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Fracture Resistance of Mandibular Molars with Occlusal Class I Amalgam Preparations
A R T IC L E S
Fracture resistance of mandibular molars with occlusal Class I amalgam preparations G erald J. Re, DMD R ich a rd N. D raheim , DDS B a rr y K. N orling, PhD
The internal angles in a m andibular m olar with a Class I occlusal amalgam preparation could affect the strength of the teeth. A n experim ent was done to com pare the fra ctu re strength o f natural and acrylic resin teeth with either rounded or sharp line angles.
I n am algam cavity preparations, rounded line angles rather than sharp lin e an g les have been ad vocated because of the lower concentration of stress at the rounded line angle.1'3 This project proposes to evaluate in v itro the effect on tooth fractu re stren gth of rounded vs sharp line angles in m andibular molars with Class I occlusal amalgam preparations.
developmental grooves. Group 2 was prepared with a no. 330 plain fissure bur (Midwest American) and group 3 was prepared with a no. 56 plain fissu re bur (Fig 1). T he radius of curvature of the no. 330 bur was 0.36 mm.
A ll p r e p a r a tio n s w ere approximately 0.5 mm into dentin, and a spray of water and air was used with an ultra-speed handpiece during preparation. Also, 50 acrylic resin m and ibu lar left first m olars were prepared as discussed except that 1.5 mm w as e s t a b l i s h e d fro m th e cavosurface margin to the pulpal floor at all marginal and intercuspal ridges to maintain uniform depth of all test teeth. Pulpal floors were flat; therefore, for large occlusal preparations, the depth at triangular ridges neared 2 mm. All prepared teeth were restored w ith a h ig h -co p p e r-co n ten t allo y (P h a s e a llo y ) th a t w as c a rv e d to
Group 4 330
Materials and methods To ev alu ate the e ffe ct of various anatomic variations of natural teeth on the independent variables of size and internal line angle shape, both natural and melamine-formaldehyde (Ivorine) teeth (acrylic resin teeth) were used. A total of 50 noncarious, mandibular first, second, and third molars were o b ta in ed and kept m o ist in 10% formalin solution. Teeth were selected random ly and separated into five groups of ten. Group 1 consisted of sound teeth. Groups 2 and 3 comprised teeth prepared with occlusal Class I preparations, a fourth the width of the distance between summits of cuspal t i p s , b i s e c t e d b y th e p r im a r y 580 ■ JADA, Vol. 103, October 1981
Group
Group 5
Group 3
56
56
UNRESTORED
1/4 INTERCUSPAL WIDTH
1/2 INTERCUSPAL WIDTH
Fig 1 ■ S ch e m a tic d raw in g o f m ethod o f p re p a ra tio n s o f n a tu ra l and a c r y lic resin teeth.
ARTICLES
reproduce existing anatomic contours. Natural teeth were mounted just before testing in low-fusing metal alloy. The flat base of each acrylic resin tooth essentially was parallel to the tooth’s occlusal plane. All acrylic resin teeth were placed on a flat surface, and acrylic resin was flowed around each tooth to the junction of the apical and middle thirds to maintain it in a stable upright position. Occlusal forces were applied parallel to the long axis of the anatomic crown of the teeth using a ball bearing, 7132 inch in diameter. The ball bearing was allowed to seek its most stable position, at or near the central fossa of the tooth. A universal testing machine (Instron) was used to fra c tu re th e te e th at 1 m m /m in crosshead speed. Fractured teeth were examined and the severity of fracture was classified according to the system of Talim and Gohil.4 That is, Class I fractures are of enamel, Class II fractures are of enamel and dentin, and Class III fractures are of enamel, dentin, and pulp. E a c h to o th a d d itio n a lly w as classified as restorable or unrestorable according to whether the fracture extended less than or greater than 2 mm b e lo w th e c e m e n to e n a m e l junction.
Results Fracturing loads for the natural teeth are shown in Table 1. Unprepared teeth fractured under a mean load of 3,187 N (Newton). Teeth with small o cclu sa l p rep aratio n s and round internal line angles fractured under a mean load of 3,682 N whereas those with small occlusal preparations and sharp internal line angles fractured under a mean load of 3,630 N. Those with large occlusal preparations and round internal line angles fractured under a mean load of 3,758 N, and teeth with large occlusal preparations and sh a rp in te rn a l lin e a n g le s fractured under a mean load of 3,310 N. M ultiple range an alysis by the S tu d e n t, N ew m an , K eu ls m ean c o m p a r i s o n t e s t s h o w e d no statistically significant difference in strength of unrestored teeth and the four treatment groups at the 0.05 level of confidence. At the same level of co n fid e n ce, tw o-w ay an aly sis of v a ria n c e sh ow ed no sig n ifica n t difference in strength because of the
Table 1 ■ Loads required to fracture natural m andibular m olars. Group Group 1 (no preparation) Group 2 (sm all occlu sal preparation w ith round line angles) Group 3 (sm all occlusal preparation w ith sharp line angles) Group 4 (large occlu sal preparation w ith round line angles) Group 5 (large occlu sal preparation w ith sharp line angles)
M ean (N)*
M ean Ub)
SD+ (N)
SD+ (lb)
3 ,1 8 7
716
1 ,1 3 6
255
3,6 8 2
828
961
216
3 ,6 3 0
816
718
1 61
3 ,7 8 5
851
1 ,2 2 5
275
3 ,3 1 0
744
518
116
*Newton. tStan dard deviation.
T able 2 ■ Loads required to fracture acrylic resin molars. Group Group 1 (no preparation) Group 2 (sm all occlu sal preparation w ith round line angles) Group 3 (sm all occlu sal preparation w ith sharp line angles) Group 4 (large occlu sal preparation w ith round line angles) Group 5 (large occlu sal preparation w ith sharp line angles)
M ean (N)*
M ean (lb)
SD+ (N)*
SD+ (lb)
2 ,7 3 5
615
191
43
2 ,311
519
464
104
2 ,4 7 0
555
522
117
3 ,5 6 0
800
476
107
3 ,5 1 0
789
766
17 2
H om ogenous subsets*
*Newton. tStan dard deviation. tS u b sets are not statistically different at 0.05 level o f confidence.
shape of the internal line angles. Results of the fracturing forces for the acrylic resin teeth are summarized in Table 2. Unprepared acrylic resin teeth fractured under a mean load of 2,735 N. Teeth with small occlusal preparations and round internal line angles fractured under a mean load of 2 ,3 1 1 N whereas those teeth with small o cclu sa l p rep aratio n s and sharp internal line angles fractured under a mean load of 2,470 N. Those with large o cclu sal p rep aration s and round internal line angles fractured under a mean load of 2,560 N, and teeth with large occlusal preparations and sharp internal line angles fractured under a mean load of 3,510 N. M ultiple range analysis by the aformentioned mean comparison test show ed no sign ifican t statistical d iffe re n ce in s tre n g th b etw een unrestored teeth and those with small resto ra tio n s at the 0 .0 5 level of confidence. Similarly, there was no difference between teeth with large restorations; however, the strengths of the teeth with large restorations were significantly greater than those with the small or no restorations.
At the same level of confidence, two-way analysis of variance showed no significant difference in strength because of the shape of the internal line angles. All acrylic resin specimens, except for two teeth in group 5, fractured beyond restorability (Fig 2, 3) as previously defined. Likewise, natural teeth were evaluated for restorability (Table 3). In group 1, five teeth were restorable. In the treatment groups, group 2 had 6 restorable teeth; group 3 had 3 restorable teeth; group 4 had 1; and group 5 had 3. The most severe fracture, the Class III vertical complete, occurred most often in group 4 , that with large occlusal amalgam preparations and round line angles. Of the total Class III vertical complete fractures (27) that occurred, surprisingly, two (one in group 2 and one in group 5) were restorable (Table 3). The least damaging fracture, the Class II oblique complete, occurred most often in group 1 of unprepared teeth. Of the total (15) that occurred, tw o ( b o t h i n g r o u p 5) w e r e unrestorable (Table 3).
Re-Draheim-Norling : RESISTANCE OF MANDIBULAR MOLARS WITH CLASS I PREPARATIONS ■ 581
A R T IC L E S
F ig 3 ■ M esial v iew o f tooth dep icted in third p h otograp h in F ig u re 2.
Fig 2 ■ Pred o m in ately lin g u al cu spal failu re in u n p rep ared and p re p a re d teeth. G roup 1 consisted of sound teeth. G roups 2 and 3 w ere o cclu sal Class I p rep aratio n s, one fou rth w idth o f distan ce betw een su m m its of cu sp al tips; group 2 teeth w ere prep ared w ith no. 3 3 0 p lain fissure bur and group 3 teeth w ere p rep ared w ith no. 56 p lain fissure bur. G roups 4 and 5 w ere C lass I p re p a ra tio n s m ore th an half in tercu sp al d istan ce, bisected by p rim ary developm ental grooves (group 4 , no. 3 3 0 , and group 5, no. 56 p lain fissure burs).
Discussion The results do not substantiate the widely held assumption that sharp internal line angles, in comparison with round line angles, lessen fracture
strength of restored teeth, at least as concerns occlusal Class I amalgams in mandibular molars. Sim ilarly, data c o n t r a d ic t th e c o n t e n t i o n th a t conservative amalgam preparations increase strength.
This latter contention was studied by Re and Norling .5 In that study using only preparations with rounded line angles, natural teeth with medium and large occlusal restorations fractured at higher loads than those with small re sto ra tio n s or u n resto red teeth . Concurring results were obtained in this study for a cry lic resin teeth, re g a r d le s s o f w h e th e r sh a rp or rounded line angles were used in preparation. The sam e d ifferen ces were not confirmed in this study for the natural teeth. Although the mean fracture load for teeth with large restorations (3,785 N) w a s g r e a t e r t h a n t h a t f o r u n p rep ared te e th (3 ,1 8 7 N), the d iffe re n c e s w ere n ot s ig n ific a n t statistically. The lack of significance may reflect o n ly th e d iffic u lty o f o b ta in in g uniform samples of natural teeth and
Table 3 ■ T ype a n d severity of fracture of m an d ib u lar m olars.* Class II—Oblique* Group
Complete
Group 1 Restorable (5)t Unrestorable (5) Group 2 Restorable (6) Unrestorable (4) Group 3 Restorable (3) Unrestorable (7) Group 4 Restorable (1) Unrestorable (9) Group 5 Restorable (3) Unrestorable (7)
Incomplete
Complete
Class III—ObliqueH
Incomplete •
Complete
Incomplete
Class III—Verticali! Complete
Incomplete
5 5 4
1
1
4 2
1 2
5
1 9 2 2
*No Class I or Class II root fractures, t Total. tFracture in enamel or dentin. ''Fracture in enamel, dentin, and in pulp. 582 ■ JADA, Vol. 103, October 1981
Class II—V erticalt
1
1 2
2
A RTICLES
Table 4 ■ Com parison of m ean loads (m easured in N ewtons) required to fracture natural and acrylic resin molars.
N atural Re and Norling, 1 9 8 0 (lb) Ivorine (ib)
No preparation
Sm all o cclu sal, rou nd line angles
Large o cclu sal, round line angles
2 ,5 1 7 (566) 2 ,7 3 5 (615)
2 ,4 3 3 (547) 2 ,3 1 1 (519)
3 ,4 6 4 (7 7 9 ) 3 ,5 6 0 (8 0 0 )
the need for developing adequate in vitro models. Initially it was assumed that acrylic resin was an unlikely model because of its homegeneity. The structural inhomogeneities of natural teeth were assumed to have a major role in determining fracture response and stre n g th s. S u rp risin g ly , the fracture loads for the acrylic resin teeth in this study paralleled closely those of the natural teeth in the previous study (Table 4). Mean fracture loads for natural teeth and acrylic resin teeth (respectively) were 2,517 and 2,735 N for unprepared teeth, 2,433 and 2,311 N for those, with small preparations with round line angles, and 3,464 and 3 , 5 6 0 N for t h o s e w i t h l a r ge preparations with round line angles. Evaluation of the data in Table 3 shows no predictible trend relating t y p e , s e v e r i t y of f r a c t u r e , a nd restorability to unprepared teeth, to minimally prepared teeth with sharp or round internal line angles, or to
' extensively prepared teeth with sharp or round internal line angles.
Conclusion The finding6 that large occlusal Class I restorations weaken premolars is not confirm ed for natural mandibular m o la rs b e ca u s e no s ta tis tic a lly significant difference was found in the m e a n lo a d s n e e d e d to f r a c tu r e m a n d ib u l a r m o la r s t h a t w e re unprepared, minimally prepared, or p re p a re d w ith a la rg e o c c lu s a l amalgam restorations. Similarly, there was no statistically significant change in the amount of force necessary to fracture natural mandibular molars prepared with sharp vs round internal line angles (a no. 56 vs a no. 330 bur). Duplication of the experiment in a c ry lic resin m an d ib u lar m olars confirmed the results of a previous experiment by Re and Norling as well as the results relating to sharp vs round
internal line angles found in this experim ent on natural mandibular m o la r s . A c r y l i c r e s in te e th of homogeneous shape and substance o v erco m e som e of the sam plin g p ro b lem s in h e re n t in o b tain in g su itab le hum an teeth for testin g purposes; however, further testing of this material is indicated.
A ll th e au th ors are w ith th e dep artm en t o f resto rativ e den tistry, U n iversity o f T e x a s Dental S ch o ol a t San A n ton io, 7 7 0 3 Floyd Curl Drive, S an A n ton io, 7 8 2 8 4 . Dr. Re is assistant professor, d ivision o f op erative dentistry; Dr. D raheim is a s s is ta n t p r o fe s s o r, d iv isio n of o p e ra tiv e d en tistry; and D r. N orling is a ssociate professor, d iv isio n o f b iom aterials. A d d ress requests for reprints to Dr. Re. 1. G r a n a th , L .E . P h o t o e l a s t i c m o d e l exp erim en ts on C lass II ca v ity resto ratio n s of dental am algam , O dont R ev 1 6 ( 9 ) :6 - 3 8 ,1 9 6 5 . 2. G uard, W .F .; H aack, M .S.; and Ireland, R.L. P h o to e la s tic s tr e s s a n a ly s is o f b u c c o lin g u a l se c tio n s o f C lass II ca v ity re sto ra tio n s. JADA 5 7 :6 3 1 -6 3 5 , 1 9 5 8 . 3 . J u d e s , H . I m p ro v e d c a v i t y d e s ig n fo r am algam restorations. Israel J D ent M ed 2 4 :2 8 -3 0 , 1975. 4 . T alim , S .T ., and G ohil, K .S. M anagem ent of co ron al fractures of perm anent posterior teeth. J P ro sth et Dent 3 1 ( 2 ) :1 7 2 - 1 7 8 ,1 9 7 4 . 5. R e, G .J., an d N orlin g,U .K . F o rces required to cra ck unfilled a n d filled m olar teeth. J Dent Res 5 9 (lssu e A ) :3 5 1 ,1 9 8 0 . 6 . M ondelli, J ., and others. F ra ctu re strength of h u m an te e th w ith cav ity p re p a ra tio n s.} Prosthet D ent 4 3 ( 4 ) :4 1 9 - 4 2 2 ,1 9 8 0 .
R e-D raheim -N orling: RESISTANCE OF MANDIBULAR MOLARS WITH CLASS I PREPARATIONS a 583
Fracture resistance of mandibular molars with occlusal Class I amalgam preparations G erald J. Re, DMD R ich a rd N. D raheim , DDS B a rr y K. N orling, PhD
The internal angles in a m andibular m olar with a Class I occlusal amalgam preparation could affect the strength of the teeth. A n experim ent was done to com pare the fra ctu re strength o f natural and acrylic resin teeth with either rounded or sharp line angles.
I n am algam cavity preparations, rounded line angles rather than sharp lin e an g les have been ad vocated because of the lower concentration of stress at the rounded line angle.1'3 This project proposes to evaluate in v itro the effect on tooth fractu re stren gth of rounded vs sharp line angles in m andibular molars with Class I occlusal amalgam preparations.
developmental grooves. Group 2 was prepared with a no. 330 plain fissure bur (Midwest American) and group 3 was prepared with a no. 56 plain fissu re bur (Fig 1). T he radius of curvature of the no. 330 bur was 0.36 mm.
A ll p r e p a r a tio n s w ere approximately 0.5 mm into dentin, and a spray of water and air was used with an ultra-speed handpiece during preparation. Also, 50 acrylic resin m and ibu lar left first m olars were prepared as discussed except that 1.5 mm w as e s t a b l i s h e d fro m th e cavosurface margin to the pulpal floor at all marginal and intercuspal ridges to maintain uniform depth of all test teeth. Pulpal floors were flat; therefore, for large occlusal preparations, the depth at triangular ridges neared 2 mm. All prepared teeth were restored w ith a h ig h -co p p e r-co n ten t allo y (P h a s e a llo y ) th a t w as c a rv e d to
Group 4 330
Materials and methods To ev alu ate the e ffe ct of various anatomic variations of natural teeth on the independent variables of size and internal line angle shape, both natural and melamine-formaldehyde (Ivorine) teeth (acrylic resin teeth) were used. A total of 50 noncarious, mandibular first, second, and third molars were o b ta in ed and kept m o ist in 10% formalin solution. Teeth were selected random ly and separated into five groups of ten. Group 1 consisted of sound teeth. Groups 2 and 3 comprised teeth prepared with occlusal Class I preparations, a fourth the width of the distance between summits of cuspal t i p s , b i s e c t e d b y th e p r im a r y 580 ■ JADA, Vol. 103, October 1981
Group
Group 5
Group 3
56
56
UNRESTORED
1/4 INTERCUSPAL WIDTH
1/2 INTERCUSPAL WIDTH
Fig 1 ■ S ch e m a tic d raw in g o f m ethod o f p re p a ra tio n s o f n a tu ra l and a c r y lic resin teeth.
ARTICLES
reproduce existing anatomic contours. Natural teeth were mounted just before testing in low-fusing metal alloy. The flat base of each acrylic resin tooth essentially was parallel to the tooth’s occlusal plane. All acrylic resin teeth were placed on a flat surface, and acrylic resin was flowed around each tooth to the junction of the apical and middle thirds to maintain it in a stable upright position. Occlusal forces were applied parallel to the long axis of the anatomic crown of the teeth using a ball bearing, 7132 inch in diameter. The ball bearing was allowed to seek its most stable position, at or near the central fossa of the tooth. A universal testing machine (Instron) was used to fra c tu re th e te e th at 1 m m /m in crosshead speed. Fractured teeth were examined and the severity of fracture was classified according to the system of Talim and Gohil.4 That is, Class I fractures are of enamel, Class II fractures are of enamel and dentin, and Class III fractures are of enamel, dentin, and pulp. E a c h to o th a d d itio n a lly w as classified as restorable or unrestorable according to whether the fracture extended less than or greater than 2 mm b e lo w th e c e m e n to e n a m e l junction.
Results Fracturing loads for the natural teeth are shown in Table 1. Unprepared teeth fractured under a mean load of 3,187 N (Newton). Teeth with small o cclu sa l p rep aratio n s and round internal line angles fractured under a mean load of 3,682 N whereas those with small occlusal preparations and sharp internal line angles fractured under a mean load of 3,630 N. Those with large occlusal preparations and round internal line angles fractured under a mean load of 3,758 N, and teeth with large occlusal preparations and sh a rp in te rn a l lin e a n g le s fractured under a mean load of 3,310 N. M ultiple range an alysis by the S tu d e n t, N ew m an , K eu ls m ean c o m p a r i s o n t e s t s h o w e d no statistically significant difference in strength of unrestored teeth and the four treatment groups at the 0.05 level of confidence. At the same level of co n fid e n ce, tw o-w ay an aly sis of v a ria n c e sh ow ed no sig n ifica n t difference in strength because of the
Table 1 ■ Loads required to fracture natural m andibular m olars. Group Group 1 (no preparation) Group 2 (sm all occlu sal preparation w ith round line angles) Group 3 (sm all occlusal preparation w ith sharp line angles) Group 4 (large occlu sal preparation w ith round line angles) Group 5 (large occlu sal preparation w ith sharp line angles)
M ean (N)*
M ean Ub)
SD+ (N)
SD+ (lb)
3 ,1 8 7
716
1 ,1 3 6
255
3,6 8 2
828
961
216
3 ,6 3 0
816
718
1 61
3 ,7 8 5
851
1 ,2 2 5
275
3 ,3 1 0
744
518
116
*Newton. tStan dard deviation.
T able 2 ■ Loads required to fracture acrylic resin molars. Group Group 1 (no preparation) Group 2 (sm all occlu sal preparation w ith round line angles) Group 3 (sm all occlu sal preparation w ith sharp line angles) Group 4 (large occlu sal preparation w ith round line angles) Group 5 (large occlu sal preparation w ith sharp line angles)
M ean (N)*
M ean (lb)
SD+ (N)*
SD+ (lb)
2 ,7 3 5
615
191
43
2 ,311
519
464
104
2 ,4 7 0
555
522
117
3 ,5 6 0
800
476
107
3 ,5 1 0
789
766
17 2
H om ogenous subsets*
*Newton. tStan dard deviation. tS u b sets are not statistically different at 0.05 level o f confidence.
shape of the internal line angles. Results of the fracturing forces for the acrylic resin teeth are summarized in Table 2. Unprepared acrylic resin teeth fractured under a mean load of 2,735 N. Teeth with small occlusal preparations and round internal line angles fractured under a mean load of 2 ,3 1 1 N whereas those teeth with small o cclu sa l p rep aratio n s and sharp internal line angles fractured under a mean load of 2,470 N. Those with large o cclu sal p rep aration s and round internal line angles fractured under a mean load of 2,560 N, and teeth with large occlusal preparations and sharp internal line angles fractured under a mean load of 3,510 N. M ultiple range analysis by the aformentioned mean comparison test show ed no sign ifican t statistical d iffe re n ce in s tre n g th b etw een unrestored teeth and those with small resto ra tio n s at the 0 .0 5 level of confidence. Similarly, there was no difference between teeth with large restorations; however, the strengths of the teeth with large restorations were significantly greater than those with the small or no restorations.
At the same level of confidence, two-way analysis of variance showed no significant difference in strength because of the shape of the internal line angles. All acrylic resin specimens, except for two teeth in group 5, fractured beyond restorability (Fig 2, 3) as previously defined. Likewise, natural teeth were evaluated for restorability (Table 3). In group 1, five teeth were restorable. In the treatment groups, group 2 had 6 restorable teeth; group 3 had 3 restorable teeth; group 4 had 1; and group 5 had 3. The most severe fracture, the Class III vertical complete, occurred most often in group 4 , that with large occlusal amalgam preparations and round line angles. Of the total Class III vertical complete fractures (27) that occurred, surprisingly, two (one in group 2 and one in group 5) were restorable (Table 3). The least damaging fracture, the Class II oblique complete, occurred most often in group 1 of unprepared teeth. Of the total (15) that occurred, tw o ( b o t h i n g r o u p 5) w e r e unrestorable (Table 3).
Re-Draheim-Norling : RESISTANCE OF MANDIBULAR MOLARS WITH CLASS I PREPARATIONS ■ 581
A R T IC L E S
F ig 3 ■ M esial v iew o f tooth dep icted in third p h otograp h in F ig u re 2.
Fig 2 ■ Pred o m in ately lin g u al cu spal failu re in u n p rep ared and p re p a re d teeth. G roup 1 consisted of sound teeth. G roups 2 and 3 w ere o cclu sal Class I p rep aratio n s, one fou rth w idth o f distan ce betw een su m m its of cu sp al tips; group 2 teeth w ere prep ared w ith no. 3 3 0 p lain fissure bur and group 3 teeth w ere p rep ared w ith no. 56 p lain fissure bur. G roups 4 and 5 w ere C lass I p re p a ra tio n s m ore th an half in tercu sp al d istan ce, bisected by p rim ary developm ental grooves (group 4 , no. 3 3 0 , and group 5, no. 56 p lain fissure burs).
Discussion The results do not substantiate the widely held assumption that sharp internal line angles, in comparison with round line angles, lessen fracture
strength of restored teeth, at least as concerns occlusal Class I amalgams in mandibular molars. Sim ilarly, data c o n t r a d ic t th e c o n t e n t i o n th a t conservative amalgam preparations increase strength.
This latter contention was studied by Re and Norling .5 In that study using only preparations with rounded line angles, natural teeth with medium and large occlusal restorations fractured at higher loads than those with small re sto ra tio n s or u n resto red teeth . Concurring results were obtained in this study for a cry lic resin teeth, re g a r d le s s o f w h e th e r sh a rp or rounded line angles were used in preparation. The sam e d ifferen ces were not confirmed in this study for the natural teeth. Although the mean fracture load for teeth with large restorations (3,785 N) w a s g r e a t e r t h a n t h a t f o r u n p rep ared te e th (3 ,1 8 7 N), the d iffe re n c e s w ere n ot s ig n ific a n t statistically. The lack of significance may reflect o n ly th e d iffic u lty o f o b ta in in g uniform samples of natural teeth and
Table 3 ■ T ype a n d severity of fracture of m an d ib u lar m olars.* Class II—Oblique* Group
Complete
Group 1 Restorable (5)t Unrestorable (5) Group 2 Restorable (6) Unrestorable (4) Group 3 Restorable (3) Unrestorable (7) Group 4 Restorable (1) Unrestorable (9) Group 5 Restorable (3) Unrestorable (7)
Incomplete
Complete
Class III—ObliqueH
Incomplete •
Complete
Incomplete
Class III—Verticali! Complete
Incomplete
5 5 4
1
1
4 2
1 2
5
1 9 2 2
*No Class I or Class II root fractures, t Total. tFracture in enamel or dentin. ''Fracture in enamel, dentin, and in pulp. 582 ■ JADA, Vol. 103, October 1981
Class II—V erticalt
1
1 2
2
A RTICLES
Table 4 ■ Com parison of m ean loads (m easured in N ewtons) required to fracture natural and acrylic resin molars.
N atural Re and Norling, 1 9 8 0 (lb) Ivorine (ib)
No preparation
Sm all o cclu sal, rou nd line angles
Large o cclu sal, round line angles
2 ,5 1 7 (566) 2 ,7 3 5 (615)
2 ,4 3 3 (547) 2 ,3 1 1 (519)
3 ,4 6 4 (7 7 9 ) 3 ,5 6 0 (8 0 0 )
the need for developing adequate in vitro models. Initially it was assumed that acrylic resin was an unlikely model because of its homegeneity. The structural inhomogeneities of natural teeth were assumed to have a major role in determining fracture response and stre n g th s. S u rp risin g ly , the fracture loads for the acrylic resin teeth in this study paralleled closely those of the natural teeth in the previous study (Table 4). Mean fracture loads for natural teeth and acrylic resin teeth (respectively) were 2,517 and 2,735 N for unprepared teeth, 2,433 and 2,311 N for those, with small preparations with round line angles, and 3,464 and 3 , 5 6 0 N for t h o s e w i t h l a r ge preparations with round line angles. Evaluation of the data in Table 3 shows no predictible trend relating t y p e , s e v e r i t y of f r a c t u r e , a nd restorability to unprepared teeth, to minimally prepared teeth with sharp or round internal line angles, or to
' extensively prepared teeth with sharp or round internal line angles.
Conclusion The finding6 that large occlusal Class I restorations weaken premolars is not confirm ed for natural mandibular m o la rs b e ca u s e no s ta tis tic a lly significant difference was found in the m e a n lo a d s n e e d e d to f r a c tu r e m a n d ib u l a r m o la r s t h a t w e re unprepared, minimally prepared, or p re p a re d w ith a la rg e o c c lu s a l amalgam restorations. Similarly, there was no statistically significant change in the amount of force necessary to fracture natural mandibular molars prepared with sharp vs round internal line angles (a no. 56 vs a no. 330 bur). Duplication of the experiment in a c ry lic resin m an d ib u lar m olars confirmed the results of a previous experiment by Re and Norling as well as the results relating to sharp vs round
internal line angles found in this experim ent on natural mandibular m o la r s . A c r y l i c r e s in te e th of homogeneous shape and substance o v erco m e som e of the sam plin g p ro b lem s in h e re n t in o b tain in g su itab le hum an teeth for testin g purposes; however, further testing of this material is indicated.
A ll th e au th ors are w ith th e dep artm en t o f resto rativ e den tistry, U n iversity o f T e x a s Dental S ch o ol a t San A n ton io, 7 7 0 3 Floyd Curl Drive, S an A n ton io, 7 8 2 8 4 . Dr. Re is assistant professor, d ivision o f op erative dentistry; Dr. D raheim is a s s is ta n t p r o fe s s o r, d iv isio n of o p e ra tiv e d en tistry; and D r. N orling is a ssociate professor, d iv isio n o f b iom aterials. A d d ress requests for reprints to Dr. Re. 1. G r a n a th , L .E . P h o t o e l a s t i c m o d e l exp erim en ts on C lass II ca v ity resto ratio n s of dental am algam , O dont R ev 1 6 ( 9 ) :6 - 3 8 ,1 9 6 5 . 2. G uard, W .F .; H aack, M .S.; and Ireland, R.L. P h o to e la s tic s tr e s s a n a ly s is o f b u c c o lin g u a l se c tio n s o f C lass II ca v ity re sto ra tio n s. JADA 5 7 :6 3 1 -6 3 5 , 1 9 5 8 . 3 . J u d e s , H . I m p ro v e d c a v i t y d e s ig n fo r am algam restorations. Israel J D ent M ed 2 4 :2 8 -3 0 , 1975. 4 . T alim , S .T ., and G ohil, K .S. M anagem ent of co ron al fractures of perm anent posterior teeth. J P ro sth et Dent 3 1 ( 2 ) :1 7 2 - 1 7 8 ,1 9 7 4 . 5. R e, G .J., an d N orlin g,U .K . F o rces required to cra ck unfilled a n d filled m olar teeth. J Dent Res 5 9 (lssu e A ) :3 5 1 ,1 9 8 0 . 6 . M ondelli, J ., and others. F ra ctu re strength of h u m an te e th w ith cav ity p re p a ra tio n s.} Prosthet D ent 4 3 ( 4 ) :4 1 9 - 4 2 2 ,1 9 8 0 .
R e-D raheim -N orling: RESISTANCE OF MANDIBULAR MOLARS WITH CLASS I PREPARATIONS a 583