Clinical acceptance of single-unit crowns and its association with impression and tissue displacement techniques: Findings from the National Dental Practice-Based Research Network

Abstract

Statement of problem.

The definitive impression for a single-unit crown involves many material and technique factors that may affect the success of the crown.

Purpose.

The purpose of this prospective cohort study was to determine whether impression technique (tray selection), impression material, or tissue displacement technique are associated with the clinical acceptability of the crown (CAC).

Material and methods.

Dentists in the National Dental Practice-Based Research Network (N=205) documented details of the preparation, impression, and delivery of 3730 consecutive single-unit crowns. Mixed-model logistic regression analyses were used to evaluate associations between impression techniques and materials and the CAC and to assess associations between the presence of a subgingival margin with the displacement technique and the outcome variables CAC and number of impressions required.

Results.

Of the 3730 crowns, 3589 (96.2%) were deemed clinically acceptable. A significant difference in the CAC was found with different impression techniques (P<001) and different impression materials (P<.001) The percentage of the CAC for digital scans was 99.5%, 95.8% for dual-arch trays, 95.2% for quadrant trays, and 94.0% for complete-arch impression trays. Although no statistically significant difference was found in the CAC produced with dual-arch trays without both mesial and distal contacts, crowns fabricated under these conditions were less likely to achieve excellent occlusion. The percentage of the CAC for digital scans was 99.5%, 97.0% for polyether impressions, 95.5% for polyvinyl siloxane impressions, and 90.5% for other impression materials. Accounting for the location of the margin, the use of a dual-cord displacement technique was significantly associated with lower rates of requiring more than 1 impression (P=.015, OR=1.43).

Conclusions.

Dual-arch trays produced clinically acceptable crowns; however, if the prepared tooth was unbounded, the occlusal fit was more likely to have been compromised. Digital scans produced a slightly higher rate of CAC than conventional impression materials. The use of a dual-cord technique was associated with a decreased need to remake impressions when the margins were subgingival.

INTRODUCTION

Single-unit crowns have proven to be a reliable restorative treatment, with estimated 5-year survival rates of 95.7% (metal-ceramic), 93.8% (zirconia), and 96.6% (lithium disilicate and leucite).¹ In order to achieve this successful outcome, many steps are required to produce a crown that is deemed acceptable at the time of delivery. From the perspective of the clinician, proper tooth preparation and accurate impressions are perhaps the most critical. In a recent study of crown and fixed partial denture impressions sent to 4 commercial laboratories, 86% of impressions contained at least 1 detectable error, and 55% of impressions contained a critical error related to the finish line.² The errors created while making an impression can be related to tray selection, material choice, and tissue displacement.

Broadly, 2 different tray types can be used for single crown impressions: single- and dual-arch trays. Complete single-arch trays can record the complete dentition in a rigid mold which aids the articulation of casts. As these trays capture the teeth on the side of the mouth contralateral to the crown preparation, occlusal interferences may be avoided. Partial single-arch trays may also be used, but this technique presents challenges in mounting.³ Dual-arch trays (closed mouth technique) are faster to use and more comfortable for the patient and use less material than complete-arch trays.⁴ Additionally, a dual-arch tray may more accurately capture the relationship of the arches in maximum intercuspation without the errors introduced by the external articulation of opposing casts.⁵ However, the flexure of the dual-arch tray that may occur during closing from inadvertent contact with the alveolus can distort an impression.⁶ A 2009 survey of impressions of fixed restorations sent to a commercial dental laboratory observed that over 70% were made with a dual-arch impression tray. Of the dual-arch trays, only 55.7% recorded a single unit bounded by intact teeth that was opposing an intact tooth.⁷ A 2013 survey of impressions of fixed restorations sent to 4 different commercial laboratories discovered that 62% of impressions were made with a dual-arch tray and many violated the requirements for their use. Of these dual-arch impressions, 26% did not record an unprepared tooth anterior and posterior to the preparation, 14% did not register canine teeth, and 9% were not made in maximum intercuspation.² There is little clinical evidence of the effects of violating the requirements for the use of a dual-arch tray.

Polyvinyl siloxane (PVS) impression materials are the most commonly used type of material for obtaining a definitive impression for fixed restorations.⁸ Polyether impression materials have similar properties to those of PVS materials, but they are differentiated by a higher flow,⁹ inherent hydrophilicity,¹⁰ and lower tear strength.¹¹ In a clinical trial conducted with dental students, impressions made with polyether and PVS materials resulted in similar success rates when used with complete-arch trays, but PVS materials demonstrated a higher success rate when used with dual-arch trays.¹² The most common operator error for both types of impressions was inadequate gingival displacement. Digital scanning techniques have been reported for 12% of definitive impressions.⁸ Recent reviews have reported that digital scans produce similar^13–15 or better¹⁶ marginal fit than conventional impressions.

Several methods of displacing gingival tissue can be used alone or in combination before making a definitive impression, including displacement cord, injectable displacement pastes, and soft tissue lasers. A previous survey reported that 92% of dentists used displacement cords and 20.2% used lasers as an adjunct.¹⁷ Another recent survey reported that a dual-cord technique was used by 35% of respondents, single-cord technique by 35%, and an injectable displacement technique by 16%.⁷ One clinical evaluation determined that an injectable displacement paste created more horizontal displacement of the sulcus (0.26 ±0.02 mm) than a displacement cord (0.21 ±0.01 mm),¹⁸ whereas another trial reported that cord produced more displacement (0.74 mm) than 2 types of paste (0.48 and 0.41 mm).¹⁹ A clinical trial determined that impregnated displacement cord, displacement paste, or both used in combination produced perfect or acceptable impressions. Nonimpregnated displacement cord produced a lower quality of impressions.²⁰ Displacement may be particularly important in crown preparations with subgingival margins. A clinical study reported that margins located more than 2 mm below the gingival tissue led to inferior impression quality.²¹ A possible disadvantage of displacement cord is that a recent clinical trial reported that the use of a double-cord technique led to a higher rate (32%) of minor gingival recession (0.1 to 0.5 mm) than the use of an injectable displacement paste (8%).²²

A previous questionnaire study surveyed dentists in the National Dental Practice-Based Research Network regarding impression material and technique preference.⁸ Additionally, previous studies have examined errors detected in definitive impressions related to material and technique choice.^2,7 Ultimately, the clinician should be informed which aspects of impression making affect the outcome of the definitive crown. The purpose of this study was to determine whether impression technique (tray selection) is associated with clinical acceptability of the crown (and more specifically whether the presence of mesial and distal contacts is associated with crown acceptability and occlusal fit with the use of dual-arch trays); whether the choice of impression material is significantly associated with the ultimate clinical acceptability; and whether the displacement technique is associated with clinical acceptability or the number of impressions required to obtain an acceptable impression, while accounting for the presence of a subgingival margin. The null hypotheses were that impression technique (tray selection) would not be associated with clinical acceptability of the crown (and more specifically that the presence of mesial and distal contacts would not be associated with crown acceptability and occlusal fit with the use of dual-arch trays); whether the choice of impression material would not be significantly associated with the ultimate clinical acceptability of the crown; and whether the displacement technique would not be associated with clinical acceptability of the crown or the number of impressions required to obtain an acceptable impression, while accounting for the presence of a subgingival margin.

MATERIAL AND METHODS

This study was conducted by dentists in the National Dental Practice-Based Research Network (PBRN; “network”). The network is a consortium of dental practices and dental organizations focused on improving the scientific basis for clinical decision-making.²³ This network study comprised a prospective cohort study focused on the acceptability of crowns made in routine clinical practice. Network Regional Coordinators (RCs) were asked to recruit 200 dentists to participate in this study from within the network. Data were collected about each practitioner using the network’s Enrollment Questionnaire. This questionnaire is publicly available at http://nationaldentalpbrn.org/study-results/factors-for-successful-crowns.php. Questionnaire items documenting test and retest reliability were taken from previous practice-based studies of dental care.^24,25 Dentists enrolled in the network were eligible for this study if they met all of these criteria: completed an Enrollment Questionnaire; were currently practicing and treating patients in the United States; were in the network’s “full” network participation category; completed the Stage 1 questionnaire²⁶; and reported doing at least 7 crowns in a typical month. Practitioners were required to complete human subjects training and secure the participation of at least 1 dental laboratory for the technical evaluation of the crown preparations done in the study.

The network’s applicable Institutional Review Boards approved the study, and all participants provided informed consent. The study was launched March 1, 2016; clinician training was completed by August 15, 2016; all patients had been enrolled by December 31, 2016; and patient follow-up was closed on February 28, 2017. Once they had agreed to participate, dentists were trained in the proper conduct of the study protocol by RCs. Once a clinician began the study, he or she was asked to complete patient enrollment within 3 months. Each clinician’s goal for enrollment was 20 patients. Dentists or their practices were remunerated $50 for conducting informed consent, enrolling the patient, completing the study’s data forms, and $25 for completing the insertion visit data forms and communicating regularly with the RC about any data monitoring issues. All data forms are publicly available at http://nationaldentalpbrn.org/study-results/factors-for-successful-crowns.php.

Clinicians recruited patients from among their current patients needing a single-unit crown. Clinicians were asked to recruit patients consecutively and to record the number of patients recruited for the study and those who declined, and if they declined, why. To be eligible, patients had to meet the following criteria: 18 years old or older; able to provide informed consent; and in need of a single-unit crown on a natural tooth. Abutments for fixed partial dentures were not permitted in this study, nor were patients who needed multiple single-unit crowns done in a single appointment. This was done to eliminate the possibility that providing multiple units might affect the impression or ultimate clinical acceptability of the crown at cementation.

Once enrolled in the study, the clinicians prepared the tooth for a crown and completed a crown preparation data form regarding clinical aspects of the procedure, including impression technique (quadrant tray, complete-arch tray, dual-arch tray, or digital scan), impression material (PVS, polyether, digital scan, or other), and tissue displacement technique (none, single cord, dual cord, injectable tissue displacement material, dual impression technique, gingival troughing, tissue displacement cap, and/or other). Clinicians were asked if the tooth to be crowned had a mesial contact and/or a distal contact, or no proximal contact. Clinicians were asked to estimate the deepest margin of the preparation (above the crest of the gingival tissue, at the crest of the gingival tissue, 1 mm below the gingival tissue, 2 mm below the gingival tissue, or 3 mm or more below the gingival tissue). Finally, clinicians were asked the number of impressions required to obtain an acceptable definitive impression. Clinicians were asked to place the crown within 6 weeks of preparation.

At the time of insertion, clinicians were asked to evaluate the crown clinically and report this information on a second insertion data form. The primary outcome for this study was clinical acceptability of the crown as judged by the treating clinician. A secondary outcome measured was an assessment of the occlusion of the crown after adjustments (excellent, good, or acceptable). Another secondary outcome was the number of impressions required to obtain acceptability (1 or more than 1).

Overall percentages were calculated as descriptive statistics across all practitioners. The associations between impression technique, clinical acceptability of the crown, and impression material and clinical acceptability were evaluated using mixed-model logistic regression analysis (α=.05). A random effect term was included to account for correlation due to clustering of patients within practitioners. Because crowns and patients were clustered within practitioners, outcomes from patients treated by the same practitioner are not independent. The crown preparations impressed with a dual-arch tray were categorized as having both a mesial and distal contact and not having both contacts (either mesial contact only, distal contact only, or no proximal contact). Clinical scenarios with both mesial and distal contacts were compared with those without both contacts for the outcomes clinical acceptability and occlusal assessment using mixed-model logistic regression analysis (α=.05). Occlusal assessment was collapsed into 2 groups, excellent and not excellent (good or acceptable).

Mixed-model logistic regression analyses were used to evaluate multivariable associations between the presence of a subgingival margin and tissue displacement technique and the outcome variables, clinically acceptable crown and number of impressions. Models using subgingival margin as the sole predictor variable were evaluated, followed by models for each tissue displacement technique, accounting for the effect of margin location. The comparison group for each tissue displacement technique included all observations for which the tissue displacement technique currently being evaluated was not checked. In other words, these analyses compared observations in which a particular tissue displacement technique was checked versus all observations for which that technique was not checked, including those for which no technique was checked. The presence of a subgingival margin was defined by categorizing margins located 1 mm below the gingival tissue, 2 mm below the gingival tissue, or 3 mm or more below the gingival tissue. Adjusted odds ratios (OR) were calculated, accounting for the effects of clustering and of margin location, in the analysis of tissue displacement techniques.

RESULTS

A total of 205 dentists recruited at least 1 patient and crown into this study. The characteristics of these dentists are presented in Table 1. Most were male (73%) and the owner of a private practice (76%). These dentists represented each of the network’s 6 regions. A large majority work full-time. Two dentists were prosthodontists; the rest reported being general practitioners. The majority had been in practice for over 20 years, and 28 clinicians reported practicing less than 10 years.

Table 1.

Characteristics of participating dentists

Characteristics	Number1 (n=205)	Percent (%)
Sex
Male	148	73
Female	54	27
Years Since Dental School
Graduation	28	14
<10	54	26
10-19	42	21
20-29	80	39
30+
Type of Practice
Owner of Private Practice	153	76
Associate in Private Practice	22	11
Health Partners2	8	4
Permanente2	8	4
Other	9	5
Network Region3
Western	28	14
Midwest	34	17
Southwest	39	19
South Central	46	23
South Atlantic	32	16
Northeast	25	12
Time Commitment
Full time	179	89
Part time (<32 hours)	23	11
Race
White	165	81
Black/African-American	11	5
Asian	19	9
Other	8	4
Ethnicity
Hispanic	7	4
Non-Hispanic	192	96
Private Insurance Status
<40% Private Insurance	3	2
40-79% Private Insurance	87	44
80%+ Private Insurance	107	54
Practice Location Type
Urban/ Inner City	27	13
Urban (not inner city)	55	27
Suburban	96	47
Rural	25	12

¹Due to missing values, not all columns add to 100%.

²Either HealthPartners Dental Group in greater Minneapolis, MN or Permanente Dental Associates in greater Portland, OR.

³Reported on Enrollment Questionnaire as state, subsequently categorized into 1 of the 6 regions of network.

Of the 3883 patients who were approached to participate in the trial, 3806 (98%) provided informed consent. Of the 3730 patients who had crowns which were evaluated for clinical acceptability, slightly more were females than males (56% versus 44%). Most of these patients were white (87%), and the remainder of the patients were African American (7%), American Indian or Alaska Native (3%), Asian (1%), Pacific Islander (<1%) or multiple races (1%). Most of the patients (83%) had at least some college or associate degree education. Most had some form of dental insurance, although 20% reported no insurance. The average age of the patients was 55 ±15 years old, with a range of between18 and 100 years old.

A total of 3589 of the 3730 crowns in this study were deemed clinically acceptable for cementation, a 96.2% acceptance rate. Of the impressions made for these crowns, 87.0% were deemed acceptable at the first attempt. At the time of impression, 58.9% of the crown preparations had subgingival margins, and 46.5% of the preparations had some bleeding during the impression. The percentage and number of overall crowns produced with each type of impression technique, impression material, and tissue displacement technique are presented in Table 2.

Table 2.

Percentage and number of crowns produced by type of impression technique used to produce crown, impression material used, and tissue displacement technique used (N=3730)

	Crowns, % (n)
Impression technique
Dual-arch tray	62.0 (2311)
Quadrant tray	16.9 (629)
Digital scan	16.3 (606)
Complete-arch tray	4.9 (184)
Impression material
PVS	71.9 (2681)
Digital scan	16.2 (605)
Polyether	10.8 (402)
Other	1.1 (42)
Tissue displacement technique
Single cord	33.8 (1262)
Dual cord	31.0 (1158)
Injectable tissue displacement material	18.7 (698)
None	11.6 (434)
Tissue displacement cap	10.1 (378)
Dual impression	8.2 (304)
Gingival troughing	6.2 (231)
Other	4.5 (167)

Note that multiple selections were possible for tissue displacement technique, so percentages total over 100%.

The number and percentage of clinically acceptable crowns for each type of impression technique used are presented in Table 3. A statistically significant difference was found in the clinical acceptability of crowns produced with different impression techniques (P<.001). The percentage of clinically acceptable crowns for digital scans was 99.5%, 95.8% for dual-arch trays, 95.2% for quadrant trays, and 94.0% for complete-arch impression trays.

Table 3.

Percentage and number of clinically unacceptable crowns by type of impression technique used (N=3730)

Impression technique	Clinically acceptable crowns, % (n/N)
Digital scan	99.5 (603/606)a
Dual-arch tray	95.8 (2214/2311)b
Quadrant tray	95.2 (599/623)b
Complete-arch tray	94.0 (173/184)b

P<.001, Mixed-model logistic regression accounting for clustering by clinician.

Means with different letter superscripts differ significantly, P<.05, by Tukey test.

Among the 2311 crowns with a dual-arch tray, 1589 indicated that both mesial and distal contacts were present. Of the records in which a dual-arch tray was used to record a tooth without mesial and distal contacts, 96.5% of the crowns were deemed clinically acceptable, and 95.5% of crowns were clinically acceptable in clinical scenarios with mesial and distal contacts. Having both contacts present was not significantly associated with the clinical acceptability of the crown (P=.218). Of the records in which a dual-arch tray was used to record a tooth without mesial and distal contacts, 57.9% of the crowns reported an excellent occlusal fit, which was significantly lower than the 66.2% of crowns with mesial and distal contacts with excellent occlusal fit (P=.001).

The number and percentage of clinically acceptable crowns for each type of impression material used are presented in Table 4. A statistically significant difference was found in the percentages of clinical acceptability of crowns produced with different impression materials (P<.001). The percentage of clinically acceptable crowns for digital scans was 99.5%, 97.0% for polyether impressions, 95.5% for PVS impressions, and 90.5% for other impression materials. For 41 of the 42 crowns in which “other” was selected as the impression material, a vinyl polyether silicone (VPES) material was used.

Table 4.

Percentage and number of clinically acceptable crowns for each type of impression material used (N= 3730)

Impression material	Clinically acceptable crowns, % (n/N)
Digital scan	99.5 (602/605) a
Polyether	97.0 (390/402) a,b
PVS	95.5 (2559/2681) b
Other	90.5 (38/42) c

P<.001, Mixed-model logistic regression accounting for clustering by clinician.

Means with different letter superscripts differ significantly, P<.05, by Tukey test.

The number and percentage of clinically acceptable crowns for each type of tissue displacement technique are presented in Table 5. Crowns impressed without the use of a tissue displacement material or technique showed higher rates of acceptable crowns, with unadjusted acceptance rates of 98.4% versus 95.9% for crowns using any material or technique (P=.012, OR=2.14). No other tissue displacement technique was associated with a higher rate of acceptable crowns. The results of the mixed-model logistic regression analyses found no association between the presence of subgingival margins and the clinical acceptability of the crown, irrespective of tissue displacement technique. Of clinically acceptable crowns, 58.9% (2111/3585) had subgingival margins, and 41.1% (1474/3585) had supragingival margins.

Table 5.

Percentage and number of clinically unacceptable crowns by type of tissue displacement technique used

Tissue displacement technique	Clinically acceptable crown, % (n/N)	P
Injectable tissue displacement material	94.4 (656/695)	.066
Dual impression	96.0 (291/303)	.904
Single cord	95.8 (1200/1253)	.469
Dual cord	95.8 (1106/1154)	.561
Tissue displacement cap	96.0 (362/377)	.793
Other	97.6 (162/166)	.311
Gingival troughing	97.8 (226/231)	0.4439
None	98.4 (423/430)	0.0098

Tissue displacement technique categories not mutually exclusive.

N=number of crowns for which specific tissue displacement technique was reported.

P values are for associations of each technique separately with clinically acceptable crown.

The number and percentage of clinical scenarios requiring 1 impression for each type of tissue displacement technique are presented in Table 6. Accounting for subgingival margins, the use of a dual-cord tissue displacement technique was significantly associated with lower rates of having more than 1 impression made (P=.015, OR=1.43). No other tissue displacement technique was associated with the rate of requiring more than 1 impression. Controlling for tissue displacement technique, the mixed-model logistic regression analyses showed the presence of subgingival margins to be significantly associated with the number of impressions made (P<001, OR=1.47). The odds ratio for the association indicates that patients with the deepest part of the margin below the gingival crest have higher rates of requiring more than 1 impression. Of the crowns with a subgingival margin, 85.1% (1875/2203) required only 1 impression, whereas 89.9% (1378/1533) of those without subgingival margins required only 1 impression. Note that the total sample size for this comparison was 3736 crowns of 3740 that had no missing values for number of impressions.

Table 6.

Percentage and number of clinical scenarios requiring 1 impression for each type of tissue displacement technique

Tissue displacement technique	Only one impression required, % (n/N)	P
Injectable tissue displacement material	86.1 (600/697)	.328
Dual impression	82.2 (250/304)	.456
Dual cord	83.4 (961/1153)	.008
Tissue displacement cap	84.9 (321/378)	.345
Gingival troughing	85.2 (196/230)	.257
Single cord	88.6 (1116/1259)	.233
Other	89.8 (149/166)	.471
None	93.8 (407/434)	.012

Tissue displacement technique categories not mutually exclusive.

N=number of crowns for which specific tissue displacement technique reported.

P values for associations of each technique versus all others in prediction of use of only 1 impression.

DISCUSSION

Based on the results of this study, the null hypothesis that impression technique (tray selection) is not associated with crown acceptability was rejected. The null hypothesis that the presence of mesial and distal contacts is not associated with occlusal fit with the use of dual-arch trays was rejected. The null hypothesis that the presence of mesial and distal contacts is not associated with crown acceptability was accepted. The null hypothesis that impression material is not significantly associated with crown acceptability was rejected. The null hypotheses that the displacement technique is not associated with crown acceptability or the number of impressions required to obtain an acceptable impression, while accounting for the presence of a subgingival margin, were rejected.

In this study, 87% of impressions were deemed acceptable after the first attempt even as most were made on subgingival preparations and many with bleeding present. Many factors may prevent a dentist from remaking an impression, such as time, cost, patient comfort, and soft tissue health. Therefore, dentists will accept some errors in their impression. A previous study reporting thresholds of acceptability of definitive impressions²⁷ demonstrated that few dentists (14%) will tolerate voids on the finish line, whereas many (47%) will accept voids away from the finish line. Nonclinical factors were also associated with accepting errors in an impression, such as how busy the practice was. Within a commercial dental laboratory, independent evaluators determined 86% of crown and fixed partial denture impressions contained at least 1 detectable error and 55% contained a critical error related to the finish line.² This high error rate seemingly contradicts the high clinical acceptance rate of single-unit crowns reported in this study (96.2%). These observations suggest that either errors viewed in an impression are not critically important for achieving a well-fitting crown, that clinicians are not aware of these errors, or that clinicians are tolerant of these errors.

The first aim of this study was to determine the association between crown acceptability and different impression techniques (tray selection). The results indicate that digital scans produced a higher percentage of acceptable crowns than complete-arch trays, dual-arch trays, and quadrant trays. Complete-arch, dual-arch, and quadrant trays all produced similar rates of clinically acceptable crowns. Previous studies have reported similar accuracy with the use of dual-arch and complete-arch impression trays.^28–30 Dual-arch trays were used for most impressions (62.0%), followed by quadrant trays (16.8%), and then complete-arch trays (4.9%). Of the dual-arch impressions, 31.3% did not fulfill the requirement to have both mesial and distal contacts for the prepared tooth. This value is within the range of 26% to 44.3% of definitive impressions in violation of this rule previously reported.^2,7 The theory for requiring teeth mesial and distal to the prepared tooth with a dual-arch tray is that these teeth help to create occlusal stability between the maxillary and mandibular casts during mounting. Additionally, the occlusal stops from the adjacent teeth can help prevent flexure of the impression in a nonrigid dual-arch tray. Despite the violation of this requirement, no difference was found in the clinical acceptability of the crowns produced with dual-arch trays without either mesial or distal contacts; however, crowns fabricated under these conditions were less likely to achieve an excellent occlusion assessment (66.2% with both contacts and 57.8% without). When faced with a fully or partially unbounded tooth preparation, the clinician must balance the time saving achieved with a dual-arch tray with the slightly compromised outcome of the crown. An absolute contraindication to the use of a dual-arch tray is recording the most posterior tooth in the arch.³¹ This information was not recorded in the current study.

The second aim of this study was to determine the association between crown acceptability and different impression materials. PVS materials were used for most of the definitive impressions (71.9%), then digital scans (16.2%), polyether (10.8%), and other (1.1%). A slight improvement was noted in crown acceptability for polyether materials (97.0%) over PVS materials (95.5%). With additional surfactants that have been added to PVS materials to improve their hydrophilicity⁹ and the availability of extra light body PVS materials, the properties of PVS and polyether materials have become relatively similar. Additionally, clinicians used the impression material they were currently using in their office. As the dentist was experienced with this material, they could be expected to know the techniques necessary to obtain a satisfactory impression with that type of material. This aspect of the study design also may have contributed to the relatively similar performance of both materials. In all but 1 of the clinical scenarios in which ‘other’ was selected as the impression material, a VPES material was used. This is a relatively new material that is claimed to be a hybrid of polyether and PVS. Laboratory testing of this material showed acceptable dimensional stability,³² hydrophilicity,^33,34 and flow,³³ but less consistent detail reproduction.³² There was a lower rate of clinically acceptable crowns with VPES (90.5%); however, only a small number of clinicians (1.1%) used this material and failures may have been related to other factors in their technique.

Digital scans had a higher rate of crown acceptability than conventional impression materials. There are several possible explanations. Although some have claimed a similar accuracy for digital and conventional impression making,³⁵ there is little evidence that digital scanning technology is equally or more accurate than conventional impression material for short dentate spans. More likely, the use of digital impressions and digital crown production eliminates some of the errors that can be introduced during the conventional impression making and laboratory production processes, including improper tray selection, violation of working and/or setting time, errors in cast or die, and improper application of die spacer. Systematic reviews suggest a similar level of marginal and internal fit of crowns fabricated from conventional impressions and digital scans.^13–15 Second, crown acceptability may have been higher with digital scans as the majority of the resulting crowns were fabricated by in-office milling. As these crowns were fabricated in the clinician’s office, the dentist would have more control to locate the crown margins and design the crown to the desired specifications. Furthermore, the clinician may have had a bias towards accepting the crown, as errors in fabrication would be the responsibility of the clinician rather than an external laboratory.

The third aim of this study was to determine whether tissue displacement technique is associated with the clinical acceptability of a crown. To examine the effects of tissue displacement technique, a regression analysis was performed to account for the existence of subgingival margins. Additionally, the number of impressions required to achieve an acceptable definitive impression was also used as a secondary outcome. Irrespective of tissue displacement technique, preparing a tooth with a subgingival margin did not cause a higher incidence of producing a clinically unacceptable crown. However, crowns with subgingival margins had a higher incidence of requiring more than 1 attempt to achieve an acceptable definitive impression.

Regarding tissue displacement technique, the only tissue displacement technique associated with higher rates of acceptable crowns was the use of no tissue displacement. Although this result seems counterintuitive, it is likely the result of clinicians choosing this technique based on the clinical presentation of the tooth preparation. While this regression analysis accounted for the presence of subgingival margins, only 28.6% of the impressions made without tissue displacement were subgingival, whereas 58.6% to 88.4% of the impressions made with gingival troughing or cord technique were subgingival. Additionally, only 32.0% of impressions made without tissue displacement had some bleeding around the preparation and 49.4% to 53.3% of impressions made with gingival troughing or cord technique presented with bleeding. The choice not to use tissue displacement could also have been related to the gingival biotype or gingival health of the patient, which was not recorded on the data collection form. It is also possible that clinicians who were less judicious with tissue displacement were also less discriminating in determining crown acceptability.

The use of a dual-cord technique was the only tissue displacement technique associated with lower rates of requiring more than 1 impression. In this technique, 1 cord is removed immediately prior to making the impression, while the other cord is left in the sulcus. In the single cord technique, no cord is left in the sulcus. The cord that remains in the sulcus in a double-cord technique prevents recoil of the cuff of the gingival tissue and serves as a sulcus liner to block fluid exudate and tearing of the epithelium when the outer cord is removed.³⁶ As errors at the margin are the most common cause of inadequate impressions,^2,12 these benefits of the double cord likely lead to lower rates of making supplemental definitive impressions.

There were limitations of this study. The evaluation of the crowns during insertion was subjective, as the evaluations were all performed by different clinicians who may have held themselves to different standards when deciding whether cementation was warranted given all possible considerations on a given clinic day. This limitation underscores the practicality of practice-based research, in which straightforward indicators of clinical performance are emphasized. Future research could evaluate whether long-term clinical outcomes differ as a result of different interclinician standards. Additional limitations include the potential for bias toward finding the crown clinically acceptable, as these crowns were all evaluated by the clinician who prepared and made the impression of the prepared teeth. Ideally, crowns would be evaluated by independent clinicians; however, this is not practical in a practice-based research study. Another limitation is that clinicians could indicate the use of multiple tissue displacement techniques. The option to record multiple selections complicated the statistical analysis. Additionally, although network practitioners have much in common with dentists at large,^37,38 it may be that their crown procedures are not representative of a wider group of dentists. Network members were not recruited randomly, so factors associated with network participation (such as, an interest in clinical research) may make network dentists unrepresentative of dentists at large. While it cannot be asserted that network dentists are entirely representative, it can be stated that they have much in common with dentists at large, while also offering substantial diversity in these characteristics. This assertion is warranted because substantial percentages of network dentists are represented in the various response categories of the characteristics in the Enrollment Questionnaire, because findings from several network studies document that network general dentists report patterns of diagnosis and treatment similar to those determined from non-network general dentists,^39–42 and because network dentists are similar to non-network dentists using the 2010 ADA Survey of Dental Practice.⁴³

CONCLUSIONS

Within the limitations of this practice-based study, the following conclusions were drawn:

1. Dual-arch trays produced clinically acceptable crowns; however, if the prepared tooth was unbounded, the occlusal fit was more likely to have been compromised.

2. Digital scanning produced a slightly higher rate of clinically acceptable crowns than conventional impression materials.

3. The use of a dual-cord technique was associated with a decreased need to remake impressions when the margins were subgingival.

CLINICAL SIGNIFICANCE.

Dual-arch trays can be recommended for single tooth crowns; however, the tooth preparation should be bounded anteriorly and posteriorly. Digital scans show promise for producing more acceptable crowns. Keeping crown margins supragingival reduces the risk of requiring an additional impression, but if a subgingival margin is required, a double-cord tissue displacement technique seems advisable.