October 28, 2002
Operative IV - Section 6
Melissa McCartney, Brian McDonald, Sachin Mehta, Krista Miller,
Todd Miller, Stephanie Miner, Peter Miskovich, Stephen Moore, Nathan Mork
In dental school, every student is taught the proper techniques to prepare teeth that need indirect restorations because of caries, fractures, missing teeth, or other reasons. However, we are taught to prepare ivorine teeth with entirely intact tooth structure. It is quite a different situation when one begins the process of preparing a tooth for an indirect restoration in the "real world". Tooth structures are missing and sometimes very little is remaining for us to work with. Now a whole new set of decisions needs to be made before we begin to prepare for the final restoration. Is there enough tooth structure left for my restoration to be successful? What and how much tooth structure do I need to replace? How does this tooth function in relation to the other teeth? In other words, how strong does my replacement have to be? How long does it have to last? How long will it be before the final restoration is placed? How important is esthetics here? Finally, what material do I use that will satisfy all of these requirements? The short answer here is that no material will satisfy all of the questions and concerns. "...No material may be considered ideal on the basis of its physical properties and characteristics. Furthermore, it could be considered unlikely that there will even be an ideal restorative material capable of truly replacing lost tooth tissues, and thereby fully restoring the form, function, and appearance of diseased and damaged teeth. As a consequence, clinicians should have an informed understanding of the advantages and limitations of alternative materials for specific applications and with due regard to specific environmental circumstances, modify their clinical technique accordingly to enhance the best possible clinical outcome." (12). In other words, we can not give you a simple solution by giving you a type of material and the type of foundation to use in every situation. What we will give you is a summary of literature that is out there right now comparing the different materials to be used in different situations. Your duty, as the clinician, will be to integrate our findings into your specific clinical scenarios and keep up on the current research because the materials of today may not be the materials of the future.
Restoration type based on indications
Fillers
A filler should be used only when a small amount of tooth structure is missing. The purpose of a filler is not to provide strength or resistance to the preparation, but rather to alter the shape of the preparation. This is most often done with the purpose of abolishing undercuts in the preparation. A class five restoration needing replacement is a fine example of a situation where a filler would be necessary. Fillers should not be utilized when more than 50% of the coronal tooth structure is missing, as they do not provide the necessary strength for the crown preparation (7).
Buildups
It is appropriate to use a build-up when more than 50% of the coronal tooth structure is gone, or if there is not a 2-3 mm collar of tooth structure left behind on the gingival segment of the tooth preparation (7). The purpose of a buildup is different than that of a filler. A buildup is used when the remaining tooth structure would not provide adequate strength and/or resistance to the tooth preparation. Ideally, the strength of the buildup material should be similar to that of tooth structure.
Posts and Cores
A tooth may need a post and core if it has been endodontically treated and there is less than one half of the coronal tooth structure remaining. This post will serve to attach the core material to the root. If a tooth has been treated endodontically, and significant coronal damage has not occurred, than a post and core should generally not be used, as the tooth still possesses most of its inherent strength (9). An exception to this rule comes when the tooth may have excessive forces placed on it. Such a situation could be a canine in a patient with canine guidance, a bruxing patient, a fixed prosthesis abutment, or the support for a removable prosthetic appliance.
Another situation that calls for the use of a post and core is when there is no remaining coronal tooth structure, and only the root remains. A post and core with anti-rotational elements such as channels or pins should be used in this situation. Crown lengthening will often be used in concert with a post and core on such a tooth.
It should be stressed that a post does not contribute to the strength of the remaining tooth. On the contrary, posts may actually weaken some teeth (8). The purpose of a post is to provide retention to the crown by joining the root to the coronal portion of the preparation.
Obviously, these are basic descriptions and guidelines, not absolute rules. The practitioner must take into consideration on what tooth the crown is to be placed. If the patient is a bruxer, the tooth preparations will require much more strength and resistance than those of nonaggressive chewers (7). Similarly, if the tooth being prepared is to be used as an abutment for a fixed prosthesis or as support of a removable one, maximal strength and resistance are both desirable and a buildup may be indicated, even if more than half of the coronal tooth structure is remaining. The patient's occlusion must be evaluated to determine the amount of forces to be placed on the tooth in question. Also, the final restoration must be taken into consideration. It has to be determined whether the indirect restoration is made of a material that will mask any underlying filler, build-up, or core, and whether a specific type is indicated. The strength of that final restorative material also needs to be evaluated to decide what to place as a foundation.
Materials used as foundations
The optimal foundation material should have physical properties similar to those of tooth structure (15). Several different materials are presently being used as a core buildup on teeth that are severely broken down by excessive wear, caries, or trauma (19). The three materials most often used for direct application are amalgam, composite, and glass-ionomer. The indirect method of a cast dowel and core is also widely used. This method has the highest mean fracture load of any of the materials used for buildups (4). Despite this, the direct methods, now available in response to the vast improvements in bonding over the last few years, have increased popularity because of their reduced chair time and lower cost to the patient. No matter what the material and advances made, all types of fillers, build-ups, and posts and cores are subject wear, breakdown, fractures, loosening, and bond failure.
Amalgam
Core buildups in amalgam have a long history of use in buildups and
offer high compressive strength and ease of manipulation. Other
advantages of amalgam are that it is not especially technique sensitive,
it is strong in bulk section, corrosion products seal it, and it can be
'glued' into place with cements and resins. Disadvantages of amalgam
are the 24 hours of set time before tooth preparation, that it is weak
in thin section, and its mercury content may be of concern to some
patients. There may also be problems with its potential electrolytic
action between core and metal crown, and with its lack of intrinsic
adhesiveness. Recommendations for the use of amalgam are for posterior
core build-up and interim restorations for posterior teeth. Because
amalgam is weak in thin section, it is not recommended for a core
material in anterior teeth (28). Amalgam may also be hard to mask under
esthetics restorations, especially in the anterior.
When comparing the physical properties of amalgam, the compressive strength of amalgam is higher than composites and glass-ionomer. The elastic modulus, the relative stiffness of the material within the elastic range, was significantly higher than all the other materials tested and had the closest value to that of dentin (20 GPa). On the other hand, the tensile strength of amalgam is low due to its brittleness. The flexural strength tests are to analyze how cracks, voids, and other surface flaws can influence the fracture strength of brittle materials, and amalgam tested low in this category. It is interesting to note that if sound tooth structure is considered to be at 100% strength, after restoring the buildup with amalgam the strength was 65% of sound tooth structure. This value was higher than both composites and glass-ionomers. However, after the buildup was prepared for the FGC, the strength went down to 29%, 14% lower than composites (5). Based on this finding, amalgam is good for buildups that do not need immediate preparation. It would be good for those clinical situations that require time between the buildup and the preparation/placement of the final restoration.
Composite
Composites have become widely used by dentists as a direct method of foundation buildups. Reasons for this are their strength, their ability to be used in thinner sections when compared to amalgam, and their fast setting time enabling immediate preparation. Composites are also chosen because they can bond directly to the tooth structure and they avoid the controversy of mercury associated with amalgam. Disadvantages of composites are their high technique sensitivity, increased procedural time, and the importance of absolute isolation. It can also be hard to distinguish the tooth from the composite core during preparation. Because of the bonding techniques associated with their application, they should only be used when contamination can be properly controlled and shrinkage can be minimized as much as possible. It is an excellent material for buildups for anterior and posterior teeth if proper isolation is assured (28).
When comparing the physical properties of composites, the compressive strength of composites was higher at one hour than amalgam, but less after one hour. The pure hybrid composites were stronger in compressive strength than the composites with titanium or ceramic fillers. As far as elastic modulus, the pure hybrid composites were the lowest and the titanium and ceramic filled composites were slightly higher. The tensile strength of composites was significantly higher than amalgam and glass-ionomer with the pure hybrid composite being higher than the filled composites. Flexural strength and flexural modulus for composites were higher than amalgam and glass-ionomer especially when considering the pure hybrid composite (12). When fracture testing was performed, the hybrid composites were second to amalgam with 52% remaining strength after being built up and this was only reduced to 43% after the preparation was completed as compared to a 36% reduction in the amalgam foundation preparation. The two composites with titanium and ceramic fillers had lower values of 34% and 28% strength respectively remaining after preparation but also had little change in their strength values following preparation (5). This shows that there is less reduction in strength when preparing composite buildups compared to the preparation of the amalgam foundation.
A characteristic of composites that can not be overlooked is their low modulus of elasticity. Because composites are so flexible, they are able to flex with the force and cause shearing stresses on the cement interface and the posts themselves. It is an easy way of blaming the cement or the post for the fracture when really it could be the low modulus of elasticity (17).
Glass- Ionomers
In every study that was read, glass ionomers were not recommended as buildup materials based on their weak physical properties. Advantages of using glass-ionomer are its intrinsic adhesive properties, fluoride release, and a similar coefficient of thermal expansion to teeth. The disadvantages are its weakness when compared to amalgam and composites, and its tendency to crack which is worsened by early instrumentation. Silver containing materials are sometimes added to glass-ionomer to improve the strength but that actually offers little improvement in the physical properties. Glass-ionomer works well as a filler but relies on having sufficient dentin to support the crown (28). Glass-ionomers are strong when subjected to compressive forces, but weak in tension and shear (23). The compressive strength was significantly lower than composite and amalgam. The elastic modulus was higher than the hybrid composite but lower than amalgam and the titanium and ceramic-filled composites. The tensile strength was the lowest of the three tested as well. As far as flexural strength and modulus, glass-ionomer was the lowest of the three (12). Because of its weak physical properties, glass-ionomers should not be used as a buildup material and reserved for the use as a filler material (18).
Cast Gold
Cast gold is regarded as the standard when it comes to post and core materials. It offers strength, does not absorb water, and has a coefficient of thermal expansion very close to that of dentin. Its downfalls are its price, the indirect process, and the substantial degree of coronal destruction needed (18). When a post is required and the removal of sufficient coronal tooth structure to allow for cast gold is not significant, cast gold is the best material for a buildup. Its tensile strength is ten times that of amalgam, composite, and glass-ionomer (18).
Types of posts
The primary function of a post is to provide retention for a core, which replaces lost coronal tooth structure and retains the final restoration. There are many different types of posts that can be used. A recent study published in CRA news reviewed the advantages and disadvantages of various post types (13).
When considering the points listed above, it is important to recognize that all posts perform well in some areas and selection should be dictated by patient's needs and what material the dentist is most proficient in using. It is also important to note again that the cast gold post and core, which was not compared in this study, has been regarded as the "gold standard" in post-and-core restorations due to its superior success rate (3,14). Heydecke et al., compared the fracture strength of zirconia posts with composite or ceramic cores and titanium posts with composite cores to the fracture strength of gold posts and cores after dynamic loading (16). No significant differences were detected among the groups. The use of zirconia posts resulted in a lower number of catastrophic root fractures. However, this was not statistically significant. The results of the study led the authors to suggest that zirconia posts with ceramic cores can be recommended as an alternative to cast posts and cores. Further, if a chairside procedure is preferred, zirconia or titanium posts with composite cores can be used (16).
With respect to all other posts, ceramic posts are the strongest, however this material was also shown to have the most fractures under extreme force whereas no other posts fractured or bent visibility (6). This material also has the highest cost. When comparing strength, metal posts (pure titanium, titanium alloy and stainless steel) perform best, but this material is more likely to cause to fracture of the tooth under load than non-metal posts (13). In fact, metal posts tend to fracture the root more often than non-metal post types, except for the carbon/quartz matrix type (47% and 60% relative overall fracture) (13). In addition, metal posts are contraindicated for people with metal allergies. As an alternative, carbon based posts should be considered as the strength of this material is adequate. The use of tooth colored posts should be considered when esthetics are a primary consideration.
The strength of the post and the type of core buildup material are important factors in the overall success of a restoration. Another key element is retention of the post in the canal. (6-10) Factors affecting retention include whether or not the post is bonded or threaded and the length and diameter of the post. Research has shown that the most retentive posts are threaded, parallel-sided posts screwed into tapped canals; while serrated, parallel-sided, cemented posts provided intermediate retention, and smooth-sided tapered posts were the least retentive (26). Overall, the length of the post has a significant effect on its retention. The more apically the post is placed in the root canal, the more retentive it becomes (26,10).
Benefits of a tapered post include preservation of tooth substance in the fragile apical area and the advantageous clinical situations of conforming to the root and canal configuration of endodontically treated teeth. There are ways to increase the retention of tapered posts (21). As with all posts, for tapered, unthreaded posts, it has been shown that retention is affected strongly by an increase in the length (approximately 100%) of the post more than by an increase in the diameter (approximately 60%) (20). When considering cements, it appears that the effect of cement type is only significant with tapered posts. Zinc phosphate cement has been shown to be the most retentive, carboxylate cement exhibits intermediate retention, and the epoxy cement is least retentive (26). For the other post designs, cement type appears to have no significant effect on retentive capacity. Further, Nergiz et al. have demonstrated that when a tapered post is used, roughening the dentin canal wall, as well as sandblasting and grooving the post, can provide statistically significant additional resistance to dislodgment (21). Under extreme force, it has been shown that all post types experience a fracture of the core buildup from the post to some degree.
Summary
Our summary of the literature has given you a lot of information on many different materials used as the foundation for an indirect restoration. Out of all this information, there are a few take home messages we would like to leave you with.
One obvious finding in our research is that glass-ionomers should be used on a very limited basis for core buildups and be saved for use as a filler material. The physical properties of glass-ionomers are too weak to support the tooth and the crown when used as a buildup material.
Amalgam is effective for buildups that will be functioning in the mouth for a longer length of time without the final restoration in place. Its speed of placement, less sensitivity to moisture contamination, strength, and long track record of success makes it a good choice in the posterior.
Composite is a good material to use for buildups in both the posterior and the anterior. If adequate isolation is obtained, the ability of preparing the tooth in the same appointment is a nice advantage of composites. Composites are a more dependable option when the material must be used in thinner sections and when esthetics is a concern. Composites are easier to mask than amalgam underneath the final restorations. They have good physical properties for buildups other than their low elastic modulus. Bonding and isolation need to be closely monitored when using this material. A reliable post and cement system are also necessary when using composites as a core buildup because it places the forces onto the post and cement and may result in failure.
All the different types of posts have advantages and disadvantages. However, if there is adequate tooth structure remaining after creating the post space, cast gold post and cores are still regarded by most to be the "gold standard". The physical properties give them strength, durability, and compatibility with the oral environment. If chairside procedures are required, zirconia or titanium posts with composite cores are a good option. These tooth-colored options are also necessary when esthetics is a concern. As far as the general properties go, metal posts are stronger but tend to fracture the root more while nonmetal are not as strong but tend to fracture the root less. Retention has been found to increase with greater length of the posts. Threaded, parallel-sided posts screwed into tapped canals are the most retentive type of post. Cement types only appear to be significant when using tapered posts with zinc phosphate cement being the most retentive. Other factors such as patient allergies and the clinicians comfort level when using the materials always need to be considered.
Using the specific material properties mentioned in the paper we have given some logical opinions of what materials may be better for anterior vs. posterior restorations, those exposed to little vs. much force, those that need to be prepped immediately or those that must have time before placement of the final restoration, etc. However, you are the doctors. Knowing the basic properties and functions of the materials will allow you to help to make the decision of which material to use in each clinical situation and subsequently inform the patient. You will see variations in the patients' needs, the immediate environment, the amount of tooth structure left, and the esthetic and functional needs in different clinical cases. Keeping these basic guidelines in mind you will choose what is best for that particular situation that day and use the one that works best for you and your patient. Just remember that there will be "new and improved" materials and methods coming out all the time and it is up to you to keep up on the literature and use this basis to evaluate the materials and methods of the future.
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