Abstract
Purpose:
To: (1) determine which preparation techniques clinicians use in routine clinical practice for single-unit crown restorations; (2) test whether certain practice, dentist, and patient characteristics are significantly associated with these techniques.
Methods:
Dentists in the National Dental Practice-Based Research Network participated in a questionnaire regarding preparation techniques, dental equipment used for single-unit crown preparations, scheduled chair time, occlusal clearance determination, location of finish lines, magnification during preparation, supplemental lighting, shade selection, use of intraoral photographs, and trimming dies. Survey responses were compared by dentist and practice characteristics using ANOVA.
Results:
Of the 2132 eligible dentists, 1,777 (83%) responded to the survey. The top two margin configuration choices for single-unit crown preparation for posterior crowns were chamfer/heavy chamfer (65%) and shoulder (23%). For anterior crowns, the most prevalent choices were the chamfer (54%) and the shoulder (37%) configurations. Regarding shade selection, a combination of dentist, assistant, and patient input was used to select anterior shades 59% of the time. Photographs are used to communicate shade selection with the laboratory in about half of esthetically demanding cases. The ideal finish line was located at the crest of gingival tissue for 49% of respondents; 29% preferred 1 mm below the crest; and 22% preferred the finish line above the crest of tissue. Average chair time scheduled for a crown preparation appointment was 76 ± 21 minutes. Practice and dentist characteristics were significantly associated with margin choice including practice type (p < 0.0001), region (p < 0.0001), and years since graduation (p < 0.0001).
Conclusions:
Network dentists prefer chamfer/heavy chamfer margin designs, followed by shoulder preparations. These choices were related to practice and dentist characteristics.
Keywords: Crowns, PBRN, survey
Dentists use crowns for a variety of reasons to restore the dentition to a state of physiological health.1,2 The indications for crowns are diverse, and may include factors such as endodontic treatment, cracks and fractures of the tooth, large restorations, esthetic shortcomings, or significant caries.1,3,4 When planning crowns, there is variation among dentists in the likelihood of recommending a crown. Although consensus exists in some areas (posterior endodontic treatment), variation dominates in others (size of an existing restoration).3,4 In fact, evidence suggests recommendations for crowns may be influenced by factors unrelated to tooth and patient variables.1
Dentists must make a variety of clinical decisions when fabricating crowns for their patients. Depending on the restorative material, historic guidelines suggest an optimal design for tooth preparation based on fundamental principles for predictable successful prosthodontic treatment.5–13 These principles of tooth preparation may include preservation of tooth structure, retention and resistance form, marginal integrity, structural durability, and esthetic considerations.5–13 The internal line angles from axial wall to shoulder junction should be smooth, as studies have shown that sharp internal line angles may be associated with marginal fit discrepancies or fracture of the restorative material.13,14 Adequate occlusal reduction is also considered an important aspect of tooth preparation.15 Published principles for tooth preparation suggest the following tooth reductions for metal-ceramic crowns: 1.5 to 2.0 mm of occlusal reduction; 1.5 to 2.0 mm of axial reduction; and approximately 10° to 20° of total occlusal convergence.16–20
Margin integrity has been found to be an essential factor in evaluating preparations for an indirect restoration and delivery of a crown with accurate fit.15,21–23 Prior studies have reported that a marginal opening of 100 μm was consequential when determining clinical acceptability.15,21 Poor margins may cause greater incidence of gingival inflammation, recurrent caries, plaque/calculus accumulation, and cement disintegration.12 At the insertion appointment, an open or defective margin may compel a dentist to re-impress and remake the crown.
Among the myriad choices and clinical factors that guide a dentist in recommending a crown, preparing a tooth for the crown, and then fabricating and inserting the restoration, relatively little is known regarding a general consensus for accepted best practices, or even what are commonly used techniques among practicing dentists. The purpose of this study was to investigate current techniques used by dentists to fabricate crowns in the United States. Furthermore, a potential association between dentist characteristics, practice characteristics, and preparation techniques was investigated. By recognizing clinically acceptable practices used by peers, dentists are able to evaluate their own clinical procedures and make changes where appropriate to increase treatment predictability.
MATERIALS AND METHODS
This study was based on a questionnaire completed 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.24 Detailed information about the network is available at its website.25 The network’s applicable Institutional Review Boards approved the study; all participants provided informed consent after receiving a full explanation of the procedures. The methods for conducting this study are previously reported,26 and reiterated here for the convenience of the reader.
Enrollment questionnaire
As part of the enrollment process, practitioners completed an enrollment questionnaire that described themselves, their practice(s), and their patient population. This questionnaire is publicly available at http://www.nationaldentalpbrn.org/study-results/ under the heading “Factors for Successful Crowns.” The questionnaire solicited information about practitioner, practice, and patient characteristics. Questionnaire items, which had documented test/re-test reliability, were taken from the authors’ previous work in a practice-based study of dental care.27,28 The majority of respondents completed the questionnaire online, although a paper option was available.
Study questionnaire development
The questionnaire for this study was developed by a study group of the authors, dentists with clinical expertise, statisticians, and laboratory technicians. Its purpose was to measure current practices in treatment planning, as well as preparing and fabricating single-unit crowns on natural teeth. The survey was reviewed by IDEA Services (Instrument Design, Evaluation, and Analysis Services, Westat, Rockville, MD), a group with expertise in questionnaire development and implementation, as well as National Institute of Dental and Craniofacial Research (NIDCR) program officers and practitioners with prosthodontic content expertise. After extensive internal review, IDEA Services pretested the questionnaire via cognitive interviewing by telephone with a regionally diverse group of eight practicing dentists. Cognitive interviewers probed the dentist’s comprehension of each question. The interviewers also asked practitioners to identify items of clinical interest that were not addressed in the survey. Results from the pretest prompted further modification of the questionnaire.
Dentists enrolled in the network were eligible for the study if they met the following criteria: (1) completed an enrollment questionnaire; (2) were currently practicing and treating patients in the United States; (3) were in the network’s “limited” or “full” participation category; and (4) reported on the enrollment questionnaire that they currently do at least some restorative dentistry in their practices. A total of 2299 network clinicians met these criteria.
Pre-printed invitation letters were mailed (postal) to eligible practitioners, informing them that they would receive an e-mail with a link to the electronic version of the questionnaire. At the time of the e-mail, practitioners were given the option to request a paper version of the survey, as this has been shown to improve response rates.29 Practitioners were asked to complete the questionnaire within 2 weeks. A reminder letter was sent after the second and fourth weeks to those who had not completed the questionnaire. After 6 weeks, e-mail and postal reminders were sent with a printed version of the questionnaire, and practitioners were offered the option of completing the online or paper versions. After 8 weeks, a final postal questionnaire attempt was made with a letter that also encouraged the dentist to complete the questionnaire online. Data collection was closed after 12 weeks from the original email invitation. Practitioners or their business entities were remunerated $75 for completing the questionnaire if desired. Data were collected from February 2015 to August 2015.
Questionnaire content
The first question of the survey confirmed that the invited clinician restored at least one natural tooth with a full crown restoration in a typical month. Questions from the survey reported here focused on techniques used to prepare teeth for crowns. One such question asked practitioners to describe the margin preparation used for a posterior crown on a first molar, and possible choices were “chamfer or heavy chamfer,” “shoulder,” “shoulder with bevel,” “knife edge,” and “other.” These represent options used for preparing the marginal detail of a crown. Generally, a chamfer has a rounded internal line angle with a continuous slope to where the finish line meets the axial wall of the preparation; a shoulder is a relatively flat ledge that meets the root surface with an cavosurface angle similar to 90°; a shoulder bevel has the addition of a brief cavosurface preparation, which creates an obtuse angle relative to the external root structure; and a knife-edge preparation uses minimal tooth preparation and a very obtuse cavosurface angle similar to the profile of the root surface itself. Other questions asked clinicians about their techniques for making crowns, such as the use of magnification, time scheduled for appointments, shade-matching techniques, and supplementary lighting use.
Statistical analyses
Power analysis was conducted based on an anticipated sample size of 1500 completed questionnaires. This sample size would yield sufficient precision to estimate response percentages within ±2.53% at the 95% confidence level. To document test/re-test reliability of the questionnaire items, 47 respondents completed the questionnaire twice online. For categorical responses, kappa and weighted kappa were used; for numeric items, Pearson’s correlation coefficient was calculated to determine test-retest reliability. Descriptive statistics are presented as counts and percentages for categorical variables, and as means and standard deviations for continuous measures. To determine if differences exist among groups responding to the survey, dentist and practice characteristics were compared to survey responses using ANOVA and multiple regression analysis. Results were considered significant at the p < 0.05 level.
RESULTS
Of the 2132 eligible dentists, 1777 (83%) completed the survey. Characteristics of the responding dentists are reported elsewhere,1 and are presented in Table 1 for the reader’s convenience. According to the results of this survey, the most-used margin configuration for crowns for posterior teeth was the chamfer/heavy chamfer (65%), followed by shoulder preparations (23%). For crowns restoring anterior teeth, the most-used margin configuration was the chamfer/heavy chamfer (54%), followed by the shoulder preparation (37%).
Table 1.
Characteristics of network dentists who participated in this survey
| Characteristics | Number1 (n = 1777) |
Percent (%) |
|---|---|---|
| Dentist characteristics | ||
| Gender | ||
| Male | 1282 | 73 |
| Female | 483 | 27 |
| Years since dental school graduation | ||
| <10 | 292 | 16 |
| 10–19 | 367 | 21 |
| 20–29 | 382 | 22 |
| 30+ | 733 | 41 |
| Type of practice | ||
| Owner of private practice | 1295 | 74 |
| Associate in private practice | 207 | 12 |
| Health Partners2 | 44 | 3 |
| Permanente2 | 70 | 4 |
| Public health, community | 64 | 4 |
| Academic | 48 | 2 |
| Other | 34 | 2 |
| Network region3 | ||
| Western | 292 | 16 |
| Midwest | 180 | 10 |
| Southwest | 311 | 18 |
| South Central | 330 | 19 |
| South Atlantic | 327 | 18 |
| Northeast | 337 | 19 |
| Time commitment | ||
| Fulltime | 1508 | 86 |
| Part time (<32 hrs) | 253 | 14 |
| Specialty status | ||
| General dentist | 1719 | 97 |
| Specialist | 56 | 3 |
| Race | ||
| White | 1451 | 82 |
| Black/African-American | 77 | 4 |
| Asian | 161 | 9 |
| Other | 70 | 4 |
| Patient population characteristics | ||
| Private insurance status | ||
| <40% Private insurance | 249 | 14 |
| 40–79% Private insurance | 1017 | 58 |
| 80%+ Private insurance | 476 | 27 |
| Patient appointment regularity | ||
| <50% of patients regularly visit | 274 | 16 |
| 50–79% regularly visit | 1044 | 60 |
| 80%+ regularly visit | 428 | 25 |
Due to missing values, reported sample sizes vary.
Either HealthPartners Dental Group in greater Minneapolis, MN or Permanente Dental Associates in greater Portland, OR.
Reported on enrollment questionnaire as the state, subsequently categorized into one of the six regions of the network.
Other aspects of crown preparation were documented with this survey (Table 2). The most common handpiece reported for single-unit crown preparations was the air turbine high-speed handpiece (81%). The total scheduled chair time (±SD) for a single-unit crown preparation, impression, and provisional restoration appointment was 76 ± 21 minutes. More than half (66%) of dentists chose 2 mm occlusal as their desired clearance for a metal-ceramic (PFM) crown; 25% chose 1.5 mm occlusal clearance. When describing the ideal location for a finish line for a single-unit crown, 49% reported they place it at the crest of the gingival tissue, 29% place it 1 mm below the gingival tissue, and 22% place it above the crest of the gingival tissue. When preparing a tooth for a crown, 79% of dentists reported the use of some sort of magnification. Fifty-seven percent report the use of extra lighting other than the usual operatory overhead lamp when preparing a tooth for a crown.
Table 2.
Techniques and practices for making single-unit crowns as reported by survey respondents (n = 1777).
| Crown fabrication techniques and practices | Number1 (n =1777) |
Percent (%) |
|---|---|---|
| Handpiece used | ||
| Electric high-speed | 345 | 19 |
| Air-turbine high-speed, costing $500 or less | 678 | 40 |
| Air-turbine high-speed, costing $501 or more | 735 | 41 |
| Other | 17 | 1 |
| Occlusal clearance for PFM, molar | ||
| 1.5 mm | 450 | 25 |
| 2.0 mm | 1164 | 66 |
| More than 2 mm | 161 | 9 |
| Ideal finish line location | ||
| Above the crest of tissue | 385 | 22 |
| At the crest of tissue | 865 | 49 |
| 1 mm below gingival crest | 508 | 29 |
| 2 mm below gingival crest | 16 | 1 |
| Who selects shade, posterior tooth | ||
| Clinician | 624 | 35 |
| Assistant | 289 | 16 |
| Lab | 7 | 0.4 |
| Patient | 9 | 0.5 |
| Combination of people | 834 | 47 |
| Other | 14 | 0.8 |
| Who selects shade, anterior tooth | ||
| Clinician | 499 | 28 |
| Assistant | 90 | 5 |
| Lab | 93 | 5 |
| Patient | 12 | 0.7 |
| Combination of people | 1048 | 59 |
| Other | 33 | 2 |
| Photographs used for shade communication for esthetically demanding cases | ||
| Yes, more than 50% of the time | 616 | 35 |
| Yes, 50% of the time or less | 710 | 40 |
| No | 451 | 25 |
Due to missing values, reported sample sizes vary.
Questions regarding crown shade selection were presented in the survey (Table 2). When selecting shades for crowns, a combination of dentist, assistant, and patient input is used for posterior crowns 47% of the time; this increased to 59% for anterior crown shade selection. About 75% of dentists report using photographs to communicate shade selection with the laboratory on esthetically demanding cases. Clinicians reported they trim dies in the office 12% of the time.
When considering posterior crown margin design (independent of crown material), several dentist and practice characteristics were significantly associated with margin configuration (Table 3). Gender was significantly associated with margin design (p < 0.001), with females tending to use more shoulder preparations. Race was significantly associated with margin design (p < 0.001), with Black/African-American clinicians using less chamfer designs and more shoulder-bevel designs than other races. Years since graduation from dental school was significantly associated with margin design (p < 0.001), with more-experienced clinicians using shoulder-bevel and knife-edge margin designs more often. Other factors significantly associated with margin configuration were region and type of practice. Practitioners in the Northeast tended to use shoulder-bevel and knife-edge preparations more often than other regions of the country.
Table 3.
Dentist and practice characteristics associated with margin configuration for posterior single-unit crowns (frequency [percent])
| Characteristics for posterior crowns | Chamfer | Shoulder | Shoulder bevel |
Knife edge | P Value | Total |
|---|---|---|---|---|---|---|
| Overall | 1133 (65) | 395 (23) | 146 (8) | 74 (4) | 1748 | |
| Gender | ||||||
| Male | 828 (66) | 259 (20) | 113 (9) | 64 (5) | 0.0005 | 1264 |
| Female | 298 (63) | 132 (28) | 32 (7) | 10 (2) | 472 | |
| Race | ||||||
| White/Caucasian | 929 (65) | 313 (22) | 124 (9) | 64 (4) | 0.005 | 1430 (83) |
| Black/African-American | 42 (56) | 19 (25) | 11 (15) | 3 (4) | 75 (4) | |
| Asian | 106 (67) | 48 (30) | 1 (1) | 3 (2) | 158 (9) | |
| Other | 45 (67) | 13 (19 | 6 (9) | 3 (4) | 67 (4) | |
| Ethnicity | ||||||
| Hispanic | 59 (59) | 28 (28) | 6 (6) | 7 (7) | 0.22 | 100 (6) |
| Non-Hispanic | 1060 (65) | 365 (22) | 138 (96) | 67 (4) | 1630 (94) | |
| Years since graduation | ||||||
| Less than 5 | 48 (69) | 18 (26) | 2 (3) | 2 (3) | 0.0001 | 70 (4) |
| 5 to 15 | 295 (68) | 116 (27) | 13 (3) | 10 (2) | 434 (25) | |
| More than 15 | 787 (63) | 261 (21) | 131 (11) | 62 (5) | 1241 (71) | |
| Type of practice | ||||||
| Owner of private practice | 799 (63) | 301 (24) | 119 (9) | 58 (5) | 0.0003 | 1277 (74) |
| Associate in private practice | 140 (69) | 49 (24) | 9 (4) | 6 (3) | 204 (12) | |
| Health Partners | 39 (89) | 2 (5) | 3 (7) | 0 | 44 (3) | |
| Permanente | 58 (87) | 4 (6) | 2 (3) | 3 (4) | 67 (4) | |
| Public health, community | 35 (56) | 19 (31) | 5 (8) | 3 (5) | 62(4) | |
| Academic | 32 (70) | 6 (13) | 6 (13) | 4 (9) | 46 (3) | |
| Other | 23 (68) | 10 (3) | 10 (29) | 0 | 34 (2) | |
| Network Region | ||||||
| Western | 193 (68) | 63 (22) | 19 (7) | 9 (3) | 0.0001 | 284 (16) |
| Midwest | 137 (78) | 29 (16) | 6 (3) | 4 (2) | 176 (10) | |
| Southwest | 197 (64) | 76 (25) | 19 (6) | 18 (6) | 310 (18) | |
| South Central | 218 (66) | 66 (20) | 23 (7) | 23 (7) | 328 (19) | |
| South Atlantic | 213 (66) | 83 (26) | 16 (5) | 16 (5) | 323 (18) | |
| Northeast | 175 (54) | 78 (24) | 63 (19) | 63 (19) | 327 (19) | |
| Practice Busyness | ||||||
| Too busy | 74 (75) | 14 (14) | 7 (7) | 4 (4) | 0.23 | 99 (6) |
| Burdened | 221 (69) | 69 (22) | 20 (6) | 10 (3) | 320 (18) | |
| Balanced | 565 (63) | 211 (24) | 77 (9) | 43 (5) | 896 (51) | |
| Not busy | 272 (63) | 101 (23) | 42 (10) | 16 (4) | 431 (25) | |
| Private insurance status | ||||||
| <40% private insurance | 34 (65) | 11 (21) | 5 (10) | 2 (4) | 0.99 | 52 (3) |
| 40–79% private insurance | 484 (65) | 168 (23) | 60 (8) | 34 (5) | 746 (43) | |
| 80%+ private insurance | 605 (65) | 207 (22) | 80 (9) | 37 (4) | 929 (54) | |
| Practice hours | ||||||
| Full-time (32+hours/week) | 964 (65) | 339 (23) | 125 (8) | 58 (4) | 0.37 | 1486 (86) |
| Part-time (<32 hours/week) | 162 (66) | 50 (20) | 19 (8) | 15 (6) | 246 (14) | |
| Environment location of practice | ||||||
| Inner city of urban area | 136 (65) | 44 (21) | 15 (7) | 13 (6) | 0.23 | 208 (12) |
| Urban (not inner city) | 307 (64) | 109 (23) | 40 (8) | 21 (4) | 477 (27) | |
| Suburban | 499 (64) | 185 (24) | 75 (10) | 25 (3) | 784 (45) | |
| Rural | 187 (69) | 54 (20) | 15 (6) | 15 (6) | 271 (16) |
When considering anterior crown margin design, several factors were also associated with margin configuration types (Table 4). Gender was significantly associated with margin design (p < 0.001), with females again using more shoulder preparations than males (44% vs. 34%). Significant variations in margin design were associated with the clinician’s race, with Asian clinicians using a high percentage of shoulder designs (43%); dentists reporting Hispanic ethnicity also commonly used the shoulder preparation (49%), compared to non-Hispanic clinicians (36%). Regarding years since graduation, clinicians with more than 15 years since dental school graduation were less likely to use a shoulder preparation. Type of practice was significantly associated with margin design (p < 0.01); dentists in the Health Partners organization (a large group practice in Minnesota) used chamfer designs 77% of the time, while dentists in academic settings used the chamfer only 47% of the time. Network region and practice busyness were also significantly associated with margin configuration. Knife-edge margins, though only a small percent of overall margin designs, were concentrated in the South Central and South Atlantic regions. Clinicians in the Midwest favored the chamfer more than other regions. Dentists who classified their practice as not busy used more shoulder bevel margin designs than other practices.
Table 4.
Dentist and practice characteristics associated with margin configuration for anterior single-unit crowns (frequency [percent])
| Characteristics for anterior crowns | Chamfer | Shoulder | Shoulder bevel |
Knife edge | P Value | Total |
|---|---|---|---|---|---|---|
| Overall | 938 (54) | 645 (37) | 120 (7) | 46 (3) | 1749 | |
| Gender | ||||||
| Male | 701 (55) | 432 (34) | 96 (8) | 37 (3) | 0.0006 | 1266 (73) |
| Female | 230 (49) | 209 (44) | 23 (5) | 9 (2) | 471 (27) | |
| Race | ||||||
| White/Caucasian | 775 (54) | 518 (36) | 99 (7) | 35 (2) | 0.0007 | 1427 (82) |
| Black/African-American | 40 (52) | 21 (27) | 8 (10) | 8 (10) | 77 (4) | |
| Asian | 81(51) | 69 (43) | 7 (4) | 0 | 159 (9) | |
| Other | 35 (51) | 29 (42) | 5 (7) | 2 (1) | 69 (4) | |
| Ethnicity | ||||||
| Hispanic | 40 (41) | 48 (49) | 6 (6) | 3 (3) | 0.053 | 97 (6) |
| Non-Hispanic | 891 (54) | 589 (36) | 113 (7) | 42 (3) | 1635 (94) | |
| Years since graduation | ||||||
| Less than 5 | 38 (55) | 31 (45) | 0 | 0 | 0.0003 | 69 (4) |
| 5 to 15 | 220 (50) | 192 (44) | 21 (5) | 7 (2) | 440 (25) | |
| More than 15 | 677 (55) | 422 (34) | 99 (8) | 39 (3) | 1237 (71) | |
| Type of practice | ||||||
| Owner of private practice | 698 (55) | 452 (35) | 87 (7) | 39 (3) | 0.006 | 1276 (74) |
| Associate in private practice | 102 (50) | 89 (43) | 9 (4) | 5 (2) | 205 (12) | |
| Health Partners | 33 (77) | 9 (21) | 1 (2) | 0 | 43 (2) | |
| Permanente | 40 (59) | 25 (37) | 3 (4) | 0 | 68 (4) | |
| Public health, community | 26 (42) | 28 (45) | 8 (13) | 0 | 62 (4) | |
| Academic | 22 (47) | 17 (36) | 7 (15) | 1 (2) | 47 (3) | |
| Other | 12 (35) | 17 (50) | 4 (12) | 1 (3) | 34 (2) | |
| Network region | ||||||
| Western | 156 (55) | 107 (38) | 19 (7) | 2 (1) | 0.005 | 284 (16) |
| Midwest | 107 (61) | 59 (34) | 8 (5) | 1 (1) | 175 (10) | |
| Southwest | 147 (48) | 137 (44) | 20 (6) | 5 (2) | 309 (18) | |
| South Central | 176 (54) | 109 (33) | 26 (8) | 18 (5) | 329 (19) | |
| South Atlantic | 171 (53) | 119 (37) | 22 (7) | 12 (4) | 324 (19) | |
| Northeast | 181 (55) | 114 (35) | 25 (8) | 8 (2) | 328 (19) | |
| Practice busyness | ||||||
| Too busy | 50 (51) | 37 (37) | 10 (10) | 2 (2) | 0.01 | 99 (6) |
| Burdened | 181 (57) | 118 (37) | 13 (4) | 5 (2) | 317 (18) | |
| Balanced | 461 (51) | 333 (37) | 74 (8) | 34 (4) | 902 (52) | |
| Not busy | 245 (57) | 156 (36) | 23 (19) | 5 (1) | 429 (25) | |
| Private insurance status | ||||||
| <40% private insurance | 28 (54) | 15 (29) | 6 (12) | 3 (6) | 0.523 | 52 (3) |
| 40–79% private insurance | 408 (55) | 269 (36) | 49 (7) | 18 (2) | 744 (43) | |
| 80%+ private insurance | 495 (53) | 349 (37) | 63 (7) | 24 (3) | 931 (54) | |
| Practice hours | ||||||
| Full-time (32+hours/week) | 814 (55) | 538 (36) | 93 (6) | 40 (3) | 0.071 | 1485 (86) |
| Part-time (<32 hours/week) | 119 (48) | 100 (40) | 24 (10) | 5 (2) | 248 (14) | |
| Environment location of practice | ||||||
| Inner city of urban area | 104 (50) | 79 (38) | 13 (6) | 11 (5) | 0.239 | 207 (12) |
| Urban (not inner city) | 258 (53) | 183 (38) | 35 (7) | 7 (1) | 483 (28) | |
| Suburban | 429 (55) | 286 (37) | 49 (6) | 19 (2) | 783 (45) | |
| Rural | 145 (54) | 92 (34) | 22 (8) | 9 (3) | 268 (15) |
DISCUSSION
The results of this study showed a higher prevalence of chamfer or heavy chamfer margin configuration compared to other types of crown preparations: shoulder, shoulder with bevel, and knife edge. Margin preference varied with practice and dentist characteristics in addition to the type of crown material. In this survey, dentists were able to conceptualize using their material of choice for anterior and posterior restorations, so it is likely that material considerations were included in selecting margin configuration. Typically, a shoulder preparation is associated with all-ceramic restorations, although it can be used for a variety of crown materials. Regardless of the margin configuration, a sufficient bulk of material is required to prevent material fracture and allow adequate space for restorative materials.17,30,31 Some evidence suggests a knife-edge or feather-edge margin may be associated with less gingival recession, but more bleeding on probing.32,33 The knife-edge preparation was frequently correlated with minimally invasive crown preparations.34
Studies have shown that a clinician’s gender is associated with treatment decisions and outcomes.35–38 However, it is unclear as to why females use more shoulder preparations than males, for both anterior and posterior crowns. It is tempting to speculate that females use all-ceramic materials more often than males, but results from this survey suggested the opposite. Regarding posterior crowns, females were more likely to use metal-ceramic crowns.39 Margin design also varied based on years since graduation of the dentist, with more experienced clinicians less likely to use shoulder preparations. This may be associated with the relatively recent widespread use of all-ceramic crowns, and their associated demands for marginal bulk of material. Survey findings have shown that the leading choice of crown materials for an anterior crown is lithium disilicate, and the principal choices for posterior crowns are all-zirconia and porcelain-fused-to-metal crowns.39 The heavy chamfer margin typically used to prepare a tooth for a zirconia crown allows the restoration to have a bulk of material that is confined to the space created by the tooth preparation and withstand the forces of occlusion.40,41 Some other advantages are suggested for chamfer marginal design in the literature. Prior studies have shown the fracture resistance of deep chamfer margins was greater than that of other marginal configurations.15,42,43 Another study supported the use of heavy chamfer preparations for all-ceramic crowns, as more practices are driven by esthetics.40 Most network dentists selected this margin for anterior and posterior single-unit crowns. The advantages of heavy chamfer finish lines are that they provide superior support for all-ceramic restorations, improve seating, and provide adequate bulk of material.44
Network dentists who participated in this study reported the ideal location for a finish line for a single-unit crown is at the crest of the gingival tissue. Some reports show that equigingival margins are associated with plaque accumulation and gingival inflammation.45–48 Additionally, any amount of gingival recession can potentially cause a visible, unesthetic margin; however, it is possible that with the use of all-ceramic crowns, visible crown margins can be esthetically acceptable and finished easily to provide a smooth, polished interface at the gingival margin, reducing the undesirable attributes of equigingival margins.
In this group of dentists responding to this survey, a combination of the dentist, assistant, and patient selected the shade for posterior crowns almost half the time; for anterior crowns, dentists reported this group selects the shade 59% of the time. Twenty-five percent of the time, the dentist alone selected the shade for anterior crowns. Approximately 25% of dentists do not use photographs to communicate shade selection with the laboratory on esthetically demanding cases. It would be worth future research to investigate whether this group has a higher incidence of remakes.
This study did have certain limitations, and the interpretation of the study should take these into account, as previously reported.26 When describing margin configuration, the crown material was not specified, only that the crown was posterior or anterior. Clearly, the choice of a particular margin design was confounded by the material variable. This study used self-reported information, which may differ from actual clinical treatment behavior. Additionally, although the response rate was good, it is possible that non-respondents would have reported different behavior. Although network practitioners have much in common with dentists at large,49,50 it their crown procedures may not be representative of a wider number of dentists. Additionally, network members were not recruited randomly, so factors associated with network participation (e.g., an interest in clinical research) could have made 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: 1) substantial percentages of network general dentists are represented in the various response categories of the characteristics in the enrollment questionnaire; 2) findings from several network studies document that network general dentists report patterns of diagnosis and treatment that are similar to patterns determined from non-network general dentists51–54 and 3) the similarity of network dentists to non-network dentists using the best available national source, the 2010 ADA Survey of Dental Practice.55
CONCLUSIONS
According to the results of this study:
Network dentists preferred chamfer/heavy chamfer finish lines for anterior and posterior crowns over other margin designs.
The choice of margin design is significantly associated with certain practice and dentist characteristics.
When developing practices, a clinician benefits from knowing how other dentists have been operating and what factors may influence these decisions.
ACKNOWLEDGEMENTS
This work was supported by NIH grant U19-DE-22516. An Internet site devoted to details about the nation’s network is located at http://NationalDentalPBRN.org. We are very grateful to the network’s Regional Coordinators who followed up with network practitioners to improve the response rate (Midwest Region: Tracy Shea, RDH, BSDH; Western Region: Stephanie Hodge, MA; Northeast Region: Christine O’Brien, RDH; South Atlantic Region: Hanna Knopf, BA, Deborah McEdward, RDH, BS, CCRP; South Central Region: Claudia Carcelén, MPH, Shermetria Massengale, MPH, CHES, Ellen Sowell, BA; Southwest Region: Stephanie Reyes, BA, Meredith Bucherg, MPH, Colleen Dolan, MPH). Opinions and assertions contained herein are those of the authors and are not to be construed as necessarily representing the views of the respective organizations or the National Institutes of Health. The informed consent of all human subjects who participated in this investigation was obtained after the nature of the procedures had been explained fully.
This work was supported by NIH grantU19-DE-22516.
Footnotes
The authors deny any conflicts of interest.
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