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Post core restoration in endodontically treated posterior teeth
CLINICAL ARTICLE
Post core restoration in endodontically treated posterior teeth G. Wayne Christian, DDS;Gilbert L. Button, DDS; Peter C. Moon, PhD;Marshall C. England, DDS, MS; and Hugh B. Douglas, DDS, MSP
A n in vitro s t u d y was p e r f o r m e d o n e n d o d o n t i c a l l y t r e a t e d m a n d i b u l a r m o l a r s r e s t o r e d w i t h a m a l g a m cores, w i t h a n d w i t h o u t CI posts, in the distal canal to c o m p a r e their r e s i s t a n c e to h o r i z o n t a l forces. U n d e r m o s t c i r c u m s t a n c e s the r e s i s t a n c e to force s e e m e d a d e q u a t e for b o t h t y p e s of r e s t o r a t i o n . T h e p o s t a n d core t r e a t m e n t h a d the g r e a t e s t resistance. H o w e v e r , l o w - s t r e n g t h failures o c c u r r e d for the p o s t a n d core r e s t o r a t i o n w h e n it w a s n o t s u r r o u n d e d b y a m a l g a m in the c h a m b e r , or for the a l l - a m a l g a m core w h e n the c h a m b e r w a s s h a l l o w w i t h o u t a d e q u a t e u n d e r c u t s , a n d w h e n the t h i c k n e s s of the r o o t wall w a s r e d u c e d to o n l y 1 m m .
The restorative dentist is often confronted with an endodontically treated posterior tooth in which portions of the structure crown of the tooth are either missing or severely damaged. In these situations, some type of core restoration is indicated. In recent years, techniques and materials have been developed as an alternative to the more expensive and time-consuming cast-gold procedures. These new restorative procedures involve the use of a m a l g a m or composite resins, used either alone or in conjunction with pins and various types of posts) Restorative procedures should produce m a x i m u m tooth strength, conserve sound dentin, and prevent the fracture of the remaining tooth structure after treatment. Therefore, proper restoration of an endodontically treated posterior tooth cannot be overemphasized. 182
A pulpless tooth has special restorative problems according to Gutmann, 2 who has emphasized the importance of root anatomy, instrumentation, root axis orientation, depth of preparation, and internal shaping of the canal to avoid failure of the restoration and lo~s of the tooth. The loss of elasticity and increased brittleness of pulpless teeth because of loss of moisture 3 makes endodontically treated teeth more fragile and susceptible to fracture. 1 When pins were introduced to aid in the retention of amalgam, it was thought that the pins would strengthen the amalgam, both reinforcing the tooth and splinting it to avoid fracture. ' However, research has proved that pins can weaken amalgam restorations by acting as stress concentrators 5 under tensile stress, causing minute fracture lines
and crazing within the dentin ~;~ and leading to fracture and loss of the tooth. 9n Problems associated with the use of pins could be more severe in endodontically treated teeth; therefore, a post in the root canal is usually advisable. The objective of this study was to determine the horizontal resistance force of alternative systems to the cast-post failed. The two alternative systems evaluated were amalgam cores in posterior teeth used in conjunction with dowel posts compared with all-amalgam cores in posterior teeth. The three types of commercially available dowel posts are parallel, tapered, and threaded parallel; each generates different stress distributions in teeth. Tapered and threaded posts exhibit the highest stress toward the occlusal part of the dow-
JOURNAL OF E N D O D O N T I C S ! VOL 7, N O 4, APRIL 1981
el, while the parallel wall posts tend to distribute stress apically.:'-' C a p u t o and Standlee *:~ do not r e c o m m e n d self-threading posts because they can produce stress a n d crazing in the dentin that predispose the tooth to root fracture. Also, care m u s t be taken during the p l a c e m e n t of the tapered post to keep it from w e d g i n g into the root canal a n d causing root fracture. O n l y the t a p e r e d post was evaluated in this study. METHODS AND
MATERIALS
Twenty-five freshly e x t r a c t e d mandibular molars were used in this study. All teeth were free of caries and restorations in the cervical third of the a n a t o m i c crown; they were placed in w a t e r i m m e d i a t e l y after extraction. T o s i m u l a t e a b a d l y broken-down tooth that w o u l d require a core restoration, a n d to control as many variables as possible, the coronal portion of each tooth was ~duced to a 1 to 2 m m level a b o v e the c e m e n t o e n a m e l j u n c t i o n a n d perpendicular to the long axis of each tooth. In a d d i t i o n , all teeth were m o u n t e d in a m a c h i n e d s a m p l e holder, which established a c c u r a t e tooth position in relation to the loading pin, a n d s t a n d a r d i z e d the amount of resin e m b e d d e d each tooth. The three groups were: the contr01, five intact teeth m o u n t e d in the sample holder in u n a l t e r e d form except for a flat area (5 by 5 m m ) , which was p r e p a r e d on the buccal surface of the enamel, p e r p e n d i c u l a r to the loading pin; g r o u p A, ten teeth prepared to receive an a l l - a m a l g a m core; group B, ten teeth p r e p a r e d to receive a post in the distal canal before a m a l g a m core b u i l d u p . Teeth in g r o u p A were p r e p a r e d as follows: after c o n v e n t i o n a l root c a n a l
therapy, with use of g u t t a - p e r c h a with lateral c o n d e n s a t i o n , the canals a n d p u l p c h a m b e r were p r e p a r e d to receive an a m a l g a m core. A h e a t e d e n d o d o n t i c plugger was used to remove g u t t a - p e r c h a 3 to 4 m m into each canal. T h e mesial canals were p r e p a r e d with a no. 3 Gates G l i d d e n bur to 4 m m , followed by a no. 4 Gates G l i d d e n b u r to a d e p t h of 2 ram. A no. 4 b u r w a s used in the larger distal c a n a l to 4 m m followed by a no. 5 b u r to a d e p t h of 2 ram. U n d e r c u t s were p l a c e d a r o u n d the base of the p u l p c h a m b e r with a no. 330 bur. A c o p p e r b a n d was tightly a d a p t e d to each tooth a n d e x t e n d e d 6 m m a b o v e the p r e p a r a t i o n level of the tooth. T h e b a n d was secured with green c o m p o u n d to stabilize the m a t r i x d u r i n g c o n d e n s a t i o n . 11 Velvalloy was mixed using a 1:1 alloyt o - m e r c u r y ratio. Initially, small e n d o d o n t i c pluggers were used to condense a m a l g a m into the canals (Fig 1). T h e p u l p a n d coronal portion were built up w i t h I- to 2-ram a m a l g a m condensers. Vertical a n d lateral condensations were exerted with small increments of a m a l g a m . T e e t h in group B were p r e p a r e d in the same m a n n e r as those in g r o u p A, except that a no. 2 C I (Parkell) post was placed 6 m m into the distal canal a n d allowed to e x t e n d 2 m m above the level of the p r e p a r a t i o n . T h e CI post h a d a t a p e r of .030 inch, with a small d i a m e t e r of .045 inch. T h e post was c e m e n t e d with zinc p h o s p h a t e c e m e n t (Fig 2). Acrylic resin was used to stabilize each specimen in a m a c h i n e d s a m p l e holder. T e e t h were m o u n t e d in the holder p e r p e n d i c u l a r to the l o a d i n g pin of a universal Instron tensile testing machine. T e e t h in groups A a n d B h a d flat surfaces (5 by 5 ram) p r e p a r e d on buccal surfaces, which were p e r p e n -
Fig 1--All amalgam core.
Fig 2 - C I post in dz~'ial canal with amalgam core.
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JOURNAL OF ENDODONTICS, VOL 7, NO 4, APRIL 1981
Table 9 Summary of force at failure and fracture sites. Sample
dicular to the l o a d i n g pin. A m a chined nut was inserted between the flat surface on the s a m p l e a n d the loading pin to d i s t r i b u t e the force from the pin over a b r o a d e r a r e a on the s a m p l e a n d help p r e v e n t stresspoint c o n c e n t r a t i o n t h a t m i g h t cause p r e m a t u r e fracture. T h e Instron universal testing machine was used for all shearing tests. T h e force was directed at a 90 degree angle to the flattened area on the buccal surface of the a m a l g a m . T h e m a c h i n e was run at a crosshead speed of .02 i n c h / r a i n , a n d the force was recorded on an X Y g r a p h recorder. Force at the time of failure was d e t e r m i n e d from the g r a p h , a n d comparisons were m a d e a m o n g groups A, B, a n d the control group. RESULTS T h e n a t u r a l teeth (control group) d e m o n s t r a t e d the greatest resistance to fi'acture (mean = 269 lb) of all groups tested. T h e a m a l g a m cores with a reinforcing post ( g r o u p B) displayed greater resistance to fracture (mean = 142 lb) t h a n the alla m a l g a m core (group A), which h a d the least resistance to fracture (mean = 123 lb). All teeth in the control group displayed a n g u l a r root fractures. In general, all fractures in this g r o u p originated in the coronal third of the root surface nearest the l o a d i n g pin (buccal) a n d progressed lingually a n d apically to a point in the a p i c a l third of the root. Specific results From group A W h e n the fracture sites were analyzed according to which p a r t failed (the a m a l g a m core or the tooth), certain groupings b e c a m e a p p a r e n t (Table). T h r e e samples (6, 12, 13) 184
Control 1 2 3 4 5 Group A 6 7 8 9 10 11 12 13 14 15 Group B 16 17 18 19 20 21 22 23 24 25
Chamber depth -----4 3 4 4 3 4 3 3 3 2 3 3 4 3 4 3 3 3 3 4
Force at failure (lb) Mean = 269 250 325 255 175 330 Mean = 123 142 138 136 136 130 144 143 148 76 35 Mean = 142 84 195 212 110 225 87 160 87 96 165
Fracture site T* T T T T T At A + T A +T A A A + T A A A + T A
(155)5
(141)* (146)5 (192)5
A A A A A A A A A A
+ + + +
T T T T
+ T + T + T
*Tooth. ",Amalgam. Sin there samples, failure occuned in two s t a g s . In the first stage, the restoration was partially displaced, which would ha~e indicated failure of the restoration; in the final stage, total displacement occurred.
failed at the p r e p a r a t i o n level; two samples ( 9 , 10) showed t h a t the a m a l g a m failed at a d e p t h a p p r o x i m a t e l y 1 m m below the p r e p a r a t i o n level; three other samples (7, 8, l i) had tooth-root a n d a m a l g a m core failures, which occurred at the base of the p u l p c h a m b e r . S a m p l e s 14 a n d 15 were u n u s u a l in their resulting failures. S a m p l e 14 f r a c t u r e d with a c o m b i n a t i o n tootha m a l g a m failure at 76 lb. Close visual e x a m i n a t i o n disclosed t h a t the r e m a i n i n g tooth thickness of the lingual wall in the p u l p a l floor area was only 1 m m . S a m p l e 15 showed a m a l g a m core d i s l o d g m e n t w i t h no tooth fracture at 35 lb. S a m p l e 15 h a d the shallowest c h a m b e r a n d lacked sufficient undercuts. T h e m e a n value of group A was 123 lb. However, if samples 14 a n d 15 were excluded, the m e a n value w o u l d be increased to f40 lb.
Specific results From group B Seven teeth h a d a combination t o o t h - a m a l g a m core failure. Three samples (20, 22, 23) resulted in amalg a m failure only. F u r t h e r visual e x a m i n a t i o n of the samples after final failure a n d separation o f the core from the tooth resulted in two sets, each with a special characteristic. In five samples (16, 19, 21, 23, 24), the post i m p i n g e d on the distal wall of the p u l p c h a m b e r p r e c l u d i n g condensation of a m a l g a m distal to the post. T h e m e a n force required for failure of these samples was only 93 lb. In the other five samples (17, 18, 20, 22, 25), it was possible to condense a m a l g a m , distal to the post, d o w n to the canal orifice. These samples h a d a m e a n force at failure of 191 lb.
JOURNAL OF E N D O D O N T I C S I VOL 7, N O 4, APRIL 1981
DISCUSSION This study shows t h a t differences in preparation a n d p l a c e m e n t of post and core restorations m a y significantly affect t h e resistance to failure. The numerical d a t a i n d i c a t e t h a t the natural tooth c a n w i t h s t a n d g r e a t e r force before f r a c t u r e ( m e a n = 269 lb) than either the a l l - a m a l g a m core in group A ( m e a n = 123 lb) or t h e amalgam c o r e - C I post g r o u p B (mean = 142 lb), w h i c h agrees w i t h a similar s t u d y on m a x i l l a r y incisors by Lovdahl a n d Nicholls. ~ In e v a l u a t i n g t h e l o c a t i o n o f fractures, it was n o t i c e d t h a t six of ten samples in g r o u p A s h o w e d a m a l g a m failure with no d a m a g e to the remaining t o o t h s t r u c t u r e . T h r e e of the ten samples in g r o u p B failed with no d a m a g e to the t o o t h . T h e least d a m a g e f r o m f a i l u r e in t h e oral cavity w o u l d be o n e in w h i c h the remaining t o o t h s t r u c t u r e w o u l d be unaffected. T h e mosl f a v o r a b l e situation if failure d i d o c c u r w o u l d be that the t o o t h w o u l d r e m a i n i n t a c t and could be r e t r e a t e d . The distinct d i c h o t o m y of g r o u p B st~owed the i m p o r t a n c e of the o p e r a tive procedures in r e s t o r i n g e n d o d o n tically treated teeth. T h e strongest experimental r e s t o r a t i o n s in this study were t h e ones in w h i c h the teeth were p r e p a r e d , l e a v i n g s p a c e around the C I post for c o n d e n s a t i o n of a m a l g a m s u r r o u n d i n g the post down to the orifice. T h e s e five s a m ples had a m e a n v a l u e of 191 lb as opposed to o n l y 93 lb for t h e ones
t h a t d i d n o t a l l o w for c o n d e n s a t i o n of the a m a l g a m s u r r o u n d i n g the post.
SUMMARY A n in vitro s t u d y was c o n d u c t e d to c o m p a r e the forces r e q u i r e d to prod u c e failure On t w o types of a m a l g a m core b u i l d u p s . N a t u r a l t e e t h (controls) demonstrated greater resistance to f r a c t u r e t h a n e i t h e r of the experimental groups. The strongest r e s t o r a t i o n was a post w i t h a n a m a l g a m core in w h i c h the a m a l g a m was weU c o n d e n s e d a r o u n d the post. T h e a l l - a m a l g a m core was the n e x t m o s t resistant to f r a c t u r e . T h e least resistant r e s t o r a t i o n was the post w i t h an a m a l g a m core in w h i c h the post p l a c e m e n t p r e v e n t e d cond e n s a t i o n o f the a m a l g a m distal to the post. Dr. Christian, former resident in endodontics, Medical College of Virginia School of Dentistry, now is in private endodontic practice, Charlestown, WVa; Dr. Button is assistant professor, restorative dentistry; Dr. Moon is associate professor, restorative det:: tistry; Dr. England is associate professor and chairman, department of endoduntics; and Dr. Douglas is assistant, professor, restorative dentistry, Medical College of Virginia School of Dentistry, Virginia Commonwealth University, Richmond, Va. Requests for reprints should be directed to Dr. Button, Medical College of Virginia School of Dentistry, Virginia Commonwealth University, Richmotld, Va 23298. References
1. Johnson, J.K.; Schwartz, N.L.; and Blackwell, R.T. Evaluation and restoration of endodontally treated posterior teeth. JADA 93:597-605, 1976.
2. Gutmann, J.L. Preparation of endodontally treated teeth to receive a post-core restoration. J t'rosthct Dent 38:413-419, 1977. 3. Heifer, A.R.; Mclnick, S.; and Schilder, H. Determination of the moisture content of vital and pulpless teeth. J Oral Surg 34:661670, 1972. 4. Markley, R.L. Pin reinforcement and retention of amalgam foundations and restoraiions. JADA 56:675-679, 1958. 5. Going, R.E., and others. The strength of dental amalgam as influenced by pins. JADA 77:1331-1334, 1968. 6. Chan, K.C., and Svare, C.W. Comparison of the dentinal crazing ability of retention pins and machinists taps. J Dent Res 52:178, 1973. 7. Dihs, W.E., and others. Crazing of tooth structure associated with placement of pins for amalgam restorations. JADA 81:387-391, 1970. 8. Markley, M.R. Pin-retained and picareinforced amalgam. JADA 73:1295-1300, 1966. 9. Caputo, A.A., Standlee, J.P.; and Collard, E.W. The mechanics of load transfer by retentive pins. J Prosthet Dent 29:442-449, 1973. 10. Birtcil, R.F., and Venton, E.A. Extracoronal amalgam restorations utilizing available tooth structure for retention. J Prosthet Dent 35:171-177, 1976. 11. Kantor, M.E., and Pines, M.S. A comparative study of restorative techniques for pulpless teeth. J Prosthet Dent 38:405-412, 1977. 12. Standlee, J.P.; Caputo,A.A.; and Cotlard, E.E. Analysis of stress distribution of endodontic posts. Oral Surg 33:952-960, 1972. 13. Caputo, A.A., and Standlee, .I.P. Pins and posts-why, when and how. Dent Clin North Am 20:299-311, 1976. 14. Harrington, W.G., and others. Coinpound reinforced matrices for pin amalgams. J Prosthet Dent 41:622-624, 1979. 15. I,ovdahl, P.E., and Nicholls, J.I. Pia retained amalgam cores vs cast-gold dowelcores. J Prosthet Dent 38:507-514, 1977.
185
Post core restoration in endodontically treated posterior teeth G. Wayne Christian, DDS;Gilbert L. Button, DDS; Peter C. Moon, PhD;Marshall C. England, DDS, MS; and Hugh B. Douglas, DDS, MSP
A n in vitro s t u d y was p e r f o r m e d o n e n d o d o n t i c a l l y t r e a t e d m a n d i b u l a r m o l a r s r e s t o r e d w i t h a m a l g a m cores, w i t h a n d w i t h o u t CI posts, in the distal canal to c o m p a r e their r e s i s t a n c e to h o r i z o n t a l forces. U n d e r m o s t c i r c u m s t a n c e s the r e s i s t a n c e to force s e e m e d a d e q u a t e for b o t h t y p e s of r e s t o r a t i o n . T h e p o s t a n d core t r e a t m e n t h a d the g r e a t e s t resistance. H o w e v e r , l o w - s t r e n g t h failures o c c u r r e d for the p o s t a n d core r e s t o r a t i o n w h e n it w a s n o t s u r r o u n d e d b y a m a l g a m in the c h a m b e r , or for the a l l - a m a l g a m core w h e n the c h a m b e r w a s s h a l l o w w i t h o u t a d e q u a t e u n d e r c u t s , a n d w h e n the t h i c k n e s s of the r o o t wall w a s r e d u c e d to o n l y 1 m m .
The restorative dentist is often confronted with an endodontically treated posterior tooth in which portions of the structure crown of the tooth are either missing or severely damaged. In these situations, some type of core restoration is indicated. In recent years, techniques and materials have been developed as an alternative to the more expensive and time-consuming cast-gold procedures. These new restorative procedures involve the use of a m a l g a m or composite resins, used either alone or in conjunction with pins and various types of posts) Restorative procedures should produce m a x i m u m tooth strength, conserve sound dentin, and prevent the fracture of the remaining tooth structure after treatment. Therefore, proper restoration of an endodontically treated posterior tooth cannot be overemphasized. 182
A pulpless tooth has special restorative problems according to Gutmann, 2 who has emphasized the importance of root anatomy, instrumentation, root axis orientation, depth of preparation, and internal shaping of the canal to avoid failure of the restoration and lo~s of the tooth. The loss of elasticity and increased brittleness of pulpless teeth because of loss of moisture 3 makes endodontically treated teeth more fragile and susceptible to fracture. 1 When pins were introduced to aid in the retention of amalgam, it was thought that the pins would strengthen the amalgam, both reinforcing the tooth and splinting it to avoid fracture. ' However, research has proved that pins can weaken amalgam restorations by acting as stress concentrators 5 under tensile stress, causing minute fracture lines
and crazing within the dentin ~;~ and leading to fracture and loss of the tooth. 9n Problems associated with the use of pins could be more severe in endodontically treated teeth; therefore, a post in the root canal is usually advisable. The objective of this study was to determine the horizontal resistance force of alternative systems to the cast-post failed. The two alternative systems evaluated were amalgam cores in posterior teeth used in conjunction with dowel posts compared with all-amalgam cores in posterior teeth. The three types of commercially available dowel posts are parallel, tapered, and threaded parallel; each generates different stress distributions in teeth. Tapered and threaded posts exhibit the highest stress toward the occlusal part of the dow-
JOURNAL OF E N D O D O N T I C S ! VOL 7, N O 4, APRIL 1981
el, while the parallel wall posts tend to distribute stress apically.:'-' C a p u t o and Standlee *:~ do not r e c o m m e n d self-threading posts because they can produce stress a n d crazing in the dentin that predispose the tooth to root fracture. Also, care m u s t be taken during the p l a c e m e n t of the tapered post to keep it from w e d g i n g into the root canal a n d causing root fracture. O n l y the t a p e r e d post was evaluated in this study. METHODS AND
MATERIALS
Twenty-five freshly e x t r a c t e d mandibular molars were used in this study. All teeth were free of caries and restorations in the cervical third of the a n a t o m i c crown; they were placed in w a t e r i m m e d i a t e l y after extraction. T o s i m u l a t e a b a d l y broken-down tooth that w o u l d require a core restoration, a n d to control as many variables as possible, the coronal portion of each tooth was ~duced to a 1 to 2 m m level a b o v e the c e m e n t o e n a m e l j u n c t i o n a n d perpendicular to the long axis of each tooth. In a d d i t i o n , all teeth were m o u n t e d in a m a c h i n e d s a m p l e holder, which established a c c u r a t e tooth position in relation to the loading pin, a n d s t a n d a r d i z e d the amount of resin e m b e d d e d each tooth. The three groups were: the contr01, five intact teeth m o u n t e d in the sample holder in u n a l t e r e d form except for a flat area (5 by 5 m m ) , which was p r e p a r e d on the buccal surface of the enamel, p e r p e n d i c u l a r to the loading pin; g r o u p A, ten teeth prepared to receive an a l l - a m a l g a m core; group B, ten teeth p r e p a r e d to receive a post in the distal canal before a m a l g a m core b u i l d u p . Teeth in g r o u p A were p r e p a r e d as follows: after c o n v e n t i o n a l root c a n a l
therapy, with use of g u t t a - p e r c h a with lateral c o n d e n s a t i o n , the canals a n d p u l p c h a m b e r were p r e p a r e d to receive an a m a l g a m core. A h e a t e d e n d o d o n t i c plugger was used to remove g u t t a - p e r c h a 3 to 4 m m into each canal. T h e mesial canals were p r e p a r e d with a no. 3 Gates G l i d d e n bur to 4 m m , followed by a no. 4 Gates G l i d d e n b u r to a d e p t h of 2 ram. A no. 4 b u r w a s used in the larger distal c a n a l to 4 m m followed by a no. 5 b u r to a d e p t h of 2 ram. U n d e r c u t s were p l a c e d a r o u n d the base of the p u l p c h a m b e r with a no. 330 bur. A c o p p e r b a n d was tightly a d a p t e d to each tooth a n d e x t e n d e d 6 m m a b o v e the p r e p a r a t i o n level of the tooth. T h e b a n d was secured with green c o m p o u n d to stabilize the m a t r i x d u r i n g c o n d e n s a t i o n . 11 Velvalloy was mixed using a 1:1 alloyt o - m e r c u r y ratio. Initially, small e n d o d o n t i c pluggers were used to condense a m a l g a m into the canals (Fig 1). T h e p u l p a n d coronal portion were built up w i t h I- to 2-ram a m a l g a m condensers. Vertical a n d lateral condensations were exerted with small increments of a m a l g a m . T e e t h in group B were p r e p a r e d in the same m a n n e r as those in g r o u p A, except that a no. 2 C I (Parkell) post was placed 6 m m into the distal canal a n d allowed to e x t e n d 2 m m above the level of the p r e p a r a t i o n . T h e CI post h a d a t a p e r of .030 inch, with a small d i a m e t e r of .045 inch. T h e post was c e m e n t e d with zinc p h o s p h a t e c e m e n t (Fig 2). Acrylic resin was used to stabilize each specimen in a m a c h i n e d s a m p l e holder. T e e t h were m o u n t e d in the holder p e r p e n d i c u l a r to the l o a d i n g pin of a universal Instron tensile testing machine. T e e t h in groups A a n d B h a d flat surfaces (5 by 5 ram) p r e p a r e d on buccal surfaces, which were p e r p e n -
Fig 1--All amalgam core.
Fig 2 - C I post in dz~'ial canal with amalgam core.
183
JOURNAL OF ENDODONTICS, VOL 7, NO 4, APRIL 1981
Table 9 Summary of force at failure and fracture sites. Sample
dicular to the l o a d i n g pin. A m a chined nut was inserted between the flat surface on the s a m p l e a n d the loading pin to d i s t r i b u t e the force from the pin over a b r o a d e r a r e a on the s a m p l e a n d help p r e v e n t stresspoint c o n c e n t r a t i o n t h a t m i g h t cause p r e m a t u r e fracture. T h e Instron universal testing machine was used for all shearing tests. T h e force was directed at a 90 degree angle to the flattened area on the buccal surface of the a m a l g a m . T h e m a c h i n e was run at a crosshead speed of .02 i n c h / r a i n , a n d the force was recorded on an X Y g r a p h recorder. Force at the time of failure was d e t e r m i n e d from the g r a p h , a n d comparisons were m a d e a m o n g groups A, B, a n d the control group. RESULTS T h e n a t u r a l teeth (control group) d e m o n s t r a t e d the greatest resistance to fi'acture (mean = 269 lb) of all groups tested. T h e a m a l g a m cores with a reinforcing post ( g r o u p B) displayed greater resistance to fracture (mean = 142 lb) t h a n the alla m a l g a m core (group A), which h a d the least resistance to fracture (mean = 123 lb). All teeth in the control group displayed a n g u l a r root fractures. In general, all fractures in this g r o u p originated in the coronal third of the root surface nearest the l o a d i n g pin (buccal) a n d progressed lingually a n d apically to a point in the a p i c a l third of the root. Specific results From group A W h e n the fracture sites were analyzed according to which p a r t failed (the a m a l g a m core or the tooth), certain groupings b e c a m e a p p a r e n t (Table). T h r e e samples (6, 12, 13) 184
Control 1 2 3 4 5 Group A 6 7 8 9 10 11 12 13 14 15 Group B 16 17 18 19 20 21 22 23 24 25
Chamber depth -----4 3 4 4 3 4 3 3 3 2 3 3 4 3 4 3 3 3 3 4
Force at failure (lb) Mean = 269 250 325 255 175 330 Mean = 123 142 138 136 136 130 144 143 148 76 35 Mean = 142 84 195 212 110 225 87 160 87 96 165
Fracture site T* T T T T T At A + T A +T A A A + T A A A + T A
(155)5
(141)* (146)5 (192)5
A A A A A A A A A A
+ + + +
T T T T
+ T + T + T
*Tooth. ",Amalgam. Sin there samples, failure occuned in two s t a g s . In the first stage, the restoration was partially displaced, which would ha~e indicated failure of the restoration; in the final stage, total displacement occurred.
failed at the p r e p a r a t i o n level; two samples ( 9 , 10) showed t h a t the a m a l g a m failed at a d e p t h a p p r o x i m a t e l y 1 m m below the p r e p a r a t i o n level; three other samples (7, 8, l i) had tooth-root a n d a m a l g a m core failures, which occurred at the base of the p u l p c h a m b e r . S a m p l e s 14 a n d 15 were u n u s u a l in their resulting failures. S a m p l e 14 f r a c t u r e d with a c o m b i n a t i o n tootha m a l g a m failure at 76 lb. Close visual e x a m i n a t i o n disclosed t h a t the r e m a i n i n g tooth thickness of the lingual wall in the p u l p a l floor area was only 1 m m . S a m p l e 15 showed a m a l g a m core d i s l o d g m e n t w i t h no tooth fracture at 35 lb. S a m p l e 15 h a d the shallowest c h a m b e r a n d lacked sufficient undercuts. T h e m e a n value of group A was 123 lb. However, if samples 14 a n d 15 were excluded, the m e a n value w o u l d be increased to f40 lb.
Specific results From group B Seven teeth h a d a combination t o o t h - a m a l g a m core failure. Three samples (20, 22, 23) resulted in amalg a m failure only. F u r t h e r visual e x a m i n a t i o n of the samples after final failure a n d separation o f the core from the tooth resulted in two sets, each with a special characteristic. In five samples (16, 19, 21, 23, 24), the post i m p i n g e d on the distal wall of the p u l p c h a m b e r p r e c l u d i n g condensation of a m a l g a m distal to the post. T h e m e a n force required for failure of these samples was only 93 lb. In the other five samples (17, 18, 20, 22, 25), it was possible to condense a m a l g a m , distal to the post, d o w n to the canal orifice. These samples h a d a m e a n force at failure of 191 lb.
JOURNAL OF E N D O D O N T I C S I VOL 7, N O 4, APRIL 1981
DISCUSSION This study shows t h a t differences in preparation a n d p l a c e m e n t of post and core restorations m a y significantly affect t h e resistance to failure. The numerical d a t a i n d i c a t e t h a t the natural tooth c a n w i t h s t a n d g r e a t e r force before f r a c t u r e ( m e a n = 269 lb) than either the a l l - a m a l g a m core in group A ( m e a n = 123 lb) or t h e amalgam c o r e - C I post g r o u p B (mean = 142 lb), w h i c h agrees w i t h a similar s t u d y on m a x i l l a r y incisors by Lovdahl a n d Nicholls. ~ In e v a l u a t i n g t h e l o c a t i o n o f fractures, it was n o t i c e d t h a t six of ten samples in g r o u p A s h o w e d a m a l g a m failure with no d a m a g e to the remaining t o o t h s t r u c t u r e . T h r e e of the ten samples in g r o u p B failed with no d a m a g e to the t o o t h . T h e least d a m a g e f r o m f a i l u r e in t h e oral cavity w o u l d be o n e in w h i c h the remaining t o o t h s t r u c t u r e w o u l d be unaffected. T h e mosl f a v o r a b l e situation if failure d i d o c c u r w o u l d be that the t o o t h w o u l d r e m a i n i n t a c t and could be r e t r e a t e d . The distinct d i c h o t o m y of g r o u p B st~owed the i m p o r t a n c e of the o p e r a tive procedures in r e s t o r i n g e n d o d o n tically treated teeth. T h e strongest experimental r e s t o r a t i o n s in this study were t h e ones in w h i c h the teeth were p r e p a r e d , l e a v i n g s p a c e around the C I post for c o n d e n s a t i o n of a m a l g a m s u r r o u n d i n g the post down to the orifice. T h e s e five s a m ples had a m e a n v a l u e of 191 lb as opposed to o n l y 93 lb for t h e ones
t h a t d i d n o t a l l o w for c o n d e n s a t i o n of the a m a l g a m s u r r o u n d i n g the post.
SUMMARY A n in vitro s t u d y was c o n d u c t e d to c o m p a r e the forces r e q u i r e d to prod u c e failure On t w o types of a m a l g a m core b u i l d u p s . N a t u r a l t e e t h (controls) demonstrated greater resistance to f r a c t u r e t h a n e i t h e r of the experimental groups. The strongest r e s t o r a t i o n was a post w i t h a n a m a l g a m core in w h i c h the a m a l g a m was weU c o n d e n s e d a r o u n d the post. T h e a l l - a m a l g a m core was the n e x t m o s t resistant to f r a c t u r e . T h e least resistant r e s t o r a t i o n was the post w i t h an a m a l g a m core in w h i c h the post p l a c e m e n t p r e v e n t e d cond e n s a t i o n o f the a m a l g a m distal to the post. Dr. Christian, former resident in endodontics, Medical College of Virginia School of Dentistry, now is in private endodontic practice, Charlestown, WVa; Dr. Button is assistant professor, restorative dentistry; Dr. Moon is associate professor, restorative det:: tistry; Dr. England is associate professor and chairman, department of endoduntics; and Dr. Douglas is assistant, professor, restorative dentistry, Medical College of Virginia School of Dentistry, Virginia Commonwealth University, Richmond, Va. Requests for reprints should be directed to Dr. Button, Medical College of Virginia School of Dentistry, Virginia Commonwealth University, Richmotld, Va 23298. References
1. Johnson, J.K.; Schwartz, N.L.; and Blackwell, R.T. Evaluation and restoration of endodontally treated posterior teeth. JADA 93:597-605, 1976.
2. Gutmann, J.L. Preparation of endodontally treated teeth to receive a post-core restoration. J t'rosthct Dent 38:413-419, 1977. 3. Heifer, A.R.; Mclnick, S.; and Schilder, H. Determination of the moisture content of vital and pulpless teeth. J Oral Surg 34:661670, 1972. 4. Markley, R.L. Pin reinforcement and retention of amalgam foundations and restoraiions. JADA 56:675-679, 1958. 5. Going, R.E., and others. The strength of dental amalgam as influenced by pins. JADA 77:1331-1334, 1968. 6. Chan, K.C., and Svare, C.W. Comparison of the dentinal crazing ability of retention pins and machinists taps. J Dent Res 52:178, 1973. 7. Dihs, W.E., and others. Crazing of tooth structure associated with placement of pins for amalgam restorations. JADA 81:387-391, 1970. 8. Markley, M.R. Pin-retained and picareinforced amalgam. JADA 73:1295-1300, 1966. 9. Caputo, A.A., Standlee, J.P.; and Collard, E.W. The mechanics of load transfer by retentive pins. J Prosthet Dent 29:442-449, 1973. 10. Birtcil, R.F., and Venton, E.A. Extracoronal amalgam restorations utilizing available tooth structure for retention. J Prosthet Dent 35:171-177, 1976. 11. Kantor, M.E., and Pines, M.S. A comparative study of restorative techniques for pulpless teeth. J Prosthet Dent 38:405-412, 1977. 12. Standlee, J.P.; Caputo,A.A.; and Cotlard, E.E. Analysis of stress distribution of endodontic posts. Oral Surg 33:952-960, 1972. 13. Caputo, A.A., and Standlee, .I.P. Pins and posts-why, when and how. Dent Clin North Am 20:299-311, 1976. 14. Harrington, W.G., and others. Coinpound reinforced matrices for pin amalgams. J Prosthet Dent 41:622-624, 1979. 15. I,ovdahl, P.E., and Nicholls, J.I. Pia retained amalgam cores vs cast-gold dowelcores. J Prosthet Dent 38:507-514, 1977.
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