Crown Preparation Design: An Evidence-Based Review Jason W. Soukup, DVM Summary:

Evidence-based clinical guidelines for full metal crown preparation design are seldom encountered in the veterinary literature. The veterinary literature regarding prosthodontic treatment in companion animals is sparse and consists primarily of case reports and expert opinion based primarily on clinical experience. The goal of this manuscript is to present concepts of evidence-based practice, review common recommendations on crown preparation design from the veterinary literature, and to provide an assessment of the evidence for or against those recommendations. J Vet Dent 30(4); 214-219, 2013

Introduction

The problem was best articulated by Roudebush, Logan and Hale1: “The prevailing system of veterinary medical education and the practice of continued learning are not based on rigorous assessment of evidence for or against particular management options, including many aspects of veterinary dentistry”. One such aspect of veterinary dentistry that has traditionally not been firmly rooted in evidence-based practice is prosthodontics; in particular preparation design for the application of full metal crowns in dogs. Evidence based medicine is the incorporation of best research evidence, clinical expertise and patient values (in veterinary medicine this is more appropriately referred to as client values) into the clinical decision making process. The best clinical decisions are made when all three aspects are fully considered. The clinician must successfully incorporate research evidence with his/her own clinical expertise and experience. The ultimate decision will also be guided by the preferences, concerns, and expectations of the client and the veterinarian’s obligation to respect the interest of the patient (health status, prognosis, benefits and risks of each treatment option). When making diagnostic and treatment decisions, veterinarians should strive to seek and apply information derived from systematic, controlled clinical studies, with appropriate statistical analysis.

Evidence Levels

In order for a clinician to use solid evidence in the decision making process, he/she must have an appreciation for what constitutes best research evidence. Several methods for evaluating research evidence have been presented.1,6-8 An evidence pyramid is one such method (Fig. 1). The top of the pyramid represents sources of the strongest evidence and includes randomized controlled clinical studies and systematic reviews. These are followed by sources of weaker evidence, such as epidemiologic studies, models of disease, case series, case reports and textbooks (non-peer reviewed ideas and opinions). As one progresses up the pyramid, the amount of available literature decreases. An alternative method for assessment of evidence is a 214

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scheme derived from a classification system proposed for use in veterinary clinical nutrition (Table 1).6 This scheme has also been applied to oral homecare products.1 In this classification scheme, grade I represents the evidence with the highest quality and is comprised of randomized, controlled clinical studies in the target species. Grade IV represents the evidence with the lowest quality and is comprised of evidence from textbooks, editorials, proceedings, etc. Most available veterinary literature on crown preparation design in dogs would fall into grade III or IV.

Common Clinical Recommendations and Associated Evidence for Veterinary Crown Preparation Design Recommendation #1 - A Convergence angle of 5-14 degrees should be utilized to achieve acceptable retention/resistance form (Table 2 and Fig. 2).2,3,9-12 A recommendation that is perpetuated in the veterinary literature is that axial wall reduction should be performed in order to achieve parallelism. This recommendation appears to be based on very early human studies. In an in vitro study performed in 1955, it was shown that if a tensile force were applied, the amount of force required to separate a restoration cemented to a die with zinc phosphate luting cement would drop as the convergence angle (CA) increased.13 This relationship was found to be hyperbolic with a precipitous drop in retention as the CA increased from 5° to 20° and a plateau in retention between 20° and 90°. These findings were confirmed in subsequent in vitro studies. However, if the inner surface of the cast restoration was etched (sandblasted) there was a 2-3-fold increase in the amount of force required to separate the restoration from the die. This latter finding has essentially been forgotten and, thus, has not been factored into present recommendations on acceptable CA. Sandblasting the inner surface of a metal crown is common practice today and should be factored into the equation when determining an acceptable CA. In an in vitro investigation into the effect of CA on resistance form, dies were prepared that had a length of 3.5-mm, a diameter of 10-mm, and varying CA between 10-22°.14 The ability of the crown to resist shear forces was tested and it was found that the amount of force required to displace the crown decreased as the CA increased, consistent with previous findings.13 However, the mean force required to displace the crown at a CA of 22° (consistent with what is achievable in dogs12) was 1386 N. It has been shown that the amount of force a dog generates during encouraged chewing behavior, primarily representing occlusal stress at the restoration-cement-tooth interface, is in the range of 20-937 N.15 It has also been shown that the range of bitingpulling force generated on canine teeth, primarily representing shear stress at the restoration-cement-tooth interface, by military dogs is 180-1120 N.16 The forces dogs have been shown to generate in the referenced studies are lower than the forces required to displace a crown on a tooth that has unfavorable physical characteristics.

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Figure 1

Evidence Pyramid. This representation presents the levels of evidence in a semi-quantitative manner. The apex of the pyramid represents the most desirable (and least available) evidence and the base represents the least desirable (and most available) evidence.

Figure 2

Comparison of convergence angle (CA) and taper. Taper (T) is the angle between one axial wall of the preparation and the long axis of the preparation. CA (CA) is the angle between two opposing axial walls of a preparation and equals the sum of the taper of two opposing axial walls. (From: Soukup JW, Snyder CJ, et al. Achievable CA and the effect of preparation design on the clinical outcome of full veneer crowns in dogs. J Vet Dent 2011; 28:72-82. Reprinted with permission.)

Table 2 (From: Evidence-based practice in the health sciences: Evidence-based nursing tutorial. Information Services Department of Library of the Health Sciences-Chicago, University of Illinois at Chicago. Available at http://ebp.lib.uic.edu/nursing/node/12. Accessed on June 26, 2012. Reprinted with permission.)

Table 1

Commonly recommended axial wall inclinations (taper/ convergence angle [CA]) in veterinary dentistry. Veterinary Recommendation Source

Reference Provided

Reference # 2 CA = 6° None

Evidence Grade Grade IV Textbook/expert opinion

Evidence Grades. Grade I is the most desirable evidence and grade IV is the least desirable evidence.

Reference # 9 Taper = 5-7° Human textbook Grade IV chapter Textbook/expert opinion, studies in other species

Evidence Grade I

Evidence Guidelines Evidence obtained from at least 1 properly randomized controlled clinical study that used the treatment in the target species with animals that had developed the disease/condition naturally

Reference # 10 CA = 5-10° None

Grade IV Textbook/expert opinion

Reference # 3 CA = 10° None

Grade IV Textbook/expert opinion

Reference # 11 Taper = 6° None

Grade IV Textbook/expert opinion

II

Evidence obtained from randomized controlled clinical studies conducted in a laboratory setting that used the treatment in the target species with animals that had developed the disease/condition naturally

Reference # 12 CA < 25-30°; but N/A - dependent on height and original study diameter of preparation

Grade III – Clinical study

III Evidence obtained from 1 or more of the following*: • At least 1 properly designed clinical study without randomization • Cohort or case-controlled analytic studies • Studies that used acceptable models of disease or simulations in the target species • Case series • Dramatic results from uncontrolled studies IV Evidence obtained from 1 or more of the following: • Opinions based on clinical experience (textbooks, monographs, or proceedings) • Descriptive studies • Studies conducted in other species • Pathophysiologic justification • Reports of expert committees *Data published in peer-reviewed journals is preferred.

Only one veterinary study exists assessing CA in dogs and it suggested that parallelism was not as important to success as previously reported.12 The mean CA of clinically successful crowns was 25.7°. It also showed that the necessary CA for a successful outcome was not dictated solely by CA. It was also dependent on other physical parameters of the preparation (i.e. height and diameter of the tooth). Near parallelism can rarely be achieved clinically (0-13% of the time)12,17-19 and achievable angles range from 16-27° in humans17-21 and 8-70° in dogs12. In addition, most CA studies have focused on the CA between the opposing axial walls of a preparation. By implementing internal grooves, the “effective” CA can be decreased and, thus, improve the resistance/retention form (Fig. 3).22,23 Parallel, vertical, 1-mm deep proximal grooves placed into a cast/die in vitro were tested and evaluated.22 These grooves provided complete resistance to dislodgement in dies with unfavorable preparation design properties (height of 3 to 4-mm and taper of 10-15°), whereas, without the grooves, the same design failed to provide dislodgement resistance. Similar J VET DENT Vol. 30 No. 4 Winter 2013

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Figure 3

Internal grooves placed in a crown preparation with poor resistance/retention features (low height/diameter ratio and high convergence angle [CA]). Note the wall of the internal groove intersects with the path of rotation (red arrow) reducing the “effective” CA of the preparation and prevents dislodgement of the crown when an oblique occlusal force (black arrow) is delivered.

Table 3

Commonly recommended minimum clinical height (mm) for support of crown in veterinary dentistry. Veterinary Recommendation Source

Reference Provided

Evidence Grade

Reference # 9 3 to 4-mm Human textbook Grade IV chapter Textbook/expert opinion, studies in other species Reference # 10 6 to 8-mm None

Grade IV Textbook /expert opinion

Reference # 11 4 to 6-mm None

Grade IV Textbook /expert opinion

Reference # 25 1/3 of clinical crown None

Grade IV Descriptive/case report

Reference # 26 1/3 of clinical crown None should be available

Grade IV Textbook/expert opinion

Reference # 12 H/D should be > 1.6 N/A – original study

Grade III Clinical study

H = height; D = diameter; CA = convergence angle

up to 25-30° may be clinically acceptable in dogs. However, the recommendation for CA is dependent upon other physical dimensions (height, diameter, surface area) of the preparation (see below). There is also grade III and IV evidence that when using resin-based cements even larger CAs are often clinically acceptable.

(From Blair FM, Wassell RW, et al. Crowns and other extra-coronal restorations: preparations for full veneer crowns. Br Dent J 2002; 192:561-571. Reprinted with permission.)

axial wall grooves improved resistance to dislodgement over a control group in dies prepared with extremely unfavorable design properties (H/D ratio near 0.25 and CA of 20°).23 It is also important to note that the comparison 1955 study used a luting cement of zinc phosphate.13 Zinc phosphate cements are no longer utilized due to their poor bonding properties compared to more modern cements. The effect of various cements (zinc phosphate, glass ionomer-based, resin-based) was compared on the retention of crowns cemented onto dies with varying physical properties (height, diameter, CA).24 The amount of force required to dislodge a crown cemented onto a die with a CA of 24° and a height of 4-mm was increased 3.5 times when a resin-based cement was utilized instead of zinc phosphate. Even when a 70° CA was used, the force required to dislodge the crown was 600 N. In a recent study, it was found that the use of resin-based cements allowed the use of high (up to 70°) CAs.12 Therefore, the use of more modern materials must be taken into consideration when determining an acceptable CA. Summary: There is only grade IV evidence for the use of a CA of 5-12°. There is grade III evidence that achieving the commonly recommended CA of 5-12° is very difficult and rarely achieved. There is grade III evidence that suggests CAs 216

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Recommendation #2 - A Minimum 3 to 8-mm (or 1/3) Clinical Crown Must Exist to Support a Full Metal Crown (Table 3):9-12,25,26 Most veterinary references have historically made recommendations that fail to consider the interdependency of CA, tooth diameter, tooth height, and tooth surface area. However, the human literature emphasizes this point. It has been shown that, for a given base diameter and CA, an increase in height resulted in increased retention.27 Others have elaborated on the roles of height, base diameter, and CA in preventing restoration dislodgement (Fig. 4).14 For a given tooth diameter and CA, preparation height is directly proportional to dislodgement resistance. Conversely, for a given height and diameter, CA is inversely proportional to dislodgement resistance. A tailored guideline that considers all 3 physical properties for crown preparation in dogs has been provided.12 Each preparation has a unique H/D ratio and preparations with H/D ratios < 1.6 may be more likely to fail compared to those with H/D ratios > 1.6. This study also showed that there is no minimum height necessary, per se. However, for a given diameter, as the height decreases, a smaller CA becomes more critical for resistance/retention form. A preparation with higher surface area allows for a greater surface area for the formation of an adhesive bond between the tooth and the crown cement. A direct correlation has been shown between surface area and dislodgement resistance in an in vitro

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Figure 4

Figures depicting the relationship between height (H), diameter (D) and convergence angle (CA). (A) If, for a given D (10-mm), CA (10°), and H (3-mm), line AB is equal to or longer than line AC, casting will dislodge. (B) If height is increased to 4.2-mm, casting will not dislodge because of interference with side BC. (C) If diameter is decreased to 6-mm, casting will not dislodge because of interference with side BC. (D) If CA is reduced to 5°, casting will not dislodge because of interference with side BC.

(Modified with permission from Weed RM, Baez RJ. A method for determining adequate resistance form of complete cast crown preparations. J Prosthet Dent 1984; 52:330-334.)

study.27 The dislodgement resistance was evaluated for a tensile load applied to a cast crown cemented to a machined aluminum alloy die with zinc phosphate cement. The force required to dislodge the crown from the die was directly correlated with the surface area. It has been suggested that this relationship is also true in dogs.a In an in vivo study, the surface area of clinically prepared dies for canine teeth in dogs was measured with a 3-D scanner. The surface area was recorded and compared to the clinical outcome (dislodgement, no dislodgement, crown fracture) of the crown. It was shown that the lower the surface area, the more likely the crown was to be dislodged (poor resistance/retention form). A preparation’s surface area is largely dictated by the physical dimensions of the preparation (height, diameter, CA). However, these physical dimensions do not necessarily restrict the surface area available for cement contact. Internal features (grooves, boxes, etc.) may be placed into the preparation to increase a preparation’s surface area.22,23 In addition to altering the effective CA, as described above, this technique also increases the available surface area for adhesive bonding of the cement. Although not recommended by this author, another method of increasing height, and thus surface area, is to place the margin subgingivally.28,29 The decision to place a margin subgingivally must be considered carefully. The potential quantitative benefit of an increase in retention and resistance form must be weighed against the potential negative consequences to periodontal health

Table 4

Commonly recommended maximum crown height (length) in veterinary dentistry. Veterinary Recommendation Source

Reference Provided

Reference # 9 < 2/3 of the original None Grade IV crown length Textbook/expert opinion Reference # 10 < 2/3 of the original None Grade IV crown length Textbook/expert opinion Reference # 26 Slightly shorter than None Grade IV the lost crown Descriptive/case report Reference # 12 H/D ratio < 1.6 N/A original study Grade III – Clinical study

(see recommendation #4 below). An alternative method to maximize crown height without the negative consequences to the periodontium from a sub-gingival margin is to perform a crown lengthening procedure.9 Summary: There is grade IV evidence for the recommendation of a minimum crown height as reported in an absolute unit (mm). There is grade III evidence that suggests crown heights of less than 1/3 of the original height may lead to higher rates of crown failure of the canine tooth in dogs. There is also grade III evidence that crown treated teeth with H/D ratios < 1.6 may have higher failure rates. It is accurate to note that retention and resistance improves with increasing height of the preparation. However, given the broad range of clinical tooth presentations (breed, size, fracture versus non-vital tooth) a veterinary dentist may encounter, and in light of the interdependencies described here, making the statement that a minimum height of 3 to 8-mm of clinical crown is required for adequate resistance/retention may be misleading. Recommendation #3 - Final Cast Crown Height Should Not Exceed 2/3 the Original Crown Length (Table 4):9,10,12,25 This recommendation has been made in order to take the tooth out of occlusion and thus reduce the forces placed upon the restoration. It is argued that doing so would decrease the chance of clinical failure (crown dislodgement, crown fracture). However, there does not appear to be an evidence-based origin for this recommendation. A literature search revealed only one study that comments on any association between tooth height and the clinical outcome. Another study failed to show any positive association between increasing height and crown dislodgement.12 In fact, the opposite association was found to exist. A crown was more likely to be dislodged when the H/D ratio was < 1.6. However, the study also revealed that preparations with H/D ratios > 1.8 may be more likely to fracture. Considering height alone to evaluate possible future clinical outcome has been shown to be oversimplified. All physical parameters of a preparation play a role in the clinical outcome and the clinician must consider the interplay between those parameters when making preparation design decisions. a,12,14,30,31 Summary: There is grade IV evidence for the recommendation that a final cast crown height > 2/3 the original crown height may lead to clinical failure. On the contrary, there is grade J VET DENT Vol. 30 No. 4 Winter 2013

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Table 5

Common recommendations regarding the placement of the crown margin in veterinary dentistry. Veterinary Source

Recommendation

Reference Provided

Evidence Grade

Larato DC. Effect of cervical margins on gingiva. J Calif Dent Assoc 1969; 45:19. Loe H. Reactions of marginal periodontal tissues to restorative procedures. Int Dent J 1968; 18:759.

Grade IV – Textbook/expert opinion, studies in other species

Margin placement [should be] 1-2 mm coronal to the free gingival margin

None

Grade IV – Textbook/expert opinion

[Margin] should be seated 1-2 mm above the gingival margin

None

Grade IV – Textbook/expert opinion

Reference #9 Studies…report increases in gingival inflammation and periodontitis in areas of sub-gingival restorative materials Sub-gingival margin should only be used in specific cases where good oral hygiene will be practiced Reference #11 Reference #26

III and grade IV evidence that taller preparation heights aid in resistance/retention form and generally improve clinical success. However, there is grade III evidence that suggests preparations with H/D ratios > 1.8 may have higher crown fracture rates. Recommendation #4 – The crown margin should be placed supragingivally (Table 5):9,11,26 Most veterinary references recommend a supragingival margin, except when otherwise dictated by esthetics or the need for increased height for improved retention/resistance form. This recommendation appears to be firmly based on clear evidence in both dogs and humans. Subgingival margins are associated with increased plaque accumulation32, gingival inflammation33-38, deep pocket formation33,34,37,39, attachment loss33,34, and gingival recession40. The clinical and histologic consequences of both subgingival and supragingival crown margins to normal control teeth has been compared in adult dogs.28 The study reported increased clinical gingivitis and higher degrees of histologic inflammatory changes associated with subgingival margins. There was a direct relationship between the depth of margin within the sulcus and the degree of inflammation. The deeper the margin within the sulcus, the more pronounced the inflammation. In addition, gingival recession was seen with subgingival margins. Degree of gingivitis and histologic evidence of inflammation for supragingival finishes were minimal and consistent with the normal control teeth. Placing the margin in the subgingival space increases the likelihood of compromising biologic width.41 Two studies in beagle dogs have demonstrated the negative periodontal consequences of compromising biologic width. Restorative margins at the alveolar crest were associated with 5-mm of alveolar bone resorption.42 Margins placed 4-mm coronal to the alveolar crest caused minimal resorption. In addition, significant gingival recession and bone loss compared to a control (3.16 and 1.17-mm versus 0.5 and 0.15-mm, respectively) occurred in class V restorations with the apical extent at the alveolar crest.43 The human literature supporting this recommendation is much more abundant. One important 10-year longitudinal study revealed that gingival recession was common among 218

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patients with subgingival margins in that only slightly > 50 % of the original subgingival margins remained subgingival at the end of the study.33,34 In contrast, one human study showed that a slightly subgingival margin did not have a significant deleterious effect on the gingiva.44 However, an intensive hygiene program directed by professionals was required to achieve these results. The reality in veterinary patients is that hygiene is moderate at best and the same positive results should not be expected. Summary: There is grade III and IV evidence supporting the recommendation that a crown preparation margin should be placed supragingivally whenever possible in order to preserve the health of the periodontium.

Conclusions

The best evidence available shows that retention and resistance form generally increases with a lower CA. Also, the larger the H/D ratio, the better the retention and resistance form. However, CA, tooth height, tooth diameter, and tooth surface area are inherently interdependent. This intimate relationship, along with the morphology of the canine tooth in the dog and the use of axial grooves generally allows for larger CAs than have historically been considered ideal. In addition, the best evidence shows that H/D ratio can be used as a reliable method for a clinical assessment of future adhesive/cohesive failure and tooth fracture. Although minor increases in retention and resistance form may be gained with subgingival preparations, the best evidence suggests that this practice is not usually necessary and the gains in retention and resistance form are minimal when compared to the potential deleterious effects on the periodontium. Evidence-based recommendations regarding veterinary crown therapy are limited. However, given the recent interest in the practice of evidence-based medicine, the paucity of prosthodontics research in veterinary dentistry will likely improve. In the meantime, this review should serve to guide veterinary dentists in designing a crown preparation with features that both increase resistance and retention form and avoid deleterious effects on periodontal health. ___________________________________________________ a



Riehl JR, Soukup JW. (2012). The effect of surface area on the clinical outcome of full veneer crowns of the canine teeth in dogs. Unpublished data.

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Author Information

Section of Dentistry and Oral Surgery, Department of Surgical Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, 2015 Linden Drive, Madison, WI, 53706. Email: [email protected]

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21. Kent WA, Shillingburg HT, et al. Taper of clinical preparations for cast restorations. Quintessence Int 1988; 19:339-345. 22. Woolsey GD, Matich JA. The effect of axial grooves on the resistance form of cast restorations. J Am Dent Assoc 1978; 97:978-980. 23. Proussaefs P, Campagni W, et al. The effectiveness of auxillary features on a tooth preparation with inadequate resistance form. J Prosthet Dent 2004; 91:33-41. 24. El-Mowafy OM, Fenton AH, et al. Retention of metal ceramic crowns cemented with resin cements: effects of preparation taper and height. J Prosthet Dent 1996; 76:524-529. 25. van Foreest A, Roeters J. Evaluation of the clinical performance and effectiveness of adhesively- bonded crowns on damaged canine teeth of working dogs over a two- to 52-month period. J Vet Dent 1998; 15:13-20. 26. Visser CJ. Restorative dentistry: crown therapy. Vet Clinics North Amer Small Anim Pract: Canine Dentistry 1998; 28:1273-1284. 27. Kaufman EG, Coehlo DH, et al. Factors influencing the retention of cemented gold castings. J Prosthet Dent 1961; 11:487-502. 28. Karlsen K. Gingival reactions to dental restorations. Acta Odontol Scand 1970; 28:895-904. 29. Reitemeier B, Hänsel K, et al. Effect of posterior crown margin placement on gingival health. J Prosthet Dent 2002; 87:167-172. 30. Parker MH, Gunderson RB, et al. Quantitative determination of taper adequate to provide resistance form: concept of limiting taper. J Prosthet Dent 1988; 59:281-288. 31. Parker MH, Calverley MJ, et al. New guidelines for preparation taper. J Prosthod 1993; 2:61-66.

8. Pitak-Arnnop P, Hemprich A, et al. Evidence-based oral and maxillofacial surgery: some pitfalls and limitations. J Oral Maxillofac Surg 2011; 69:252-257.

32. Kosyfaki P, del Pilar Pinilla Martin M, et al. Relationship between crowns and the periodontium: a literature update. Quintessence Int 2010; 41:109-122.

9. Wiggs RB, Lobprise HB. Operative dentistry: crowns and prosthodontics. In: Wiggs RB, Lobprise HB, eds. Veterinary dentistry: principles and practice. 1st ed. Philadelphia: Lippincott-Raven, 1997; 395-434.

33. Valderhaug J, Birkeland JM. Periodontal conditions in patients 5 years following insertion of fixed prostheses. Pocket depth and loss of attachment. J Oral Rehabil 1976; 3:237-243.

10. Harvey and Emily. Restorative dentistry. In: Harvey CE, Emily PP, eds. Small animal dentistry. 1st ed. St. Louis: Mosby, 1993; 213-265.

34. Valderhaug J. Periodontal conditions and carious lesions following the insertion of fixed prostheses: a 10-year follow-up study. Int Dent J 1980; 30:296-304. 35. Gemalmaz D, Ergin S. Clinical evaluation of all-ceramic crowns. J Prosthet Dent 2002; 87:189-196.

11. Holmstrom SE, Fitch PF, Eisner ER. Restorative dentistry. In: Holmstrom SE, Fitch PF, Eisner ER, eds. Veterinary dental techniques for the small animal practitioner. 3rd ed. Philadelphia: Saunders, 2004; 415-497.

36. Bader JD, Rozier RG, et al. Effect of crown margins on periodontal conditions in regularly attending patients. J Prosthet Dent 1991; 65:75-79.

12. Soukup JW, Snyder CJ, et al. Achievable CA and the effect of preparation design on the clinical outcome of full veneer crowns in dogs. J Vet Dent 2011; 28:72-82.

37. Kancyper SG, Koka S. The influence of intracrevicular crown margins on gingival health: preliminary findings. J Prosthet Dent 2001; 85:461-465.

13. Jørgensen KD. The relationship between retention and CA in cemented veneer crowns. Acta Odontol Scand 1955; 13:35-40.

38. Larato DC. Effects of artificial crown margin extension and tooth brushing frequency on gingival pocket depth. J Prosthet Dent 1975; 34:640-643.

14. Weed RM, Baez RJ. A method for determining adequate resistance form of complete cast crown preparations. J Prosthet Dent 1984; 52:330-334.

39. Orkin DA, Reddy J, et al. The relationship of the position of crown margins to gingival health. J Prosthet Dent 1987; 57:421-424.

15. Lindner DL, Marretta SM, et al. Measurement of bite force in dogs: a pilot study. J Vet Dent 1995; 12:49-52.

40. Padbury Jr A, Eber R, et al. Interactions between gingiva and the margin of restorations. J Clin Periodontol 2003; 30:379-385.

16. Hamel L, Le Brech C, et al. Measurement of biting-pulling strength developed on canine teeth of military dogs. J Vet Dent 1997; 14:57-60.

41. Nevins M. Skurow HM. The intracrevicular restorative margin, the biologic width, and the maintenance of the gingival margin. Int J Periodontics Restorative Dent 1984; 3:31-49.

17. Noonan JE, Goldfogel MH. Convergence of the axial walls of full veneer crown preparations in a dental school environment. J Prosthet Dent 1991; 66:706-708.

42. Parma-Benfenati S, Fugazzotto PA, et al. The effect of restorative margins on the postsurgical development and nature of the periodontium. Part II. Anatomical considerations. Int J Periodontics Restorative Dent 1986; 6:65-75.

18. Nordlander J, Weir D, et al. The taper of clinical preparations for fixed prosthodontics. J Prosthet Dent 1988; 60:148-151. 19. Rafeek RN, Marchan SM, et al. Abutment taper of full cast crown preparations by dental students in the UWI School of Dentistry. Eur J Prosthodont Rest Dent 2006; 14:63-66. 20. Ohm E, Silness J. The CA in teeth prepared for artificial crowns. J Oral Rehabil 1978; 5:371-375.

43. Tal H, Soldinger M, et al. Periodontal response to long-term abuse of the gingival attachment by supra-crestal amalgam restorations. J Clin Periodontol 1989; 16:654-659. 44. Carnevale G, di Feba G, et al. A retrospective analysis of the perio-prosthetic aspect of teeth prepared during periodontal surgery. J Clin Periodontol 1990; 17:313-316.

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Crown preparation design: an evidence-based review.

Evidence-based clinical guidelines for full metal crown preparation design are seldom encountered in the veterinary literature. The veterinary literat...
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