Determination of the optimal photoinitiator concentration in dental composites based on essential material properties.

OBJECTIVES: The aim of this study was to determine the concentrations of the photosensitizer (camphoroquinone, CQ) and coinitiator (ethyl-4-dimethylaminobenzoate, EDMAB) that resulted in maximum conversion but generated minimum contraction stress in experimental composites. METHODS: Experimental composites were prepared with an identical resin formulation [TEGDMA:UDMA:bis-GMA of 30.25:33.65:33.65]. Five groups of resin were prepared at varied CQ concentrations (0.1, 0.2, 0.4, 0.8 and 1.6wt% of the resin). Five subgroups of resin were prepared at each level of CQ concentration, by adding EDMAB at 0.05, 0.1, 0.2, 0.4 and 0.8wt% of the resin, resulting in 25 experimental resins. Finally, strontium glass ( approximately 3microm) and silica (0.04microm) were added at 71.5 and 12.6wt% of the composite, respectively. Samples (n=3) were then evaluated for Knoop hardness (KHN), degree of conversion (DC), depth of cure (DoC) and contraction stress (CS). RESULTS: There was an optimal CQ and EDMAB concentration that resulted in maximum DC and KHN, beyond which increased concentration resulted in a decline in those properties. KHN testing identified two regions of maxima with best overlaps occurring at CQ:EDMAB ratio of 1.44:0.42 and 1.05:1.65mol%. DC evaluation showed one region of maximum, the best overlap occurring at CQ:EDMAB ratio of 2.40:0.83mol%. DoC was 4mm. Overall, maximum CS was attained before the system reached the maximum possible conversion and hardness. SIGNIFICANCE: (1) Selection of optimal photoinitiator/amine concentration is critical to materials' formulation, for excessive amounts can compromise materials' properties. (2) There was no sufficient evidence to suggest that contraction stress can be reduced by lowering CQ/EDMAB concentration without compromising DC and KHN.

Factors affecting photopolymerization stress in dental composites.

Polymerization stress development results from the complex interplay of volumetric shrinkage, reaction kinetics, and viscoelastic properties. The objective of this study was to examine the relationships among volumetric shrinkage, degree of conversion, rate of polymerization (RP(max)), and stress development for 2 model bis-GMA-based composites. Three irradiances were used--220, 400, or 600 mW/cm(2)--with exposure times adjusted to deliver the same radiant energy. Volumetric shrinkage was determined with a mercury dilatometer, degree of conversion and RP(max) by differential scanning calorimetry (DSC), and polymerization stress with a low-compliance device (Sakaguchi et al., 2004b). Results indicated that polymerization reaction rate and shrinkage were not correlated. Irradiance was directly related to polymerization reaction rate and to stress development. The group with the highest stress/degree of conversion exhibited the lowest RP(max), so it can be assumed, within the limitations of this study, that the conversion was most closely related to stress development.

Critical configuration analysis of four methods for measuring polymerization shrinkage strain of composites.

OBJECTIVES: To compare four methods for measuring polymerization shrinkage strain of composites and to develop a rational basis for comparing data from different methods and laboratories. METHODS: Dilatometry, modified bonded disk, strain gage, and a new linear transducer method were used to measure polymerization shrinkage strain of a model composite under similar irradiation conditions. The resin consisted of an untinted resin (50:50 BISGMA/TEGDMA, 0.7% CQ, 0.35% DMAEM, 0.05% BHT) filled with 5% fumed silica and 67 wt% untreated hybrid filler. Specimens (n = 10) were exposed for 60 s at 600 mW/cm2 and then monitored for 300 s. Specimen volumes were 8 mm3 for the strain gage method, 25 mm3 for the linear transducer and dilatometer methods and 43 mm3 for the bonded disk method. The degree of constraint applied to the specimens by each method was calculated and compared. Shrinkage strain values at 60 and 300 s were tested for significance at p = 0.05 using one-way ANOVA and Tukey's test. RESULTS: Shrinkage strain magnitudes at 60 and 300 s for the four methods were significantly different (p < 0.01) The modified bonded disk method measured the highest shrinkage value and exhibited the highest degree of specimen constraint. There was a 5 s delay after light activation before strain was detected by the strain gage. SIGNIFICANCE: Specimen constraint differed in all four methods and was linearly correlated with shrinkage strain magnitude when the degree of constraint was less than 42%.

Prediction of composite elastic modulus and polymerization shrinkage by computational micromechanics.

OBJECTIVES: The objective of this study was to simulate the elastic modulus and polymerization shrinkage of a light activated polymer matrix composite using a generalized method of cells (GMC) micromechanics model. Two hypotheses were tested: (1) the micromechanics model provides estimates of elastic modulus vs filler fraction with greater accuracy than the rule of mixtures, Hashin-Shtrikman and phenomenological models; (2) Micromechanics Analysis Code/Generalized Method of Cells accurately simulates experimental benchmarks of polymerization shrinkage strain. METHODS: The study applied mathematical algorithms to a representative volume element of a model polymer composite to yield value estimates of the elastic modulus and contraction strain. Mechanical properties of the composite constituents were derived from thermomechanical and dynamic mechanical analysis of BisGMA and TEGDMA filled and unfilled resins. Data from the micromechanics model were compared to results of other analytical methods as well as experimental benchmarks. RESULTS: Predictions of elastic modulus vs filler fraction from the micromechanics model provided greater accuracy than the rule of mixtures and the Hashin-Shtrikman models. Predictions of polymerization shrinkage strain were within 13% of experimental values. SIGNIFICANCE: The elastic micromechanics model presented accurately predicted elastic modulus and polymerization shrinkage strain as a function of filler fraction, superior to other analytical methods.

Contraction force rate of polymer composites is linearly correlated with irradiance.

OBJECTIVES: The force developed during cure of a composite represents the potential loads that can be induced into the dental adhesive and tooth structure that in turn affects the integrity of the dental adhesive and tooth. The purpose of this study was to evaluate the dependence of polymerization contraction force development on light energy density (product of irradiance and time). METHODS: Contraction force during polymerization was measured with a low compliance test fixture in which the composite specimen was placed between a glass plate and steel rod. The steel rod passed through a washer-type load cell that measured force development during cure. Six irradiance levels were evaluated as well as a 'pulse-delay' method. A generic composite consisting of a 1:1 blend of BisGMA and TEGDMA resin and 67 wt% unsilanated hybrid filler with 5 wt% fumed silica was used for all experiments. Contraction force was collected for 550 s. The first derivative of contraction force with respect to time (dF/dt) was calculated. Net contraction force at 550 s and max[dF/dt] was statistically analyzed as a function of irradiance and energy density (product of irradiance and time) with one-way ANOVA and Tukey's post-hoc test at the 0.05 level of significance. RESULTS: Contraction force increased most rapidly immediately following light activation. Force resulting from the pulse-delay method was significantly different from all other methods (p < 0.001). Force resulting from irradiation at 600 mW/cm2 was significantly different (p < 0.01) from all other methods and 500 mW/cm2 was significantly different from 100 and 200 mW/cm2. Maximum df/dt (max[dF/dt] over full range of time) was linearly related to irradiance, linear regression r2 = 0.98. All pairs of irradiance were significantly different except pulse-delay and 200-300 mW/cm2 and 300 and 400 mW/cm2. SIGNIFICANCE: The pulse-delay method demonstrated contraction force rates lower than what would be expected using energy considerations and lower force rates at each of the two light exposures than their single exposure counterparts. Since the adhesive resin and dentin are viscoelastic and thus strain rate dependent, time dependent contraction force should be an important consideration.

Reduction of polymerization contraction stress for dental composites by two-step light-activation.

OBJECTIVES: The goal of this study was to assess the reduction of polymerization contraction stress of composites during a two-step light-activation process and to relate this reduction to the process of polymerization shrinkage and specimen thickness. METHODS: Three test procedures were performed to compare two-step light-activation with delay with one-step continuous irradiation of composites: polymerization contraction stress using a closed-loop servohydraulic testing instrument, polymerization shrinkage by a mercury dilatometer, and degree of conversion by FTIR. For the one-step continuous curing method, the samples were light-activated for 60s at 330 mW/cm(2). For the two-step curing method, a 5s light exposure at 60 mW/cm(2) was followed by 2 min without light exposure, and then a second light exposure for 60s at 330 mW/cm(2). The same light parameters were used for measurements of stress, shrinkage, and degree of conversion. Three composites, Heliomolar, Herculite and Z100 were evaluated. The contraction stress experiments were repeated with varying thickness for Herculite using the one-step and two different two-step techniques. RESULTS: Polymerization contraction stress 10 min after light-activation was significantly reduced (P<0.05) by the two-step method: 29.7% for Heliomolar, 26.5% for Herculite, and 19.0% for Z100. Total volumetric shrinkage and degree of conversion were not significantly different for composites cured by the two different techniques. Increasing the thickness of the composite sample reduced the measured contraction stress, especially for one of the two-step curing methods. SIGNIFICANCE: A combination of low initial energy density followed by a lag period before a final high-intensity light irradiation provides a reduction of polymerization contraction stresses in dental composites. The stress reductions cannot be attributed to reductions in degree of conversion or unrestrained volumetric shrinkage.

Effect of light power density on development of elastic modulus of a model light-activated composite during polymerization.

PURPOSE: Elastic modulus development during polymerization of a composite is a measure of the polymerization maturity and the restoration's ability to transfer stress to enamel and dentin. The characteristics of elastic modulus development in real time during cure are largely unknown. The purpose of this study was to evaluate the effect of light power density and total energy density on the early development of elastic modulus for a light-activated composite. METHODS: Cylindrical specimens of a model hybrid composite were tested in flexure in a dynamic mechanical analyzer (DMA). Specimens were light-activated (Variable Intensity Polymerizer, Bisco, Itasca, Illinois) for 60 seconds. Elastic modulus was measured continuously for 5 minutes from the start of light activation. Development of elastic modulus was assessed for six different light power densities and two reduced power density levels given at longer exposure duration to provide similar energy density values. One-way analysis of variance with Tukey's post hoc comparison test was used to evaluate significant differences of elastic modulus at p = .05. RESULTS: The rates of elastic modulus development and final moduli were dependent on the light power density applied. Composite specimens cured by equivalent energy densities using short times and high power density or long times and low power density produced equivalent elastic moduli. Elastic moduli for emitted power densities between 400 and 600 mW/cm2 (160-260 mW/cm2 measured at the specimen surface) were not significantly different (p > .05). CONCLUSIONS: Light power densities greater than 160 mW/cm2 measured at the specimen surface resulted in elastic moduli that were not significantly different. Equivalent energy densities produced comparable elastic moduli.

Radicular temperatures associated with thermoplasticized gutta-percha.

Thermoplasticized gutta-percha has been used to obturate root canals. The continuous wave of condensation technique uses the System B Heat Source with the choice of different-sized pluggers. The purpose of this study was to measure the temperatures within the root canal and on the root surface at different radicular levels while using the System B Heat Source. Fine, Fine-Medium, and Medium pluggers were evaluated at temperature settings of 200 degrees C, 250 degrees C, and 300 degrees C. The Obtura II gutta-percha delivery system following the manufacturer's instructions and ultrasonically thermoplasticized gutta-percha were used for comparative purposes. The highest mean temperature change on the internal root surface was 74.19 degrees C with the system B at the 6 mm level (6 mm coronal to working length) when the Fine-Medium plugger was set at 300 degrees C. The lowest mean temperature change on the internal root surface was 2.09 degrees C at the 0 mm level (at working length) when the F plugger was set at 200 degrees C. With the Obtura II, the lowest mean internal temperature change was 5.22 degrees C at the 0 mm level, whereas the highest mean internal temperature change was 26.63 degrees C at the 6 mm level. With ultrasonic lateral compaction the lowest mean internal temperature change was 5.01 degrees C at the 0 mm level, whereas the highest mean internal temperature change was 28.95 degrees C at the 6 mm level. At no time did the System B, the Obtura II, or ultrasonic delivery of warm gutta-percha exceed an increase of 10 degrees C at any thermocouple level on the external root surface.

Correlation of noncarious cervical lesion size and occlusal wear in a single adult over a 14-year time span.

STATEMENT OF PROBLEM: Noncarious cervical lesions are described as having a multifactorial cause, with occlusal trauma and toothbrush abrasion frequently mentioned as major factors. Finite element modeling studies have demonstrated a relocalization of occlusal stresses to the cervical area due to flexure of the crown. This may cause microcracking, especially under tensile stresses, that will lead to a loss of enamel and dentin in the cervical region. Clinical confirmation of an occlusal cause for noncarious cervical lesions has been difficult to obtain. PURPOSE: This study investigated whether occlusal wear was correlated with an increase in the size of noncarious cervical lesions. MATERIAL AND METHODS: Loss of contour at occlusal and cervical sites on 3 teeth of a single individual was measured using digital and visualization techniques at 3 time intervals over a 14-year time span. The 1983 baseline casts and 1991, 1994, and 1997 clinical impressions of a single adult patient with existing noncarious cervical lesions were replicated in epoxy. Surfaces of all replicas were digitized with a contact digitizing system. Sequential digitized surfaces were fit together and analyzed using AnSur-NT surface analysis software. Clinical losses of surface contour by volume and depth of the left mandibular first molar and first and second premolars were recorded. RESULTS: Nine measurements of cervical volume loss (range 0.9 to 11.5 mm(3)) and 9 corresponding measurements of occlusal volume loss (range 0.39 to 7.79 mm(3)) were made. The correlation between occlusal and cervical volume loss was strong (r(2)=0.98) and significant (P<.0001). CONCLUSION: For the single adult patient in this study, there was a direct correlation between occlusal wear and the growth of noncarious cervical lesions.

A review of the curing mechanics of composites and their significance in dental applications.

Much of the technique sensitivity associated with polymer matrix composites is a direct result of their curing shrinkage. Challenges with marginal integrity, adaptation of proximal contact, and residual stress are related to this intrinsic property. There are many test methods described in the literature that measure various aspects of polymerization contraction. Some measure total contraction, which is the sum of pre- and postgelation shrinkage, whereas others are sensitive only to postgelation deformation, which occurs after the onset of measurable stiffness. Development of methods to compensate for curing shrinkage is best described on the basis of an understanding of the polymerization mechanics. The distinction between total and postgelation contraction, and recognition of limitations of test methods are important considerations when interpreting literature data before selecting a restorative material.

Reduced light energy density decreases post-gel contraction while maintaining degree of conversion in composites.

The objective of the study was to evaluate the relationship between curing light intensity and (1) linear post-gel polymerization contraction strain, and (2) degree of conversion of a dental composite. Cylindrical specimens of a dental resin composite were cured from a distance of 7 mm for 40 s at four attenuated light intensities (71%, 49%, and 34% of control intensity and for 20 s at 71% plus 20 s at 100% intensity). A group cured at full intensity served as a control. Degree of conversion (DC) was measured at the top and bottom and linear contraction strain was measured at the bottom of the composite samples. DC at the sample top was significantly different (P < 0.05) between all groups except the 71% and 49% intensity groups. At the sample bottom, DC resulting from the two highest intensities (71% and 100%) were not significantly different from each other (P > 0.05). All other groups were significantly different from each other (P < 0.05). DC for the sample cured at two light intensities was not significantly different from those cured at the lower intensity or higher intensity for 40 s (P > 0.05). The sample cured with two intensities showed a 21.8% reduction from the contraction strain predicted by a light energy density calculation. Application of light at less than the maximum intensity of the curing light resulted in significant reduction of polymerization contraction strain without significantly affecting the degree of conversion.

Stress transfer from polymerization shrinkage of a chemical-cured composite bonded to a pre-cast composite substrate.

OBJECTIVES: (1) To develop and test a strain gauge-based method for evaluating the strain transferred through a bonded interface to a deformable substrate; and (2) to develop and test a finite element (FE) model for evaluating the stress development in a chemical-cured composite during polymerization. METHODS: A generic light-cured resin composite was used to fabricate a rectangular plate with an internal slot filled with a chemical-cured composite. Strain gauges on the surface of the composite in the channel and on the plate adjacent to the channel-plate interface were used to record strain continuously up to 500 s after mixing the composite paste. A quadrant three-dimensional (3D) finite element model used strains measured on the channel to simulate the experimental conditions. The model was used to estimate stresses in the channel and at the bonded interface. RESULTS: Strain in the plate reached a plateau 200 s after mixing the composite. Strain of the composite paste in the channel continued to rise with time but at a steadily decreasing rate. Maximum principal stress predicted by the FE model on top of the plate, on top of the channel and within the channel was 5.12 MPa, 3.78 MPa, and 5.26 MPa, respectively. SIGNIFICANCE: Stresses were effectively transferred through the bonded interface in this test configuration, and results were in close agreement with previously reported literature values for polymerization contraction stresses generated in composite configurations with similar bonded to unbonded surface ratios.

Analysis of strain gage method for measurement of post-gel shrinkage in resin composites.

OBJECTIVE: The objective of this study was to refine a strain gage method for measuring polymerization contraction of resin composites and to isolate the net post-gel contraction by identifying factors contributing to the measured strains. The hypothesis to be tested was that carefully controlled strain gage measurements of composite polymerization could isolate post-gel contraction events. METHODS: Composite was placed on a biaxial strain gage and light-cured. This method enabled real-time registration of the progress of shrinkage strain, corresponding to elastic modulus development. Strain from the two axes of the strain gage were averaged and plotted as a function of time. A representative curve was calculated from the mean of ten measurements. The following factors influencing the total contraction measurement were evaluated: thermal expansion of the gage, thermal expansion of the composite due to the exothermic reaction and exposure to the curing light, and adhesion of the composite to the gage. These parameters were measured so that the net deformation of the composite during polymerization could be calculated. RESULTS: Parametric studies of pre-cured and photointiator-free materials confirmed the hypothesis that strain gages measure post-gel contraction. Thermal artifacts were measured and subtracted from the total strain output. SIGNIFICANCE: Strain gages are suitable for measuring the clinically significant phase of composite polymerization contraction.

Testing mode and surface treatment effects on dentin bonding.

The goal of this project was to evaluate the effect of the following variables on shear dentin-bonding test results: mode of testing (cyclic fatigue versus static loading), surface treatments (32% phosphoric acid, 10% phosphoric acid, and no treatment [unetched]), and type of shear test (traditional planar versus push-out). All teeth were stored in distilled water and tested in a shear mode at a loading rate of 2 mm/ min. The specimens were loaded in static or cycled for 1000 cycles using a staircase approach or until fracture, whichever occurred first. On samples with etched dentin surfaces, the push-out test did not demonstrate a significant difference in measured bond strength when compared with results from the planar test, although sample preparation was more labor-intensive. The bond strength resulting from cyclic fatigue of the etched specimens was approximately 51% of the static loading value. Ten percent phosphoric acid was as effective as 32% phosphoric acid for dentin bonding. Finite-element analysis indicated that the traditional planar shear test produces flexure of the specimen and high tensile stress magnitudes within the resin bonding layer. The push-out test produces elevated compressive stresses localized in the composite along the circumference of the punch. Shear stresses in the resin bonding layer are similar for both testing methods at the same loading element contact force.

Thermal expansion coefficient of dental composites measured with strain gauges.

OBJECTIVES: A simple test method was developed to determine the coefficient of thermal expansion of prevailing restorative resin composites and to study the transient behavior as a function of temperature and repeated thermocycles. METHODS: Strain gauges were used to determine the thermal expansion for seven commonly used restorative resin composites by measuring the instantaneous strain along with temperature change. The temperature was measured by means of a thermocouple, the tip of which was embedded in the composite. The differences among the test groups were analyzed using ANOVA, followed by Scheffé's multiple comparisons test. RESULTS: The coefficient of thermal expansion determined for the composites tested was: 22.5 +/- 1.4 x 10(-6)/degree C (Z-100), 23.5 +/- 1.4 x 10(-6)/degree C (P-50), 32.6 +/- 1.6 x 10(-6)/degree C (Herculite XR), 34.1 +/- 1.8 x 10(-6)/degree C (APH), 35.4 +/- 1.4 x 10(-6)/degree C (Conquest), 41.6 +/- 1.5 x 10(-6)/degree C (Silux Plus), 44.7 +/- 1.2 x 10(-6)/degree C (Heliomolar). The coefficient was almost linear in the considered temperature range (26-75 degrees C) for all composites (r > 0.99) and decreased with each consecutive thermocycle (p < 0.1). SIGNIFICANCE: Thermally induced loads, introduced into restored teeth by the mismatch of the coefficient of thermal expansion of the tooth and the restorative material, may be related to microleakage and wear problems. A highly filled hybrid composite such as Z-100 had a coefficient of thermal expansion closest to that of the tooth crown, confirming other studies which demonstrated the benefits of high filler loading in matching the properties of the dental hard tissues.

Does an incremental filling technique reduce polymerization shrinkage stresses?

It is widely accepted that volumetric contraction and solidification during the polymerization process of restorative composites in combination with bonding to the hard tissue result in stress transfer and inward deformation of the cavity walls of the restored tooth. Deformation of the walls decreases the size of the cavity during the filling process. This fact has a profound influence on the assumption--raised and discussed in this paper--that an incremental filling technique reduces the stress effect of composite shrinkage on the tooth. Developing stress fields for different incremental filling techniques are simulated in a numerical analysis. The analysis shows that, in a restoration with a well-established bond to the tooth--as is generally desired--incremental filling techniques increase the deformation of the restored tooth. The increase is caused by the incremental deformation of the preparation, which effectively decreases the total amount of composite needed to fill the cavity. This leads to a higher-stressed tooth-composite structure. The study also shows that the assessment of intercuspal distance measurements as well as simplifications based on generalization of the shrinkage stress state cannot be sufficient to characterize the effect of polymerization shrinkage in a tooth-restoration complex. Incremental filling methods may need to be retained for reasons such as densification, adaptation, thoroughness of cure, and bond formation. However, it is very difficult to prove that incrementalization needs to be retained because of the abatement of shrinkage effects.

Elongation and preload stress in dental implant abutment screws.

A common problem associated with dental implant restorations is loosening of screws that retain the prosthesis to the implant. A method was developed to determine initial preload on UCLA-type abutment screws by measuring elongation after applying known tightening torques with a digital torque gauge. Loosening torque was also measured after tightening to 32 N-cm torque for gold alloy abutment screws and 20 N-cm for titanium abutment screws. Gold alloy and titanium abutment screws were each used to secure a gold UCLA hexed abutment to a titanium implant. Stresses and forces were calculated from the elongation measurements for three regions of each screw. Elongation of the screws after applying the manufacturer's recommended tightening torques were within the elastic range. Induced stresses were 57.5% and 56% of the yield strengths for gold alloy and titanium, respectively. Tightening of screws beyond recommended levels may be possible without producing plastic deformation. At manufacturer's recommended torques, mean preload was 468.2 (+/- 57.9) N using gold alloy screws and 381.5 (+/- 72.9) N with titanium screws.

Nonlinear contact analysis of preload in dental implant screws.

Clinical studies indicate that loosening or fracture of dental implant prostheses occurs in 5% to 45% of cases during the first year. The nature of loosening or displacement of prosthetic components is complex, since it involves cycling fatigue, oral fluids, and varied chewing patterns and loads. A finite element contact analysis method was used to study the load-transfer mechanism between prosthetic components caused by torque application to the threaded fasteners used for assembly. Screw elongation is achieved while allowing for elastic recovery of the screw to produce a clamping force on the fastened elements. Clamping forces were additive along the axis of the prosthetic components. When the gold retaining screw is fastened into the abutment screw, clamping force on the implant is increased at the expense of decreasing the clamping force at the abutment screw-abutment interface by 50%. Maximum tensile stresses in the screws after preload were less than 55% of the yield stress.

Digital imaging of occlusal contacts in the intercuspal position.

PURPOSE: The purpose of this study was to develop an approach to the measurement of occlusal contact area and location using digitized video images of occlusal records. MATERIALS AND METHODS: Five occlusal records in the intercuspal position were made using a polyvinylsiloxane material on five subjects with intact, natural dentition. In regions of occlusal contact, the material showed a minimal film thickness without perforation. A dental cast of the mandibular arch was video digitized and followed by digitization of each of the five occlusal records in place on the cast. An impression of a calibration stepwedge was video digitized to provide the relationship between impression material thickness and pixel density. RESULTS: Contact surface areas ranged from 0.02 to 3.16 mm2 between subjects. The contact positions on a single tooth determined in five records from a single individual showed coefficients of variation between 7.4% to 36.1%. Large variations in contact size were found in this group of five records from a single individual (coefficient of variation ranged from 10.8% to 156.7%). The large difference in contact size between records may be due to variations in biting force at the time the records were made. When the cast position was changed and records redigitized, the mean area of the contact was not significantly different (P > .20) from measurements at the original position. CONCLUSIONS: For the small sample evaluated, a large variation in occlusal contact size was found in the five records. The occlusal contact location was consistent in the five records. The measurement method developed seems to provide reliable measures of occlusal contact surface area and location.

Nonlinear finite element contact analysis of dental implant components.

The treatment design of most dental restorations is largely empirical and based on the experience of the individual practitioner. Because the biomechanical aspects of implant-supported restorations are difficult to assess on an individual basis, there is a possibility for compromised biomechanical performance of the implant-retained restoration to achieve satisfactory esthetics and phonetics. Through repeated loading cycles, the restoration or its components may fatigue and fail. This study evaluated the biomechanical behavior of the crown component relative to the gold retaining screw and abutment under load to provide insight into the mechanism of loosening and fracture of the retaining screw. A two-dimensional finite element model of the dental implant components was developed for nonlinear contact analysis. A simulation of tightening of the retaining screw was followed by axial loading of a cusp tip on the implant-supported crown. Loading of the cusp tip resulted in separation of the contact between (1) the gold retaining screw and abutment, and (2) the crown and the abutment. Repeated loading and unloading cycles resulted in alternating contact and separation between the retaining screw head base and the crown. Clinical findings of screw loosening and failure probably result from these separation events and from elevated strains in the screw as demonstrated by the model.