Effect of abutment finish lines on the mechanical behavior and marginal fit of screw-retained implant crowns: An in vitro study.

STATEMENT OF PROBLEM: The design of the implant-abutment connection has been widely researched, but the impact of different crown-abutment geometries remains unclear. PURPOSE: The purpose of this in vitro study was to evaluate the effect of different crown-abutment margin geometries on the mechanical behavior and fit of screw-retained implant-supported single-crown restorations by using mechanical static and fatigue tests and mastication simulation. MATERIAL AND METHODS: A total of 45 cobalt-chromium premolar-shaped metal frameworks were fabricated for single-unit implant-supported screw-retained restorations on stock abutments and internal hexagon Ø4.25×11-mm cylindrical implants. They were divided into 3 groups according to margin geometry: S, shoulder; C, chamfer; and F, feather-edge. Three static load until fracture and 24 dynamic load tests were performed by using the International Organization for Standardization 14801:2016 standard (ISO 14801:2016) (number of cycles limit: 5×106 cycles, frequency: 6 Hz). The ProFatigue software program was used to optimize the procedure (S, n=12 specimens; C, n=7 specimens; and F, n=5 specimens). Six additional specimens from each group were subjected to a mastication simulation (limit number of cycles: 1×106 cycles, cyclic loading from Pmin=30 N to Pmax=300 N, frequency: 6 Hz). Results from the fatigue tests were reported descriptively, and the Fisher exact test was used to analyze the difference in failure modes. Data from maximum misfit were evaluated by photogrammetry and statistically analyzed with the Anderson-Darling test and the Kruskal-Wallis and Dunn multiple comparison tests (α=.05). RESULTS: The fatigue limit was 456 N for group S, 512 N for group C, and 514 N for group F. The mean ±standard deviation misfit was 2.6 ±0.1 µm for group S, 3.8 ±1.1 µm for group C, and 3.6 ±0.8 µm for group F. Differences in misfit between groups S and C and between groups S and F were statistically significant (P<.05). CONCLUSIONS: Crown-abutment connections with chamfer and feather-edge margins showed better mechanical behavior, while shoulder margin exhibited better fit. However, high levels of fit were achieved for all the evaluated geometries.

Probabilistic Assessment of Fracture Toughness of Epoxy Resin EPOLAM 2025 Including the Notch Radii Effect.

Many design scenarios of components made of polymer materials are concerned with notches as representative constructive details. The failure hazard assessment of these components using models based on the assumption of cracked components leads to over-conservative failure estimations. Among the different alternative approaches proposed that are based on the apparent fracture toughness, KcN is considered. In so doing, the current deterministic underlying concept must be replaced by a probabilistic one to take into account the variability observed in the failure results in order to ensure a reliable design. In this paper, an approach based on the critical distance principle is proposed for the failure assessment of notched EPOLAM 2025 CT samples with each different notch radii (ρ) including a probabilistic assessment of the failure prediction. First, each apparent fracture toughness is transformed into the equivalent fracture toughness for ρ=0 based on the critical distances theory. Then, once all results are normalized to the same basic conditions, a Weibull cumulative distribution function is fitted, allowing the probability of failure to be predicted for different notch radii. In this way, the total number of the specimens tested in the experimental campaign is reduced, whereas the reliability of the material characterization improves. Finally, the applicability of the proposed methodology is illustrated by an example using the own experimental campaign performed on EPOLAM 2025 CT specimens with different notch radii (ρ).

Effect of region-dependent viscoelastic properties on the TMJ articular disc relaxation under prolonged clenching.

The disc of the temporomandibular joint (TMJ) is located between the mandibular condyle and temporal bone, and has an important load-bearing and stress absorbing function. The TMJ disc presents viscoelastic characteristics that are largely dependent on its collagen fibre and proteoglycan composition and organization. The purpose of this study is to investigate the possible effects of region-specific dynamic viscoelastic properties on stress relaxation during prolonged clenching. Two finite element models were used to compare the stress distribution within the TMJ disc, namely, one with uniform disc material property and another one with region-specific disc material properties. Similar results were observed in both models with slight differences in the location of maximum stress. Larger stresses were observed in all cases for the model with uniform disc material property. Moreover, the higher values for the model with uniform disc material property appeared in the lateral region, while in the model with region-specific disc properties, these values moved to the lateral and central region. This investigation confirms that both models are sufficiently accurate to investigate stress distribution in the TMJ disc, and, particularly, the model with the region-specific disc material properties ensure better simulations of the TMJ disc behaviour.

Optimized Planning and Evaluation of Dental Implant Fatigue Testing: A Specific Software Application.

Mechanical complications in implant-supported fixed dental prostheses are often related to implant and prosthetic design. Although the current ISO 14801 provides a framework for the evaluation of dental implant mechanical reliability, strict adherence to it may be difficult to achieve due to the large number of test specimens which it requires as well as the fact that it does not offer any probabilistic reference for determining the endurance limit. In order to address these issues, a new software program called ProFatigue is presented as a potentially powerful tool to optimize fatigue testing of implant-supported prostheses. The present work provides a brief description of some concepts such as load, fatigue and stress-number of cycles to failure curves (S-N curves), before subsequently describing the current regulatory situation. After analyzing the two most recent versions of the ISO recommendation (from 2008 and 2016), some limitations inherent to the experimental methods which they propose are highlighted. Finally, the main advantages and instructions for the correct implementation of the ProFatigue free software are given. This software will contribute to improving the performance of fatigue testing in a more accurate and optimized way, helping researchers to gain a better understanding of the behavior of dental implants in this type of mechanical test.

Effects of loading direction in prolonged clenching on stress distribution in the temporomandibular joint.

Parafunctional habits, such as bruxism and prolonged clenching, have been associated with dysfunctional hyperactivity of the masticatory muscles, including the lateral pterygoid muscle. The resultant loading to the temporomandibular joint (TMJ) is subject to the degradation of bone, cartilage and disc in the TMJ. In this study, we examined the effect of clenching direction on the stress distribution in the TMJ. In this line, we hypothesised that asymmetrical clenching involved in parafunction might result in increased stresses on the TMJ disc as well as on the condylar and temporal articular surfaces. The distribution of stress for various directional loadings was analysed using a three-dimensional finite element model of the TMJ, with viscoelastic properties for the disc. The numerical results revealed that load direction influenced the amount and distribution of stresses on the disc surfaces. In particular, the lateral region of the disc suffered higher stress values. Moreover, the results showed a significant stress relaxation in the disc that revealed its capacity for stress energy dissipation. From the present study, it can be established that during prolonged clenching, the higher stresses are concentrated in the lateral region, which could imply that TMJ disorders related to damage or wear in the disc and the condylar cartilage, overall, occur when lateral dysfunctional displacements are present.

A Probabilistic Approach to Assessing and Predicting the Failure of Notched Components.

This work presents a probabilistic model to evaluate the strength results obtained from an experimental characterisation program on notched components. The generalised local method (GLM) is applied to the derivation of the primary failure cumulative distribution function (PFCDF) as a material property (i.e., independent of the test type, load conditions and specimen geometry selected for the experimental campaign), which guarantees transferability in component design. To illustrate the applicability of the GLM methodology, an experimental program is performed using specimens of EPOLAM 2025 epoxy resin. Three different samples, each with a specific notch geometry, are tested. As a first scenario, a single assessment of each sample is obtained and the PFCDFs are used to perform cross predictions of failure. Some discrepancies are noticeable among the experimental results and cross-failure predictions, although they are within the expected margins. A possible reason for the disagreement can be assigned to the inherent statistical variability of the results and the limited number of tests per each sample. As a second scenario, a joint assessment of the three samples is performed, from which a unique PFCDF is provided, according to the GLM. In the latter case, a more reliable assessment of the experimental results from the geometry conditions is achieved, the suitability of the selected driving force is verified, and the transferability of the present material characterisation is confirmed.

Viscoelastic properties of the central region of porcine temporomandibular joint disc in shear stress-relaxation.

In this study, shear relaxation properties of the porcine temporomandibular joint (TMJ) disc are investigated. Previous studies have shown that, in fatigue failure and damage of cartilage and fibrocartilage, shear loads could be one of the biggest contributors to the failure. The aim of the present study is to develop an evaluation method to study shear properties of the disc and to do a mathematical characterization of it. For the experiments, twelve porcine discs were used. Each disc was dissected from the TMJ and, then, static strain control tests were carried out to obtain the shear relaxation modulus for the central region of the discs. From the results, it was found that the disc presents a viscoelastic behavior under shear loads. Relaxation modulus decreased with time. Shear relaxation was 10% of the instantaneous stress, which implies that the viscous properties of the disc cannot be neglected. The present results lead to a better understanding of the discs mechanical behavior under realistic TMJ working conditions.

Dynamic and stress relaxation properties of the whole porcine temporomandibular joint disc under compression.

In this study, the dynamic and static compressive properties of the whole porcine temporomandibular joint (TMJ) disc were investigated. The aim of the study was to develop a new simple method for the evaluation of joint viscoelasticity, enabling examination of the load-bearing capacity and joint flexibility of the entire disc. For the experiments, a novel testing fixture that reproduces the condylar and fossa surfaces of the TMJ was developed to replicate TMJ disc geometry. Ten porcine discs were used in the experiments. Each disc was dissected from the TMJ and sinusoidal compressive strain was applied to obtain the storage and loss moduli. Static strain control tests were carried out to obtain the relaxation modulus. The result of static and dynamic tests indicated that the whole disc presented viscoelastic behavior under compression. Storage and loss moduli increased with frequency and the relaxation modulus decreased over time. The loss tangent showed less frequency dependence, with values ranging from 0.2 to 0.3, suggesting that the viscous properties of the disc cannot be neglected. These results provide a better understanding of whole disc mechanical compression behavior under realistic TMJ working conditions.