Tooth Fracture in Vivo and in Vitro

7/30/2019 Tooth Fracture in Vivo and in Vitro

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J. Dent. 1992; 20: 131-l 39 131

Review

Tooth fractureF. J. T. BurkeDepartment of Restorative Dentistry,

ABSTRACT

in vivo and in vitroUniversity Dental Hospital of Manchester, UK

The incidence, causes and methods of investigating tooth fracture are reviewed. This is a problem ofincreasingclinical significance, with many predisposing factors. Large restorations and extensive cari ous lesions tend tobe associated with most fractures, with fracture incidence being higher in first permanent molars than othertooth types, especially in the lower jaw. Tooth anatom y influences fracture incidence. as does the functiona lforce applied to cusps. Fracture risk in restored teeth may be reduced by cuspal coverage. Traditional toothfracture investigations using destructive techniques provide valuable information; however, replica and non-destructive techniques are also of value.KEY WORDS: Tooth fracture, Review

J. Dent, 1992; 20: 13 l-l 39 (Received 15 January 1991; reviewed 6 March 1991; accepted 19September 199 1)

Correspondence should be addressed to: Mr F. J. T. Burke, Department of Restorative Dentistry, UniversityDental Hospital of Manchester, Higher Cambridge Street, Manchester M 15 6FH, UK.

TOOTH FRACTURE IN WV0The potentially weakening effect of dental caries and itstreatment, alongside the effect of tooth surface loss(attrition, erosion and abrasion) (Eccles, 1982 ), maypredispose to tooth fracture.

Tooth fracture may be complete, where p art of the toothbecomes detached from the remainder, or incompletewhere the fractured portion remains in situ. While thecomplete fracture may be clinically obvious, an incompletefracture often may be more subtle, and therefore presentsa more difficult diagnostic problem. The symptom sassociated with incomplete tooth fracture have beendescribed as the cracked tooth syndrome by Cameron(1964) and Stanley (1968). The incidence of incompletetooth fracture ha s not been evaluated clinically, althoughin a survey of patients referred to endodontists fortreatment, it was considered that incomplete toothfracture may be present in 20 per cent of such patients(Braly and Maxw ell, 1981).

Fracture of the cusp of a tooth has been considered to bea common clinical problem (Braly and Maxw ell, 1981;Cave1 et al., 1985). It would appear that with patientskeeping more of their teeth for longer periods of time, theproblem of tooth fracture is of increasing importance(Liebow, 1976). Fractures of teeth may vary in severityfrom the minimal enamel fracture, to fracture of a wholecusp or longitudinal fracture, which may lead to loss of the@ 1992 Butterworth-Heinemann Ltd.0300-5713/92/030131-09

tooth. Intermediate cases may be seen, such as crackedcusps associated with large restorations, and these havebeen reported to have an increasing incidence (Fisher,1982). Indeed, tooth fracture has been implicated as amajor reason for replacement of up to 13 per cent ofamalgam restorations and may be responsible for 5 percent of restorations placed in posterior teeth (Mjiir, 1981;Charbeneau and Klausner, 1984). Such cuspal fracturesare considered to frequently present a difficult restorativeproblem, because the fracture may often extend sub-gingivally (Lagouvardos et al., 1989).

Causes of tooth fractureThe most commo n causes of tooth fracture have beenidentified as high impact forces caused by biting on a hardobject or uncontrolled contact of opposing teeth. Thecause o f tooth fracture was investigated in India by Talimand Gohil(1974), who suggested a classification for toothfracture (Table I). They found that sudden biting on a hardobject w as the most common cause of fracture, with stoneparticles and nuts being particularly implicated. How ever,other factors may predispose to tooth fracture. Theseinclude excessive contact of posterior tooth cusps duringeccentric jaw movement, large internally retained restora-tions, wear, malocclusion, dehydration due to endodontictherapy, and steep cusp inclines and/or deep grooves in


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132 J. Dent. 1992; 20: No. 3

Table 1. Classification of fractured teeth (from Talim andGohil, 1974)Class I Fracture involving enamel

A Horizontal or obliqueB Vertical

1. Complete2. IncompleteClass I/ Fracture involving enamel and dentine without

involving pulpA Horizontal or obliqueB Vertical

1. Complete2. Incomplete

C/ass Ill Fracture of enamel and dentine involving theA HorizontalB Vertical

1. Complete2. Incomplete

Class IV Fracture of the rootsA Vertical or oblique

1. Involving the pulp2. Not involving the pulp

B Horizontal1. Cervical third2. Middle third3. Apical third

the occlusal morphology (Helfer et al., 1972; Braly andMaxw ell, 1981; Ketterl, 1983; Gheretal., 1987). How ever, ithas been considered that large restorations and cariouslesions seem to be associated with most fractures (Eakle etal., 1986).

Effect of ageWith regard to the age at which tooth fracture mostfrequently occurs, Cameron considered that fracturesoccurred most frequently in patients over the age of 50years, and indeed, 80 per cent of the cases which hereviewed were 40 years or older (Cam eron, 1964), whileSnyder found that most fractures occurred in patientsaged between 30 and 59 years (Snyder, 1976). Talim andGohil considered that most tooth fractures o ccurred inpatients who were middle-aged or older (Talim and Gohil,1976). How ever, these figures are at variance with the morerecent work of Eakle et al. (1986) w ho found that 66 percent of the patients who suffered com plete and incompletetooth fractures were less than 40 years of age. Anotherrecent study of posterior tooth fractures at the Universityof Athens show ed that 82 per cent of the fractures occurredin patients less than 49 years of age (Lagouvard os et al.,1989). This trend may simply indicate that differing agegroups of patients attended the clinics where the investiga-tions took place: alternatively, these results may indicate atrend towards tooth fracture in younger age groups.

Effect of tooth typeWith rega rd to tooth typ e, the incidence of fracture hasbeen shown to be higher in first permanent molars than

other teeth, especially in the lower jaw, possibly because oftheir high susceptibility to caries and their subsequentrestoration (Cave1 et al.. 1985). Indeed, fractures ofmandibular first permanent molars accounted for 27 percent of all posterior tooth fractures in a study of 191patients (with 206 fractured teeth) by Eakle et al. (1986).The high incidence of fracture of mandibular first molarswas explained by Cameron as being due to the increasedleverage (nutcracker effect) on these teeth, as they wereconsidered to be close to the fulcrum of masticatorymovement (Cameron, 1976). The highest frequency offracture (43 per cent) w as also seen in first permanentmolars in the study by Lagouv ardos et al. (1989) againwith fracture incidence being higher in the lower than theupper arch. This is in contrast to fracture incidence inpremolar teeth, where the highest incidence has beenreported to be in the maxillary arch (Lagouv ardos et al.,1989). This finding was also observed in the study by Eakleand Maxw ell, where fractures were seen in 49 maxillarypremolars and only 12 mandibular premolars (Eakle etal., 1986).

It has also been noted that the lingual cusps of lowermolars tend to fracture more readily than the buccal cusps(Cave1 et al., 1985; Eakle et al., 1986) while in maxillarymolars, buccal and lingual cusps fracture w ith almostequal frequency. It was also found that in maxillarypremolars, the lingual cusps fractured only slightly morefrequently than the buccal cusps (Eakle et al., 1986).How ever, in the study by Cave1 et al., 62 per cent o f themaxillary premolar cusp fractures occurred in the non-functional buccal cusp (Cave1 et al., 1985). Conversely, inthe University of Athens study, the frequency of fracturesdid not appear to be related to functional or non-functional cusps, nor to teeth with vital or non-vital pulps(Lagouvardos et al.. 1989).

The frequency of cuspal fracture and its relationship totooth anatomy has been investigated by Khera et al.(1990). These worke rs examined serial sections of upperand lower molar and premolar teeth, and analysed thespecimens for difference in cuspal wid th, difference incuspal angular inclination, difference in enamel thicknessand difference in angular inclination. Results showed thatthe functional cusps of the maxillary molars and of all ofthe mandibular posterior teet h were significantly widerthan the non-functional cusps, although maxillarypremolars had smaller functional cusps. It has beensuggested that functional cusps of restored teeth fractureless frequently than non-functional cusps, this also beingseen in maxillary premolars, where the smaller functionalcusps fracture less often (Cave1 et al., 1985; Eakle et al.,1986). This m ay be correlated with the results o f Kheraet al. (1990) with regard to cuspal dimension. Theseworkers show ed that all of the functional cusps weresignificantly larger in buccolingual dimensions than thenon-functional cusps, with the exception of maxillarypremolars. A further comparison of fracture frequencyand enamel thickness concluded that thicker enamelmade the cusps of molars stronger (Khera et al.. 1990).


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Table II. Investigations1956-90

Burke: Review of tooth fracture 133

involving tooth fracture reported in the dental literature

ReferenceMethod of application

fractureCrosshead speed

per min.Vale, 1956 Steel ball 3/l 6 in. diameterVale, 1959 3/l 6 in. steel ballMondelli et al., 1980 Steel sphere 4 mm diameterRe and Norling, 1980 5.56 mm ball bearingLarson et al., 198 1 Steel sphere 3/l 6 in.Re et al., 198 1Re et al., 1982Blaser et al., 1983Simonsen et al., 1983Landy and Simonsen

et al., 1984Morin et a/., 1984Reel and Mitchell

et al., 1984Mishell and Share1984Eakle and Braly, 1985Eakle, 1985aEakle, 1985bBakke et al., 1985Joynt et al., 1985Eakle, 1986aEakle, 1986bWatts, 1986Schultz et al., 1986Stampalia et a/., 1986Watts et al., 1987

(4.76 mm) diameter7/32 in. ball bearing7/32 in. ball bearingMetal bar 3/l 6 in. wideN/SN/SSteel sphere 6.3 mm diameterMetal ball 4.7 mm diameter

N/SBall bearing l/8 in. diameterBall bearing 3/l 6 in. diameterN/SN/SN/SBall bearing 3/l 6 in. diameter4.76 mm ball bearing8 mm bail bearing

Joynt et al., 1987Oliveira et a/., 1987Jensen et al., 1987Sheth et al., 1988Weiczkowski et al.,

1988Joynt et al., 1989

20 lb per second under closed conditions3.9-5 mm bar 5cm8 mm ball bearing 1 cm4 mm ball bearing2 Metal rods 0.01 cm

1.985 mm in. diameterSpecially designed bar 0.5 cmN/S 0.05 cm5 mm.. . . . sphere 0.5 cm2 Metal rods 1.985 mm 0.01 cm

diameter2 Metal rods approx. 2 mm

diameterReel and Mitchell,

1989Reagan et a/., 1989Purk et al., 1990aPurk et al., 1990bSorensen and Engelman,

1990Kane et al., 1990Dietschi et al., 1990Burke et al., 1990N/S, not stated.

Metal ball 4.7 mm diameterN/S0.79 mm2 cast premolar tip0.79 mm2 cast premolar tipN/S

45 bevel2 mm sphere4 mm bar

N/SN/S0.5 mm1 mm

N/S1 mm1 mm

IOmm0.05 mm

N/SN/S

0.05 mm

0.25 mm20 mm

5mm5mm

N/S0.5 mm5mm5mm1 mm

0.01 cm0.508 mm0.13 mm0.5 mm0.5 mm2.54 mm

2mm1 mm1 mm

How ever, notwithstanding factors such as the above, itwas still considered that the extent o f the carious lesionand of the intracoronal restoration will exert an effect onthe likelihood of fracture (Khera et al., 1990).

currently available, classic designs of Class II cavity arestill being cut (Hood , 1990).

It has been stated by Hoo d (1990) that the eventualconsequence of the use of cavity designs introduced byBlack in 1895 is fracture of one of the cusps. Thisphenomenon, which is often attributed to ageing, is theresult of overextended cavity designs. It is therefore feltsurprising that with all the modern restorative techniques

INVESTIGATIONS OF TOOTH FRACTURERESISTANCEOne of the earliest investigations of the influence of cavitydesign on tooth fracture resistance was reported by Vale in1956. In this study, contralateral pairs of premolars were


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134 J. Dent. 1992; 20: No. 3

Fig. 7. Diagrammatic representation of the application of a Fig. 2. Diagrammatic representation of the application of acompressive force to a restored tooth, without direct applica- compressive force to a restored tooth, with the force beingtion of the force to the restoration. applied directly to the restoration.

used, one of each pair acting as control, and the otherbeing prepared with a Class II cavity. The teeth w ere thenfractured by a compressive load applied to a steel ballcentred in the occlusal fossa. The results showed that:

1. When the isthmus width w as one-quarter of theintercuspal width, the fracture force was the same forcontrol teeth and prepared teeth.

2. When the isthmus width was one-third of theintercuspal width, the fracture force was two-thirds that ofthe intact controls.

3. There was no difference between the fracture forcefor restored and prepared/unrestored teeth.

4. Teeth restored with gold overlays were twice as strongas unrestored teeth with the same cavity preparation.

A similar technique has been utilized to examine theeffect of restorative techniques on the fracture resistanceof teeth in many subsequent investigations (Table II). Inthe investigations quoted in this table, a universal testingmachine was used to deliver a compressive force to theocclusal surfaces ofthe teeth to be tested, with various steelspheres or bars being u sed to apply the compressive forceat varying crosshead speeds. In some investigations theball bearing was allowed to find its most stable position ator near the central fossa of the tooth (R e et al., 1982). whilein other experiments the bearing is held in a speciallydesigned testing hea d (Eakle and Braly, 1985). In otherinvestigations, two bearings are used to apply thecompressive force (Weiczkow ski et al., 1988; Joynt et al.,1989) while other w orkers prepare the points of contact ofthe tooth to prevent slipping of the ball bearing (Morin etal.. 1984; Eakle, 1986a. b). These measures were taken tostabilize the position of the sphere on the tooth (a problemwhich may, in part, be overcome by the use of a steel bar)and to ensure that the sphere or bar is positioned so that itcontacts only the buccal and lingual cusp inclines, rather

than the restoration. Under these conditions, when a forceis applied to the tooth, th e buccal and lingual cusps areplaced under compressive stress and deform outward witha resultant tensile stress at the tooth/restoration interface.How ever, if the force is applied to the restoration alone, acompressive force is applied to the restoration and therebytransmitted to tooth substance. Accordingly there wouldstill be tensile stress at the restoration/tooth interface (Figs1. 2).Alternative non-destructive techniques have beendeveloped to investigate the stresses occurring in teethfollowing cavity preparation and restoration. An earlystudy, which examined such stresses using a non-destructive technique, was conducted by Noonan (1949).In this investigation, varying designs of cavity were cut insheets of Bakelite and packed with dental amalgam .When the amalgam had set, loads were applied and themodels were then examined in a polariscope.

Hood et al. in 1975 used a photoelastic technique toobserve the stresses in three different pontic designs.utilizing two-dimensional photoelastic sheets. Theseworke rs em ployed a technique similar to the one used toexamine stress distribution in Class V restorations in 1972(Hood , 1972). Two-dimensional photoelastic models werealso used in a study of stresses in inlay and onlaypreparations (Fisher et al., 1975).

The setting up of a mathem atical model is an alternativemethod of analysis of stress in fractured teeth (Bell et al.,1982), a method also put forward by Peters (1981) and usedin subsequent investigations (deVree et al., 1984). Finiteelement analysis was also used in a study by Khera et al.(1988). How ever, the development and use of strain gauges(Malcolm, 1973; Malcolm and Hood, 1977) appears to givesensitive estimation of stresses in teeth. In these studies,subfracture loads were applied through a steel ball in theocclusal fossa, while the strain g auges applied to the


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buccal and lingual cusps measured cusp movement.These results we re expressed in a relative stiffness ratio(RS), where the stiffness of the intact tooth is 1. Values ofgreater than 1 indicate a rigidity greater than that of theintact tooth , while lesser values indicate a reducedstiffness. A similar bonded strain gauge technique hasalso been utilized by Douglas (1985).

Effect of cavity d imensionThe importance of conservative cavity preparation, whilenot extensively practised until recent times, has beenadvocated in the past as a method of preserving toothstrength (Bronner, 1930; Markley, 1951). How ever, it wasthe pioneering work of Vale (1956) which co nfirmed thewisdom of conservative cavity preparation by showing adecrease in the strength of a prepared tooth wh en thewidth of the isthmus was extended from one-quarter toone-third of the distance between the buccal and lingualcusp-tips. Nevertheless. while Vale did not demonstrate adecreased strength in the most minimal cavity prepara-tions, Mondelli et al. (1980) did demonstrate a decrease instrength when te eth were prepared, even with a narrowClass I preparation. Their investigation of upper pre-molars also showe d that, in all preparations, the narrowerthe isthmus, the greater the load required to cause fracture.This was found to be statistically true for Class Ipreparations of various dimensions. How ever, for Class IIpreparations, only in those with an isthmus of one-quarterthe intercuspal distance was the strength statisticallysuperior to the other dimensions. On analysis of theirresults, Mondelli et al . (1980) suggested that cast restora-tions with cuspal protection were indicated when theocclusal isthmus measured one-half or more of theintercuspal distance.

A study by Larson et al. (1981) partly confirmed thework of Mondelli et al. (1980) and indicated that theextension of a preparation to involve proximal boxes doesnot significantly reduce t he strength of the tooth, providedthat only a minimal amount of dentine is removed.Indeed, it is suggested that breaking the continuity of theenamel weakens the tooth and that this step should beavoided if possible (Hood , 1990).

Upper premolar teeth were used in a study by Blaser etal. (1983). These worke rs found that the loss of strength inteeth with wide occlusal isthmus preparations was not assevere as previously reported. They considered thatnarrow isthmus/deep pulpal floor preparation had agreater weakening effect than the wide isthmus andshallow floor preparation. Furthermore, they noted thatlarger teeth may resist fracture better than small teeth.

The effect of cavity w idth and dep th on fractureresistance was also investigated by Re et al. (1982). Withregard to facie-occlusolingual (FOL ) restorations, theyfound that there appeared to be a relationship between theextent of a restoration and the ability to restore thestrength of a fractured tooth. The greatest strength andlowest susceptibility to unrestorable fracture wa s seen in

teeth with narrow/shallow restorations. Width aloneappeared to have little effect on restorability, since sixteeth with narrow/deep preparations and five with wide/deep preparations were unrestorable. More severefractures were seen to occur with deeper restorations.

The effect of the use of sharp line angles on stressconcentration has also been examined (Eakle and Braly,1985). This study indicated that, while concentrations ofstress occurred around sharp line angles, there was not asignificant weakening of the tooth by using sharp ratherthan rounded line angles. How ever, no hand instrumentswere used to sharpen the angles. and the preparationswere fairly conservative in that the occlusal isthmus wasless than one-quarter of the intercuspal width. Never-theless, w hen the effect of sharp internal angles wasexamined in Class I cavities in molars, it was consideredthat there was no reduction in strength when comp aredwith cavities w ith rounded internal angles (Re andNorling, 1980).

Other studies have investigated the effect of restorativematerials on tooth fracture resistance (Stampalia et al.,1986; Watts. 1986; Joynt et al., 1987; Sheth et al.. 1988; Reeland Mitchell, 1989). From these studies it may beconcluded that preparation of teeth for restorationreduces resistance to fracture and that in this respectthere is no difference between amalgam and compo sitematerials (Joynt et al., 1987). Furthermore, cusp reinforce-ment (Morin et al., 1984) and improved resistance tofracture (Ea kle, 198 5a, 1986b) may be demonstrated usinga bonded compo site restorative technique. Indeed, ahybrid com posite material has been shown to restore thestrength of teeth w ith Class I cavities to a similar strengthlevel as sound teeth (Watts et al.. 1987). Another studycomp ared the fracture strength of teeth with weakenedmarginal ridges restored with Class I composite restora-tions and those restored with Class II compo sites andamalgams (Purket al., 1990a). These w orkers showed thatin the Class II amalgam restorations only the materialfractured, while in the Class II compo site specimens, six ofthe 25 teeth also show ed a fracture. It was thereforeconsidered that. in cases wh ere the risk of restorationfracture was high, the use of amalgam rather thancomposite may be desirable (Purk er al.. 1990a, b).

It has been found that alternative restorative techniquesmay also restore the fracture resistance of teeth. Forexample, MO D amalgam overlays have been shown toproduce fracture resistance similar to that of unpreparedcontrols, w hile MO D gold overlays enhanced fractureresistance to a level much greater than that of theunprepared control teeth (Salis et al., 1987). The tooth-strengthening effect of overlay placement has also beendemonstrated with indirect composite restorations, whenthe incorporation of 2 mm cuspal overlays in a compositeinlay preparation increased the fracture strength to avalue equivalent to that of sound teeth (Burke, 1991).

The effect of fissure sealant application on fractureresistance has been investigated by Schultz et al. (1986).These work ers concluded that pit and fissure sealants


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136 J . Dent. 1992; 20: No. 3

do not affect the fracture resistance of maxillary firstpremolars, but that preparation of one or more proximalboxes significantly reduces strength of teeth which havehad sealant applied to the occlusal fissure; however, therewas no difference in fracture resistance when one or morethan one box was prepared.

Effect of endodontic treatmentThe fracture of teeth at some time subsequent to root canaltreatment is a common clinical observation (Lewinsteinand Grajow er, 1981) with Gher et al. (1986) in a study o fthe clinical features associated with tooth fracture, findingthat 71 per cent of the fractured teeth w hich they examinedhad been root tilled. This increased risk of fracture hasbeen attributed to several factors:

1. The loss of tooth substance due to carious destruc-tion necessitating root canal therapy (C ortade andTimmermans, 1971).

2. The effect of the endodontic access cavity prepara-tion. This has been investigated by How e and McKendry(1990) who compared the effect of MO D cavity prepara-tions with endodontic access preparations and found nosignificant difference in fracture resistance between teethwith only an endodontic access cavity and those preparedfor conservative MO D amalgam restorations. How ever,teeth prepared with both MOD and endodontic accesscavities had their fracture resistance reduced to 5.5per centof the value of those with only one o f these cavitypreparations.

3. The effect on the physical properties of dentine.The effect of root canal treatment on dentine hardness hasbeen investigated by Lewinstein and Grajowe r (1981) in astudy of 16 vital and 3 2 root-filled teeth which had beenextracted. Their results indicated that root canal therapydid not affect the hardness of dentine, even after periods of5-10 years. Healey , in 1960, wrote that the coronal portionof treated pulpless teeth was more brittle and fragile thanwhen th e pulp was vital, and considered that this wasoften attributed to decreased moisture content. Th is viewwas confirmed, in part, by Helfer et al. (1972) who show edthat there was 9 per cent less moisture in the calcifiedtissues of pulpless dogs teeth than in those of vital teeth.These w orkers also found that there was greater loss ofmoisture in anterior teeth than posterior teeth, and that itwas only after the twelfth week following pulp extirpationthat the difference in percentage loss of moisture becamemore consistent. The strength and toughness of dentinefrom root-treated teeth has been investigated by Carter etal . (1983) who, using a punch shear test, reported a 14 percent reduction in strength and toughness and concludedthat dentine from root-treated molars w as weak er andmore brittle, compa red with samples from teeth with vitalpulps.

The fracture resistance of endodontically treatedpremolar teeth was investigated by Hansen (1988) in aretrospective survey. It was found that of the 181 teeth

which were filled with MO D amalgams, 13 per centfractured within the first year after treatment and one-third fractured within 3 years. By comparison, none of the40 root-tilled premolars filled with MO D resin restora-tions fractured during a similar period. When observedover a 3-lo-year interval, the resin-restored premolarsalso had a better survival rate, although this differencewas not statistically significant. In the amalgam group, thebuccal cusp failed in the upper premolar in 52 per cent ofcases while in the lower first premolar, only fractures ofthe lingual cusp w ere seen. Only th ree of the 107 fractureswere vertical, leading to loss of the tooth. How ever, theseresults differ from those obtained in the in vitro study byStampalia et al. (1986) whose work showed verticalfracture in 21 of 22 teeth tested on a universal testingmachine. This difference in results may be explained notonly by the difference in forces applied in the laboratoryand in the clinical situation, but also by the differingcavity dimensions in the two investigations, It may also beexplained by the fact that in vitro testing of a rigidlysupported tooth ignores the resiliency of the periodontalligament (Craig, 1985).

The restoration and prevention of cusp fracture of root-filled teeth has been the subject of some de bate, withpreservation of all remaining sound dentine beingconsidered to be a primary objective and with full occlusalcoverage being the minimal acceptable restoration(Johnson et al., 1976) or a restoration such as an inlaywhich provides cuspal protection (Hood, 1985). However,dentists may not find it acceptable to place a finalrestoration upon a root-tilled tooth until t here is radio-graphic evidence of periapical healing, perhap s 6-12months after root canal treatment. As Hansen (1988) hasdemonstrated, during that period there is a considerablepossibility of cuspal fracture in premolar teeth restoredwith amalgam . Accordingly, consideration should begiven to the provision of bonded composite resin restora-tions as an interim measure, while apical healing is beingascertained.

INVESTIGATION OF FORC ES GENERA TEDDURING MASTICATIONThe studies which were noted in Table ZZ employed toothfracture resistance as a means of observing the effect oftooth cavity preparation/restoration on the strength o f atooth. How ever, while these studies are of relevance incomparing the effect of varying restorative techniques, ifthey are to have clinical relevance, the forces utilizedshould be within the limits which may be obtained withthe natural dentition. It is therefore of interest to observethe results of investigations where the bite force wasmeasured.

Studies indicate that different persons chew at differentspeeds, and that this is related to the type of food beingchewe d, with one cycle of mastication normally takingbetween half and three-quarters of a second. Furthermore,the rate of loading of specimens has been considered to


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have an effect on results obtained in transverse testing(Jones et al., 1970).

The measurement of forces within the oral cavity hasevolved from the use of crude methods where devices w ereplaced between the teeth, to more sophisticated apparatuswith leads to recording equipment more recently beingreplaced by radiotelemetry metho ds. The investigation ofmasticatory forces has been carried out by three methods(Bates et al.. 1975):

1. With measuring devices between the teeth.2. With transducers within restorations.3. With transducers in dentures at the denture/mucosa

interface.Variations in masticatory forces produced by the oral

musculature have been shown by most worke rs, given thatthe limiting factors are the muscular pow er that thesubject can produce, alongside their pain threshold, andin the case of dentures, the stability and retention of theappliance. The bite force for denture wearers has beenshown to be less than that for dentate subjects, even forsatisfactory as well as unsatisfactory dentures (Haraldsoneta]., 1979). Age, per se, has been shown to have little effecton masticatory function/bite force (Carlsson, 1984). Whileresults may tend to show an age-linked effect, it isconsidered to be the age-related impairment of thedentition which results in most of the decline in mastica-tory efficiency (Carlsson, 1984). In general. it has beenfound that the magnitude of occlusal forces is greater inposterior regions o f the mouth than anterior regions, andthat biting forces are greater than chewing and swallowingforces (Lundgren and Laurell, 1986). It may thereforefollow that the larger bite forces m ay be more likely tocause tooth fracture.

Few method s of measurement of bite force have beendeveloped for routine clinical use, despite the recognizedimportance of maintaining or restoring acceptable masti-catory function. Indeed, bite force measurements may beused as an indicator of masticator-y function. Earliest biteforce recordings were carried out by Howell and Manley(1948) and Anderson (1953, 1956) who, using a straingauge in an inlay, found a whole too th maximum load of14.9 kg. The greatest recorded masticator-y load in a fulldenture was found to be 12 kg, in a study using a straingauge inserted in a denture premolar (Yurkstas andCurby, 1953). Another early study examined the effect ofmodifying the occlusion on masticatory stress, and foundthat when the tooth surface was artificially raised abov ethe general occlusal level by 0.5 mm, the loads w ere twicethe normal level (Anderson and Picton, 1958).

Remov able dentures are usually associated with areduction in bite force which is most pronounced in fulldentures (Haraldson et al., 1979) but also present inpartial dentures, as demonstrated by Watt et al., (1958).These worke rs reported average bite loads of 21.7 kg fornatural teeth, 11.2 kg for tooth-borne dentures, and 7.4 kgfor tissue-borne dentures. Conversely, fixed prostheses arenot usually associated with such a reduction. In healthy

subjects with a good dentition, the maximal bite force hasbeen calculated to average 300-500 N, but with greatindividual variation (Carlsson, 1974: Bates et al.. 1975).When bite force was measured in two matche d groups,one dentate and the other treate d by osseointegratedimplants, there were no statistically significant differences.The maximal bite force in the implant group, measuredwith a bite fork between opposing teeth, was between 42and 41 2 N. with a median value of 143 N, while the rangein the dentate group varied from 103 to 368 N (Carlssonand Haraldson, 1985). Another study of 19 oral implantpatients show ed maximal bite force measurements ofbetween 410 and 90 N in men and 230 and 40 N in wom en,although the mean values were not statistically different(Haraldson and Carlsson. 1977). The bite force of patientsbeing treated for dysfunction of masticatory system wasmeasured by Helkimo et al. (1975). They recorded amaximum bite force value of 48 kg for control patients intheir investigation. In another study of 100 male dentalstudents, bite force measurements were recorded andrelated to mandibular dysfunction symptom s and toothwear (Helkimo and Ingervall, 1978). The mean bite forcemeasured was found to be 471 N, with a range of 191-802 N, no difference having been noted in bite force insubjects with different dysfunction indices, although biteforce measurements increased with increasing tooth wear.Occlusal forces were measured by a sound transmissiontechnique in studies by Gibbs et al. (198la, b). In thesestudies. it was found that the maximum biting forceduring clenching averaged 162 lb (73 kg) for 20 subjects,and ranged from 55 to 280 pounds (25-127 kg), but nocorrelation was found between either sex or age.

CONCLUSIONTooth fracture is an increasingly common occurrence,with permanent molar teeth showing the highest incidenceof fracture clinically. Among premolars, maxillary teethfracture mo re often than mandibular teeth. Tooth fractureresistance has been an established metho d ofinvestigationof the effect of cavity/restoration design on tooth strengthfor over three decades. While alternative non-destructiveexperimental method s may give valuable information onthe stresses and strains generated by restorative tech-niques, tooth fracture resistance remains an importantmetho d of investigation of any new restorative technique.Since values of biting force of up to 800 N have beenmeasured clinically, experimental forces of this value maybe seen to be of clinical relevance.

ReferencesAnderson D. J. (1953) A method of recording masticatory

loads. J. Dent. Res. 32, 785-789.Anderson D. J. (1956) Measurement of stress in mastication I.

J. Dent. Res. 35, 664-670.Anderson D. J. and Picton D. C. A (19 58) Masticatory

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Tooth Fracture in Vivo and in Vitro

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