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.
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