A previous paper has described the relevant physical properties of vital versus non-vital teeth; the structures of the tooth used to manage stress and strain; the formation of dentinal cracks and propagation of fracture planes through dentin; and an analysis of forces placed on human teeth and their effects on the tooth.1 Included in this paper was a description of normal and paranormal forces caused by occlusion.
Briefly, the magnitude of normal masticatory forces ranges from 2-40 pounds (9-180N) with a duration of from 0.25-0.33 seconds.2 Maximal biting force has been measured in young subjects and falls between 115-120 pounds (516-532N).3 The presence of restorations does affect the bite force. Gender differences do occur. When these forces are calculated as a force per area and then converted to international units, a force of 205 pounds affecting a point of contact 1/32 inch square places 45.23 MPa of force. Normal chewing The Effect of Occlusal Forces on Restorations force using the same area of contact results in a force of 8.826 KPa, well below the modulus of elasticity of dentin. (Please see Table I for a listing of normal and paranormal occlusal forces.)
When posterior teeth are lost and the proprioception is altered, the maximal bite force also goes down.4,5 Clenching force on one tooth is reported to be up to ten times greater on the canine than maximum biting forces distributed in a balanced way.6,7 Maximum biting forces exerted by the muscles are exerted in the maximum intercuspal position and are distributed according to distance from the condyles, with the second molar taking 55% of the maximum force and the incisors taking 20% of the force.8
What effects do these forces — whether from normal chewing, single tooth bruxing, multiple tooth bruxing, or clenching — have on the teeth? In a photoelastic study using approximately 4.4 lbs (2kg) of force on a tooth in a vertical load, it was found that distal incline planes or slopes of cusps and lingual incline planes or slopes of the buccal cusps received the greatest force on mandibular molars. The magnitude of the stress is increased considerably when the occlusion is flat plane. This demonstrates the advisability of maintaining occlusal points of contact with opposing teeth, rather than areas of occlusal contact, to decrease the stress on the teeth.9 Non-axial forces have been implicated as creating a greater risk for fatigue fractures of pulpless teeth, especially those reconstructed with dowel and cores. The authors recommend that a favorable occlusal design of the prosthesis is more important for the survival of structurally compromised pulpless teeth than is the type of post used.10
Using extracted teeth prepared for endodontic access with MOD cavity preparations, teeth were stressed with a load, either continuously or cyclically. Continuous loading resulted in progressive cuspal displacement, both time- and load-dependent. It took 20 minutes for the tooth to recover from the deformation. Cyclic loading resulted in cumulative increase in cusp displacement, but only to a very small extent: 1 micron. The conclusion is that continuous loading as in clenching or bruxing is more destructive than cyclic loading as in chewing.11
Using newly extracted premolars, MOD cavity preparations were made with varying widths of the isthmus and depth, and the teeth were stressed elastically. Measurements were made of the deflection of the buccal and lingual cusps to the force. They found that the outward bending of the facial and lingual walls was about 0.15 micron per Newton. They also confirmed earlier studies by finding that the outward bending was exaggerated as the cavity depth and width increased. The authors surmise that increase in depth and width (buccolingually) will likely lead to greater microleakage.12
It is apparent from the above quoted research that occlusal forces can bend teeth to some degree. The deformation is normally elastic. However, continuous loading, especially in restored teeth, can cause permanent deformation, leaving dentinal cracks and tears. With continued use and aging, these dentinal cracks can propagate, causing the fracture of a part of the tooth. As quoted above, bruxing, clenching, and lateral forces are more destructive, especially to lingual cusps of mandibular teeth.
Mastication of food improves with placement of dental restorations to restore carious and broken teeth and implant placement for missing teeth. Different types of food — i.e., dry and hard products — require more chewing cycles before swallowing compared to soft and moist foods.13 Xerostomia dramatically affects the ability to chew and swallow. Xerostomic patients require more chewing cycles and a smaller food bolus, with reduced muscle activity during mastication.14 Occlusal forces on implants must be controlled to prevent eccentric contacts in parafunctional movement, as it is thought those forces can lead to marginal angular bone defects around two-stage implants,15,16 decreasing implant longevity.17
Reduced periodontal support is associated with decreased total biting force and increased biting pressure (defined as force per 1 mm2 of occlusal contact area).18 In a clinical study of 50 patients comparing a single-tooth dental implant to its contralateral natural tooth and an endodontically treated molar to its contralateral natural tooth, it is reported that the endodontically treated molars were statistically the same as the natural teeth in maximum biting force, chewing efficiency, and areas of contact and near contact, whereas implants are reported with significantly lower values for maximum biting force, chewing efficiency, and areas of contact and near contact.19
Occlusal force against amalgam has an effect on the marginal adaptation of the material directly related to its creep value. The higher the creep value of the amalgam, the greater the marginal gap formation, proportional to the square of the occlusal force.20 Increased gap formation exceeding 0.4 mm can lead to secondary caries.21 Fracture resistance of an amalgam restoration is unrelated to the placement of a base of 0.5 mm thickness.22 Bonding an amalgam improves the resistance to axial forces, improving marginal adaptation over time.23 Bonding also has been shown to strengthen the adjacent tooth structure.24 High copper and dispersed phase amalgams can be more resistant to occlusal force than conventional amalgam.25 The wear resistance of amalgam is affected by the size, shape, and composition of the particles of the amalgam, with Dispersalloy showing the greatest wear resistance in a two-body, cyclic variable loading pattern similar to masticatory load. Amalgam is superior in wear resistance compared to all composites except microfill composites, which is comparable to amalgam in wear.26
Composite materials are affected by cyclic loading (60N for 150,000 cycles in the first study and 80N for 200,000 cycles in the second study, forces well within normal masticatory force ranges) in that their microtensile bond strength decreases over time.27,28 In vivo gap-free margins decrease significantly after six years in the mouth, due in part to hydrolytic degradation of the bonding mechanism to dentin and enamel.29 Class II composites, where a contoured matrix system is used compared to a straight matrix band (Toffelmire burnished against the adjacent surface), have significantly higher fracture strength, preventing marginal ridge fracture.30
In Table II, the coefficient of friction is listed, showing gold and chromium-nickel alloys have the lowest coefficient of friction against bovine enamel and amalgam with a higher value. Porcelain becomes much more abrasive under wet conditions.31
Wear of composite materials depends upon the type, size, and distribution of the filler particles, although wear loss is in a roughly linear relationship to the applied load.32 In two-body in vitro microfill composites wore as well as enamel, and hybrid and silorane composites had an intermediate amount of wear. The greatest wear was with the compomers.33 Nanofilled composites due to higher filler loading are generally superior to hybrid composites in resisting wear.34 As stated previously, composite restorations do not resist wear as well as amalgam restorations.
Porcelain materials present two problems associated with occlusal forces: fracture of the porcelain, which is dependent upon the size and direction of the force (e.g., normal chewing versus bruxing), the type of porcelain (e.g., feldpathic, versus lithium disilicate, versus zirconia), and the time of force application35; and wear of the material and its antagonist, whether natural enamel or other restorative materials. This is dependent upon the type of porcelain, quantity and timing of force, glazed versus polished porcelain, and the nature of the antagonist. In vitro study compared the load to fracture values of all ceramic crowns compared to unprepared teeth. The ProCAD/Cerec 3 was equivalent to the unprepared teeth and significantly stronger than either IPS Empress and IPSe.max Press, all with values that exceed even bruxing forces.36 However, with porcelain, contact fatigue plays an important role in leading to fracture. Once the porcelain contact area has been damaged, radial cracks appear which propagate and weaken the porcelain significantly. After loads of 200N (a force between normal mastication and bruxing force), degradation can occur in esthetic porcelains of micaceous glass ceramics in as few as 104 cycles, compared to damage to alumina and zirconia porcelains at 500N and 106 cycles.37
There is considerable argument in the literature as to whether porcelain materials should always be left in a glazed state38 or if polishing after glazing is an acceptable procedure.39 Glazing porcelain imparts a very smooth, amorphous surface and offers some significant advantage by filling in all of the porosity, cracks, and crazing created by contouring the porcelain with rotary instruments. This decreases the rate of wear on opposing teeth, as glazed porcelain does less damage to opposing teeth than unglazed porcelain.40 Polishing after adjustment on a glazed porcelain surface can lead to a smooth surface that causes no more wear of the opposing teeth than a glazed porcelain surface.41The smoother the surface of the porcelain, the stronger the material is in biaxial strength. Among porcelains, differences in biaxial strength can be attributed to stress concentration of an applied load due to the roughness of the surface caused by mechanical finishing or chemical action. Wear of the porcelain materials has been measured in vitro. Castable glass ceramic has a mean wear of 59 microns versus 21.8 microns for pressed glass ceramic corresponding to approximately five years of use. Their antagonists showed enamel wear of 74.6 microns, pressed glass ceramic 153.2 microns, and 61.5 microns for feldpathic porcelain.43 An in vivo study examined wear in one year of the antagonist against e.max Press (Ivoclar Vivadent). The authors stated the annual wear of enamel to be 38 microns. They found the maximum annual wear rate to be 88.3 microns against this porcelain material.44
Implant-supported porcelain-fused-to-metal-crowns with a screw retention and cemented have been tested in vitro to determine if the screw opening affected the fracture resistance of the porcelain. Screw-retained implant-supported crowns had significantly lower fracture resistance compared to cement-retained crowns.45
Finite Element Analysis
Finite element analysis (FEA) is a computer simulation using a mathematical model of tooth and restorative material with appropriate mechanical and physical properties included in the model. It is used to predict the effect of stress/strain on teeth and restorative materials. In a study that correlated an FEA study with microscopy of extracted teeth, when tensile forces were applied to the tooth, inner dentin displays high strains (deformations), whereas outer dentin shows high stress, meaning there is less likelihood of root fracture. In a post-endodontic restoration with a post and core and loss of the inner dentin, the fracture resistance is lowered.46 In vitro testing of porcelain and composite onlay restoration of endodontically treated molars found with a force of 200N (a force between normal mastication and bruxing) similar stress distributions and forces in the range of 24-26 MPa for both restorative materials. At 700N (a force near the maximum daytime biting force), the porcelain onlay showed stress peaks 30% higher than the composite onlay, with force concentrations at the cement-enamel junction of 95MPa. The composite onlay showed stress at the cement enamel junction of 47MPa.47 This outcome in part is expected due to the differences of the modulus of elasticity, porcelain being considerably higher modulus (more brittle, less able to withstand deformation) compared to composite onlays. Excessive stress on such restorations can lead to marginal breakdown.
In an FEA study wherein Class I and Class II composite resin restorations were tested and the modulus of elasticity was altered as an independent variable, 100N of force was applied. When the restorations were bonded, maximum stresses in enamel were at the occlusal margins (7-11MPa) and in the dentin at the pulpal floor (from 3.4-5.5MPa). Stresses decreased with increasing modulus of elasticity. In a non-bonded test, the stresses were higher in dentin and lower in enamel, and the modulus of elasticity change had less effect. Marginal stresses were 6MPa in a bonded test and 3MPa in a non-bonded test.48 Similar studies confirm that the stresses are concentrated at the margins for amalgam and composite restorations.49,50 In a slightly different FEA study that looked at MOD gold inlays, the stresses accumulated at the margin and increased as the depth of the cavity form increased. Dentin stresses increased as the axial walls moved pulpally and as the depth increased. Increasing the isthmus width decreased the stress distribution at enamel walls.51 In another FEA study that looked at indirect restorations and the concentration of stress in the luting media, composite materials with four different modulus of elasticity values (5.4, 9.5, 14.5, 21 GPa) and a porcelain inlay with the highest modulus of elasticity (65GPa) were tested. It was reported that the increase in modulus of elasticity lowered the stresses felt by the luting media and on the shear stresses on the cement-tissue adhesive interface.52
What these studies all point out is that occlusal stress is transmitted through the restorative material into the enamel and dentin. These stresses affect the materials themselves and can affect durability. The greater these occlusal stresses are, the more likely they will lead to marginal failures, fracture and wear of dental materials and dentinal tearing and ultimately, tooth fracture.
CAD/CAM ceramic MOD inlays were tested for microleakage, comparing unlined luted ceramic inlays with lined (with dentin bonding and flowable resin prior to impression) and luted ceramic inlays. They tested both with force (80N at 2.5cycles/sec for 250,000 cycles) and without for microleakage. They reported that the lined ceramic inlay had significantly less microleakage. Loading did not affect the microleakage between the loaded and unloaded groups.53
V-shaped Class V cavities in vitro were restored with composite material, both lined with a flowable composite with low elastic modulus (increased flexibility) and with a high elastic modulus and with no liner. The specimens were tested for microleakage both loaded and unloaded. Occlusal force increased leakage in those cavities lined with either flowable material compared to no liner.54
In vivo research evaluated the postoperative sensitivity of composite restorations after grouping them for cavity type, cavity dimension, and indication for treatment. A total of 600 teeth in 231 patients were evaluated two weeks following treatment for sensitivity. The clinical cavity depth was significant for sensitivity, with the deepest cavities having the greatest sensitivity, either to masticatory force, temperature, or sharp/dull pain.55 This is understandable because bonding to the deepest layers of dentin closest to the pulp is made more difficult by the decrease in the amount of dentinal tubules near the pulp. This decreases the amount of available calcified dentin (peri-tubular dentin) and makes the bond more tenuous.56 In one finding, the research indicates that shear bond strengths above 21MPa lead to less microleakage, especially on cementum margins. In another study, a correlation was found that showed the stronger the shear bond strength to dentin, the greater the ability of the bonding agent to wet the dentin, the less microleakage occurred.58 Theoretically, this would be due to a more robust and deeper dentin hybrid layer engaging more collagen.
Fruits et al show that in Class V restorations restored with composite resin, the choice of material affects the microleakage and retention of the restoration. They theorized that the forces that concentrate on the cervical aspect of the tooth from occlusal loading would affect retention and microleakage depending upon the flexibility of the resin, whether hybrid composite, microfilled composite, or flowable composite resin. They found that the more flexible microfilled and flowable composite resins were better able to resist microleakage after stress testing than the hybrid composite resin.59
An in vitro evaluation of microleakage in Class II composite restorations found that dentin margins leaked significantly more than enamel margins, and that whether lined or unlined, occlusal force caused significantly more leakage than controls.60
In vitro Class V cavities with enamel and dentin margins were restored with composite and tested with 100N X 10,000 cycles versus 250N X 10,000 cycles. Gingival margins exhibited significantly greater microleakage than enamel margins, and 100N force significantly reduced microleakage compared to 250N of force.61,62
An in vivo test in dogs looked at the movement of dentin fluid in dentinal tubules and found that Class V composite restorations exhibited greater fluid flow than amalgam or untreated teeth when placed under masticatory stress.63
An FEA study tested two shapes of Class V cavities – saucer-shaped and V-shaped - and two modes of bonding, one with no hybrid layer and one with a hybrid layer. They found that the hybrid layer acted as a stress absorber, and that cavity shape altered distribution of stress and strains.64
In vitro studies of NCCL wedgeshaped lesions show bending under occlusal loads of 150N increases the gap formation at cervical restorations compared to unstressed teeth, causing microleakage at the gaps.65 Similar studies show increased microleakage at cervical margins compared to incisal/occlusal margins irrespective of type of composite (packable versus flowable composite)66 (Figure 1).
Flexure of facial cusps may increase with the added presence of Class I composites or amalgam restorations. Depth and width of the occlusal restoration affected the force of the bending on the Class V by 1-67% for Class I composites and for amalgam by 9-228% in a FEA study.67
In vitro first premolars were prepared for MO and MOD cavity and restored with bonded composite resin. The intercuspal distance measurements were made before and after restoration and with an occlusal load of 150N. Cuspal deflection produced by polymerization shrinkage and by occlusal force was significantly greater in MOD versus MO composite restorations.68
An FEA study tested ceramic inlay with composite inlay with occlusal contacts placed in different areas. Occlusal contacts on either restorative material resulted in the least amount of cuspal deformation. For the composite restoration, occlusal contacts were least favorable on the restoration margin compared to either enamel or restoration contact alone.69
Fracture mechanisms of teeth may be described as either catastrophic or fatigue fracture. Catastrophic fractures occur when an overwhelming force is applied and the tooth is broken, as in a blow to the centrals, severing the crown from the root or chipping the tooth. Most often, teeth undergo what is called fatigue fracture. The mechanics of such a fracture are different from catastrophic fracture. Fatigue is a time-dependent property, wherein stress applied at a lower energy than what would cause a fracture is applied cyclically. Using both in vitro and in vivo models, researchers have found that there is a positive correlation between creep rate and stress, and that loading with a constant force over time showed stress relaxation. This confirms earlier research that teeth are viscoelastic.70 Such stress, cyclically applied, can cause flaws in dentin, caused also by tooth preparation, and may contribute to crack propagation. Sub- surface cracks in dentin of 25 microns have been tested in finite element analysis (FEA) to propagate and cause fracture estimated more than 25 years later. Cracks of 100 microns were shown to propagate and reduce the fatigue life fracture to an estimated five years.71
Using extracted teeth prepared for endodontic access with MOD cavity preparations, teeth were stressed with a load, either continuously or cyclically. Continuous loading resulted in progressive cuspal displacement both time- and load-dependent. It took 20 minutes for the tooth to recover from the deformation. Cyclic loading resulted in cumulative increase in cusp displacement, but only to a very small extent: 1 micron. The conclusion is that continuous loading as in clenching can be more destructive than cyclic loading as in chewing because of an increase in the fatigue of the stressed teeth over time.72
Fatigue fractures have been implicated in vertical fractures of roots of endodontically treated teeth73 and vital teeth.74,75 Incidence of tooth fracture goes up with large restorations and large carious lesions.76 Teeth with restorations that are bonded generally have more favorable stress distributions, most similar to unrestored teeth, compared to those teeth with restorations that are unbonded; and these teeth have generally greater fracture resistance.77,78,79,80 An in vitro study found that teeth prepared for inlays compared to direct composite restorations have lower fracture strength due to increased tooth structure loss.81
All porcelain crowns adhesively luted on prepared teeth in vitro were tested for fracture strength, and the fracture resistance was from 2895 to 4173N (far in excess of bruxing forces). Lithium di-silicate had higher fracture strengths than leucitereinforced ceramic crowns.82 In vitro testing of metal ceramic crowns with metal margins versus metal ceramic crowns with all- porcelain margins; stressed under a cyclic loading of 200N force of more than 600,000 cycles; crowns cemented using a resinreinforced glass ionomer cement, showed that the metal ceramic crowns with metal margins had significantly greater fracture resistance than metal ceramic crowns with all-porcelain margins.83 Partial coverage cast gold and ceramic and laboratory processed resin restorations were cemented to teeth in vitro and subjected to masticatory stress of 49N applied over 1.2 million cycles and then thermal cycled between 5 and 55 C. Marginal fit of all of the restorations was evaluated. The marginal adaptation after masticatory stress and thermal cycling was rated as superior for cast gold partial coverage restorations luted conventionally and statistically similar to Targis (laboratory processed composite), IPS-e-max Press, and IPS Empress porcelains, all luted adhesively. These were all superior to and statistically better than partial coverage restorations made from ProCAD/Cerec 3.84 In another in vitro experiment, the authors tested partial coverage inlay and onlay designs using IPS e.max Press porcelain. All of the specimens were stressed with occlusal forces at 49N for 1.2 million cycles and 5,500 thermal (5-55Co) cycles. The marginal integrity of all specimens, regardless of cavity design, decreased significantly due to the fatigue-type stresses placed upon them.85
Citations in this article show that occlusal forces all have an effect on restoration durability. These forces cause greater microleakage and greater degradation of marginal fit over time that can lead to secondary caries due to gaps at the margin. Occlusal forces can cause stress and strain in enamel and dentin, leading to fatigue-type fractures made worse by tooth preparation which can cause tears in dentinal structures. The greater the amplitude and frequency of these forces, and the more these occlusal forces are directed laterally (i.e., not parallel to the long axis of the tooth), the more destructive to restorative materials and teeth they become. The forces are sufficient in amplitude to bend tooth structure and propagate dentinal tears. The forces are sufficient to fracture composite and amalgam. The durability of direct restorative materials is less than indirect restorations because indirect restorations have greater strength properties. The more extreme forces of bruxing and clenching undoubtedly shorten the lifespan of any restoration in the mouth, even indirect restorations. No research has indicated by how much the durability of a restoration is affected, but research consistently confirms that occlusal forces contribute to the degradation of restorative materials and the fatigue and fracture of teeth.
What’s a Dentist To Do?
Management of the patient’s occlusion is a part of the overall management of the patient so as to make and keep them healthy. Diagnosis of tooth wear as described in a previous article on tooth wear86 suggests the following for diagnosis:
“If the wear is considered pathologic, some form of intervention is recommended to prevent further wear, minimize damage or protect the teeth. Interventions depend on the etiology of the wear and because wear can be multifactorial, a range of interventions may be necessary. Abrahamsen87 has described a methodology used to determine which etiologies may be causing the tooth wear. He suggests hand held study casts are necessary to make the determination so that the occlusal aspects can be clearly seen.
1. If the wear is greater in the anterior than the posterior and the wear facets match up on opposing casts, the wear is from bruxism.
2. Where there is more anterior than posterior wear and the lingual surfaces of the maxillary anterior teeth are worn smoothly from gingival tissue, and the lingual surfaces of maxillary posterior teeth seem affected, the cause is acid regurgitation. In this situation the worn surfaces of the casts will not match up.
3. If the posterior teeth have greater wear than the anterior teeth and cupping or cratering is present with the mandibular first molar most severely affected, the cause is from swishing of acidic drinks. In this instance the worn surfaces of the casts do not coincide and the edges of the enamel look sharp.
4. If the posterior teeth have greater wear than the anterior teeth and cupping or cratering is present but there is even posterior wear on all of the upper and lower teeth, the wear is created by fruit mulling, where acidic fruits are kept in contact with the teeth for extended periods while chewing.
5. If the anatomic details on the teeth appear faded with a sandblasted appearance or the facial surfaces of the lower canines and premolars have cervical notches, the cause is toothbrush/dentifrice abrasion. In this case, the worn surfaces of the casts will not match up.
6. This author also suggests a miscellaneous category, but does not enumerate it. Abfraction lesions and other forms of tooth wear not described above would fit into this ill-defined category. Since abfraction is caused in part by occlusal stress, the tooth with the facial NCCL would also have an occlusal wear facet that would fit the opposing teeth.”
Once diagnosed, bruxism causing tooth wear can be managed.86 “Bruxism is thought to affect 5-20% of a normal population. Normal loss of enamel due to natural wear is estimated to be 10- 20 microns per year. Bruxers exhibit three to four times the normal wear. Interocculsal appliances are recommended to limit wear, especially in the mouths of nocturnal bruxers. Severe occlusal wear seen in posterior teeth and anterior teeth may require protection from fracture, restoration of lost vertical dimension, improvement in occlusal guidance by restoration of all the teeth with combinations of crowns and/or onlays, or veneers to restore appropriate function, esthetics and guidance, lessening the stress on the individual teeth and the periodontal support mechanisms.”
Because the enamel is hard and brittle, and because the enamel prisms are organized perpendicular to the external surface, the physical properties are maximized against opposing surfaces. As a result, enamel is built to withstand a lifetime of normal forces without being breached.93
Recognition early in the adult life of a patient of tooth wear caused by bruxing which shortens the canines in patients with anterior guidance should be addressed early with a conservative composite restoration to prevent the patient from wearing his or her dentition into a group function (Figure 2). Group function puts significant lateral stress on posterior teeth, frequently leading to increased tooth fracture.86 (Figures 3,4)
Tooth preparations for unrestored carious lesions for all direct restorative materials should be lesion-specific and bonded. That is, the outline and extension of the margins are dictated by the extension of the caries and convenience form. The smallest, most conservative restoration should be placed to maintain as much tooth strength as possible.94 This improves restoration and tooth durability (Figure 5).
Occlusion should be analyzed when providing a definitive diagnosis so that the appropriate differential restorative choices can be made to engineer choices that increase durability of both tooth and restorations. Indirect restorations will usually result in greater durability compared to direct restorations. Indirect restorations are usually chosen when sufficient tooth structure is missing or significant stress will be applied to either the tooth or the restoration. Controlling paranormal forces can improve restoration and tooth durability.
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*Dr. Larson is Associate Professor, Department of Restorative Sciences, Division of Operative Dentistry, University of Minnesota School of Dentistry, Minneapolis, Minnesota 55455. Email is email@example.com