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Retention and resistance in preparations for extracoronal restorations. Part II: Practical and clinical studies
Retention and resistance in preparations for extracoronal restorations. Part II: Practical and clinical studies C. P. Owen, B.D.S., M.Sc. Dent.* University
of the Western Cape, Faculty of Dentistry,
I
Tygerberg,
n 1956 Jorgensen’ first tested the relationship between retention and convergence angle in cemented complete veneer crowns. He used cones 8 mm in diameter at the base and 8 mm high. The convergence angle varied from 5 to 45 degrees in steps of 5 degrees. Instead of cast crowns, brass caps were used with the same base diameter, cemented with the same brand of zinc phosphate cement. To avoid errors in the cementation pressure, a vent was made in the cylindrical bore of the caps to provide an outlet for entrapped air. The maximum retentive value (81.3 gm/mm2) was obtained with the 5-degree convergence angle. The minimum value (57.4 gm/mm2) was with 45 degrees. At 10 degrees the retentive value was 41.4 gm, just about half that at 5 degrees. The relation between retentive force and convergence angle was found to be a hyperbola (Fig. l), with the formula (y - a) . x = K, where y = retentive force in grams per square millimeter, x = convergence angle, a = the constant 5.5, and K = the constant 380. Kaufman et a1.2investigated, as part of a larger study, the retention of metal dies with controlled variations in height, angle of convergence, and diameter. Castings were made with a controlled technique, and the resultant castings cemented to die assemblies were subjected to unseating with tensile forces. With dies of constant taper, analysis showed an obvious but not uniformly proportional increase in retentive ability as the height increased. This is understandable inasmuch as, in any converging cylinder, with each successive millimeter of height the diameter becomes smaller and the area is reduced. This variation was not evident when the retentive surfaces approached parallelism. Thus with a convergent preparation the gingival area contributes the greater retention. Kaufman et al. also found a linear increase in retention as the preparation increased in diameter. As in the case of inlays, the general stress distribution in complete crowns has been investigated by two-
*Prosthodontic
148
Registrar.
South Africa
dimensional photoelastic stress analysis. Craig et al3 tested crowns on tipped molar abutments as well as on normally aligned molars. The maximum compressive stresseson the interior of crowns were generally found to be on the reduced cusp surface of the restorations. When the crowns were loaded bilaterally (to simulate oral conditions) maximum tensile stresses were observed in the central fossa and along the axis of symmetry of the crown. Tensile stresseswere also observed at the cervical margins with high compressive loads and a cusp angle of 39 degrees. Craig et a1.3 drew the following conclusions: (1) Multiple-point contact of an antagonistic tooth reduces stress concentrations near the central fossa of the restored tooth. (2) Full shoulders (or their equivalent) on complete crowns are recommended, and in instances of proximal caries, proximal boxes and bevels are suggested to increase the bulk in the critical cervical margin. This redistributes the tensile stresses developed. (3) High compressive stresses (which may induce postoperative pain) on the interior surface of the crown may be avoided by rounding reduced cusps. (4) Deep developmental grooves carved near the center of the tooth should be avoided, because they will tend to produce deleterious stress concentrations. This also applies if cusps of teeth are reduced and then protected. Lorey and Myers4 used tensile forces to study the retentive qualities of five clinical preparations and a technique die. The force required to break the cement was used for a comparison of retention. The taper and axial length of the preparations were standardized, and a variety of different preparations were tested. In general, the authors found that retentive values increased with increased surface area of the preparation. Thus complete crown preparations gave the maximum retentive value for the clinical preparations. The best value was shown, for all practical purposes, by a partial veneer crown with cingulum pin and a pinledge with tapered pin. Uncemented castings gave no indication of the retentive values obtained after cementation. A popular misconception is that uncemented castings can be retentive. Reisbick and Shillingburg studied various features of AUGUST
1986
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RESISTANCE
partial veneer crowns and complete crown preparations on molars. For partial veneer crowns they tested for presence or absence of grooves and proximal boxes. They found that occlusal and axial reduction alone gave insufficient retention compared with the presence of grooves or boxes. Proximal boxes gave the best results, statistically different from the preparations that had one or two proximal grooves on each side. The data obtained for the preparation with the proximal boxes were statistically similar to those obtained for a complete crown on a premolar preparation. The molar complete crown preparation tested only length and presence or absence of grooves. The results showed that no additional retention was gained from the placement of grooves, but a significant difference existed between a 6 mm- and 2.8 mm-long preparation with the same degree of taper. Ohm and Silness6 pointed out that up to the time of their study no measurements of convergence angles on dies used for clinical work had been reported. Because they had found that loosening of single crowns in their clinic was infrequent, they assumed that the convergence angles used must have been sufficient to secure adequate retention. They measured convergence angles and bevel angles on dies of preparations made for complete crowns with an acrylic facing. They found that for vital teeth, the mean size of the convergence angles varied between approximately 19 and 27 degrees. For endodonticallytreated teeth with a cast gold core, the mean values varied between approximately 12 and 37 degrees. The bevel angles were generally larger than the core angles. The authors made no clinical attempt to assessthe fit of the crowns subsequently made on the dies measured, but reiterated their belief that the magnitude of angles encountered must have been appropriate for single crowns. Similar results were given by Eames et al.’ who measured 50 dies taken at random in a commercial dental laboratory. The mean convergence angle was approximately 20 degrees. Macks investigated the clinical feasibility of preparing crowns with small convergence angles. His theoretic investigations were described in Part I of this article; the clinical simulation and investigation are now reviewed. Mack’s intention was to use single potential variables to assessthe effect on the convergence angle. The variables were distance from dentist to tooth, form of vision used, and tooth position in the arch. By this means it was hoped that the practical results might be correlated with the theoretic values for taper. No attempt was made to modify or reduce tapers once formed, and the dentist maintained as fixed a viewpoint and position as possible. Finer tapers could readily have been achieved in the clinical situations, but the forming of fine tapers was not the purpose of the simulation. The results therefore THE
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Fig. 1. Relation between retention per unit area and convergence angle in cemented veneer crowns. X, Calculated values outside experimental range. (Redrawn from Jorgensen KD: The relationship between retention and convergence angle in cemented veneer crowns. Acta Odontol Stand 13:35, 1956.)
indicate relative differences in taper achieved under specific conditions, with the modification between each group being, as far as possible, of a single factor only. The results were as follows: 1. Mesiodistal tapers (average 22 degrees 33 minutes) were less than buccolingual tapers (average 25 degrees 17 minutes) on nearly every preparation. The more anterior canine teeth (average 22 degrees 11 minutes) had lesser angles of taper than more distal molar teeth (average 25 degrees 39 minutes). 2. The angulation of walls of individual teeth was improved when the tooth was to be a single complete crown preparation (average 18 degrees 59 minutes) rather than when several crowns were aligned to be abutments for a fixed partial denture (average 28 degrees 51 minutes). 3. The taper was less for crowns prepared under binocular vision (average 20 degrees 31 minutes) than for those prepared under monocular vision (average 27 degrees 20 minutes). All crowns prepared at short (15 cm) dentist-object distances (to simulate the closest scrutiny possible) were undercut to the line of withdrawal (average -5 degrees 41 minutes). In the clinical investigation Macks made measurements of the taper on inlay and crown dies of clinical restorations constructed for patients, as had Ohm and Silness6. He found that the tapers produced in nearly every crown preparation were greater than the recom149
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mended ideal. The average of all the clinical complete crown measurements was 16 degrees 34 minutes, a value less than the average for inlay preparations (21 degrees 59 minutes). Evaluation of the results suggests that finer tapers will be achieved when axial or opposing walls are prepared at the greatest distance from the tooth consistent with visual accuracy. Fine tapers are achieved when monocular vision is used to prepare inlays or crowns mesial to the canine, and when binocular vision is used for extracoronal preparations distal to the canine or for alignment of multiple extracoronal retainers. The grossest tapers were formed between buccolingual surfaces of teeth and between opposite walls of widely separated teeth; undercuts may be formed between the distal surfaces of widely separated teeth and when close binocular vision is used to prepare anterior teeth. Critical appraisal of these tapers is important in the clinical situation. It is probable, therefore, that the limit to the fineness of taper that may be achieved is set by (1) dentist experience, (2) the type of preparation to be formed, (3) its location within the oral cavity, and (4) the requirement that prepared surfaces should not be undercut to the line of withdrawal. Mack8 drew some further important conclusions: (1) The tapers achieved in practice (approximately 17 degrees) bear little resemblance to those advocated in textbooks. (2) Both in theoryand in practice, tapers of the generally recommended 5 degrees are difficult to achieve. This may be possible with the aid of specialized jigs. Woolsey and Matich9 investigated the effect of axial grooves on resistance form. Fifteen sets of castings were made to machined stainless steel dies of 5, 10, and 15 degrees of taper and of 3,4,6,8, and 10 mm preparation length. Each casting was loaded vertically on its sloped surface in an attempt to unseat the casting. Only the castings of 3 and 4 mm preparation length and 10 and 15 degrees of taper were unseated. Two experiments were then performed on these four sets of dies to test parallel proximal and parallel buccolingual grooves. It was found that dies with grooves on the proximal surfaces provided complete resistance to horizontal dislodgment. Dies with grooves on the buccal and lingual surfaces provided only partial resistance. Potts et al.” tested retention and resistance for test dies of different designs. The preparations had axial walls 6 mm in length with a 6-degree taper. Axial grooves, when present, were approximately 5.5 mm long and 1 mm in diameter. Although it is not stated by the authors, their photographs show that all the preparations had a shoulder-type finishing line. They found that the addition of grooves to partial veneer preparations did not significantly augment retention; neither did extending axial surface coverage augment retention, unless that extension was total. This finding was evident for both a 150
three-quarter and a seven-eighths design series. Extending surface coverage to a complete crown design more than doubled the retention shown by any of the partial veneer designs used. The addition of grooves gave insignificant gains in retention. A possible explanation given by the authors is that a groove adds little to the total surface area of the preparation. This observation might also explain the insignificant increase in retention that was observed as axial surface coverage increased from a three-quarter to a seven-eighths preparation. However, the authors point out that the relatively small increase in surface area between a seven-eighths and a complete crown preparation does not explain their observation of significantly greater retention associated with complete coverage. Obviously, then, factors other than surface area alone are involved. They concluded that the primary function of the traditional proximal groove in the partial veneer crown preparation is resistance. Combining these features, that is, grooves and coverage of the distal half of the buccal surface, produced a cumulative effect on resistance. Thus addition of grooves, extension of axial surface coverage, or both produced small increases in retention values but marked increases in resistance. Chan et al.” determined the effect of circumferential cement keys on the retention of cemented complete crowns with both good and poor retentive form. Teeth were prepared with 7- or 30-degree tapering walls. Auxiliary retention in the form of circumferential grooves was placed in three ways: (1) a control with no auxiliary grooves, (2) one groove in the crown, (3) one groove in the tooth, and (4) opposing grooves in the crown and tooth. The authors found that a single groove placed either in the tooth or in the crown was ineffective in increasing the retention over that of the control with no grooves. However, opposing grooves placed in the tooth and crown so that a cement key formed around the circumference of the tooth and within the crown effectively increased retention. The retention of 30-degree tapered crowns was almost doubled by the cement key, and the retention of 7-degree crowns was tripled. A remarkable finding was that the 30-degree crowns with cement keys had a retentive strength equal to that of the 7-degree crowns without grooves. Circumferential cement keys thus provide a clinically useful increase in retention to highly tapered crowns. Kishimoto et a1.12 after their study on mesialocclusal-distal onlays, made a test die for each of four partial veneer crown designs with axial walls 6 mm in length and with a taper of 6 degrees. The crowns were cemented with zinc phosphate cement. Their study also showed that grooves generally produced little added retention because they add little to the surface area: marked improvement in retention was attained only by doubling the number of grooves or by using boxes. Whether this improvement was achieved by increasing AUGUST
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Table I. Partial veneer crowns Contribution to Preparation Proximal
Retention
features
boxes
Better than proximal Better than
Resistance
one or two grooves nothing
Source Reisbick
Proximal grooves Axial coverage
Did not improve Improved only when coverage is total
Markedly
Proximal boxes Two proximal grooves One proximal groove V-shaped proximal groove
Improved Improved Poor in comparison Did not improve
Improved Improved Poor in comparison Poor
and
increased
Potts
Shillingbut@
et al.‘o
Kishimoto
I_-.-
et al.lz
.._
the surface area or by further geometrically limiting the freedom of displacement was not clear to the authors. Grooves in the lingual position gave greater retention than grooves in the classic position in the buccal third of the proximal surfaces. Resistance of the partial veneer crown did not seem to be sensitive to the geometry as long as the groove or box offered a vertical, flat surface to counteract nonaxial forces. Of the types of forces tested in this study, a V-shaped groove was clearly inferior. A round groove with a lingual wall perpendicular to lingually directed forces was superior to the V groove, the slanting lingual wail of which was less able to withstand those forces. A preparation with two grooves on each proximal surface and one in which boxes were substituted for grooves were equally resistant and retentive, so that if additional retention and resistance are required when the proximal surfaces are free of caries and restorations, it is more conservative to use the partial veneer crown design with four grooves. The mesial-occlusal-distal onlay and partial veneer crown with V grooves had the least retention and resistance of all the preparations. Although either of these preparations could be used for single tooth restorations, their use as retainers is therefore questionable.
reviews illuminate several important factors. Factors affecting intracoronal restorations were previously reported.14
DISCUSSION
Complete-coverage
Retention is considered to be the prevention of removal of a cast restoration along its path of insertion; resistance can be defined as preventing dislodgment by apical or oblique forces and preventing any movement of the restoration under occlusal forces.13 Original classic descriptions of preparations for cast restorations have often been based more on empiricism than on sound scientific research principles. However, as this review series has shown, good evidence now exists as to what aspects of preparation design will contribute to adequate retention and resistance. As yet, however, no one has defined what is “adequate.” Measurement of actual dislodging and unseating forces in the mouth has not yet been reported. Nevertheless, guidelines from the
Table II provides a similar summary of related preparation features as tested for complete-coverage restorations. Proximal grooves do not increase the retention of such preparations, but can be effective in aiding resistance; buccal and lingual grooves give only partial aid to resistance.“’ A significant advance in gaining retention has been shown by Ghan et al.“; circumferential grooves in both preparation and crown markedly improved retention. This finding is of great value for those teeth with short clinical crowns, large convergence angles, or both. The taper of a complete crown preparation has been the overriding feature shown to aid retention. Jorgensen’ first defined the relationship between retention per unit
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Partial veneer restorations Apart from considerations of taper, which will be discussed under the heading of Complete-coverage restorations, partial veneer preparations can incorporate additional limited features. Such features are necessary; occlusal and axial reduction alone gives insufficient retention in comparison with the addition of proximal boxes and grooves.5 Just extending axial coverage is insufficient unless that coverage is total.” In general, proximal boxes will improve both retention and resistance and give better values than single proximal grooves.5,‘2 Some authors disagree, however, over the effect of doubling the grooves: Reisbick and Shillingburg’ found the proximal box to be superior, whereas Kishimoto et al.‘* found equal resistance and retention with two grooves on each proximal surface. In light of these findings, it might be more conservative to use two grooves, provided they are not V-shaped, rather than a proximal box. Some findings from these studies are summarized in Table I.
restorations
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Table Iii Complete
full crowns Contribution
.Preparation Increased
surface
Proximal
grooves
Proximal grooves Buccal and lingual Circumferential
Retention
feature area
Did
Lorey
not
improve Effective Partial
grooves grooves
Source
Resistance
Increased
Increased
area and convergence angles, and showed that a convergence angle of 5 degrees will be twice as retentive as one of 10 degrees. A 20 degree angle gives a retention figure 62% of that at 10 degrees and 81% of that at 5 degrees. This relationship has been confirmed and clarified by other studies. At a constant taper, retention increases with increasing diameter, surface area, and length of the insertion path, although not uniformly.2~4~5 Because of such evidence it is therefore advised to prepare teeth for full coverage with the least amount of taper possible, commensurate with the technical requirements of being able to withdraw a pattern. This method is still taught in most textbooks, but as this review has shown, it is not only impractical, and even clinically impossible, but just does not happen in reality. Mack’s investigation8 pointed out the conditions necessary to prepare a tooth with minimal taper. He showed that the dentist can look directly over the preparation and see all walls, the minimum taper needs to be 5 degrees 42 minutes to confirm lack of undercuts. Such a situation is impractical, and under clinical conditions this minimum taper will increase to as much as 12 degrees. Confirmation of this figure has come from the studies of measured convergence angles of dies used for crowns subsequently fitted. Mack found that the average taper for complete crowns was 17 degrees and of inlays 22 degrees. Ohm and Silness6found angles that varied between 19 and 27 degrees for single crowns on vital teeth. Crowns on a cast gold core had angles varying between 12 and 37 degrees. Eames et al.’ measured an average angle of 20 degrees. Most crowns and inlays subsequently fitted on such dies are apparently still serviceable, a finding that again begs the question of just how much retentive force is required intraorally. It is logical to assume that the force is much less than the in vitro studies would suggest. This does not mean that teeth can be prepared with any convergence angle; rather it serves as a warning that emphasis must be placed on the correct features of a preparation and that overemphasis of minimum taper might be impractical. The studies reviewed still offer sound guidelines. Finer tapers will be achieved at the greatest distance from the tooth consistent with the visual accuracy of the dentist. Monocular vision is more effective when prepar152
to
and
Myers’
Reisbick
and
Shillingburg
Woolsey
and
Matich9
Chan
et al.”
ing anterior teeth, and binocular vision more effective for posterior teeth.* Finer tapers will also contribute to resistance form, more so for a pyramidal preparation than for a conical one.15Apparently, axial angles should not be excessively rounded. Reduced cusps should be rounded, however, to reduce compressive stress, and tensile stresses cervically can be reduced by providing adequate bulk.3
CONCLUSIONS A review of the literature has revealed confirmation of some of the empirically acquired techniques for preparing teeth to receive cast restorations, and has also shown some widely held requirements to be impractical and largely unnecessary in their emphasis. Intracoronal restorations should have rounded internal line angles. Slice preparations should incorporate proximal boxes and have adequate bulk in weak areas. It would be wise to protect cusps in all but the smallest of restorations.‘4 Partial veneer preparations rely on proximal features to improve both retention and resistance. Proximal boxes will give superior results over grooves, but two grooves in each proximal surface may be an alternative. Axial coverage extension does not increase retention unless it extends fully. Complete-coverage preparations thus rely on their size and configuration. Additional features such as axial grooves do not increase retention but may improve resistance. Only circumferential grooves in both preparation and crown will improve retention. This finding is valuable for teeth with short clinical height, large convergence angles, or both. Unless special intraoral jigs are used it is not possible to prepare teeth with a taper of less than about 12 degrees.8 Most teeth are prepared with tapers in excess of 12 degrees, and these still function adequately. It is not known what retentive figure is the minimum required clinically. REFERENCES 1.
Jorgensen KD: The relationship gence angle in cemented veneer 13~35,
between crowns.
retention and converActa Odontol Stand
1956. AUGUST
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8.
9.
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Kaufman EC;, Coelho 4G, Colin L: Factors influencing the rrtentmn of cemented gold castings. J PR~STHET DENT 11:487, 1961. Craig RG. El-Ebrashi MK, Peyton FA: Experimental stress analysis of dental restorations. Part II: Two-dimensional photoelastic stress analysis of crowns. J PROSTHET DENT 17:292, 1907. Lorey RE, Myers GE: The retentive qualities of bridge retainerr J Am Dent Assoc 76:568, 1968. Reisblck MH, Shillingburg HT Jr: Effect of preparation geometry on retention and resistance of cast gold restorations. Calif Dent Assoc J 3:51, 197’1. Ohm E, Silness J: The convergence angle in teeth prepared for artificial crowns. J Oral Rehabil 5:371, 1978. Eames WB. O’Neal SJ, Monteiro J, Miller C, Roan JD Jr, Cohen KS: Techniques IO improve the seating of castings. J Am Dent Assoc. 96:432. 1978. Mack PJ: .\ theoretical and clinical investigation into the taper nchieced on crown and inlay preparations. J Oral Rehabil 7:255, 1980. Woolsey GD, Matich JA: The elect of axial grooves on the rcslstancr lorm of cast restorations. J Am Dent Assoc 97:978, 1978.
Hypersensitive Jack T. Kramer,
10.
11.
12.
13. 14.
15.
Potts RG, Shillingburg HT Jr. Duncanson XI<, Jr: Retention and reststance of preparations for I‘.ISI rrs~or‘.~t;~;ns, .J PRWTIIE I DENT 43:303, 1980. Chan KC, Hormati AA, Boyer DR. Artx:liarv rctetltion loi complete crowns provided by cemrnt kf,y>,, j ~‘ROSt.fll:T. D5.X I 45:152, 198 I. Kishimoto M, Shillingburg H’T Jr, D~nc,mwn MC;: Inlluence of preparation features on retentrrm and iisiatance. Part 11: Three-quarter crowns. J PROSTH~.I DENT 49: 188, 198 i. Shillingburg HT Jr, Hobo S, Whitsett LD Fundamentals oi Fixed Prosthodontics. Chicago. 1978. Quintesrrnw Books. p 67 Owen CP: Factors influencing thr rwentim~ and resistance of preparations for cast intracoronal rrq:!ornIi:>ni J PKO~.I.HK.I DEN.~ 55:674, 1986. Hegdahl T, Silness .J: Preparation arras reslstine d~splaccrnent of artificial crowns. J Oral Rehabil 4:201 IO-
Rejmni rque.\f\ ir,. DR. C. P. OWEN FAC:ULTY oti DENTISTRY PRIVATE BAG X 12 TYGERBERC; 7505 REPURLIC 01: SWTH AFRICA
teeth. Part I: Etiology
D.M.D.
North Palm Beach, Fla.
P
reservation of the natural dentition in a state of comfort, health, and function is the primary goal of the dental profession. Although great progress has been made in the restoration of function and health, attention to comfort has often been overlooked. Hypersensitive teeth are a source of chronic irritation that affects eating, drinking, and breathing. Severe hypersensitivity may even result in an emotional change that causes an altered daily lifestyle. The objective of this two-part article is to present a review of hypersensitive teeth and to include the etiologies, symptoms, theories of pain production, and methods of treatment. Management of hypersensitive dentin tends to be empirical because precise knowledge about the mechanism of pain transmission within the dentin is absent. The dentin is a highly sensitive tissue that differs from other sensitive tissues in the body. In skin, for example, four different sensations, cold, heat, touch, and pain, can
Presented at the Pacific B.C.. Canada. THE
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Society
of Prosthodontists,
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Vancouver,
be perceived. In dentin, different stimuIi cause only pain. When the pulp is removed, dentin is no longer sensitive, which indicates that sensory nerves in the pulp may extend into the dentin and respond when dentin is stimulated. Although it is well established that pulpal nerves carry the sensation of dental pain to higher centers, it is still unclear how the dentin transmits the varied stimuli to these nerves. Evidence suggests that sensitive exposed dentin exhibits patent tubules. Most histologic sections indicate, however that nerve fibers from the pulp only penetrate a limited distance along some dentinal tubules.’ Graf and Galasse2 reported pain arising from exposed dentin in as many as one in seven adult patients. According to Kanapka,3 sensitive teeth afflict some 40 million Americans at one time or another and more than 10 million report chronic hypersensitivity. Little has been accomplished to alleviate this painful problem. Exposure of hypersensitive dentin arises from either the removal of the coronal enamel or a denudation of the root surface by loss of cementum and the overlying 153
of the Western Cape, Faculty of Dentistry,
I
Tygerberg,
n 1956 Jorgensen’ first tested the relationship between retention and convergence angle in cemented complete veneer crowns. He used cones 8 mm in diameter at the base and 8 mm high. The convergence angle varied from 5 to 45 degrees in steps of 5 degrees. Instead of cast crowns, brass caps were used with the same base diameter, cemented with the same brand of zinc phosphate cement. To avoid errors in the cementation pressure, a vent was made in the cylindrical bore of the caps to provide an outlet for entrapped air. The maximum retentive value (81.3 gm/mm2) was obtained with the 5-degree convergence angle. The minimum value (57.4 gm/mm2) was with 45 degrees. At 10 degrees the retentive value was 41.4 gm, just about half that at 5 degrees. The relation between retentive force and convergence angle was found to be a hyperbola (Fig. l), with the formula (y - a) . x = K, where y = retentive force in grams per square millimeter, x = convergence angle, a = the constant 5.5, and K = the constant 380. Kaufman et a1.2investigated, as part of a larger study, the retention of metal dies with controlled variations in height, angle of convergence, and diameter. Castings were made with a controlled technique, and the resultant castings cemented to die assemblies were subjected to unseating with tensile forces. With dies of constant taper, analysis showed an obvious but not uniformly proportional increase in retentive ability as the height increased. This is understandable inasmuch as, in any converging cylinder, with each successive millimeter of height the diameter becomes smaller and the area is reduced. This variation was not evident when the retentive surfaces approached parallelism. Thus with a convergent preparation the gingival area contributes the greater retention. Kaufman et al. also found a linear increase in retention as the preparation increased in diameter. As in the case of inlays, the general stress distribution in complete crowns has been investigated by two-
*Prosthodontic
148
Registrar.
South Africa
dimensional photoelastic stress analysis. Craig et al3 tested crowns on tipped molar abutments as well as on normally aligned molars. The maximum compressive stresseson the interior of crowns were generally found to be on the reduced cusp surface of the restorations. When the crowns were loaded bilaterally (to simulate oral conditions) maximum tensile stresses were observed in the central fossa and along the axis of symmetry of the crown. Tensile stresseswere also observed at the cervical margins with high compressive loads and a cusp angle of 39 degrees. Craig et a1.3 drew the following conclusions: (1) Multiple-point contact of an antagonistic tooth reduces stress concentrations near the central fossa of the restored tooth. (2) Full shoulders (or their equivalent) on complete crowns are recommended, and in instances of proximal caries, proximal boxes and bevels are suggested to increase the bulk in the critical cervical margin. This redistributes the tensile stresses developed. (3) High compressive stresses (which may induce postoperative pain) on the interior surface of the crown may be avoided by rounding reduced cusps. (4) Deep developmental grooves carved near the center of the tooth should be avoided, because they will tend to produce deleterious stress concentrations. This also applies if cusps of teeth are reduced and then protected. Lorey and Myers4 used tensile forces to study the retentive qualities of five clinical preparations and a technique die. The force required to break the cement was used for a comparison of retention. The taper and axial length of the preparations were standardized, and a variety of different preparations were tested. In general, the authors found that retentive values increased with increased surface area of the preparation. Thus complete crown preparations gave the maximum retentive value for the clinical preparations. The best value was shown, for all practical purposes, by a partial veneer crown with cingulum pin and a pinledge with tapered pin. Uncemented castings gave no indication of the retentive values obtained after cementation. A popular misconception is that uncemented castings can be retentive. Reisbick and Shillingburg studied various features of AUGUST
1986
VOLUME
56
NUMBER
2
RETENTION
AND
RESISTANCE
partial veneer crowns and complete crown preparations on molars. For partial veneer crowns they tested for presence or absence of grooves and proximal boxes. They found that occlusal and axial reduction alone gave insufficient retention compared with the presence of grooves or boxes. Proximal boxes gave the best results, statistically different from the preparations that had one or two proximal grooves on each side. The data obtained for the preparation with the proximal boxes were statistically similar to those obtained for a complete crown on a premolar preparation. The molar complete crown preparation tested only length and presence or absence of grooves. The results showed that no additional retention was gained from the placement of grooves, but a significant difference existed between a 6 mm- and 2.8 mm-long preparation with the same degree of taper. Ohm and Silness6 pointed out that up to the time of their study no measurements of convergence angles on dies used for clinical work had been reported. Because they had found that loosening of single crowns in their clinic was infrequent, they assumed that the convergence angles used must have been sufficient to secure adequate retention. They measured convergence angles and bevel angles on dies of preparations made for complete crowns with an acrylic facing. They found that for vital teeth, the mean size of the convergence angles varied between approximately 19 and 27 degrees. For endodonticallytreated teeth with a cast gold core, the mean values varied between approximately 12 and 37 degrees. The bevel angles were generally larger than the core angles. The authors made no clinical attempt to assessthe fit of the crowns subsequently made on the dies measured, but reiterated their belief that the magnitude of angles encountered must have been appropriate for single crowns. Similar results were given by Eames et al.’ who measured 50 dies taken at random in a commercial dental laboratory. The mean convergence angle was approximately 20 degrees. Macks investigated the clinical feasibility of preparing crowns with small convergence angles. His theoretic investigations were described in Part I of this article; the clinical simulation and investigation are now reviewed. Mack’s intention was to use single potential variables to assessthe effect on the convergence angle. The variables were distance from dentist to tooth, form of vision used, and tooth position in the arch. By this means it was hoped that the practical results might be correlated with the theoretic values for taper. No attempt was made to modify or reduce tapers once formed, and the dentist maintained as fixed a viewpoint and position as possible. Finer tapers could readily have been achieved in the clinical situations, but the forming of fine tapers was not the purpose of the simulation. The results therefore THE
JOURNAL
OF PROSTHETIC
DENTISTRY
%d200
--
$60
‘60 ‘40 1: 120
100
60
60
40
20
Fig. 1. Relation between retention per unit area and convergence angle in cemented veneer crowns. X, Calculated values outside experimental range. (Redrawn from Jorgensen KD: The relationship between retention and convergence angle in cemented veneer crowns. Acta Odontol Stand 13:35, 1956.)
indicate relative differences in taper achieved under specific conditions, with the modification between each group being, as far as possible, of a single factor only. The results were as follows: 1. Mesiodistal tapers (average 22 degrees 33 minutes) were less than buccolingual tapers (average 25 degrees 17 minutes) on nearly every preparation. The more anterior canine teeth (average 22 degrees 11 minutes) had lesser angles of taper than more distal molar teeth (average 25 degrees 39 minutes). 2. The angulation of walls of individual teeth was improved when the tooth was to be a single complete crown preparation (average 18 degrees 59 minutes) rather than when several crowns were aligned to be abutments for a fixed partial denture (average 28 degrees 51 minutes). 3. The taper was less for crowns prepared under binocular vision (average 20 degrees 31 minutes) than for those prepared under monocular vision (average 27 degrees 20 minutes). All crowns prepared at short (15 cm) dentist-object distances (to simulate the closest scrutiny possible) were undercut to the line of withdrawal (average -5 degrees 41 minutes). In the clinical investigation Macks made measurements of the taper on inlay and crown dies of clinical restorations constructed for patients, as had Ohm and Silness6. He found that the tapers produced in nearly every crown preparation were greater than the recom149
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mended ideal. The average of all the clinical complete crown measurements was 16 degrees 34 minutes, a value less than the average for inlay preparations (21 degrees 59 minutes). Evaluation of the results suggests that finer tapers will be achieved when axial or opposing walls are prepared at the greatest distance from the tooth consistent with visual accuracy. Fine tapers are achieved when monocular vision is used to prepare inlays or crowns mesial to the canine, and when binocular vision is used for extracoronal preparations distal to the canine or for alignment of multiple extracoronal retainers. The grossest tapers were formed between buccolingual surfaces of teeth and between opposite walls of widely separated teeth; undercuts may be formed between the distal surfaces of widely separated teeth and when close binocular vision is used to prepare anterior teeth. Critical appraisal of these tapers is important in the clinical situation. It is probable, therefore, that the limit to the fineness of taper that may be achieved is set by (1) dentist experience, (2) the type of preparation to be formed, (3) its location within the oral cavity, and (4) the requirement that prepared surfaces should not be undercut to the line of withdrawal. Mack8 drew some further important conclusions: (1) The tapers achieved in practice (approximately 17 degrees) bear little resemblance to those advocated in textbooks. (2) Both in theoryand in practice, tapers of the generally recommended 5 degrees are difficult to achieve. This may be possible with the aid of specialized jigs. Woolsey and Matich9 investigated the effect of axial grooves on resistance form. Fifteen sets of castings were made to machined stainless steel dies of 5, 10, and 15 degrees of taper and of 3,4,6,8, and 10 mm preparation length. Each casting was loaded vertically on its sloped surface in an attempt to unseat the casting. Only the castings of 3 and 4 mm preparation length and 10 and 15 degrees of taper were unseated. Two experiments were then performed on these four sets of dies to test parallel proximal and parallel buccolingual grooves. It was found that dies with grooves on the proximal surfaces provided complete resistance to horizontal dislodgment. Dies with grooves on the buccal and lingual surfaces provided only partial resistance. Potts et al.” tested retention and resistance for test dies of different designs. The preparations had axial walls 6 mm in length with a 6-degree taper. Axial grooves, when present, were approximately 5.5 mm long and 1 mm in diameter. Although it is not stated by the authors, their photographs show that all the preparations had a shoulder-type finishing line. They found that the addition of grooves to partial veneer preparations did not significantly augment retention; neither did extending axial surface coverage augment retention, unless that extension was total. This finding was evident for both a 150
three-quarter and a seven-eighths design series. Extending surface coverage to a complete crown design more than doubled the retention shown by any of the partial veneer designs used. The addition of grooves gave insignificant gains in retention. A possible explanation given by the authors is that a groove adds little to the total surface area of the preparation. This observation might also explain the insignificant increase in retention that was observed as axial surface coverage increased from a three-quarter to a seven-eighths preparation. However, the authors point out that the relatively small increase in surface area between a seven-eighths and a complete crown preparation does not explain their observation of significantly greater retention associated with complete coverage. Obviously, then, factors other than surface area alone are involved. They concluded that the primary function of the traditional proximal groove in the partial veneer crown preparation is resistance. Combining these features, that is, grooves and coverage of the distal half of the buccal surface, produced a cumulative effect on resistance. Thus addition of grooves, extension of axial surface coverage, or both produced small increases in retention values but marked increases in resistance. Chan et al.” determined the effect of circumferential cement keys on the retention of cemented complete crowns with both good and poor retentive form. Teeth were prepared with 7- or 30-degree tapering walls. Auxiliary retention in the form of circumferential grooves was placed in three ways: (1) a control with no auxiliary grooves, (2) one groove in the crown, (3) one groove in the tooth, and (4) opposing grooves in the crown and tooth. The authors found that a single groove placed either in the tooth or in the crown was ineffective in increasing the retention over that of the control with no grooves. However, opposing grooves placed in the tooth and crown so that a cement key formed around the circumference of the tooth and within the crown effectively increased retention. The retention of 30-degree tapered crowns was almost doubled by the cement key, and the retention of 7-degree crowns was tripled. A remarkable finding was that the 30-degree crowns with cement keys had a retentive strength equal to that of the 7-degree crowns without grooves. Circumferential cement keys thus provide a clinically useful increase in retention to highly tapered crowns. Kishimoto et a1.12 after their study on mesialocclusal-distal onlays, made a test die for each of four partial veneer crown designs with axial walls 6 mm in length and with a taper of 6 degrees. The crowns were cemented with zinc phosphate cement. Their study also showed that grooves generally produced little added retention because they add little to the surface area: marked improvement in retention was attained only by doubling the number of grooves or by using boxes. Whether this improvement was achieved by increasing AUGUST
1986
VOLUME
56
NUMBER
2
RETENTION
AND
RESISTANCE
Table I. Partial veneer crowns Contribution to Preparation Proximal
Retention
features
boxes
Better than proximal Better than
Resistance
one or two grooves nothing
Source Reisbick
Proximal grooves Axial coverage
Did not improve Improved only when coverage is total
Markedly
Proximal boxes Two proximal grooves One proximal groove V-shaped proximal groove
Improved Improved Poor in comparison Did not improve
Improved Improved Poor in comparison Poor
and
increased
Potts
Shillingbut@
et al.‘o
Kishimoto
I_-.-
et al.lz
.._
the surface area or by further geometrically limiting the freedom of displacement was not clear to the authors. Grooves in the lingual position gave greater retention than grooves in the classic position in the buccal third of the proximal surfaces. Resistance of the partial veneer crown did not seem to be sensitive to the geometry as long as the groove or box offered a vertical, flat surface to counteract nonaxial forces. Of the types of forces tested in this study, a V-shaped groove was clearly inferior. A round groove with a lingual wall perpendicular to lingually directed forces was superior to the V groove, the slanting lingual wail of which was less able to withstand those forces. A preparation with two grooves on each proximal surface and one in which boxes were substituted for grooves were equally resistant and retentive, so that if additional retention and resistance are required when the proximal surfaces are free of caries and restorations, it is more conservative to use the partial veneer crown design with four grooves. The mesial-occlusal-distal onlay and partial veneer crown with V grooves had the least retention and resistance of all the preparations. Although either of these preparations could be used for single tooth restorations, their use as retainers is therefore questionable.
reviews illuminate several important factors. Factors affecting intracoronal restorations were previously reported.14
DISCUSSION
Complete-coverage
Retention is considered to be the prevention of removal of a cast restoration along its path of insertion; resistance can be defined as preventing dislodgment by apical or oblique forces and preventing any movement of the restoration under occlusal forces.13 Original classic descriptions of preparations for cast restorations have often been based more on empiricism than on sound scientific research principles. However, as this review series has shown, good evidence now exists as to what aspects of preparation design will contribute to adequate retention and resistance. As yet, however, no one has defined what is “adequate.” Measurement of actual dislodging and unseating forces in the mouth has not yet been reported. Nevertheless, guidelines from the
Table II provides a similar summary of related preparation features as tested for complete-coverage restorations. Proximal grooves do not increase the retention of such preparations, but can be effective in aiding resistance; buccal and lingual grooves give only partial aid to resistance.“’ A significant advance in gaining retention has been shown by Ghan et al.“; circumferential grooves in both preparation and crown markedly improved retention. This finding is of great value for those teeth with short clinical crowns, large convergence angles, or both. The taper of a complete crown preparation has been the overriding feature shown to aid retention. Jorgensen’ first defined the relationship between retention per unit
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Partial veneer restorations Apart from considerations of taper, which will be discussed under the heading of Complete-coverage restorations, partial veneer preparations can incorporate additional limited features. Such features are necessary; occlusal and axial reduction alone gives insufficient retention in comparison with the addition of proximal boxes and grooves.5 Just extending axial coverage is insufficient unless that coverage is total.” In general, proximal boxes will improve both retention and resistance and give better values than single proximal grooves.5,‘2 Some authors disagree, however, over the effect of doubling the grooves: Reisbick and Shillingburg’ found the proximal box to be superior, whereas Kishimoto et al.‘* found equal resistance and retention with two grooves on each proximal surface. In light of these findings, it might be more conservative to use two grooves, provided they are not V-shaped, rather than a proximal box. Some findings from these studies are summarized in Table I.
restorations
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Table Iii Complete
full crowns Contribution
.Preparation Increased
surface
Proximal
grooves
Proximal grooves Buccal and lingual Circumferential
Retention
feature area
Did
Lorey
not
improve Effective Partial
grooves grooves
Source
Resistance
Increased
Increased
area and convergence angles, and showed that a convergence angle of 5 degrees will be twice as retentive as one of 10 degrees. A 20 degree angle gives a retention figure 62% of that at 10 degrees and 81% of that at 5 degrees. This relationship has been confirmed and clarified by other studies. At a constant taper, retention increases with increasing diameter, surface area, and length of the insertion path, although not uniformly.2~4~5 Because of such evidence it is therefore advised to prepare teeth for full coverage with the least amount of taper possible, commensurate with the technical requirements of being able to withdraw a pattern. This method is still taught in most textbooks, but as this review has shown, it is not only impractical, and even clinically impossible, but just does not happen in reality. Mack’s investigation8 pointed out the conditions necessary to prepare a tooth with minimal taper. He showed that the dentist can look directly over the preparation and see all walls, the minimum taper needs to be 5 degrees 42 minutes to confirm lack of undercuts. Such a situation is impractical, and under clinical conditions this minimum taper will increase to as much as 12 degrees. Confirmation of this figure has come from the studies of measured convergence angles of dies used for crowns subsequently fitted. Mack found that the average taper for complete crowns was 17 degrees and of inlays 22 degrees. Ohm and Silness6found angles that varied between 19 and 27 degrees for single crowns on vital teeth. Crowns on a cast gold core had angles varying between 12 and 37 degrees. Eames et al.’ measured an average angle of 20 degrees. Most crowns and inlays subsequently fitted on such dies are apparently still serviceable, a finding that again begs the question of just how much retentive force is required intraorally. It is logical to assume that the force is much less than the in vitro studies would suggest. This does not mean that teeth can be prepared with any convergence angle; rather it serves as a warning that emphasis must be placed on the correct features of a preparation and that overemphasis of minimum taper might be impractical. The studies reviewed still offer sound guidelines. Finer tapers will be achieved at the greatest distance from the tooth consistent with the visual accuracy of the dentist. Monocular vision is more effective when prepar152
to
and
Myers’
Reisbick
and
Shillingburg
Woolsey
and
Matich9
Chan
et al.”
ing anterior teeth, and binocular vision more effective for posterior teeth.* Finer tapers will also contribute to resistance form, more so for a pyramidal preparation than for a conical one.15Apparently, axial angles should not be excessively rounded. Reduced cusps should be rounded, however, to reduce compressive stress, and tensile stresses cervically can be reduced by providing adequate bulk.3
CONCLUSIONS A review of the literature has revealed confirmation of some of the empirically acquired techniques for preparing teeth to receive cast restorations, and has also shown some widely held requirements to be impractical and largely unnecessary in their emphasis. Intracoronal restorations should have rounded internal line angles. Slice preparations should incorporate proximal boxes and have adequate bulk in weak areas. It would be wise to protect cusps in all but the smallest of restorations.‘4 Partial veneer preparations rely on proximal features to improve both retention and resistance. Proximal boxes will give superior results over grooves, but two grooves in each proximal surface may be an alternative. Axial coverage extension does not increase retention unless it extends fully. Complete-coverage preparations thus rely on their size and configuration. Additional features such as axial grooves do not increase retention but may improve resistance. Only circumferential grooves in both preparation and crown will improve retention. This finding is valuable for teeth with short clinical height, large convergence angles, or both. Unless special intraoral jigs are used it is not possible to prepare teeth with a taper of less than about 12 degrees.8 Most teeth are prepared with tapers in excess of 12 degrees, and these still function adequately. It is not known what retentive figure is the minimum required clinically. REFERENCES 1.
Jorgensen KD: The relationship gence angle in cemented veneer 13~35,
between crowns.
retention and converActa Odontol Stand
1956. AUGUST
1986
VOLUME
56
NUMBER
2
RETENTION
2.
3.
4. i
6. 7
8.
9.
AND
RESISTANCE
Kaufman EC;, Coelho 4G, Colin L: Factors influencing the rrtentmn of cemented gold castings. J PR~STHET DENT 11:487, 1961. Craig RG. El-Ebrashi MK, Peyton FA: Experimental stress analysis of dental restorations. Part II: Two-dimensional photoelastic stress analysis of crowns. J PROSTHET DENT 17:292, 1907. Lorey RE, Myers GE: The retentive qualities of bridge retainerr J Am Dent Assoc 76:568, 1968. Reisblck MH, Shillingburg HT Jr: Effect of preparation geometry on retention and resistance of cast gold restorations. Calif Dent Assoc J 3:51, 197’1. Ohm E, Silness J: The convergence angle in teeth prepared for artificial crowns. J Oral Rehabil 5:371, 1978. Eames WB. O’Neal SJ, Monteiro J, Miller C, Roan JD Jr, Cohen KS: Techniques IO improve the seating of castings. J Am Dent Assoc. 96:432. 1978. Mack PJ: .\ theoretical and clinical investigation into the taper nchieced on crown and inlay preparations. J Oral Rehabil 7:255, 1980. Woolsey GD, Matich JA: The elect of axial grooves on the rcslstancr lorm of cast restorations. J Am Dent Assoc 97:978, 1978.
Hypersensitive Jack T. Kramer,
10.
11.
12.
13. 14.
15.
Potts RG, Shillingburg HT Jr. Duncanson XI<, Jr: Retention and reststance of preparations for I‘.ISI rrs~or‘.~t;~;ns, .J PRWTIIE I DENT 43:303, 1980. Chan KC, Hormati AA, Boyer DR. Artx:liarv rctetltion loi complete crowns provided by cemrnt kf,y>,, j ~‘ROSt.fll:T. D5.X I 45:152, 198 I. Kishimoto M, Shillingburg H’T Jr, D~nc,mwn MC;: Inlluence of preparation features on retentrrm and iisiatance. Part 11: Three-quarter crowns. J PROSTH~.I DENT 49: 188, 198 i. Shillingburg HT Jr, Hobo S, Whitsett LD Fundamentals oi Fixed Prosthodontics. Chicago. 1978. Quintesrrnw Books. p 67 Owen CP: Factors influencing thr rwentim~ and resistance of preparations for cast intracoronal rrq:!ornIi:>ni J PKO~.I.HK.I DEN.~ 55:674, 1986. Hegdahl T, Silness .J: Preparation arras reslstine d~splaccrnent of artificial crowns. J Oral Rehabil 4:201 IO-
Rejmni rque.\f\ ir,. DR. C. P. OWEN FAC:ULTY oti DENTISTRY PRIVATE BAG X 12 TYGERBERC; 7505 REPURLIC 01: SWTH AFRICA
teeth. Part I: Etiology
D.M.D.
North Palm Beach, Fla.
P
reservation of the natural dentition in a state of comfort, health, and function is the primary goal of the dental profession. Although great progress has been made in the restoration of function and health, attention to comfort has often been overlooked. Hypersensitive teeth are a source of chronic irritation that affects eating, drinking, and breathing. Severe hypersensitivity may even result in an emotional change that causes an altered daily lifestyle. The objective of this two-part article is to present a review of hypersensitive teeth and to include the etiologies, symptoms, theories of pain production, and methods of treatment. Management of hypersensitive dentin tends to be empirical because precise knowledge about the mechanism of pain transmission within the dentin is absent. The dentin is a highly sensitive tissue that differs from other sensitive tissues in the body. In skin, for example, four different sensations, cold, heat, touch, and pain, can
Presented at the Pacific B.C.. Canada. THE
JOURNAL
Coast
OF PROSTHETIC
Society
of Prosthodontists,
DENTISTRY
Vancouver,
be perceived. In dentin, different stimuIi cause only pain. When the pulp is removed, dentin is no longer sensitive, which indicates that sensory nerves in the pulp may extend into the dentin and respond when dentin is stimulated. Although it is well established that pulpal nerves carry the sensation of dental pain to higher centers, it is still unclear how the dentin transmits the varied stimuli to these nerves. Evidence suggests that sensitive exposed dentin exhibits patent tubules. Most histologic sections indicate, however that nerve fibers from the pulp only penetrate a limited distance along some dentinal tubules.’ Graf and Galasse2 reported pain arising from exposed dentin in as many as one in seven adult patients. According to Kanapka,3 sensitive teeth afflict some 40 million Americans at one time or another and more than 10 million report chronic hypersensitivity. Little has been accomplished to alleviate this painful problem. Exposure of hypersensitive dentin arises from either the removal of the coronal enamel or a denudation of the root surface by loss of cementum and the overlying 153