Fracture resistance of lower molars with varying faciocclusolingual amalgam restorations

Fracture resistance of lower faciocclusolingual amalgam Gerald J. Re, D.M.D.,*

Barry K. Norling,

Ph.D.,**

molars with restorations and Richard

varying

N. Draheim,

D.D.S.*

University of Texas Health Science Center, Dental School, San Antonio, Tex.

C

lass I carious lesions in lower molars occur frequently. These cavities may involve the facial, occlusal, and lingual developmental grooves of the tooth. Empirically, faciocclusolingual (FOL) amalgam restorations would appear to have a “stapling” effect in resisting fracture of the tooth mesiodistally. Silvestri and Singh’ have even recommended the placement of FOL amalgam restorations as a means of temporary stabilization of lower molars with vertical incomplete mesiodistal fractures. The purpose of this research was to determine whether the FOL amalgam would enhance resistance to fracture of previously sound lower molar teeth by preparing these teeth with varying width and depth FOL preparations and then fracturing the teeth by occlusal loading.

MATERIAL

AND

METHODS

Sixty caries-free, lower molars were obtained from the oral surgery department and kept moist in 10% formalin. The teeth were selected at random and separated into six groups of 10 teeth each. The first group consisted of sound, unrestored teeth. Groups two through five consisted of teeth with narrow/shallow, narrow/deep, wide/shallow, and wide/deep restorations (Fig. 1). All FOL preparations were bisected by the primary faciolingual developmental grooves. The widths of narrow and wide preparations were one-fourth and two-thirds the distances between summits of cuspal tips, respectively. The occlusal portions of shallow preparations were prepared to the depth of the head length of a No. 330 plain fissure bur, approximately 1.7 mm. Axial depths of shallow preparations were the

*Assistant Professor, Division of Operative Dentistry. **Associate Professor, Division of Biomaterials.

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diameter of a No. 245 plain fissure bur, approximately 0.8 mm at the gingival extent of the preparation. The gingival walls were 1 mm occlusal to the cementoenamel junction (CEJ). Occlusal depths of deep preparations were approximately 3 mm, while axial depths were two diameters of a No. 245 bur, approximately 1.6 mm at the gingival extent (1 mm occlusal to the CEJ) of the preparation. Axial walls followed the external contours of the tooth. For comparison, a sixth group was prepared for shallow mesiocclusodistal (MOD) restorations with cusps reduced to allow approximately 2 mm of cuspal amalgam coverage. Water and air spray were used on the ultraspeed handpiece during preparation, and all prepared teeth were restored with a high-copper amalgam alloy (Phasealloy, Phasealloy Inc., El Cajon, Calif.). Just prior to testing, the teeth were mounted in low-fusing metal alloy. Occlusal forces were applied parallel to the long axis of the anatomic crowns of the teeth using a 7h2 inch ball bearing. The ball bearing was allowed to seek its most stable position at or near the central fossa of the tooth. An Instron Universal Testing Machine (Model 1125, Instron Corp., Canton, Mass.) was used to fracture the specimens at 1 mm/min crosshead speed. Fractured teeth were visually examined, and the severity of fracture was classified according to the system of Talim and Gohil.2 In this system, a Class I fracture involves enamel, a Class II involves enamel and dentin, and a Class III involves enamel and dentin extending into the pulp chamber. In addition, each tooth was classified as restorable or unrestorable according to whether the fracture extended less than’or greater than 2 mm below the CEJ.

RESULTS The resultant fracture loads are summarized in Table I. Unrestored controls fractured under a mean load of 2,600 N. Teeth with narrow/shallow, narrow/

0022-3913/82/0505i8

+ 04$00,40/O 0 1982 The C. V. Mosby Co.

FRACTURE

RESISTANCE

OF LOWER

MOLARS

Fig. 1. Schematic representation

Table I. Forces required

to break

Group one (no preparation) Mean, N (pounds) Standard Deviation, (pounds) Homogeneous subsets’ *Brackets

molar teeth (in newtons)

Group two (narrow/shallow FOL)

2,600

indicate

Group three (narrow /deep FOL)

2,787 (627) 592 (133)

(585) N

683 (154)

JOURNAL

Group four (wide/shallow FOL)

2,365

(532) 759 (171)

2,317 (521) 817

Group five (wide/deep FOL)

I

1,924 (433) 740

(184)

OF PROSTHETIC

six

sll*w1 1,148 (258) 380

(167)

I subsets which

Group

(totrl covwzrge

!@J)

I I

are not statistically

1 different

at the 0.05 level by Student,

deep, wide/shallow, and wide/deep FOL restorations fractured under loads of 2,787,2,365,2,317, and 1,924 N, respectively. Teeth with MOD restorations with cuspal coverage did not fracture; all fractures were within the amalgam restorations. The mean fracture load was 1,148 N. A total of 21 Class III fractures (into the pulp chamber) occurred, all of which were unrestorable (Table II). The greatest incidences (six each) occurred in the unrestored teeth and in those with narrow/deep restorations. Class II fractures (into dentin) occurred in 19 teeth, three of which were unrestorable. The highest incidence (eight) occurred in the narrow/shallow group. The greatest incidence of unrestorable fractures (eight) occurred in the unrestored teeth; the lowest (zero) occurred with the full cuspal coverage (MOD) group for which all fractures were within the restoration. Multiple range analysis by the Student, Neuman, Keuls Multiple Range test at the 0.05 level resulted in three statistically homogeneous subsets (Table I). The weakest group, that with the full occlusal coverage, was significantly weaker than the others. The remaining groups fell into two homogeneous subsets with consid-

THE

of specimen preparation.

DENTISTRY

h’euman,

Keuls Multiple

__-_--..Range test

erable overlap. Only the weakest and the strongestof the prepared teeth, the wide/deep and narrow/ shallow, respectively, did not share subsets with the remaining groups.

DISCUSSION In the restoration of any tooth with minimal carious involvement, it is reasonableto question whether the tooth is better protected with a minimal conservative preparation and restoration or whether increased strength will result from the placement of a larger restoration with a greater bulk of amalgam. The question is particularly pertinent for FOL restorations where the restoration presentsat least the potential for providing a stapling effect, protecting the tooth against the most commonly occurring mesiodistalfracture. This investigation attempts to addressthis question by assessingthe effect of FOL restorations with varying widths and depths on the fracture strengths of previously sound teeth. The results suggest that a narrow/shallow FOL restoration strengthens a previously sound tooth, although not significantly. Increasing the width or depth individually did not significantly weaken the teeth. However, the combination of increased width and depth resulted in a

519

RE, NORLING,

Table II. Type and severity

Group two (narrow/shallow FOL)

Group three (narrow/deep FOL)

Group four (wide/shallow FOL)

Group six (total coverage shallow)

Group five (wide/deep FOL)

R

U

R

U

R

U

R

U

R

-

-

-

-

1

-

-

1

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

2 -

2 -

7 1

-

3 -

-

-

-

-

-

-

-

-

-

-

-

-

-

4 -

-

-

-

-

2

-

2

-

2 -

-

-

-

-

-

3

2 -

10

6

7

5

5

10

R*

U*

-

-

-

2

1

--

-

-

5 -

-

8

*R = Restorable; U = unrestorable according tFracture in enamel only. *Fracture in enamel and ,dentin. §Fracture in enamel and dentin, into pulp. IiTooth did not fracture. There were no Class IV root fractures.

-

8

2

to criteria

described

1 4

1 1

1

2

1 4

3

1

U

-

in text.

significant decrease in fracture strength. Teeth with the clinically more common MOD restorations with full cuspal coverage were significantly weaker than the unprepared controls or any of the groups with FOL restorations. However, the MOD restoration was most effective in preventing fracture of the tooth since fracture occurred only within the restoration. It has been our clinical impression that unrestored teeth fracture vertically when they fracture. Bales3 and Maxwell and Braly4 observed that unrestored teeth tend to fracture along major developmental grooves and that fracture often extends toward the pulp. Maxwell and Braly4 also observed that minimally restored teeth suffer the more severe vertical fracture. In a study of 33 unrestored teeth, Talim and Gohi identified 20 with vertical fractures, 10 of which exposed the pulp. (They included seven additional teeth with cracks caused by accidental falls-an uncommon etiology of the cracked-tooth syndrome.) 520

DRAHEIM

of fracture

Group one (no preparation)

Class It Oblique Complete Incomplete Vertical Complete Incomplete Class 114 Oblique Complete Incomplete Vertical Complete Incomplete Class 1115 Oblique Complete Incomplte Vertical Complete Incomplete Restoration Fracture11 Total

AND

In previous studies, Re et aL5-’ assessed the extent and severity of fractures in teeth with different types and sizes of restorations. In their studies on Class I occlusal and Class II amalgam restorations, no clear pattern developed relating severity of fracture to the extent of restoration. For FOL restorations, however, there does seem to be a relationship between the extent of a restoration and the ability to restore a fractured tooth. Two teeth with narrow/shallow preparations and three teeth with wide/shallow preparations were unrestorable. In contrast, six teeth with narrow/deep preparations and five with wide/deep preparations were unrestorable. Thus, width alone seems to have little effect on restorability, whereas more severe fractures occurred with deeper restorations. Fortunately, the group with the greatest fracture strength, the teeth with narrow/shallow preparations, also had the lowest incidence of fractures classed as MAY

1982

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FRACTURE

RESISTANCE

OF LOWER

MOLARS

unrestorable. Therefore, it appears that conservative FOL restorations are justified from the standpoints of both increased fracture strength of the tooth and increased restorability should fracture occur. SUMMARY

AND

fractures 31:172,

CONCLUSION

The effects of varying the width and depth of FOL amalgam restorations on tooth fracture strength and severity of fracture were evaluated. The greatest strength and lowest susceptibility to unrestorable fracture occurred with teeth with narrow/shallow restorations. Teeth with wide/deep restorations had the lowest fracture strength and the greatest incidence of unrestorable fractures.

6. 7.

I. 2.

THE

Silvestri. A. R., and Singh, I.: Treatment rationale posterior teeth. J Am Dent Assoc 972306, 1978. Tallm, S. T.. and Cohil, K. S.: Management

JOURNAL

OF PROSTHETIC

DENTISTRY

of fractured of coronal

posterior

teeth.

J PRO\rHET.

bN’[

Bales, D. J.: Pain and the cracked tooth. J Indiana Dent Assoc 54:15. 1975. Maxwell, E. H.. and Braly, B. V.: Incomplete tooth fracture. Prediction and prevention. Calif Dent hssoc J 9:51, 1977. Re, G. J., and Norling, B. K.: Forces required ICI g.rdck unfilled and filled molar teeth. ,J Dent Res 59:351 i WXi c.%bstr No. 334). Kc. G. J., Draheim. R. N., and No&y. H K. t;tacture resistance of mandibular molars with occlusai (11.~~ 1 amalyam preparatmns. J Am Dent Assoc 103:5X0, 198; Re, G. J., Norling, B. K., and Draheim. R. N.. Fracturing Class II amalgam preparations in molar trrrh with axial forces (In press.)

Kc/ml/ reylds 10: DR.

REFERENCES

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GERALD

J. RE

?‘HE UNIVERSITY DENTAL SCMX~L

7703 FLOYII CURL SAN I\NTOtilO, TX

OF rr~~~~

HEALTH

SCIENCE

CEM

EK

DR. 78284

521