July 1, 2000
by Keith Titley, BDS, Dip Paedo, MScD, FRCD(C), and Richard Caldwel
In an editorial for the 1999 inaugural issue of the Consultants Special edition of Oral Health Watson1 enumerated some of the limitations of resin composites as restorative materials. These included polymerisation shrinkage and its contribution to micro-leakage leading to post-operative sensitivity and eventual endodontic treatment, mechanical abrasion and wear during function, and chemical degradation in the oral environment. In the same issue Kanca2 noted that ‘resin adhesives are used to seal tooth structure and affix the resin composite to the tooth.’ He also noted that resin composites shrink upon polymerisation with the result that stress is put on the resin composite-adhesive interface.
D’Souza and Brown3 evaluated the ‘packable composites’ and questioned whether they represented a new advance in resin composite technology or whether they were simply a ‘new wrinkle’. This inaugural issue also included a paper in which Jackson and Soll4 stressed the need to follow the manufacturers’ instructions and to be meticulous in application techniques to maximise clinical success and longevity of the resultant restoration. This article will discuss some of the factors that have had, and will continue to have, an influence in the development and testing of adhesives and resin composites and what to look for when selecting an adhesive or resin composite.
CLINICALLY DERIVED DATA
Reviews of the literature reveal that the majority of the information on the properties of adhesive and resin composite materials is derived from in vitro laboratory investigations and that there is a dearth of in vivo clinical data. This latter observation underscores the difficulties associated with clinical studies in that with our highly mobile populations it is difficult to generate sufficient numbers of subjects who will return for long-term follow up. Studies such as these require large numbers of patients because of the drop out factor and to provide statistically significant results. The richest source of in vivo data probably exists with private general practitioners with well-established practices that have a following of loyal patients.
Provided that the technique is standardised in accordance with the manufacturers instructions, and parameters for the measurement of success are clearly defined highly significant amounts of clinical data could be collected which would be of value to both clinicians and to manufacturers. Another problem associated with clinical trials is that, in response to the professions’ demands, adhesive resin and resin composite formulations are constantly changing so that at the end of a trial on a specific material it may no longer exist in the same formulation as it did at the beginning. A recent example of this is the combining of primer and adhesive in the so-called single-bottle fifth generation adhesives.
LABORATORY DERIVED DATA
In contrast to in vitro investigations, data derived from in vitro laboratory investigations can be assembled more quickly and economically provided that the laboratory is well-equipped and ready sources of human and bovine tooth material are available. There is, however, a disquieting tendency to report on data which has been derived over a 24 hour or 7-14 day period. When it is considered that restorations are expected to endure over a period of years in the hostile environment of the oral cavity these periods of time are clearly inadequate. This is particularly critical where dentin-bonding agents are concerned with respect to the maintenance, over time, of the integrity of the hybridised layer formed by the penetration of resin adhesive into the de-mineralised collagen of the conditioned dentin surface. If it is not completely infiltrated by the adhesive resin the collagen is subject to hydrolysis by oral fluids resulting in a weakened bond. This is also an area in which micro-leakage occurs.
It should also be noted that the International Organisation for Standardisation (ISO) has issued a series of technical reports in which are outlined uniform testing protocols.5 If all laboratories followed the suggested protocols the practitioner would have a standard set of criteria with which to evaluate a new material when it reached the marketplace. It is important, however, that as new testing methods evolve they are allowed to replace those that have become outdated.
As a result the practitioner must be appraised of not only the new products but also have an appreciation of the test procedures outlined in the product monographs. Product monographs usually contain results derived from not only in-house testing but also from independent laboratories which publish their results in the current literature. The practitioner should ensure that the latter information has been taken from peer reviewed journals, has a sound scientific basis, and is not anecdotal in nature. It is no longer sufficient to take the words of the salesperson that a new and improved product should replace the tried and tested one that is currently in use.
SOME RESULTS OF INVESTIGATIONS IN OUR LABORATORY
In our laboratory we have recently completed a study, using bovine dentin, in which we examined the shear bond strengths of two fourth generation adhesives where the primer and adhesive were in separate bottles, and three fifth generation single bottle adhesives. Shear bond strengths were recorded at 24-hours and after 7 and 270 days. The results showed that over a 7 day period there were no significant changes but over 270 days there were significant decreases in bond strengths with the single bottle systems decreasing the most. Although these shear bond strengths may still be clinically acceptable the indications are that they will probably continue to decrease over time. These findings are similar to those of Meiers and Young6 who showed a similar effect over a 24-month period.
The oral cavity is an extremely hostile environment and, depending on what is being ingested, is subject to frequent changes in pH and temperature. In an attempt to mimic temperature changes it is suggested that thermal cycling should be an integral part when testing the properties of restorative materials in the laboratory. The thermal cycling protocol presently recommended by the ISO is that specimens be exposed to 500 cycles between 5C and 55C with dwell times of at least 20 seconds in each water bath and a transfer, or air time, of between 5 and 10 seconds. A survey of the current literature reveals there is considerable variation between laboratories for each of the suggested parameters particularly with respect to the number of cycles which can vary from 100 to as many as 50,000 or more.
We recently carried out an investigation using bovine dentin and the recommended ISO protocol for thermal cycling with two fourth generation and three fifth generation single bottle adhesives.7 The specimens were thermal cycled after 24-hours or 6.5 days and they, and a control group, were shear bond tested after a total of 7 days had elapsed after the initial bonding procedure. The results obtained were equivocal and showed that the shear bond strengths were dependent not only on the individual adhesive but also on the length of time of thermal cycling before shear bond testing. In an extensive review of the current literature Gale and Darvell8 reported that the variations in regimens reported was large and made comparison of reports difficult. As a result they concluded that thermal cycling is an ‘irrelevancy with spurious legitimacy, or is at best premature, because the validating preconditions have yet to be met.’ Our results would appear to provide support for this observation.
Polymerisation shrinkage occurs in all photo-polymerised resin composites and this affects their physical properties and the marginal integrity of the resultant restorations. It has recently been suggested that this shrinkage can be minimised by pre-polymerising resin composite at low visible light intensity followed by a final higher cure at a higher intensity without significantly affecting its physical properties or ma
rginal integrity. This two step method of polymerisation has been termed dual-cure.
We recently completed an investigation using human third molar teeth in which we compared the shear bond strength of cylinders of Z100 resin composite of varying thickness that had either been dual-cured or single cured and bonded to dentin with either fourth generation Scotchbond Multipurpose, or single bottle fifth generation Single Bond adhesive resins9 (3M, Co. MN. USA.). The results obtained indicated that no more than 3mm increments of resin composite, but more preferably 2.0mm, should be dual-cured or single cured at one time particularly when the bond is mediated with the single bottle adhesive. Preliminary hardness tests also indicated there were no differences either at the top or at the resin tooth interface for resin composite of this thickness. These results are similar to those reported by Kanca10 and by Price et. al.11
WHAT DOES ALL THIS MEAN?
The dental marketplace contains a plethora of restorative materials all of which claim to be new and improved and ready to replace materials that a practitioner is not only satisfied with, but has used with long term clinical success. It is important that the new material has a ‘track record’ and this should include both in vitro and in vivo tests over several months rather than days.
The tests that we have carried out in our laboratory make a small contribution to the overall understanding of a limited number of resin adhesives and composites; they are by no means exhaustive but represent what we have achieved on a limited budget. As Watson1 noted in his editorial in the inaugural consultants’ issue, polymerisation shrinkage, the degree of polymerisation, and mechanical wear and chemical degradation from enzymes and other chemicals found in saliva require meticulous research in order to produce a good replacement for both silver amalgam and current resin composites. He rightly concludes that this research is costly.
The data bases, however, on adhesives and resin composites continues to expand and the choice of material is very much up to individual practitioners. Careful evaluation of product monographs should enable them to select the best available material for specific restorations. It must also be remembered that following the manufacturers’ instructions and practising with a meticulous technique will go a long way towards ensuring clinical success and, ultimately, patient satisfaction.
r. Keith Titley is Oral Health’s Paediatric consultant.
Oral Health welcomes this original article.
1.Watson, P. Future in biomaterials. Editorial. Oral Health, 89(7): 3, 1999.
2.Kanca, J. Adhesive dentistry. Where are we? Oral Health, 89(7): 7-9, 1999.
3.D’Souza, NL., Brown, JW. Packable Composites, new wave in posterior composite resins or new wrinkle? Oral Health, 89(7): 46-50,1999.
4.Jackson, R., Soll, J. The importance of technique in eliminating post-operative sensitivity. Oral Health, 89(7): 71-75,1999.
5.ISO Technical Report. Dental materials-Guidance on testing of adhesion to tooth structure. ISO TR 11405, First edition, 1994.
6.Meiers, JC., Young, D. Two year composite to dentin shear bond strengths. J. Dent. Res. Special Issue. 79: 509, Abstract 2925, 2000.
7.Caldwell, R., Kulkarni, GV., Titley, K. Thermocycling and shear bond strength-a standardized approach. J. Dent. Res. Special Issue. 79: 509, Abstract 2923, 2000.
8.Gale, MS., Darvell, BW. Thermal cycling procedures for laboratory testing of dental restorations. J. Dent. 27: 89-99, 1999.
9.Titley, K., Caldwell, R., Kulkarni, GV. Resin thickness and single vs dual cure photopolymerization: effects on SBS. J. Dent. Res. Special Issue. 79: 509, Abstract 2922, 2000.
10.Kanca, J. The effect of thickness and shade on the polymerization of light-activated posterior composite resins. Quint. Int. 17(12): 809-811, 1986.
11.Price, RB., Doyle, G., Murphy, D. Effects of composite thickness on the shear bond strength to dentin. J. Can. Dent. Assoc. 66: 35-39, 2000.