Relative Clinical Success of Bis-Acryl Composite Provisional Crowns

The requirements for a single-tooth provisional restoration have been adequately described1-3 and differ little from the requirements for definitive crowns.4 In the 1980s and 1990s, the use of tooth-colored temporaries gained popularity.1,5,6 Materials for the production of provisionals and their properties have steadily improved. Polymethyl methacrylate (PMMA) was replaced by the more expensive bis-acryl composite.7-10 Bis-acryl composites show good fit, moderate color retention, and strength, but few in vivo studies have been performed.4,10-12 In 2000, Luthardt et al noted that there are no clinically controlled comparative studies of the clinical performance of bis-acryl composite provisionals and definitive conventional crowns (fixed partial dentures [FPDs]).4 In 2007, a working group headed by Hyde replaced the word “temporary” with the word “provisional.” The term “provisional” is intended to convey a statement about this type of restoration's function as an important step in the supply chain toward final restoration.8-13

Today, bis-acryl composite is a reliable material for the efficient and direct production of provisional restorations.8 It has better color stability than PMMA or polyethyl methacrylate (PEMA).4,14 Bis-acryl composites have become commercially popular, especially because of their ease of use, handling properties, and superior mechanical properties.8,15-18 However, the precondition for the use of chemically cured material in direct chairside procedures is that clinical crowns be practically undisturbed. A rubber impression must be taken before tooth preparation. This serves as an impression tray for the bis-acryl composite. If the clinical crown anatomy is disturbed, a rubber impression cannot be taken. In such cases, transparent strip crowns are used, especially in the repair of primary teeth.19-21 Similar to the manufacturing process of these strip crown provisionals is the development of single-unit, self-supporting, malleable, light-curing bis-acryl composite single-tooth crowns. These can be adapted and modeled to the prepared abutment tooth and light-cured afterwards.

In this investigation, Protemp™ 4 (3M ESPE), a chemically cured bis-acryl composite used in the conventional manufacture of chairside provisional, and Protemp Crown™ (3M ESPE), a single-unit, self-supporting single-tooth crown, were used for each manufacturing method. Protemp material and its further-developed derivatives is a restorative material that has been clinically accepted for more than 15 years.4,10 According to the manufacturer, Protemp Crowns facilitate the repair of teeth whose impression cannot be used for the subsequent preparation of a provisional crown, and manufacturing time is said to be less than 4 minutes.22 The crowns are claimed to have sufficient marginal fit, interproximal contact, surface structure, and occlusal abrasion after 1 year. The manufacturer recommends Protemp Crowns as long-term provisionals for an in situ period of up to 2 years.22

The clinical requirements for provisional crowns are similar to those of definitive crowns, and clinical investigations of provisional single-tooth crowns are rare.4 Considering recent demographic changes and the subsequent effects on health financing,23 the indications that bis-acryl composite crowns may be a longer-lasting alternative for restoration supply should be considered.

The purpose of this study was to investigate whether a chairside fabricated bis-acryl composite single-tooth crown is comparable to a conventional, dental laboratory-produced single crown in terms of clinical success. The present study observed the differences in the evaluations given by two dentists (one with little experience [< 3 years professional experience] and one with a higher level of experience [> 3 years professional experience]) of each provisional type based on ratings from five examination parameters in the FDI criteria. The focus of the investigation was to explore if the two manufacturing options for the provisionals differ from one another and particularly to evaluate the performance of the relatively new procedure of manufacturing single-unit, self-supporting, single-tooth provisional crowns. To obtain a better understanding of patient perceptions, the patients' judgment of the provisionals and the definitive crowns was documented and analyzed.

The present investigation is a comparative, monocentric clinical study. It compares two types of possible application modes for preparing chairside bis-acryl composite provisionals, a flowable chemically curing material (Protemp 4) and a light-curing material (Protemp Crown). Bis-acryl composite is well known,4,8,10 and, according to the manufacturer, certified for clinical use as a material for the fabrication of provisional single-tooth crowns or bridges with a retention period up to 2 years or for long-term temporization (for Protemp 4 and Protemp Crown). Institutional approval for the study was obtained from the relevant ethics commission at Witten/Herdecke University (reference number 08/2013). Patients from the dental clinic of Witten/Herdecke University were asked to participate in the clinical study. Women and men (ages 18 to 80) with indications for the restoration of a single-tooth crown in the molar or premolar area in the maxilla or mandible could participate. The patients were enrolled into the study after they were informed about the study procedure orally and in writing and gave their written consent to participate. Data were collected anonymously using a CRF (case report form), which was created specifically for the trial.

Inclusion criteria were premolars and molars with the need for a single-crown restoration (vital and nonvital abutment tooth). Exclusion criteria were abutments for removable partial dentures (RPDs), aggressive periodontitis, and abutment heights less than 4 mm. Every participant and every other patient not included in the test groups, received complete dental diagnostics and treatment. This treatment included standardized intra- and extraoral examination, an oral hygiene program (if needed), periodontal and restorative treatment (if needed), and pre-prosthetic diagnostics such as radiographs and pulpal testing. Abutment teeth were prepared following the tooth preparation guidelines of Shillingburg.2 The occlusal reduction was at least 1.5 mm and axially at least 1 mm to 1.2 mm.

The test groups received chairside-fabricated, single-tooth crowns (test group 1 = Protemp 4; test group 2 = Protemp Crown). The control group received conventional fabricated single-tooth crowns. The sample population was 24, with 12 patients in each test group.

The distribution of teeth fulfilling the inclusion criteria for the two test groups was binary and depended on the condition of the tooth. The allocation followed a certain protocol. If the criteria were met, the tooth shape was intact, and the occlusal plus approximal contacts had been fulfilled, then an impression of the unprepared tooth could be taken to generate an impression tray using a-silicon (Silagum; DMG, dmg-dental.com). Teeth that met these requirements were allocated to test group 1. If only one criterion was not fulfilled, the tooth was allocated to test group 2. The light-curing bis-acryl composite crown is a self-supporting, malleable, single-tooth crown that can be placed and modeled on the prepared tooth without the need for a silicon impression tray.

After tooth preparation, an impression was taken and the facebow record and maxillomandibular relationship was recorded. Then, the abutment tooth was cleaned using water and cotton pellets and then restored with the bis-acryl composite crowns cemented with non-eugenol zinc oxide cement (Temp Bond NE; 3M ESPE). With the fabrication of the conventional single tooth, tooth-colored crowns were created in the dental laboratory after mounting the casts in an articulator. These crowns are cemented with a self-adhesive luting cement (Rely X™ Unicem; 3M ESPE) or a zinc oxide phosphate cement (Harvard Cement; Harvard Dental International, harvard-dental-international.de).

Immediately after cementing either the provisional or the definitive crown, the evaluation was performed. Dentist 1 and dentist 2 evaluated the crowns independently after dentist 1 treated the patient. Every provisional was prepared by dentist 1, independent of the dentist preparing the tooth.

To generate a comparable evaluation of the groups, FDI criteria were applied. The FDI criteria originally consisted of 16 criteria from which specific categories could be developed and then used for the intended purpose.24 In this study, two esthetic and three functional parameters were used as the clinical examination guidelines to evaluate the direct and indirect restorations.24 The following esthetic property subcategories were chosen: color match, translucency, and esthetical anatomical form. Also, the following functional subcategories were chosen: marginal adaptation, proximal anatomical form, and contact point and contour.

The evaluation results from a binary regimen allowed for clinical classification of the results. A grade of 1 to 3 implied clinical sufficiency. Clinically insufficient classification (failure) is represented by a grade of 4 or 5 (Table 1). The worst score within the criteria prevailed and generated the overall ranking of the restoration. According to Hickel et al, not all failures call for the replacement of the restoration.24 Table 1 shows the selected criteria as adapted from the original.

Every restoration (within the test group and control group) was evaluated by two dentists (a highly experienced dentist and a less experienced dentist) independently. The bis-acryl composite crowns (test groups 1 and 2) were all prepared by the less experienced dentist (dentist 1). In the follow-up to this evaluation, a pilot study was performed to calibrate the two practitioners in relation to the evaluation criteria, as recommended by Hickel et al.24

The patients' perspectives were also evaluated. The patients were asked to assess the delivered crowns (provisionals and final crown) on a scale from one to four, with a score of one being the best assessment and a score of four being the worst assessment.

Ample time is necessary to prepare the conventional provisional crowns. Utley described a production time as long as 15 minutes until cementation.25 Concerning the relatively new Protemp Crowns, the manufacturer indicates that 4 minutes is needed until cementation.22 The time needed to prepare and polish the provisional was measured from the time the bis-acryl composite material was inserted into the mouth until cementation.

Statistical analysis was performed in collaboration with the Institute of Medical Biometry and Epidemiology of Witten/Herdecke University. Due to the relatively small sample size of this study, the collected data were subjected to descriptive statistical analysis using McNemar's Test (α = 0.05) and analyzed for inter-rater agreement and Cohen's kappa coefficient (SPSS Version 23 and R Version 3.2.2).

No patients were lost during follow-up. The loss of the provisional or possible replacement was included in the evaluation because the primarily fabricated provisional had already been evaluated. A measurement of the occurrence of provisional loss would not influence the evaluation.

Bis-acryl composite crowns are clinically not as successful as conventional laboratory-fabricated crowns (significant rejection of null hypothesis stratified by dentist to a Bonferroni corrected level of 2.5%; (McNemar) P both < .001. Test for binary outcome 0 ≙ clinically insufficient, ie, score 4 or lower in at least one criterion and 1 ≙ clinically sufficient, ie, score 3 or higher in all criteria. Dentist 2 evaluated the provisionals more harshly than dentist 1. Dentist 1 scored the provisionals with score a score of 4 or worse in 79% of the evaluated cases, while dentist 2 did so in 87.5% of the evaluated cases. The definitive crowns were scored as clinically insufficient in only 25% (dentist 1) and 21% (dentist 2) of all cases. Neither of the dentists scored a provisional crown as clinically sufficient or a final crown as clinically insufficient for the same patient.

When comparing the evaluations of the two dentists, the statistical analysis (Cohen's kappa as a measurement of inter-rater agreement) shows that there is a greater consistency between the dentists in their scores for the provisional (κ = 0.407, CI = (-0.06; 0.87)) than their scores for the final crown (κ = 0.176, CI = (-0.26; 0.61)). Of the provisionals, 75% were consistently scored as clinically insufficient (scores 4 or 5) and 8% were consistently scored as clinically sufficient (3 or better). Between the two practitioners, 83% of the scores were consistent. Concerning the definitive crown, 71% of the scores were consistent between the dentists (63% clinically sufficient and 8% clinically insufficient). Figure 1 shows the evaluation of the dentists for the definitive crown and provisional crown.

With regard to the separate criteria, the agreement between the two dentists was less than 50% in each case. The highest consistency was found in the assessment of the crown margin at 46%.

In particular, in the color match, crown margin, and esthetic anatomical form criteria, dentist 2 gave worse ratings than dentist 1. Within the esthetical criteria, dentist 2 gave worse ratings in 79% of the cases than dentist 1 (Figure 2). Otherwise, no single criterion can be found that is explicitly responsible for the insufficient results.

Test group 1 received better scores than test group 2. Dentist 1 gave better scores in three out of five criteria. Dentist 2 gave test group 1 better scores than test group 2 in all five criteria (Figure 3 and Figure 4).

Not considering individual cases, no relevant difference in median production time was observed between the two groups. Approximately 10 minutes was needed to prepare the provisionals for each test group (median value 9.72 (IQR = 8.39–13.76) minutes for test group 1 and 9.25 (IQR = 7.02–16.08) minutes for test group 2.

The patients were asked to score their provisional crowns and definitive crowns. No definitive crown was given a score of four or worse. Only two provisionals were given a score of four. Overall, the patients gave both the provisionals and the definitive crowns better scores than the dentists did.

Clinical investigations analyzing the clinical success of provisionals are rare.4 Based on the authors' literature search, no comparable hypothesis in a clinical study could be found. An article from 2007 describes the increase in medical tourism among patients willing to accept uncertainties in the quality of the treatment to receive compensation for their medical care.26 Questions such as the one posed in the present study—ie, whether a chairside-fabricated bis-acryl composite single-tooth crown is comparable to a conventional, dental laboratory-produced single crown in terms of clinical success—must be investigated to ensure the availability of reliable and ethically justifiable care for patients who may struggle to afford treatment. This necessity is reinforced by the fact that the participants were more satisfied with the provisional than the dentists, which could be because patients' perception of a successful crown differs from practitioners' perception, especially with regard to esthetic perception.27 It is becoming necessary to consider offering patients a faster, more cost-effective yet still functionally sufficient and esthetically pleasing option. Therefore, the clinical success of definitive crowns and provisionals was compared in this study.

Despite the high demand for provisional crowns, both dentists evaluated the provisional worse than the conventional laboratory-produced crown. This result should be critically evaluated because the results of the study were predetermined by the study design: a chairside-fabricated bis-acryl composite crown cannot fulfill the high demands of a laboratory-produced crown (eg, crown margin or translucency of layered ceramic coating) that is fabricated in several steps.

In 2001 a research group led by Young indicated that the fabrication of a direct provisional restoration with adequate quality to ensure a healthy, functional, and esthetic dentition is a challenging task, even for the most experienced dentist.8 The chairside bis-acryl composite crowns were fabricated by the less experienced dentist. The lower level of clinical experience and experience in preparing provisionals could be a reason for the outcome. This speculation is supported in an article by Hyde et al, which demonstrated that provisional crown failures in dental school occur more frequently due to lack of experience of the practitioners.28 The authors recommend intensive training and supervision of young dentists and students in the fabrication of provisionals for FPDs. A comparison of provisionals made by less-experienced dentists and highly experienced dentists could have reinforced these recommendations but was not part of the study design. Preparation time should also be incorporated into these considerations. The average time dentist 1 needed to manufacture the provisionals was 10 minutes, which is 6 minutes more than recommended by the manufacturers. Perhaps more time should have been taken to generate a better outcome in the evaluation.

Whenever a provisional was scored as clinically insufficient in one criterion, the overall score was insufficient. Therefore, a critical evaluation was made to determine which criteria for the restoration showed a clinically insufficient score. As Hickel et al indicate, not every clinical insufficiency calls for a replacement; therefore, the respective provisional was replaced or improved.24

The manufacturer of the restoration used in test group 2 claims fast (50% reduction in manufacturing time), easy handling. This investigation did not provide consenting experience to substantiate this claim. The manufacturer also suggests that no polishing is needed after Protemp Crown preparation. Within this study it was necessary to polish after adjustment (eg, occlusal or marginal adjustment) to smooth out the roughened surface. The conventional procedure for the handling of materials for creating a provisional and the conventional chemical curing of the bis-acryl composite remains the gold standard in the supply of provisional crowns.

Future studies should generate further information for manufacturers to create materials that comply with the requirements of a definitive crown in a direct procedure to enable a fast and economical, but still clinically sufficient, treatment.

Conclusion

The chemically curing bis-acryl composite remains the material of choice for the provisional supply of teeth within the manufacturing time of the final FPD. Nevertheless, dentists should provide manufacturers with their clinical expertise to enable the continuing development and optimization of provisional materials such as Protemp Crowns. Although the self-supporting, malleable, light-curing Protemp Crown showed poorer results in clinical performance in this study, further investigations with regard to the constant optimization of dental materials should be supported.

Carla Kozmacs, Dr. med. dent.

Department of Prosthodontics and Dental Technology,  Faculty of Health, Witten/Herdecke University, Witten, Germany

Vanessa Baumann

Research Assistant, Institute for Medical Biometry and Epidemiology, Faculty of Health, Witten/Herdecke University, Witten, Germany

Oskar Bunz

Department of Prosthodontics and Dental Technology, Faculty of Health, Witten/Herdecke University, Witten, Germany

Andree Piwowarczyk, Prof. Dr. med. dent.

Department of Prosthodontics and Dental Technology, Faculty of Health, Witten/Herdecke University, Witten, Germany