaporation of chloroform under vacuum.
Another early study compared the flow of sealers on a vertical glass plate at two temperatures(21). However two levels of humidity were used in this study, so that accurate comparison of results from both temperatures is not possible.
Because viscosity is difficult to measure, viscosity-related measures are often used. ISO specification EN ISO 6876-2002(22) for endodontic sealers uses the measurement of the diameter of material flowing between two glass plates under a specified weight for a specified time. In this method, 0.05 ml of material is placed between glass plates 40 mm square and 5 mm thick, a weight of 100g is placed on top of the upper plate for 10 min and the diameter of the resultant compressed disc of sealer is measured. To comply with ISO 6876 this disc diameter should be not less than 20 mm. This method is described in several studies(23,14, 24-31,8).
Significant and non-significant variations in flow rate were demonstrated for different sealers. It was found that there was variation in restriction of seating of gutta percha points for different sealers(24). However a satisfactory recommended flow rate has not as yet been established(27) and clinical effectiveness depends on other factors as well as flow(26). Aged eugenol gives an improved flow rate which may affect adhesion(28) but this study also had variation in P:L ratio as well as age of eugenol, so again results for flow cannot be accurately compared.
A custom made capillary apparatus(8) gave viscosity related measures of volumetric flow and velocity for six sealers. It was found that the flow of sealers increased with increased applied strain and with reduced capillary internal diameter. This conforms with basic rheological behaviour for capillary or pipe flow(1). It was found that the effect of variation of P:L ratio varied with change in strain rate(8).
Capillary extrusion rheometers(16,18) (i.e. not flow within the capillary) have demonstrated that sealers are shear dependant but an ideal flow rate has not yet been determined. Extrusion through a bore(29) has shown that flow rates and working times were all similar for the tested sealers and they satisfied requirements of ISO specifications.
A rheometer with rotating spindle geometry(17) measured viscosity and showed that viscosity of sealers increases with time (because the material is setting) and decreases with increasing speed. An oscillating rheometer(14) showed variation in flow of sealers and was used to determine their working times.
Pressure and heat applied via a wire rod did not produce flow of gutta percha into lateral canals until a temperature of 47ºC is reached(32). A controlled stress rheometer, with cone and plate geometry(33) showed that increase in sealer viscosity produces increased intercanal pressure, which increases on cone insertion.
Using the ARES controlled stress cone and plate rheometer, the effect of temperature on the viscosity of sealers was investigated(19). It was found that most sealers had reduced viscosity with increased temperature and all sealers except Ketac-endo were shear thinning at 37ºC.
Temperature
ISO standard for flow of endodontic sealers does not specify a temperature for the procedure, but it is an important factor in determining and comparing flow in materials. In the studies reviewed, the authors based in Britain(8) and Sweden(14,24) have quoted 23ºC as ambient temperature, while those in US quote 25ºC(16,18). Where mouth temperature is quoted, it is always 37ºC and it is advisable that this should continue to be the experimental mouth temperature for future work in this area. It should be noted however, that in the rheological studies of other dental materials, such as impression materials, the experimental temperature used is often less than 37ºC, as it is felt that the material does not in fact reach body temperature.
Humidity
Humidity is often not considered in these studies, although it may affect the experimental results, and where it is, there is discrepancy in the humidity level reported. There is some debate as to whether relative or absolute or specific humidity should be used. It is felt now that relative humidity should not be used because of the variation caused by altitude and atmospheric pressure. The term absolute humidity has been discontinued.
Shear Rate
For some of these studies, shear rate is not applicable although it could be calculated from force of gravity, density of material and rate of movement of material on vertical glass plates. It could also be calculated using the applied force in the two plate experiment.
In other studies, using modern rheometers, which are stress controlled, the shear rate is known. However, there is variation in shear rate reported which makes accurate comparison of the studies difficult.
Viscosity
Many of these studies do not measure viscosity, using instead viscosity related measures as indicators of flow. However the newer stress and temperature controlled commercial rheometers give values of viscosity as well as other rheological parameters.
Sample size and control
These endodontic studies generally have adequate sample size and 8 out of 20 studies have used a control material. This is in good comparison to rheological studies of other dental materials, where many have a sample size of only one.
Statistical analysis
Earlier studies did not include a statistical analysis apart from one correlation study(14) but since 1995, most studies have had statistical analysis giving statistically significant results.
Future studies: in vitro and clinical
Many topics related to the flow of endodontic materials could be addressed in future studies.
1. Rheological characterisation of various endodontic materials.
2. Rheological studies using model systems which simulate clinical geometries and conditions
3. Clinical studies to test hypotheses of the effectiveness of materials in different flow categories: rate of insertion, force applied, surface geometry, preparation technique, intracanal conditions
Questions still to be addressed
1. We know that flow is affected by temperature. We need to determine how quickly sealers and other endodontic materials reach mouth temperature from ambient temperature.
2. How quickly is heat transferred via gutta percha points into sealer material in warm condensation and touch and heat techniques.
3. A quantitative study is required to determine the effect of change in humidity on the rheological properties of endodontic materials with reference to different types of rheometers, with different geometries. Does change in humidity have a significant effect on flow properties. A suitable level of humidity could be suggested for further studies.
4. For the study of the rheology of dental materials, it is necessary initially to use a wide range of shear and extensional stresses to understand the nature and behaviour of these materials. This is the rheometric characterisation of the material. On the other hand, to understand the clinical application of these materials and how their use is affected by their rheology, clinically relevant levels of shear need to be determined. How much force is applied, what is the rate of insertion, how do these change on moving from a syringe to a narrowing applicator or from insertion to lateral compaction. What is the effect of secondary flows and turbulence. Therefore, after rheometric characterisations , suitable model systems need to be developed to study the materials in clinical situations.
5. Viscosity is a very sensitive parameter and can range from very small e.g. 10000 to very large e.g. 1000000 so that log values are often used. It has been shown that it is affected by the shape of the measuring device i.e. its geometry and by temperature and humidity. Many studies do not calculate or measure viscosity. Future studies should attempt to give values of viscosity for endodontic materials at specific temperatures and in specific geometries. It would be useful if future studies adhered to temperatures of 25ºC and 37ºC.
6. Rheological properties also include elasticity, loss modulus, storage modulus and tan delta. These also affect the flow and are related to the working and setting times of the materials. These are not considered in the ISO specifications for endodontic sealers but could give a more accurate indication of when the material is changing from liquid-like behaviour to solid-like behaviour and how this affects their handling and sealing properties.
7. More work needs to be done on the flow of gutta percha points. The
temperatures achieved within the canal in warm condensation techniques should be studied.
Conclusion
It is necessary to distinguish between the rheometric characterisation of materials, with suggested clinical implications, and hypothesis based studies of rheological behaviour in clinical situations. More rheometric characterisation studies should be published to contribute to the literature on dental rheology and to inform further rheological study of endodontic materials in the clinical situation.
References
1. Barnes HA, Hutton JF, and Walters K (2001). 'Introduction to Rheology.'
(Elsevier: Amsterdam.)
2. Jensen MK, Bach A, Hassager , Skov AL. Linear rheology of cross-linked
polypropylene oxide as a pressure sensitive adhesive International Journal of Adhesion and Adhesives, Volume 29, Issue 7, October 2009, Pages 687-693
3. Morrison FA (2001) “Understanding Rheology” (OUP:Oxford)
4. Leforestier, E. Darque-Ceretti, J. M. Costini, M. Muller, M. Bolla
Adaptation of a standard adherence test to dentistry: the peeling test. Study of the interface between dentine and a one step dentine adhesive
system. International Journal of Adhesion and Adhesives, Volume 22,
Issue 1, 2002, Pages 23-35
5. Barnes HA (2000). 'A Handbook of Elementary Rheology.' (University of
Wales Institute of Non-Newtonian Fluid Mechanics: Aberystwyth, Wales.)
6. Yu. Gorbatkina, D. Pavlovskii, V. Ivanova-Mumjieva, I. Masalova.
The effect of heat treatment on the adhesion and rheological behavior of polysulfone International Journal of Adhesion and Adhesives, Volume 28, Issue 3, April 2008, Pages 135-141
7. Belén Jaúregui-Beloqui, Juan C. Fernández-García, A. César Orgilés- Barceló, M. Mar Mahiques-Bujanda, José Miguel Martín-MartínezRheological properties of thermoplastic polyurethane adhesive solutions containing fumed silicas of different surface areasInternational Journal of Adhesion and Adhesives, Volume 19, Issue 4, August 1999, Pages 321-328
8. Lacey S, Pitt Ford TR, Watson TF and Sherriff M ( 2005) A study of the rheological properties of endodontic sealers. International Endodontic Journal 38:499-504
9. J. Camps, L. Pommel, F. Bukiet, I. About
Influence of the powder/liquid ratio on the properties of zinc oxide–eugenol-based root canal sealers Dental Materials, Volume 20, Issue 10, December 2004, Pages 915-923
10. 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.
11. Siqueira JF Jr, Favieri A, Gahyva SM, Moraes SR, Lima KC and Lopes HP (2000) Antimicrobial activity and flow rate of newer and established root canal sealers. Journal of Endodontics 26(5):274-7
12. Kaplan, A. E., Ormaechea, M. F., Picca, M., Canzobre, M. C., and Ubios, A. M. (2003). Rheological properties and biocompatibility of endodontic sealers. International Endodontic Journal 36, 527-532.
13. Christophe Charton, Véronique Falk, Philippe Marchal, Fernand Pla, Pierre Colon Influence of Tg, viscosity and chemical structure of monomers on shrinkage stress in light-cured dimethacrylate-based dental resins Dental Materials, Volume 23, Issue 11, November 2007, Pages 1447-1459
14. Ørstavik, D. (1983). Physical properties of root canal sealers: measurement of flow, working time, and compressive strength. International Endodontic Journal 16, 99-107.
15. McCabe, J. F. and Arikawa, H. (1998). Rheological properties of elastomeric impression materials before and during setting. J.Dent.Res. 77, 1874-1880.
16. Uhrich, J. M., Moser, J. B., and Heuer, M. A. (1978). The rheology of selected root canal sealer cements. Journal of Endodontics 4, 373-379.
17. Vermilyea, S. G., De Simon, L. B., and Huget, E. F. (1978). The rheologic properties of endodontic sealers. Oral Surgery,Oral Medicine,Oral Pathology 46, 711-716.
18. Vermilyea, S. G., Huget, E. F., and De Simon, L. B. (1979). Extrusion rheometry of fluid materials. Journal of Dental Research 58, 1691-1695.
19. Lacey S, Pitt Ford TR, Yuan XF, Sherriff M and Watson TF (2006) The effect of temperature on viscosity of root canal sealers. International Endodontic Journal 39: 860-6
20. Weisman, M. I. (1970). A study of the flow rate of ten root canal sealers. Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology and Endodontics 29, 255-261.
21. Grossman, L. I. (1976). Physical properties of root canal cements. Journal of Endodontics 2, 166-175.
22. EN ISO 6876:2002 International Standard (2002) Dental root canal sealing materials.
23. McComb, D. and Smith, D. C. (1976). Comparison of physical properties of polycarboxylate-based and conventional root canal sealers. Journal of Endodontics 2, 228-235.
24. Ørstavik D (1982). Seating of gutta-percha points:effect of sealers with varying film thickness. Journal of Endodontics 8, 213-218.
25. Siqueira, F. J., Jr., Fraga, R. C., and Garcia, P. F. (1995). Evaluation of sealing ability, pH and flow rate of three calcium hydroxide-based sealers. Endodontics and Dental Traumatology 11, 225-228.
26. Siqueira JF Jr, Favieri A, Gahyva SM, Moraes SR, Lima KC and Lopes HP (2000) Antimicrobial activity and flow rate of newer and established root canal sealers. Journal of Endodontics 26(5):274-7
27. Ono, K. and Matsumoto, K. (1998). Physical properties of CH61, a newly developed root canal sealer. Journal of Endodontics 24, 244-247.
28. Mendonca, S. C., de Carvalho, Jr., Guerisoli, D. M., Pecora, J. D., and Sousa-Neto, M. D. (2000). In vitro study of the effect of aged eugenol on the flow, setting time and adhesion of Grossman root canal sealer. Brazilian Dental Journal 11, 71-78
29. McMichen, F. R., Pearson, G., Rahbaran, S., and Gulabivala, K. (2003). A comparative study of selected physical properties of five root-canal sealers. International Endodontic Journal 36, 629-635.
30. Gambarini G, Testarelli L, PongioneG, Gerosa R, Galiani M. Radiographic and rheological properties of a new endodontic sealer. Aust Endod J 2006; 32:31-34
31. Asgary S, Shahabi S, Jafarzadeh T, Amini S, Kheirieh S. The properties of a new endodontic material J Endod 2008; 34:990-993
32. Venturi M, Di Lenarda R, Breschi L. An ex vivo comparison of three different gutta-percha cones when compacted at different temperatures: rheological considerations in relation to the filling of lateral canals. Int Endod J. 2006 Aug;39(8):648-56.
33. Brooke, K. K. and Grace, M. G. (2000). Relationship of intracanal pressure with viscosity of endodontic sealer during warm Gutta-Percha vertical compaction. Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology and Endodontics 89, 618-622.
No comments:
Post a Comment