The purpose of this study was to determine whether varying the preload on the implant-abutment complex would affect screw loosening under simulated loading conditions. Abutment screws in sample models were tightened to 25, 30, 35, and 40 N-cm. One group of samples was allowed to stand for 3 hours after being torqued and then loosened. Another group of samples was retorqued after 10 minutes with the same initial torque value and then allowed to stand for 3 hours before loosening. For the load group of samples, the abutments were torqued into place, retorqued after 10 minutes, and a load applied for 3 hours before loosening. Cyclic loading was carried out using a servo-hydraulic testing machine with the values cycled between 1 and 26 pounds and the load applied directly to the abutments. Analysis of variance, analysis of covariance, and linear regression analysis was performed. Within the parameters of this in vitro investigation, the following recommendations can be made: (1) retightening abutment screws 10 minutes after the initial torque applications should be routinely performed and (2) increasing the torque value for abutment screws above 30 N-cm can be beneficial for abutment-implant stability and to decrease screw loosening.

One of the most serious and prevalent problems associated with the restorative aspect of dental implants is loosening and fracturing of screws. Implant screws should be retightened 10 minutes after the initial torque application as a routine clinical procedure to help compensate for the settling effect. Mechanical torque gauges should be used instead of hand drivers to ensure consistent tightening of implant components to torque values recommended by implant manufacturers.

Failure due to fatigue manifests itself in dental prostheses and restorations as wear, fractured margins, delaminated coatings, and bulk fracture. Mechanisms responsible for fatigue-induced failure depend on material ductility: Brittle materials are susceptible to catastrophic failure, while ductile materials utilize their plasticity to reduce stress concentrations at the crack tip. Because of the expense associated with the replacement of failed restorations, there is a strong desire on the part of basic scientists and clinicians to evaluate the resistance of materials to fatigue in laboratory tests. Test variables include fatigue-loading mode and test environment, such as soaking in water. The outcome variable is typically fracture strength, and these data typically fit the Weibull distribution. Analysis of fatigue data permits predictive inferences to be made concerning the survival of structures fabricated from restorative materials under specified loading conditions. Although many dentalrestorative materials are routinely evaluated, only limited use has been made of fatigue data collected in vitro: Wear of materials and the survival of porcelain restorations has been modeled by both fracture mechanics and probabilistic approaches. A need still exists for a clinical failure database and for the development of valid test methods for the evaluation of composite materials.

No abstract

Masticatory forces cause fatigue to cement-retained crowns and abutments and may adversely effect retention. The relation between the number of load cycles and the retentive forces is important. This study evaluated the effect of compressive cyclic loading on the retentive forces of a temporary cement used to retain implant crowns and the relationship between load cycles and retentive forces. Ten castings and implant abutments were cemented with zinc oxide-eugenol temporary cement. The retentive force necessary to dislodge the casting from the abutment was determined before and after the application of 2 Hz of vertical off-axis 3-mm sinusoidal-type compressive cyclic loading between 20 and 130 N for 500000, 1000000, and 5000000 cycles. These forces were equivalent to approximately 6 months, 1 year, and 5 years of human mastication. Data before and after the applied loading were analyzed with a paired sample t test (alpha = 0.05). The retentive forces of the 3 groups were analyzed by 1-way analysis of variance and post hoc by Scheffé multiple comparison (alpha = 0.05). The relation of the loading and the altered retentive forces were analyzed with the Pearson correlation coefficient. Compressive cyclic loading reduced the retentive forces significantly in all groups (P = .000). The retentive forces were reduced 16.75%, 18.73%, and 19.68% during the applied loading cycles of 500000, 1000000, and 5000000. All reduced retentive forces were not significantly different (P = .792). Although cyclic loading reduced the retentive forces, the increased cycles had little relationship (R = 0.119) to the decreased retentive forces of the temporary cement. The relationship between occlusal loading and retentive force can influence the choice of a temporary cement for a particular clinical situation.