Fatigue behavior of the resinous cement to zirconia bond.

PURPOSE: Resinous cements are widely used for luting zirconia restorations. Adhesive failures have occurred at the cement/zirconia interface, rather than at the cement/dentin interface, suggesting that the cement/zirconia bond may lack durability; however, few comprehensive, comparative evaluations of fatigue effects have been reported. The rate of fatigue-induced loss of bond strength may be a more important predictor of long-term success than a single snapshot of bond strength after an arbitrary number of thermocycles. Previous studies have failed to identify trends by investigating bond strengths at several different numbers of cycles. This may result in invalid conclusions about which cements have superior bond strengths. The purpose of this study was to investigate the effects of artificial aging by thermocycling and resinous cement type on bond strengths to zirconia. MATERIALS AND METHODS: The effect of the number of thermocycles (0, 1, 10, 100, 1000, and 10,000) on the bond strengths of five resinous cements, two of which were used with and without a primer, and an oxygen-inhibiting gel, was studied. Specimens were randomly assigned to thermocycle number/cement-type test groups. Because zirconia has a very low thermal diffusivity, exceptionally long thermocycle dwell times were used. Cylinders of zirconia were bonded end-to-end. One end of each bonded specimen was insulated, specimens were thermocycled and tested in shear, and bond strengths were calculated and analyzed. RESULTS: Two-way ANOVA revealed that the effects of cement type, the number of thermocycles, and their interaction all significantly affected bond strength (p < 0.0001). By 10,000 cycles, most cements had lost at least half of their initial bond strengths, and two cements effectively recorded zero bond strengths. Failure modes were cement specific, but adhesive modes predominated. Fatigue resistance of two cements was greatly improved by use of a primer and an oxygen-inhibiting gel, as recommended by their respective manufacturers. CONCLUSIONS: Both the type of resin cement and the number of thermocycles influenced bond strength. Fatigue through thermocycling affected different cement types in different ways. Some materials displayed more rapid loss of bond strength than others. Cements differed in their failure modes.

Microleakage resistance of minimally invasive Class I flowable composite restorations.

Minimally invasive flowable composite Class I restorations are widely used. However, flowable composites are characterized by low filler contents, modified resin formulations, low moduli of elasticity, low viscosity, generally poor mechanical properties, and decreased long-term stability. The purpose of this study was to compare the microleakage resistance of a wide variety of flowable composites used with their manufacturers' recommended bonding systems to that of a long-used and widely studied microhybrid composite when placed as minimally invasive occlusal restorations. Molar teeth were prepared in a standardized manner, restored, artificially aged, stained, sectioned, evaluated, and analyzed. Microleakage varied substantially, by a whole order of magnitude, among the material groups tested. The control group, a conventional microhybrid composite material, leaked significantly less than all the flowable composite groups. Microleakage varied very slightly among measurement site locations. Tiny microscopic bubbles were seen within many of the flowable composite specimens, as were a few voids.

Effect of compromised cortical bone on implant load distribution.

PURPOSE: To investigate photoelastically the difference in load distribution of dental implants with different implant neck designs in intact and compromised bone. MATERIALS AND METHODS: Composite photoelastic models were fabricated using two different resins to simulate trabecular bone and a 1-mm thick layer of cortical bone. The following parallel-sided, threaded implants were centrally located in individual models representing intact and compromised cortical bone: Straumann (4.1-mm diameter x 12-mm length), AstraTech (4.0-mm diameter x 13-mm length), and 3i (3.75-mm diameter x 13-mm length). The compromised cortical bone condition was simulated by contaminating a 1-mm neck portion with Vaseline to impair the implant-resin interface. Vertical and oblique static loads were applied on the abutments, and the resulting stresses were monitored photoelastically and recorded photographically. RESULTS: For the fully intact condition, the highest stresses were observed around the crest and apical region for all implant designs under vertical and inclined loads. There were no appreciable differences in magnitude or distribution between implant types. With compromised cortical bone, for all designs and load directions, higher stresses in the supporting structures were observed. Increased stresses were noted especially at the cortical bone-trabecular bone interface. Somewhat lower stress levels were observed with the 3i implant. CONCLUSIONS: The condition of implant-cortical bone contact has considerable influence on stress distribution. A compromised cortical bone condition caused higher level stresses for all implant designs tested.

Retention and load transfer characteristics of implant-retained auricular prostheses.

PURPOSE: The use of osseointegrated implants for maxillofacial prostheses reduces the need for adhesives, provides for a more stable and more esthetic prosthesis with thinner margins, and results in increased patient acceptance and confidence. The purpose of this study was to compare the retention and load transfer characteristics of differently designed implant-retained auricular prostheses. MATERIALS AND METHODS: A photoelastic model was fabricated of the auricular-temporal region of a human skull. Craniofacial implants 3.75 mm in diameter and 4 mm long were embedded in locations typically selected to retain auricular prostheses. Two retention mechanisms were evaluated on the implants: a Hader bar with 3 clips and the use of 3 Locator attachments. The retentive capacity of the prostheses was determined on an Instron test machine. Initial retention and changes with multiple removals were examined. Dislodgment forces were applied to each retentive device in the field of a circular polariscope. Resulting stresses were monitored and recorded photographically. RESULTS: The highest initial retention demonstrated by the Locator device was 12.4 +/- 0.9 lb, and the highest retention value for the Hader bar with clips was 7.5 +/- 1.1 lb. All attachments decreased in retention after multiple removals. The Locator devices produced higher peri-implant stresses compared to the Hader bar-with-clips design. CONCLUSIONS: Since higher retention is associated with higher stresses, results of this study suggest that a balance between retention and stress production is necessary in selecting a retention mechanism for the specific requirements of the patient being treated. The Locator attachment was correlated with higher retention values as well as with higher peri-implant stress compared to the Hader bar-and-clip attachment design. Retention decreased and then stabilized after multiple

Biomechanical rationale for surgically facilitated expansion of the maxilla in the cleft palate patient.

AIM: To evaluate in vitro the biomechanical effects of surgically assisted rapid palatal expansion in a photoelastic unilateral cleft palate analog by observing stresses produced during appliance activation. METHODS: A photoelastic analog of an adult skull with unilateral cleft palate was fabricated using birefringent materials to simulate bone and teeth. A customized hyrax appliance was applied to the anchor teeth and incrementally activated. Resulting stress patterns on both the cleft side and the intact side were observed and recorded photographically in the field of a circular polariscope. Subsequently, the pterygomaxillary junctions were sequentially cut, the appliance activations repeated, and the resulting stresses recorded. RESULTS: With intact pterygomaxillary junctions, the highest stresses were localized at the zygomaticomaxillary and zygomaticofrontal sutures, with higher intensity on the intact side. Stresses concentrated in the pterygoid plates evidenced resistance to the expansion forces of the hyrax appliance. On separating the defect pterygomaxillary junction, increased stresses were located from the zygomaticomaxillary suture to the zygomaticofrontal suture and at the zygomatic arches on both sides, as well as the pterygomaxillary junction of the non-defect side. After cutting both the pterygomaxillary junctions, decreased stress was located from the zygomaticomaxillary suture to the zygomaticofrontal suture, the zygomatic arch, and the frontonasal sutures on both sides. CONCLUSIONS: The pterygomaxillary junctions acted as the main resistive elements to expansion forces generated by a hyrax appliance. Separating the pterygomaxillary junctions assisted the bodily displacement of the lateral maxillary segment, which would facilitate correction of maxillary arch constrictions in the adult unilateral cleft lip patient.

Load-induced stresses in photoelastic primary canines with facial restorations.

PURPOSE: The purpose of this study was to photoelastically compare the stresses generated by loads on primary canines with facial restorations of different stiffness. METHODS: Composite photoelastic models of a typical maxillary primary canine were fabricated using individual simulant materials for enamel, dentin, periodontal ligament, and alveolar bone. Models were made with identical facial preparations included either near the cementoenamel junction or at 2 mm incisally. The model teeth were restored using the following materials of disparate elastic moduli: (1) high modulus, hybrid composite (Herculite XRV); and (2) lower modulus compomer (Dyract). Three replications of each type of restored tooth were fabricated. Simulated masticatory forces were applied on the cusp tip and the cingulum of each tooth model. The resulting stress patterns were observed and recorded photographically in the field of a circular polariscope. RESULTS: Prior to load application, similar low-level, polymerization-induced shrinkage stresses were localized at the preparation margins of all models. Under both incisal and cingular loading, the higher modulus hybrid composite tended to concentrate stress along the gingival and proximal margins more than did the lower modulus compomer. This effect was more pronounced with cingular than with incisal loading. For both preparations and restoratives, higher stresses were produced by the cingulum loading. No significant differences were observed within each group of replicated restored models. CONCLUSION: These results suggest that, regardless of preparation height, lower modulus compomer restoration of facial lesions in primary maxillary canines may reduce stress production by occlusal forces.

Comparing cutting efficiencies of diamond burs using a high-speed electric handpiece.

This study sought to compare the cutting efficiency of different diamond burs on initial use as well as during repeated use, alternating with sterilization. Long, round-end, tapered diamond burs with similar diameter, profile, and diamond coarseness (125-150 microm grit) were used. A high-torque, high-speed electric handpiece (set at 200,000 rpm) was utilized with a coolant flow rate of 25 mL/min. Burs were tested under a constant load of 170 g while cuts were made on a machinable ceramic substrate block. Each bur was subjected to five consecutive cuts for 30 seconds of continuous operation and the cutting depths were measured. All burs performed similarly on the first cut. Cutting efficiencies for three of the bur groups decreased significantly after the first cycle; however, by the fifth cycle, all bur groups performed similarly without any significant differences (p > 0.05). A scanning electron microscope revealed significant crystal loss after each use.

Dimensional accuracy analysis of implant framework castings from 2 casting systems.

PURPOSE: To compare the dimensional accuracy of implant framework castings from an argon vacuum casting machine with those from a centrifugal casting machine. MATERIALS AND METHODS: Three 4 x 10-mm external hex-type implants (3i/Implant Innovations) were embedded in an acrylic resin block 7 mm apart, with a 2 mm offset of the middle implant. Eight reference points were marked on the implant collars. Twenty implant bar frameworks were waxed with UCLA abutments, invested with a ringless system, and subjected to the same thermal cycle. Ten wax patterns were cast in gold alloy using an oxygen-propane torch and centrifugal casting system; 10 were cast using an argon vacuum casting machine (KDF; Denken). The White 1-screw technique was applied after sequentially tightening the mesial and distal abutment screws to 10 Ncm. Fit of the implant framework castings was evaluated by measuring the marginal opening between the casting and implant at the reference points. These measurements were averaged and statistically compared for differences. RESULTS: The mean marginal openings at the most distant measuring locations from the tightened retaining screw at location 1 was between 44 to 48 microm for the centrifugal system compared to between 28 to 32 mm for KDF (P < .01). For screws tightened at location 3, the mean marginal openings at the most distant measuring locations were between 40 to 51 mm for the centrifugal system compared to between 27 to 29 microm for KDF (P < .01). DISCUSSION: In comparison with the centrifugal casting and oxygen-propane system, the argon vacuum system was more accurate and user friendly and less technique-sensitive. CONCLUSION: The argon vacuum casting machine tested produced more accurate, better fitting implant-supported prosthesis frameworks than a conventional centrifugal casting system. The "1-screw" method of evaluating casting fit was most effective when either of the prostheses' end screws were tightened.

Thickness and stiffness characteristics of custom-made mouthguard materials.

OBJECTIVES: The purpose of this investigation was to determine the thickness of different types of custom-made mouthguards after processing and the deformational changes after simulated occlusal loading. MATERIALS AND METHODS: Ten mouthguards were fabricated for the same dental arch using the following materials and processing techniques: Group I: vacuum-formed (4 mm), Colored Mouthguard; Group II: vacuum-formed (4 mm), Proform; and Group III: pressure-laminated (3 + 3 mm), Drufosoft. After processing, the thickness was measured in three places: lingual cusps of the first molar, distal marginal ridge of the first premolar; and facial of central incisor. The stiffness of each group was determined by applying a simulated clenching force to the first molar lingual cusp areas on an Instron test machine with a blunt probe. The corresponding penetration was measured with a dial gauge. Thickness and force-deflection measurements of the different mouthguard groups were compared using analysis of variance and post hoc tests. RESULTS: The mean thicknesses at the molar for Groups I and II were 1.55 and 1.52 mm, respectively, and were significantly smaller then the corresponding thickness for Group III (3.48 mm). The mean thicknesses at the facial of the incisors for Groups I and II were similar (2.05 and 2.06 mm, respectively), and were significantly smaller than the corresponding thickness for Group III (3.29 mm). Groups I and II demonstrated similar stiffness that was significantly higher than that for Group III. CONCLUSION: These results show that vacuum-formed mouthguards produced smaller thicknesses than the pressure-laminated mouthguards. The pressure-laminated mouthguards produced material thicknesses that were previously shown to be adequate to protect athletes from trauma.

Composite resin bond strength to primary dentin prepared with Er, Cr:YSSG laser.

This in vitro study evaluated the shear bond strength of a hybrid composite resin bonded to primary dentin prepared with an Er, Cr:YSGG hydrokinetic laser compared to conventional bur prepared primary dentin. The results suggest that primary dentin surfaces treated with the Er, Cr:YSGG laser, with or without etching, may provide comparable or increased composite resin bond strengths depending upon bonding agent used.

Effect of Implant Diameter and Ridge Dimension on Stress Distribution in Mandibular First Molar Sites-A Photoelastic Study.

The long-term clinical success of a dental implant is dependent upon maintaining sufficient osseointegration to resist forces of occlusion. The purpose of this study was to investigate the effect of implant diameter on stress distribution around screw-type dental implants in mandibular first molar sites using photoelastic models. The design included models with different buccal-lingual dimension. Twelve composite photoelastic models were assembled using 2 different resins to simulate trabecular and cortical bone. Half of the models were fabricated with average dimensions for ridge width and the other half with narrower buccal-lingual dimensions. One internal connection implant (13 mm length) with either a standard (4 mm), wide (5 mm), or narrow (3.3 mm) diameter was embedded in the first molar position of each photoelastic model. Half the implants were tapered and the other half were straight. Full gold crowns in the shape of a mandibular first molar were fabricated and attached to the implants. Vertical and angled loads of 15 and 30 pounds were applied to specific points on the crown. Wide-diameter implants produced the least stress in all ridges while narrow-diameter implants generated the highest stress, especially in narrow ridges. It may be that the volume and quality of bone surrounding implants influences stress distribution with a greater ratio of cortical to trabecular bone, thus providing better support. Models with wide-diameter implants loaded axially had a more symmetrical stress distribution compared to standard and narrow diameter implants. A more asymmetrical stress pattern developed along the entire implant length with angled loads. Implant diameter and ridge width had considerable influence on stress distribution. Narrow-diameter implants produced more stress than wide diameter implants in all conditions tested.

Removal torque of immediately loaded transitional endosseous implants in human subjects.

PURPOSE: Transitional implants were designed to support provisional restorations and to allow for load-free osseointegration of conventional implants while a patient was provided with immediate esthetics and function. The purpose of this study was to evaluate the removal torque values of immediately loaded transitional implants in human subjects. MATERIALS AND METHODS: Thirty-one 1.8-mm-diameter transitional implants were placed in 4 patients to support provisional restorations and were subjected to immediate loading. Removal torque values were recorded using a modified ITI torque driver, which was calibrated on an Instron test machine. Torque values were subjected to analysis of variance and post hoc comparisons of means (P < .05). RESULTS: Twenty-six implants were removed intact with torque values between 10.5 and 22.9 Ncm, while 5 fractured at the bone crest at values between 27.1 and 35.4 Ncm. Mean torque values increased with time in function. These values were significantly lower in the maxilla (16.1 +/- 4.8 Ncm) than in the mandible (24.0 +/- 7.3 Ncm). DISCUSSION: Results suggest that these transitional implants may be safely removed from the maxilla after 7 to 15 months, but there is a risk of implant fracture during removal from the mandible after 10 months. CONCLUSION: Removal torque levels for these implants indicate varying degrees of integration even though they were subjected to immediate loading.

Load transfer by an implant in a sinus-grafted maxillary model.

PURPOSE: This in vitro study determined the stress distribution around an implant placed in a posterior edentulous maxillary model with simulated sinus grafts that had different degrees of stiffness. MATERIALS AND METHODS: The composite photoelastic model with a standard threaded implant consisted of simulated crestal cortical, cancellous, sinus cortical, and grafted bone. The graft maturation process and inherent graft quality were represented in the model by varying the stiffness of the graft. Prior to placement of the simulated graft, axial and inclined loads were applied to the implant The stresses that developed in the supporting structures were analyzed photoelastically. The graft was then placed and the testing procedure was repeated over 4 consecutive days, during which time the simulated graft stiffened. RESULTS: The stress analysis indicated that before placement of the simulated graft, loading on the implant transferred the highest stresses to cortical bone. The presence of the simulated graft transferred stress from the native bone simulants to the simulated grafted bone. DISCUSSION: As the stiffness of the graft increased, a more equitable stress distribution was observed in the multilayer bone surrounding the implant. CONCLUSION: Loading of an implant in a less stiff grafted sinus could lead to overloading of the native bone as well as the maturing grafted bone.

Accuracy of mechanical torque-limiting devices for implants.

PURPOSE: To examine the accuracy of 3 mechanical torque wrenches. MATERIALS AND METHODS: The torque outputs of the Nobel Biocare, Straumann ITI, and DynaTorq ITL mechanical torque-limiting devices were determined using a special setup on an Instron test machine. The devices were held in the test setup and oriented so that activation of the drivers caused a pure torsion effect. RESULTS: Significant differences generally existed between individual units and the target torque levels for the Nobel Biocare torque controller. DISCUSSION: The mean torque values of the ITI and ITL devices were within 10% of their respective target torque levels. Knowledge of applied torque levels to the screws that retain implant abutments and their attached prostheses is necessary to achieve optimal preload. The ITI and ITL devices tested in this study were capable of providing consistent torque at or near their respective targets. CONCLUSION: The torque output of each individual device deviated in varying degrees from target torque values.

Bicortically stabilized implant load transfer.

PURPOSE: Questions exist as to the potential advantages of bicortical stabilization of implants in the mandible through engagement of the lingual cortical plate. The purpose of this investigation was to determine photoelastically the effect of lingual cortical plate engagement on implant load transfer. MATERIALS AND METHODS: Composite photoelastic models of an edentulous posterior segment of a mandible were fabricated using plastics of different modulus to simulate cortical and trabecular bone. One model included a 3.75 x 15-mm threaded implant that engaged the simulated lingual cortical plate, while in the other model the implant was centrally located within the simulated trabecular bone. A metal superstructure was cast using an abutment cylinder. Simulated vertical occlusal loads were applied to the superstructure over the implant and at fixed buccal cantilever lengths. Stresses that developed within the model were monitored photoelastically and recorded photographically. Peri-implant defects were then formed in the models and the loading and recording procedures were repeated. RESULTS: Lingual cortical plate engagement generated the highest stresses at the lingual cortical plate and the buccal crestal cortical layer at the implant neck. Stress intensity within the buccal plate at the implant neck was lower than that in the centrally placed implant. In the presence of a peri-implant defect, for all load conditions, more load was borne by the trabecular bone. Increasing cantilever lengths caused asymmetric load transfer with higher maximum stresses. DISCUSSION: For both implant placements, a large portion of the applied load was taken by the crestal cortical bone simulant. Engagement of the lingual cortical plate reduced maximum stress in the crestal cortical bone by approximately 25%. With peri-implant defects, the simulated trabecular bone provided the main support of the applied load. Longer buccal cantilever lengths increased maximum stresses for all placement and crestal bone conditions. CONCLUSIONS: The results of this investigation do not indicate a clear load transfer advantage to apical engagement of the lingual cortical plate in this model.

Contraction stress of composite resin build-up procedures for pulpless molars.

PURPOSE: The purpose of this study was to photoelastically evaluate contraction stresses associated with various resin composite build-up procedures for pulpless molars. MATERIALS AND METHODS: Photoelastic models of endodontically treated mandibular molars were fabricated to simulate a preparation for a full-cast crown. The model configuration included three lateral walls, but no post space. The buildups were made with dual-curing resin composite using the following procedures: 1) bulk dual cure, 2) bulk chemical cure, 3) horizontal two increments, dual cure, and 4) indirect. Five specimens were fabricated for each condition. The stresses developed in the models were recorded photographically in the field of a circular polariscope. RESULTS: The build-up procedures tested generated widely different stress distributions and intensities. The highest stresses were seen with the bulk dual-cure method. The slower polymerization chemical-cure group developed distributions similar to the bulk dual-cure group with a significantly lower fringe order. The two horizontal increments dual-cure techniques developed an individual group of fringes for each layer. Compared with the bulk dual-cure group, stresses around the occlusal margin were reduced by incrementalization, while fringes were more closely spaced at the line angles with a slightly lower fringe order. The indirect method demonstrated the lowest stress which extended over the smallest area. CONCLUSION: Contraction stress in resin composite buildups varied significantly depending upon the procedures of fabrication. The bulk dual-cure method developed the most severe contraction stresses, while the indirect technique resulted in significantly lower contraction stresses than the other techniques tested.

Biomechanical comparison of straight and staggered implant placement configurations.

The effect of buccolingual staggered implant placement on stress distribution within the supporting structure was examined photoelastically. Two photoelastic models of a human mandible, edentulous distal to the canine, were fabricated. Three screw-type implants were embedded into the edentulous region of each model. The implants were placed in a straight line in one model and in a buccolingual staggered configuration in the other. Vertical and lateral loads were applied to a fixed partial denture superstructure. No clear biomechanical advantage to a staggered 1.5 mm buccal and lingual offset placement configuration was observed.

Effects of aging on the dimensional stability of custom-made mouthguards.

OBJECTIVE: The purpose of this investigation was to assess the effects of aging on the dimensional stability of vacuum-formed, pressure-formed, and pressure-laminated custom-made mouthguards. METHOD AND MATERIALS: A master model representing the dental arch was fabricated in acrylic resin. Two ethylene vinyl acetate thermoplastic materials were tested: Essix (regular) and Proform (prelaminated). The model was benchmarked in five places along the curvature of the arch. The model was used to fabricate 10 specimens for each of the following groups: 1 = vacuum-formed (3-mm) Essix; 2 = vacuum-formed (4-mm) Proform; 3 = pressure-formed (3-mm) Essix; 4 = pressure-formed (4-mm) Proform; 5 = pressure-laminated (3-mm + 2-mm) Essix; 6 = pressure-laminated (3-mm + 3-mm) Essix. Die stone was poured into the newly made specimens and measured. Aging of the specimens was simulated by reproducing cleaning cycles (48 hours' soaking in water at 60 degrees F and brushing 100 times with a soft denture brush in soapy water at 140 degrees F for 20 minutes). Die stone was poured into the aged specimens and measured. Initial and postaging measurements were compared. RESULTS: All specimens showed dimensional changes post-aging, mainly decreases. Pressure-laminated specimens showed the best dimensional stability. CONCLUSION: Aging induced various dimensional changes in mouthguards, depending on the materials and processing techniques used. Most of the dimensional change for all mouthguards occurred at the central incisor region, the most important area for the protection of the anterior teeth and the premaxilla. Pressure-laminated mouthguard specimens showed the smallest range of changes at the central incisor region, suggesting potentially improved fit, comfort, and protection.

Effects of incremental curing on contraction stresses associated with various resin composite buildups.

OBJECTIVE: The purpose of this study was to photoelastically evaluate contraction stresses associated with various resin composite build-up procedures, including incremental curing and the use of flowable composite for pulpless molars. METHOD AND MATERIALS: Life-sized photoelastic models of an endodontically treated molar were fabricated. The cavity represented a conservative access preparation and included four lateral walls. The following materials were used for buildup: dual-cured hybrid composite (Cleafil DC Core [DC], BIS-CORE [BC]); light-cured flowable composite (AELITEFLO LV [ALV]); and chemical-cured flowable composite (CORE-FLO [CF]). The photoelastic models were built up with the following techniques: bulk-cured (BDd [dual-cured DC], BDc [chemically-cured DC], BBd [dual-cured BC], and BC [CF]); and incremental-cured (1 mm gingivally + 3.5 mm occlusally; IAB [ALV + BC], ICB [CF + BC], and IBB [BC + BC]). Isochromatic fringes developed in the models were recorded photographically in the field of a circular polariscope, and maximum fringe order was determined. Five specimens were tested for each condition. RESULTS: Stress intensity of the build-up methods fell into two categories: high (BDd, BBd, BC, and IAB) and low (BDc, ICB, and IBB). The difference between high and low groups was statistically significant, except between BC and ICB. The maximum fringe order was developed around the point angles at the cavity floor for all the conditions tested. CONCLUSIONS: Type and setting mechanism of resin composite build-up technique had considerable influence on contraction stress. Incremental buildup using composites with low elastic modulus did not reduce contraction stress intensity compared with bulk-cured techniques.

Influence of attachment systems on load transfer of an implant-assisted maxillary overdenture.

PURPOSE: This photoelastic study compared the load transfer characteristics of 2 retention mechanisms in an implant-assisted overdenture prosthesis. MATERIALS AND METHODS: Four implants were incorporated into a photoelastic model of a moderately resorbed edentulous human maxilla. Two retention mechanisms were studied by changing components on the same model and the palateless overdenture. The retention mechanisms studied were bar splint with anterior clip and distal resilient attachments, and solitary ball/O-ring attachments. Loads, ranging from 1.4 to 14.4 kg, were applied to the palatal incline of central incisors and buccal incline of premolars with and without balancing contacts. Stresses developed around all the implants under each loading condition were photographed in the field of a circular polariscope. RESULTS: With both retention mechanisms, protrusive and laterotrusive loads without balancing contacts caused instability of the overdenture, producing minimal stress around the implants in the supporting structure. High intensity stresses indicating intrusion of the posterior implants were noted when the bar/distal resilient attachment overdenture had balancing contacts for protrusive and laterotrusive loads. The posterior implants of ball/O-ring attachment overdenture exhibited high intensity stresses indicating not only intrusion, but also bending, when the occlusion was balanced. CONCLUSIONS: Balanced occlusion was required in both retention mechanisms for stability of the implant-assisted overdenture when clinically acceptable loads were applied. The protrusive and laterotrusive loads were not distributed equitably in either mechanism, since highest stresses occurred at the posterior implants.