David Assif, D.M.D.,* Barry Raphael Pilo, D.M.D.**

L. Marshak,

with B.D.S.,**




Tel Aviv University, The Maurice and Gabriela Goldschleger School of Dental Medicine, Tel Aviv, Israel Cavity preparation causes cuspal flexure under simulated occlusal loads. During amalgam condensation, the dentist exerts forces on the tooth. After condensation, dental amalgam undergoes dimensional changes. We measured possible changes in the cuspal position of premolars during and after their restoration with dental amalgam. Strain gauges were attached to the buccal surfaces of the teeth, and a direct reading of the strain and a simultaneous time-strain curve were obtained. Measurements were taken at the onset of amalgam condensation and continued for 24 hours. The amalgam was then removed from the teeth, and a subsequent reading was made. On the basis of this model, we found that the use of amalgam as a restorative material caused a static load on the cusps of the teeth brought about their consequent permanent deformation. After amalgam removal, we observed complete elastic recovery for all the treated teeth. (J PROSTHET DENT 1990,63:258-



he crown of the tooth is weakenedby occlusaland proximal cavity preparations. Thus the tooth is prone to cuspalfracture. The width of the cavity preparation on the occlusalportion influencesthe strength of the tooth and its ability to resist loading. The more tooth structure removed from the occlusalsurfaceduring the cavity preparation, the weaker the cuspsof the tooth become,increasingthe possibility of cusp fracture.lw4 Earlier researchersrecorded measurementsof cuspal flexure under simulated occlusal loads. They found that removal of tooth structure causedcuspal flexure and that the deformation was proportional to the applied load.5*6 During amalgamcondensation,the dentist exerts forces on the to0th.l After the completion of condensation,dental amalgam undergoesdimensional changeswithin the first 24 hours, the end result of which is expansion8 It is possiblethat the condensation forces and the expanding amalgamexert pressureon the walls of the cavity. Little is known about the mechanical behavior of the crown of the treated tooth after amalgamrestorations. In this study, we used strain gaugesto measure possible changesin the cuspalposition of premolars during and after their restoration with dental amalgam.




We selectedsevenfreshly extracted, flawless,noncarious maxillary secondpremolars of similar size. From the time of extraction until cavity preparation and testing, the teeth

*Coordinator, Department of Prosthodontics; Lecturer in Prosthodontic Rehabilitation. **Instructor, Department of Prosthodontics. 10/l/15834




were stored at room temperature in physiologic saline solution. The teeth weremounted to a level of 1 mm belowthe cementoenameljunction in a block of acrylic resin. Six teeth were subjectedto’an MOD preparation. The seventh tooth wasthe control and waskept intact. A new high-speed,water-spray FG bur wasusedfor eachcavity preparation. The buccolingual width of the occlusal portion of the cavity preparation wasone-third of the intercuspal distance, and the cavity preparation wascarried 1.8 mm into the dentin. We followed the traditional method of cavity preparation for usewith amalgam.The proximal boxeswere positioned and dimensionedto represent the ideal clinical situation. The buccolingual width of eachproximal box wasthe same as that of the occlusal portion. With the use of hand instruments, one dentist completed all preparations. Strain gauges(EA-13-031CF-120,Vishay Measurements Group, Micro-Measurements Division, Raleigh, N.C.) were attached to the buccal surfacesof the teeth. This wasaccomplishedwith the useof an adhesiveespeciallydesigned for this purpose (M-bond 200, Vishay Measurements Group, Micro-Measurements Division) according to the manufacturer’s instructions (Fig. 1). The gaugewasbonded to a point that correspondedto the level of the floor of the occlusalportion of the cavity. The middle of the gaugewas slightly coronal to that level. At this location the moment is maximal, and the gaugewill measurethe averageof the strains at this particular point (Fig. 2). If a greater part of the gaugewere to be bonded below the level of the floor of the occlusalpart of the cavity, it would measurea surface in which stressesapproach zero. A gaugewasbonded to the control tooth at a level correspondingto that on the six teeth to be treated. Small, flexible jumper wires, curved to form strain-relief loops, wereusedto connect the gaugeto a bondableterminal that









1. Strain gaugeattached to buccal surface of tooth. A, Gauge; B, strain-relief loop; C, terminal; D, three-lead conductor cable. Fig. 2. Location of gaugerelated to floor of occlusalportion of cavity. Fig. 3. Measurement system: tooth and gaugeconnected to strain indicator. Fig. 4. Treated tooth after amalgamcondensation.Matrix band winding around metal pins and limiting proximal cavities without exerting pressureon tooth walls. Fig.

was installed adjacent to the gauge (Bondable terminal, CTF-GOD,Vishay Measurements Group, Micro-MeasurementsDivision) (Fig. 1). The wires connecting the gaugeto the terminal, the terminal itself, and the first part of the terminal outlet wire were bonded to the acrylic resin block with strain gaugeadhesiveto immobilize them. They were subsequentlysealedwith a transparent polyethylene coating used for protection from moisture (M-Coat A, Vishay Measurements Group, Micro-Measurements Division). A three-lead wire conductor, vinyl insulated (330 DFV, Vishay MeasurementsGroup, Micro-Measurements Division) wasusedto connect the terminal to a strain indicator (P-3500strain indicator, Vishay MeasurementsGroup, Instruments Division). The strain indicator wascalibrated to the correspondinggaugefactor, giving a direct reading of the strain in microstrain units (Fig. 3). The strain indicator wasconnectedto a pen plotter (Tek-Dyn Ltd., ElectroDynamic Industries) to provide a simultaneoustime-strain curve. To provide a reading in microstrain units, the pen plotter had beencalibrated with a specialcalibrator model 1550 (A, Vishay Measurements Group, Micro-Measure-






Fig. 5. Time-strain curve shows effect of short-acting pressureexerted by amalgamplugger on axial walls of occlusal portion of cavity-short deformation followed by complete and instant elastic recovery.






Fig. 6. Time-strain curve, sample curve of test (treated tooth No. 1). A, Beginning of condensation; B, completion of condensation; C, short relaxation; D, maximal flexure; E, small relaxation; F, permanent flexure.

Table I. Cuspal flexure (in microstrains) Tootb

No. 1

2 3 4 5 6

during different phases of the study

Flexure at completion of condensation

23 25 27 24 26 24

ments Division). Four metal pins were embedded in the acrylic resin block, directly adjacent to and parallel to the cavosurface axial line angles of the proximal boxes. A metal matrix band was threaded between the pins and the teeth to seal the outer borders of the proximal boxes, but so as not to exert pressure on the walls of the teeth. The gingival border was sealed with a wedge that was placed between the matrix and the pins. Amalgam (Spherodon, Silmet Ltd., Givatayim, Israel), triturated the same amount of time for all six teeth, was placed into each cavity to provide a smooth transition from the restoration material to tooth structure (Fig. 4). One dentist condensed the amalgam and used the same set of amalgam pluggers on all six teeth. Before amalgam condensation, pressure was exerted in an outward direction on the axial walls of the occlusal part of the cavity. An amalgam plugger was used to simulate the pressure exerted during condensation of the amalgam and possibly by the amalgam itself. A time-strain curve was registered. 260

Maximal flexure

Flexure after 12 hr

Flexure after 24 hr



46 47


35 38 39 36 31 37

43 46 44

43 39 41 40

We began recording measurements at the onset of condensation and recorded continuously for 24 hours on the strain indicator and the pen plotter. At the end of 24 hours, the measuring instruments were stopped, the amalgam res~ra~o~ were removed from the teeth, and a reading was made. We recorded the following measurements for the control tooth: 1. Occlusal pressure exerted several times at short intervals, in the middle of the central groove by means of an andgam plugger with registration of a time-strain curve 2. A 24-hour registration made in the same manner as for the six teeth that were treated EESULTS In all the treated teeth, short pressure exerted by the amalgam plugger on the axial walls of the occhrsal part of the cavity caused a deformation of the cusp. The deformation disappeared completely after the pressure was removed (that is, a complete and instant elastic recovery took MARCfI











--W_ /



-- omoISnm


I /

Fig. 7. Dimensional changes during first 24 hours after condensation.

place) (Fig. 5). We measured no strain in the control tooth during any phase of the experiment, neither during short periods of loading on the central groove nor during the 24 hours of registration. In all the treated teeth, from the onset until the completion of condensation, an increasing strain was measured, which corresponded to the outward flexure of the cusps. On completion of condensation, there was a short period of relative remission in which the flexure was slightly reduced from its value at cessation of condensation. The cusp, however, did not return to its original unstrained position. After this short relative remission, the outward cusp flexure was increased and reached a maximum after 7 hours. From that maximum, a small remission occurred for a few hours, and almost no flexural changes of consequences occurred after the first 12 hours following condensation (Fig. 6). After 24 hours, the cusp remained in permanent flexure (Fig. 6). Table I shows values of cuspal flexure during the different phases of the study for all treated teeth. At the end of 24 hours, after removal of the amalgam, complete elastic recovery was observed for all of the treated teeth.

DISCUSSION Dentin, because of its elasticity, rigidity, and low fragility, provides the firm foundation required for tooth reconstruction. The struct~al integrity of the crown of the tooth depends on the amount of dentin on the crown and the integrity of anatomic form of the crown. Teeth that have undergone amalgam restoration lose dentin because of the cavity preparation. TFiE




Vale’* 2 showed that in maxillary premolars an increase in the width of the isthmus beyond one-fourth of the intercuspal distance caused a weakening of the tooth. A marked reduction in the force required to break the tooth occurred. Larson et al3 and Mondelli et al4 confirmed these findings that occlusal cavity preparations reduce tooth strength in proportion to the width of preparation. Cuspal flexure under simulated occlusal loads was measured in earlier works. Grimaldi and Hood6 and Malcolm and Hood6 reported that removal of tooth structure caused cuspal flexure and that the deformation was proportional to the amount of tooth structure removed and to the applied load. In this study, the influence of an MOD amalgam restoration on maxillary premolars was observed from the beginning of the restoration procedure through the subsequent 24 hours. The amalgam restoration of teeth has two phases: the condensation phase and the postcondensation phase. It appears from the results of this study that flexure of the tooth cusps takes place during condensation, as well as during the period that follows condensation (Table I and Fig. 6). During the condensation phase, the dentist exerts forces on the tooth.’ These forces immediately cause stress concentrations in the cusps. These stresses are expressed as an outward cuspal flexure that gradually increases until the end of the indention phase. Additions defo~tion takes place in the cusps and can be attributed only to the dimensional changes taking place in the dental amalgam after completion of its condensation. It is an accepted theory that amalgam confined within 261


the tooth can expand in only an outward direction. This results in protrusion of the restoration from the prepared cavity. This study shows that the expanding amalgam exerts forces in a horizontal direction and hence onto the cavity walls, causing an outward flexure. The cuspal deformation thus consists of two components-an initial outward flexure caused by amalgam condensation and a continuing outward flexure that results from the dimensional changes occurring in the amalgam after condensation. The dimensional changes of amalgam during the first 24 hours after condensation have a number of phase& a short contraction followed by expansion to a maximum. Subsequently, a small contraction may take place for a few hours. After the first 12 hours following condensation, no dimensional change of consequence occurs. A comparison of the behavior of the amalgam following condensation and the behavior of the tooth following condensation reveals that the tooth cusps move in accordance with the dimensional changes taking place in the amalgam (see Results section and Fig. 6). For every contraction of the amalgam, there is a corresponding strain decrease measured in the cusp; with every amalgam expansion, there is a corresponding increase in cuspal flexure. The time-strain curve exhibited by the tooth after amalgam condensation is a mirror image of the curve of the dimensional changes of the amalgam during the first 24 hours after condensation (Fig. 7). The results of Table I reveal that all six teeth exhibited a similar amount of cuspal flexure during the various stages of the study. The minute differences recorded can be attributed to the slight variations in the size of the teeth, slight differences in the cuspal width, and the corresponding volume of amalgam within the cavity. During the 24 hours after restoration, the cusps are in a state of permanent flexure because of a permanent static load exerted by the amalgam (i.e., the tooth is in a stressed condition). The cuspal flexure is in an outward direction; thus the.chances of cusp fracture are increased. Occlusal loading affects teeth in the same manner. An undesirable feature is the probable combined effect of amalgam and ocdusal loading on production of cracks in teeth and cuspal fracture at a later stage. If only occlusal forces were responsible for cuspal fractures occasionally found in teeth with old amalgam fillings, the question arises as to why these fractures are not found in new restorations. The survival time of teeth that have been restored with amalgam may be influenced by a variety of factors. A leading factor is the amount of dentin removed from the tooth. A second factor is the type of restoration material usedamalgam. Here, the dimensional changes reported during the first 24 hours must be taken into account, as well as changes occurring later in the life of the amalgam, such as corrosion and creep. It seems that the larger the restoration the more tooth structure is destroyed. Thus, with a greater





amount of amalgam reacting on thinner tooth walls, the long-term survival of the tooth is in doubt. The third factor is occlusal loading. In light of all these factors, it would seem the one-third rule proposed by Vale19s assumes even greater significance. The stress concentration in the cusps, along with their consequent deformation, might be a source of a patient’s postoperative pain after having teeth restored with amalgam (solely or in conjunction with leakage). These findings would also seem to apply to the procedure of using amalgam (amalgam crowns, coronal-radicular amalgam restorations, etc.) to restore nonvital teeth. A nonvital tooth is more vulnerable than a vital tooth to the influence of the expanding amalgam confined within its walls. If amalgam is to be used as a core material, it should be remembered that the base for the final cast restoration is already under a permanent static load before the final restoration is put into place.




The use of amalgam as a restorative material causes a static load on the cusps of teeth and causes the consequent permanent deformation of the cusps. A logical conclusion would be that the occlusal portion of the cavity preparation should be minimized or eliminated whenever possible. If access to proximal caries can be attained from somewhere other than the occlusal surface, such an approach should be adopted. This would avoid damage to the marginal ridge and maintain occlusal structural integrity. We express our deep appreciation to Mr. Y. Dror, Director of the Department of Experimental Engineering, Vishay Israel Ltd., Holon, Israel, for invaluable assistance in the acquisition of these data. REFERENCES 1. Vale WA. Cavity preparation. Irish Dent Rev 1956;2:33-41. 2. Vale WA. Cavity preparation and further thoughts on high speed. Br Dent J 1959;107:333-45. 3. Larson TD, Douglas WH, Geistfeld RE. Effect of prepared cavities on the strength of teeth. Oper Dent 1981;6:2-5. 4. Mondelli J, Steagall L, Ishikiriama A, Navaro M, Soares FB. Fracture strength of human teeth with cavity preparation. J PROSTHET DENT 1980;43:419-22.

5. Grimaldi JR, Hood JAA. Lateral deformation of the tooth crown under axial cuspal loading. J Dent Res 1973;52:584. 6. Malcolm PJ, Hood JAA. The effect of cast restorations in reducing cusp flexibility in restored teeth. J Dent Res 1977;56207D. 7. Baskar RM, Wilson HJ. Condensation of amalgam. Br Dent J 1968;124:451-5. 8. Phillips RW. Skinner’s science of dental matariais. 7th ed. Philadelphia: WB Saunders Co, 1973;316-7. Reprint

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Cuspal flexure associated with amalgam restorations.

Cavity preparation causes cuspal flexure under simulated occlusal loads. During amalgam condensation, the dentist exerts forces on the tooth. After co...
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