Teaching the Use of Liners, Bases, and Cements: A 10-Year Follow-Up Survey of North American Dental Schools

Restorative dentistry constantly undergoes change, driven in part by new clinical applications of existing dental materials or the introduction of new materials. For example, the manufacturers of liners, bases, and cements often recommend new or modified techniques for use of their products.
Some currently available products and materials used as liners, bases, and cements were not available 10 years ago. For most clinicians, dental school provided the first introduction to these materials, and some clinicians continue to use the materials first introduced when they were in school.
Cavity liners are aqueous or volatile organic liquid dispersions of zinc oxide or calcium hydroxide that are thinly applied to the cavity surface. They usually do not have sufficient thickness, hardness, or strength to be used alone in a deep cavity.1 Liners are typically 0.5 mm thick and serve to seal the dentin on the floor and walls of the cavity against the influx of bacteria and any irritants associated with restorative materials and procedures.2,3
For deep cavities, dental schools historically taught that a base should be applied after the liner and prior to the restoration. A base used beneath the permanent restoration promotes recovery of the injured pulp and protects the pulp from thermal shock, mechanical trauma, toxic ingredients of the restorative material, galvanic shock, and microleakage.1 Further, bases can be shaped and contoured to specific forms.2
In 1994, Cox and Suzuki suggested that dentists should re-evaluate the liners and bases that they were using.4 A 1995 survey of dental schools (published in 1996) concluded that dental schools in North America do not agree on which liner or base material to use and when to use it.5
Cements serve to retain restorations or appliances in a fixed position in the mouth.6 A dental cement, also known as a luting agent, acts as a barrier against microleakage by sealing the interface between the tooth and restoration and holding them together via an attachment. This attachment can be mechanical, chemical, or a combination of the two.7 Some dental cements can be considered as temporary, short term, or weak, while others can be characterized as permanent, long term, final, or strong. Dental cements are only indicated when dealing with fixed prostheses.
This paper reports the results of a 2005 survey of dental schools in North America regarding the use of specific liners, bases, and cements that are taught for use in different clinical situations.

METHODS AND MATERIALS

In October 2005, the chairpersons of restorative dentistry departments of the 66 dental schools in the United States, Canada, and Puerto Rico received by mail a 3-page questionnaire. The packet included a cover letter that explained the survey objectives, the survey instrument, and a self-addressed return envelope. Participants were asked to complete and return the questionnaire within a 3-week period. A second mailing (with a copy of the survey included) was sent to chairpersons who had not responded to the first mailing.
Questions focused on which dental materials were taught (for the 2005-2006 academic year) in 4 distinct clinical scenarios: 1) a class II preparation within 1 mm of the pulp to be restored with amalgam; 2) a class II preparation more than 2 mm from the pulp to be restored with amalgam; 3) a class II preparation within 1 mm of the pulp to be restored with composite; and 4) a class II preparation more than 2  mm from the pulp to be restored with composite. All scenarios reflected the initial restoration of the tooth. Respondents were also asked to report the incidence of postoperative sensitivity with their current protocol. The depths of these hypothetical cavity preparations mirrored what was in the original survey, but composite restoration scenarios were not included in the 1995 survey. As a new element to the 2005 survey, dental schools were also asked about the cementation protocol taught for specific indirect restorations and any associated postoperative sensitivity. 
As in the 1995 survey, the current survey included related questions focused on cost considerations, satisfaction with the present protocol, and frequency of and responsibility for protocol review. The Statistical Package for the Social Sciences, Version 10.0 (SPSS), was used to analyze the findings and to determine any statistically significant findings.

RESULTS

 

Thirty-nine of the 66 schools responded, yielding a 59% response rate. In the 10 years since the first survey, one school had closed and three others opened (telephone conversation with the ADA Council on Dental Education, February 10, 2006). Participation in the survey was anonymous; all data are reported in the aggregate. Not every respondent answered every question. The survey gave the respondents 12 options from which to choose, including "other" (asked to specify) and ìnothingî (Table 1). Respondents interested in receiving a summary of the findings provided contact information. As in the 1995 survey, when more than one material was indicated, the first material that was listed was used to score that response.

Liners

 

Table 2 provides the most popular liners and bases taught at dental schools. In the first scenario (deep preparation with amalgam restoration), calcium hydroxide (38.5%) followed by glass ionomer (30.8%) were the most commonly taught liners. For shallow cavity preparations to be restored with amalgam, schools most often teach students to place no liner (23.1%), followed by copal varnish and glass ionomer (20.5% each).
For composite restorations in a deep cavity, the liners most frequently used were glass ionomer (35.9%) followed by calcium hydroxide (28.2%). For shallow cavity preparations restored with composite, just under half the schools (48.7%) report teaching the use of dentin bonding agents. Glass ionomer was the second most popular material (28.2%).
"Other" liners mentioned by respondents included calcium oxalate precipitate, Gluma (Hereaus Kulzer), a resin-modified glass ionomer, and what one school described as a homemade Gluma-like product.

Bases

Glass ionomer represents the most popular base under amalgam or composite in a deep cavity (56.4% and 41%, respectively). For both restorative materials in shallow preparations, schools most often teach students not to use a base (Table 3). The respondents did not indicate what, if any, other materials they use, other than what is listed in Table 1.

Cements

The survey asked the respondents which cement they were teaching for use with 9 indirect restorations (Table 4). The most frequently taught cements are prefabricated post: glass ionomers and resin (30.8% each); cast post and core: glass ionomers (38.5%); PFM crown (vital tooth): glass ionomers (64.1%); PFM crown (nonvital tooth): glass ionomers (61.5%); all-ceramic crown (anterior): resin cement (82.1%); all-ceramic crown (posterior): resin cement (76.9%); porcelain veneer: resin cement (92.3%). For provisional crowns (both vital and nonvital teeth), the most common material is a noneugenol zinc oxide product (53.8% and 57.9%, respectively).

Postoperative Sensitivity

With respect to liners and bases, only one school (from Canada) reported that more than 30% of patients experience some degree of postoperative sensitivity. Thirteen schools noted that they do not keep track of that information. All other responding schools state that less than 20% of patients suffer from postoperative sensitivity.

Satisfaction With Protocol

Respondents rated their satisfaction with their schoolís choice of liner and base on a scale from 1 (not satisfied at all) to 5 (extremely satisfied). In general, respondents were very satisfied with both the liner and base protocol used at their respective schools (mean of 4.62).  Overall satisfaction with current cementing practices was also high (mean of 4.50).

Other Factors

Cost did not appear to impact a schoolís decisions regarding the use of liners, bases, and cements. The overall importance of cost was low (mean is 2.76 measured on a 5-point scale where 1 was not at all important and 5 was extremely important). Just under two thirds (61.5%) of the schools review their liner and base protocol every year, and 25.6% do so every third year or less frequently. Cementation protocol is reviewed on a similar schedule. Fifty-nine percent of the schools review it every year, and about 10 percent (10.3%) of the responding schools review their cementation protocol every other year.
Consistent with the findings in the 1995 survey, at most schools protocol review involves a group of faculty rather than a single individual.
           
DISCUSSION

In September of 1995, a survey of all North American dentals schools examined teaching preferences and practices with respect to liners and bases.5 That study surveyed the application of liners and bases under amalgam restorations. Results indicated that teaching institutions could not agree on which material is best in a given clinical situation. This supports Christensenís view that the subject of bases and liners is confusing. Current use varies enormously, many different procedures are successful, and unanimity of opinion is not likely to be achieved.8
The current report sought to determine if North American dental schools now agree on a particular protocol with respect to liners and bases. In addition, since versions of some of these materials can also be used as a cement, this study was expanded to evaluate the cementation protocols taught at dental schools.
In the mid 1990s, for a deep cavity that was to be restored with an amalgam, 46% of the schools advocated the use of a glass ionomer as a liner followed by 25% for calcium hydroxide. Today, the results are nearly reversed, with calcium hydroxide being a more popular liner. One school noted discontinuance of calcium hydroxide in favor of mineral trioxide aggregate.
In 1995, none of the responding dental schools placed dentin bonding agents under amalgams in deep restorations. Today, the situation has changed, as approximately 10% of schools now teach that technique for both deep and shallow preparations that will be restored with amalgam. This is in contrast to composite restorations, where more schools use a dentin bonding agent as the liner (more than 25% in a deep cavity preparation and almost 50% in a shallow cavity preparation). Perhaps this is due to dentin bonding agents being part of the technique for placing the composite. Further contributing to the growing popularity of dentin bonding agents is the claim that the use of this material reduces postoperative sensitivity.9
Copal varnish, once considered the preferred material to seal dentinal tubules and reduce postoperative sensitivity, is now taught by less than 3% of dental schools as the first material placed in a deep cavity preparation that will receive an amalgam restoration. This is compared to 12% just 10 years ago. One dental school noted that they no longer teach the use of a copal varnish as a liner, based on research conducted at that school.
Depending on the clinical scenario, more than one third of dental schools still do not advocate the use of any basing material. However, when a base is used (deep preparations), glass ionomer emerges as the material of choice.
In 1995, 78% of the responding schools reported that less than 20% of their patients suffered from postoperative sensitivity after receiving a deep amalgam restoration. Today, 96% of the schools do so, which means that postoperative sensitivity is less of a problem. The same pattern follows for shallow amalgam restorations, when 10 years ago 73% of the responding schools reported that less than 20% of their patients reported sensitivity, compared to 96% of schools responding to the 2005 survey.
As previously mentioned, the 1995 survey did not offer scenarios that included composite restorations. The placement of composite restorations is now more common, so this aspect of restorative dentistry was included in the 2005 survey. The 2005 survey indicated that only 17% of the responding schools (n=24) report more than 20% of their patients having postoperative sensitivity after receiving a deep composite restoration. For those having a shallow composite restoration, 92% of the schools (n=24) reported less than 20% of patients with postoperative sensitivity.
With respect to final cementation, glass ionomer cements were the most popular material for cementation of a cast post and core restoration. Resin and glass ionomer cements are the preferred luting agents for direct post and cores. With respect to a final cementing material and postoperative sensitivity associated with vital teeth, and using the same 20% criterion as was used for liners, only one school reported a problem. This was associated with all-ceramic crowns for anterior teeth, and that school was using a resin cement. The same school reported that 20% of its patients experience postoperative sensitivity when zinc phosphate was used in conjunction with porcelain-fused-to-metal crowns.
Further analysis did not identify any particular material that was associated with either a high or low rate of postoperative sensitivity. It is also conceivable that postoperative sensitivity that occurs after both provisional and final cementation is unrelated to the cement used. Other causes could include open margins, the teeth involved being in hyperocclusion, and gingival irritation. The survey did not ask, nor did any respondents state, how long the sensitivity lasted.
For provisional cementation, products that do not contain eugenol are the most popular. Tooth vitality is not an important variable here. It is reasonable to ask if there is any relation between this practice and the belief that noneugenol materials do not interfere with resin-based final cements.10,11 One school stated that 30% of patients claim to have postoperative sensitivity when using a noneugenol cement for vital teeth. No reason was provided.
It is interesting to note that none of the schools are teaching the use of polycarboxylate-based materials. This is curious, since polycarboxylates are considered to be very biocompatible. The polyacrylate acid molecule is thought to be too large to enter the dentinal tubules.2
In 1995, the mean for the importance of the cost of materials was 2.36, which is lower than reported in the 2005 survey (2.76). The relatively low importance of cost may be related to the fact that those responsible for choosing the material(s) may not need to consider cost, as this is the responsibility of someone else, or may reflect the reduced cost offered to schools by manufacturers.
The degree of satisfaction that dental schools report with liners and bases is higher (4.62) than it was 10 years ago (4.11). This may be due to the fact that improved materials and techniques are currently available; there are now more options for clinicians. Additionally, clinicians may have a better understanding of the dynamic interaction of these materials at the dentin and pulpal levels.
Although there are no published articles reporting on surveys of private practitioners regarding the use of liners, bases, and cements, one could hypothesize that the materials and techniques taught in dental schools are being used by these graduates when they enter private practice. A survey of private practitioners is needed to determine if current clinical practice mirrors the findings of the 2005 dental school survey.

CONCLUSION

Consistent with findings a decade earlier, there is no consensus among dental schools in regard to the best materials to use as liners, bases, and cements. Dental schools report high satisfaction with their existing protocols. Since no single protocol is taught by all dental schools, clinicians must strongly consider clinical success once in practice as their primary criterion for choosing specific materials for different clinical situations. Careful reading of the dental materials literature is also strongly recommended.


References

1. Draheim RN. Cavity bases, liners and varnishes: a clinical perspective. Am J Dent. 1988;1:63-66.

2. Baum L, Phillips RW, Lund MR. Textbook of Operative Dentistry. 3rd ed. Philadelphia, Pa: WB Saunders; 1995:Chapter 7.

3. Ferracane JL. Materials in Dentistry: Principles and Applications. 2nd ed. New York, NY: Lippincott Williams & Wilkins; 2001:Chapter 4.

4. Cox CF, Suzuki S. Re-evaluating pulp protection: calcium hydroxide liners vs. cohesive hybridization. J Am Dent Assoc. 1994;125:823-831.

5. Weiner RS, Weiner LK, Kugel G. Teaching the use of bases and liners: a survey of North American dental schools. J Am Dent Assoc. 1996;127:1640-1645.

6. Craig RG, Powers JM. Restorative Dental Materials. 11th ed. St Louis, Mo: Mosby; 2001:Chapter 20.

7. Diaz-Arnold AM, Vargas MA, Haselton DR. Current status of luting agents for fixed prosthodontics. J Prosthet Dent. 1999;81:135-141.

8. Christensen GJ. To base or not to base? J Am Dent Assoc. 1991;122:61-62.

9. Farah J, Powers J. Self-etching bonding agents. The Dental Advisor. 2003;20:1-4.

10. Leirskar J, Nordbo H. The effect of zinc oxide-eugenol on the shear bond strength of a commonly used bonding system. Endod Dent Traumatol. 2000;16:265-268.

11. Peutzfeldt A, Asmussen E. Influence of eugenol-containing temporary cement on efficacy of dentin-bonding systems. Eur J Oral Sci. 1999;107:65-69.


Dr. Weiner received his DMD degree from Tufts University in 1986. He is a fellow of the Academy of General Dentistry, the American College of Dentists, and the Pierre Fauchard Academy. He has written many articles and presented numerous lectures on the topic of liners, bases, and cements. He maintains a private practice in family and cosmetic dentistry in Millis, Mass, and can be reached at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .