J!S D )A \ RESE ARCH

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Effect of endodontic access preparation on resistance to crown-root fracture Craig A. H ow e, DDS; D ouglas J. McKendry, DDS,MSD

This investigation involved the creation of coronal-radicular fractures in vitro and com­ pared the fracture resistance of intact human mandibular molars, with molars after varied tooth preparation. Forty freshly extracted, non­ car ious, nonrestored h um an m andibular molars were randomly divided into four treat­ ment groups. The molars were subjected to constantly increasing occlusal load until coro­ nal-radicular fracture occurred. Tooth prepa­ rations significantly diminished resistance to coronal-radicularfracture.

ig n s a n d sym ptom s re su ltin g from in c o m p le te fra c tu re o f a to o th has been defined as “cracked tooth syn­ d ro m e .”1 T h e p a tie n t o fte n ex p erien ces variable symptoms, often m aking accurate d iag n o sis a ch a lle n g e. B ecause p atien ts have v a rie d sym ptom s, it is te ch n ic ally inappropriate to refer to this condition as a syndrome. This study simply refers to the condition as coronal-radicular fracture. C o r o n a l-r a d ic u la r f r a c tu r e , w h e th e r co m p lete o r in c o m p le te, occurs in b o th m axillary an d m andibular prem olars and molars. T he tooth m ost often involved by this mesiodistal o rien ted coronal fracture, ex ten d in g variably dow n th e root, is th e m andibular first2-4 o r second5,6molar. T h e cause o f th e coronal-radicular frac-

ture is equivocal. N oncarious, unrestored posterior teeth occasionally fracture so it is in c o rrec t to assume th a t only extensively restored teeth are susceptible to coronalradicular fracture. Persons with heavy mas­ ticatory m usculature an d individuals with occlusal habits, such as chewing ice, often h av e f r a c tu r e d th e ir te e th . In c e rta in in stan ces, th e fra c tu re o ccu rs w hen th e p a tie n t b ites u n in te n tio n a lly o n a h a rd object. R ecent works have focused atten ­ tio n o n o cc lu sio n 7 a n d suggest occlusal adjustm ent as a treatm en t and preventive m easure to avoid co ro n al-rad icu lar frac­ ture.8

G e n e ra lly , p o s te r i o r te e th w ith endodontic trea tm e n t are at increased risk o f c o ro n a l-ra d ic u la r fra c tu re . Full cusp coverage by the p erm a n en t restoration for all posterior teeth can prevent fracture.9-11 T h e distinction betw een coronal-radicu­ la r fra c tu re an d ia tro g en ic vertical ro o t fracture is critical. Coronal-radicular frac­ ture occurs in b oth vital an d pulpless pos­ terior teeth, propagating in a crown-down d ir e c tio n w ith th e fra c tu r e lin e b e in g alig n ed m esiodistally. Iatro g en ic vertical root fracture occurs from the ro o t canal to the o u ter ro o t surface, an d the fracture is g e n e ra lly a lig n e d in a facial to lin g u a l

S

712 ■ JADA, Vol. 121, D ecember 1990

Fig 1 ■ M esial to distal orientation o f coronal-

Fig 2 ■ M esial to distal orientation o f coronal-

radicular fracture o f the mandibular molar in an

radicular fracture o f the mandibular m olar with

intact tooth.

endodontic access only.

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direction. Iatrogenic vertical ro o t fracture o c c u rs in b o th a n t e r io r a n d p o s te rio r teeth eith er at th e tim e o f ro o t canal fill­ ing o r later.1213 Ia tro g e n ic v e rtic a l r o o t fra c tu re may occur as a result o f excess spread er pres­ sure while condensing gutta-percha.14,15 A clinical rep o rt o f 32 fractured teeth listed im proper condensation o f gutta-percha as th e m ost com m on cause o f vertical ro o t fracture b u t also suggested im proper seat­ ing an d ce m e n tatio n o f posts o r pins as possible causes.16 Iatro g en ic vertical ro o t fra ctu res can be m inim ized by avoiding these clinical erro rs.17 T h e ro le o f e n d o d o n tic th e ra p y co n ­ tributing to a to o th ’s susceptibility to coro­ n a l- r a d ic u la r f r a c tu r e is u n c e r ta in . E n d o d o n tically tre a te d te e th have b ee n re fe rre d to as b ein g “b r ittle ” relative to te eth with pulpal vitality.18 A lthough con­ tr o v e r s ia l, d e n t in b r ittl e n e s s c a n be responsible for th e susceptibility to frac­ tu re o f th e endodontically treated tooth. C a rte r a n d o th e rs 19 used a p u n c h sh ear te s t a n d r e p o r te d a 14% r e d u c tio n in stre n g th an d to u g h n e ss o f d e n tin from e n d o d o n tica lly tre a te d te e th . T hey co n ­ clu d ed th a t d e n tin fro m en d o d o n tically treated m olars was bo th w eaker an d also m ore brittle com pared with samples from teeth with pu lp vitality. In this definition the authors differentiated strength (weak versu s s tro n g ) fro m p la stic ity (p lia b le versus brittle). A decrease in m oisture co n ten t may be responsible for d en tin brittleness. H eifer and others20 have rep o rted 9% less mois­ ture in the calcified tissue of pulpless dog’s teeth com pared with vital teeth. However, L e w iste in a n d G ra jo w e r21 f o u n d th a t e n d o d o n tic tre a tm e n t did n o t affect th e Vickers hardness nu m b er o f hum an cervi­ cal ro o t d e n tin . W h e th e r re d u c e d m ois­ ture co n ten t is o f clinical significance rela­ tive to f r a c tu r e r e s is ta n c e o f a to o th rem ains unresolved. T he usual cause o f degenerative pulpal disease, necessitating ro o t canal therapy, is extensive carious d e s tru c tio n o f coronal tooth substance and o f large restorations, p o ss ib ly u s in g p in r e t e n t i o n . T h e in c r e a s e d f r a c tu r e s u s c e p tib ility o f endodontically treated teeth may primarily result from the loss o f tooth structure criti­ cal to fracture resistance.22 Consequently, th e en d o d o ntically tre a te d to o th may be weakened relative to the noncarious, n on­ r e s to r e d to o th . L a rs o n a n d o t h e r s 23 r e p o r t e d th a t in c r e a s e d w id th o f th e occlusal p o rtio n o f a cavity p re p a ra tio n d im in ish e s f ra c tu re re sista n c e b u t th a t ex ten sio n o f the p re p a ra tio n to in clu d e

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Fig 3 ■ M esial to distal orientation o f coronal-

Fig 4 ■ M esial to distal orientation o f coronal-

radicular fracture o f the mandibular molar with

radicular fracture o f the mandibular molar with

an MOD cavity preparation only.

both endodontic access opening and MOD cavity preparation.

F ig 5 ■ F ra ctu re o b s e r v e d in a p r e v io u s ly

Fig 6 ■ The same tooth shown in Figure 5 with

extracted intact tooth.

fracture segm ents separated; it is apparent that the fracture line involves the pulp chamber.

the m arginal ridges an d create a shallow proxim al box did n o t w eaken th e to o th further. Blaser and others24 also concluded that teeth becom e progressively weaker as the cavity isthm us becom es wider, b u t they n o ted th a t increasin g cavity d e p th weak­ ened the tooth m ore th an increased cavity width. T his study p ro d u c e d in vitro coronalr a d ic u la r f ra c tu r e s th a t a re sim ila r in direction and propagation to the clinically observed fracture p a tte rn an d com pared the fracture resistance o f teeth p rep ared to various extents. Methods and materials Forty freshly extracted, noncarious, u n re­ stored hum an m andibular first and second m olars were o b ta in ed a n d stored in thymolized water to prevent desiccation until testing. Before teeth were accepted for this study, they w ere sc re en e d to en su re the absence o f preex istin g in co m p lete coron a l- r a d ic u la r f r a c tu r e (c ra c k s) u sin g m ethelyene blue dye an d fiber-optic trans­ illum ination. T he mass o f each tooth was determ ined

volumetrically, using a water displacem ent ap p a ratu s. As previous rese arch e rs have shown, larger teeth may be m ore fracture resistant than smaller teeth .24 T h e sample o f te eth was ran d o m ly divided in to fo u r equivalent groups with respect to the dis­ trib u tio n o f to o th mass. T h en tre a tm e n t regim ents were assigned to th e groups of teeth at random : group 1 consists o f intact teeth with n o tooth p reparation, the con­ tro l g ro u p ; g ro u p 2 has in ta c t te e th fo r e n d o d o n tic access p re p a ra tio n ; g ro u p 3 co n sists o f te e th p r e p a r e d fo r C lass II MOD am algam restorations; and g ro u p 4 is com posed o f teeth p rep ared for Class II MOD am algam restorations an d en d o d o n ­ tic access. B efore to o th p re p a ra tio n , te e th w ere m ounted in dental acrylic resin 1 mm to 2 m m below th e ce m e n to en am el ju n c tio n , with a 0.25-mm thickness o f ru b b e r base im pression m aterial to sim ulate p eriodon­ tal ligam ent as described by T ra b ert and others.25 Teeth in group 2, the end o d o n tic access g ro u p , w ere p r e p a re d u sin g th e access op en in g criteria o u tlin ed in the teaching syllabus o f the University o f W ashington, JADA, Vol. 121, Decem ber 1990 ■ 713

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d ep a rtm en t o f endodontics.26 G ro u p 3 te e th , p r e p a re d fo r Class II M O D am alg a m r e s to ra tio n s , w ere p r e ­ pared with a no. 55 fissure b u r (SS W hite C arbide) following criteria taught by the U niversity o f Iowa, d e p a rtm e n t of op era­ tive dentistry. Buccal-lingual width o f the cavity p re p a ra tio n was approxim ately 1.0 m m at all points except the intersection of m ajo r occlusal grooves. T h e buccal, lin ­ gual, a n d gingival walls o f th e proxim al b o x w ere conservatively e x te n d e d —buccally, lingually, an d gingivally beyond the fo rm e r m esial a n d distal co n tac t points. Proxim al walls and m argins were prepared parallel to the long axis o f the tooth. T he shallowest area o f the preparation was no less th a n 1.0 m m in to d en tin with dep th av e rag in g 1.5 m m . G ro u p 4 te e th , p re ­ p are d fo r Class II MOD a n d en d o d o n tic access, were prep ared according to the ref­ e re n c e d criteria, w ith cavity p re p a ra tio n p receding the endodontic access opening. All te e th w ere th e n in d iv id u ally sub­ je c te d to a constantly increasing occlusal load d irected against the lingual inclines o f th e buccal cusps and the buccal inclines o f th e lin g u a l c u s p s u n til f r a c tu r e o ccurred. Occlusal loading was perform ed using a custom -m anufactured stylus on a Instron Testing M achine (Instron Corp). Results In all four groups, mesial to distal oriented fra c tu re s o c c u rre d (Fig 1-4). Invariably, th e fracture line involved the pulp cham ­ b er (Fig 5,6) an d ex tended a variable dis­ tance th ro u g h th e ro o t until term inating on the lateral ro o t surface. O n e in ta ct to o th (in group 1) did n o t fra c tu r e a t m a x im a l lo a d in g (500 K g), using this experim ental m ethod. Descrip­ tive statistics o f th e occlusal load w hen fracture occurred for each group are dis­ played in Table 1. T he intact tooth control g r o u p ( g r o u p 1) w ith s to o d a m e a n occlusal load o f 341.4 Kg before a coronalradicular fracture. T eeth with endodontic access p re p a ra tio n only (g ro u p 2) frac­ tu red at a m ean occlusal load o f 225.5 Kg. Teeth p rep ared with a conservative Class II MOD cavity p re p a ra tio n only (g ro u p 3) fra c tu re d a t a m ean o f 222.4 Kg. T eeth with b oth endodontic access opening and a Class II M OD p r e p a ra tio n (g ro u p 4) fractured at a m ean occlusal load of 121.7

KgA statistical com parison between groups was p erfo rm ed using a one-factor analysis o f variance (ANOVA) displayed in Table 2. In ta c t te e th (gro u p 1) show ed signifi­ can t difference in observed fracture load 714 ■ JADA, Vol. 121, D ecem ber 1990

Table 1 ■ Descriptive statistics of frac­ ture load data. G roup_______ No._____ M ean:Kg 1. In tact teeth 10 341.4

SD* 106.782

2. E n d odontic access 10

225.5

62.811

3. M OD cavity 10

222.4

64.288

4. M OD an d access

121.7

50.992

10

*SD = S tan d ard deviation.

Table 2 ■ One-factor ANOVA of fracture load data. C om parison G ro u p 1 versus 2 G roup 1 versus 3 G roup 1 versus 4 G ro u p 2 versus 3 G roup 2 versus 4 G roup 3 versus 4

M ean difference: Kg 115.9* 119.0* 219.7* -3.1 103.8* 100.7*

^Significant a t P< 0.05. 4 Asterisk d en otes statistically significant difference with b o th Fisher p ro te c te d least significant differ­ en ce an d Scheffe F post-hoc tests.

from all groups of p rep ared teeth. T eeth p r e p a r e d w ith b o th e n d o d o n tic access o p e n in g a n d a Class II M OD am alg am p re p a ra tio n (g ro u p 4) were significantly w eaker th a n te e th p re p a re d w ith e ith e r pro ced u re alone (groups 2 an d 3). How­ ever, there was no significant difference (P < 0.05) b etw een th e e n d o d o n tic access opening and the MOD cavity preparation groups (group 2 and 3). In d iv id u a l to o th mass was c o rre la te d with th e observed fracture load for each sp ecim en . N o c o rre la tio n was fo u n d to ex ist (R 2 = 0.000059) w ith in tr e a tm e n t g ro u p s b etw een to o th size an d fra c tu re resistance. Discussion Tooth preparation for eith er the en d o d o n ­ tic access o p e n in g o r th e Class II MOD cavity p rep aratio n was p erfo rm ed strictly ac co rd in g to th e re fe re n c e d g u id elin es. H ow ever, g ro u p s 2, 3, a n d 4— th e p r e ­ p a re d te e th — fra c tu r e d at sig n ifican tly lower occlusal loads than the intact teeth in group 1. A m ost interesting finding o f this study was lack o f significant difference in frac­ tu r e r e s is ta n c e b e tw e e n te e th w ith an endodontic access opening (group 2) and te e th p re p a re d fo r conservative Class II

MOD am algam restoration (group 3). T he c o n c e p t th a t all e n d o d o n tic a lly tre a te d m andibular molars require full cuspal cov­ e ra g e by th e p e r m a n e n t re s to ra tio n is q u e s tio n a b le . A m a n d ib u la r m o la r with b oth m arginal ridges rem aining intact may n o t require cuspal coverage by the p erm a­ n e n t restoration. These types of m andibu­ lar molars include those n o t weakened by a previously existing incom plete coronalradicular fracture and those with marginal ridges n o t u n d e rm in e d by in ap p ro p riate end o d o n tic access opening, dental caries, o r a previously p laced re sto ra tio n . This finding m ust be interp reted in view o f the lim ita tio n s o f th e study, w h ich d o n o t account for the effect o f endodontic pro­ cedures after access opening (canal prepa­ ra tio n , in tra can a l irrig atio n , o r m edica­ tio n ) a n d d o n o t a c c o u n t fo r condensation o f the filling material. While such subsequent steps in endodontic ther­ apy are sig n ifican t relative to iatro g en ic ro o t fracture, they would n o t be expected to affect the coronal integrity o f a tooth or its resistance to coronal-radicular fracture. Ultimately, the p ro p e r p o sten d o d o n tic restoration o f a posterior tooth m ust have th e c l i n i c i a n ’s c o n s id e r a tio n o f th e p a t i e n t ’s c irc u m s ta n c e s : p e r io d o n t a l h e a lth , p ro sth etic use o f th e to o th , an d occlusal function. E ndodontic treatm ent perform ed when a to o th has n o t b een significantly weak­ e n e d by in te rp ro x im al caries o r resto ra­ tion occur in the following: teeth devital­ ized by trau m a or surgical m anipulation, teeth with deep occlusal or Class V restora­ tions, teeth with ro o t caries, and teeth th at req u ire elective devitalization because of d e n tin a l hypersensitivity. In these situ a­ tio n s , th e d e n tis t s h o u ld c o n s id e r th e a p p lic a b ility o f r e s to r in g c o n serv ativ e e n d o d o n tic access o p en in g only w ithout occlusal reduction and cusp coverage. W hen th e in teg rity o f p ro xim al to o th s tr u c tu re (g ro u p 4) h as b e e n c o m p ro ­ mised, data strongly sup p o rt the need for cu sp al co v erag e by th e p o ste n d o d o n tic re sto ra tio n . T h is may be ac co m p lish e d w ith a full-crow n casting. T h e m in im al acceptable p e rm a n e n t resto ratio n would p ro tect the u n supported cusps from coronal-radicular fracture. Conclusions A co ro n al-rad icu lar fractu re can b e cre­ ated in vitro which resembles the orienta­ tion and propagation p attern th at occurs clinically. Tooth preparation (in groups 2, 3, and 4) significantly decreased resistance to c o ro n a l-ra d ic u la r fra c tu re . O cclu sal

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endodontic access opening of the man­ dibular molar does not diminish the frac­ ture resistance significantly greater than conservative MOD cavity preparation. Based on these findings, in the majority of clinical cases the appropriate restora­ tion, after endodontic treatment, for a tooth with proxim al surfaces com pro­ m ised by caries, a previously placed restoration, or overzealous access opening would provide cuspal coverage. However, when endodontic access can be conserva­ tive and proximal tooth structure remains intact, simple restoration of the endodon­ tic access opening may be adequate.

can Association o f Endodontics 1984:16. 2. Dewberry JA. In: Weine FS, ed. Endodontic ther­ apy. 3rd ed. St. Louis: Mosby;1982:8-15. 3. Eakle WS, Maxwell EH, Braly BV. Fractures o f posterior teeth in adults. JADA 1986;112:215-8. 4. Gher ME, Dunlap RM, Anderson MH, Kuhl LV. Clinical survey o f fractured teeth. JADA 1987;114:1747. 5. H iatt WH. Incom plete crown-root fracture in pulpal-periodontal disease. J Periodontol 1973;44:36979. 6. Cameron CE. T he cracked tooth syndrome: addi­ tional findings. JADA 1976;93:971-5. 7. Swepston JH, Miller AW. The incompletely frac­ tured tooth. J Prosthet D ent 1986;55:413-6. 8. Agar JR, Weller RN. Occlusal adjustment for ini­ tial treatment and prevention o f the cracked tooth syn­ drome. J Prosthet Dent 1988;60:145-7. 9. Johnson JK, Schwartz NL, Blackwell RT. Evalua­ tion and restoration o f endodontically treated poste­ ------ !----------J liO A ----------------rior teeth. JADA 1976;93:597-605. 10. H erschm an JB, W eine FS, Strauss S, Sm ulson Information about the manufacturers o f the prod­ MH. Restoration o f the endodontically treated tooth ucts m entioned in this article may be available from and bleaching. In: Weine FS, ed. Endodontic therapy the authors. N either the authors nor the American 3rd ed. St. Louis: Mosby;1982:593. Dental Association has any commercial interests in the 11. Shillingburg HT, H obo S, Whitsett LD. Funda­ products mentioned. m entals o f fix ed prosthodon tics. 2nd ed. Chicago: Quintessence; 1981:147-58. 12. Pitts DL, Natkin E. Diagnosis and treatment o f Dr. Howe is in private practice, Permanente Dental vertical root fractures. J Endod 1983;9:338-46. Associates in Portland. Dr. McKendry is assistant pro­ 13. W alton RE, M ic h e lich RJ, Sm ith G N . T h e fessor, departm ent o f endodontics, The State Univer­ histopathogenesis o f vertical root fractures. J Endod sity o f New York at Buffalo, 240 Squire Hall, Buffalo, 1984;10:48-56. 14214. Address requests for reprints to Dr. McKendry. 14. Pitts DL, Matheny HE, Nicholls JI. An in vitro 1. Nomenclature committee. An annotated glossary study o f spreader loads required to cause vertical root fr a c tu r e d u r in g la te r a l c o n d e n s a tio n . J E n d o d o f terms used in endodontics. 4th ed. Chicago: Ameri­

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1983;9:544-50. 15. H olcom b JQ, Pitts DL, Nicholls JI. Further inves­ tigation o f spreader loads required to cause vertical root fracture during lateral condensation. J Endod 1987;13:277-84. 16. M eister F, L om m el TJ, Gerstein H. Diagnosis and possible causes o f vertical root fractures. Oral Surg Oral Med Oral Pathol 1980;49:243-53. 17. Schweitzer JL, Gutmann JL, Bliss RQ. Odontiatrogenic tooth fracture. Int Endod J 1989;22:64-74. 18. Eissmann HF, Radke RA. Pathways o f the pulp. 4th ed . C o h e n S, B u r n s RC, ed s. St. L ouis: Mosby;1987:640. 19. Carter JM, Sorensen SE, John son RR, Teitelbaum RL, Levine MS. Punch shear testing o f extracted vital and en d o d o n tic a lly treated teeth . J B iom ech 1983;16:841-8. 20. H elfer AR, Melnick S, Schilder H. Determina­ tion o f the moisture content o f vital and pulpless teeth. Oral Surg Oral Med Oral Pathol 1972;34:661-70. 21. Lewistein I, Grajower R. Root dentin hardness o f endodontically treated teeth. J Endod 1981;7:421-2. 22. Cortade GL, Timmermans JJ. Pins in restorative dentistry. St. Louis: Mosby;1971:145-72. 23. Larson TD, Douglas WH, Geistfeld RE. Effect o f prepared cavities on the strength o f teeth. Oper Dent 1981;6:2-5. 24. Blaser PK, Lund MR, Cochran MA, Potter RH. Effect o f designs o f class II preparations on resistance o f teeth to fracture. Oper D ent 1983;8:6-10. 25. Trabert KC, Caput AA, Abou-Rass M. Tooth frac­ ture— a com parison o f e n d o d o n tic and restorative treatments. J Endod 1978;4:341-5. 26. Oswald RJ, Harrington GW, Natkin E, Pitts DL. A course in endodontic technique— teaching syllabus. University o f Washington: Seattle 1985;237-45.

JADA, Vol. 121, December 1990 ■ 715

Effect of endodontic access preparation on resistance to crown-root fracture.

This investigation involved the creation of coronal-radicular fractures in vitro and compared the fracture resistance of intact human mandibular molar...
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