The prosthetic screw loosening of two-implant supported screw-retained fixed dental prostheses in the posterior region: A retrospective evaluation and finite element analysis

Influence of prosthetic index structures and implant materials on stress distribution in implant restorations: a three-dimensional finite element analysis

BACKGROUND

Mechanical complications affect the stability of implant restorations and are a key concern for clinicians, especially with the frequent introduction of new implant designs featuring various structures and materials. This study evaluated the effect of different prosthetic index structure types and implant materials on the stress distribution of implant restorations using both in silico and in vitro methods.

METHODS

Four finite element analysis (FEA) models of implant restorations were created, incorporating two prosthetic index structures (cross-fit (CF) and torc-fit (TF)) and two implant materials (titanium and titanium-zirconium). A static load was applied to each group. An in vitro study using digital image correlation (DIC) with a research scenario identical to that of the FEA was conducted for validation. The primary strain, sensitivity index, and equivalent von Mises stress were used to evaluate the outcomes.

RESULTS

Changing the implant material from titanium to titanium-zirconium did not significantly affect the stress distribution or maximum stress value of other components, except for the implant itself. In the CF group, implants with a lower elastic modulus increased the stress on the screw. The TF group showed better stress distribution on the abutment and a lower stress value on the screw. The TF group demonstrated similar sensitivity for all components. DIC analysis revealed significant differences between TF-TiZr and CF-Ti in terms of the maximum (P < 0.001) and minimum principal strains (P < 0.05) on the implants and the minimum principal strains on the investment materials in both groups (P < 0.001).

CONCLUSIONS

Changes in the implant material significantly affected the maximum stress of the implant. The TF group exhibited better structural integrity and reliability.

Single missing molar with wide mesiodistal length restored using a single or double implant-supported crown: A self-controlled case report and 3D finite element analysis

PURPOSE

Based on a self-controlled case, this study evaluated the finite element analysis (FEA) results of a single missing molar with wide mesiodistal length (MDL) restored by a single or double implant-supported crown.

METHODS

A case of a missing bilateral mandibular first molar with wide MDL was restored using a single or double implant-supported crown. The implant survival and peri-implant bone were compared. FEA was conducted in coordination with the case using eight models with different MDLs (12, 13, 14, and 15 mm). Von Mises stress was calculated in the FEA to evaluate the biomechanical responses of the implants under increasing vertical and lateral loading, including the stress values of the implant, abutment, screw, crown, and cortical bone.

RESULTS

The restorations on the left and right sides supported by double implants have been used for 6 and 12 years, respectively, and so far have shown excellent osseointegration radiographically.The von Mises stress calculated in the FEA showed that when the MDL was >14 mm, both the bone and prosthetic components bore more stress in the single implant-supported strategy. The strength was 188.62-201.37 MPa and 201.85-215.9 MPa when the MDL was 14 mm and 15 mm, respectively, which significantly exceeded the allowable yield stress (180 MPa).

CONCLUSIONS

Compared with the single implant-supported crown, the double implant-supported crown reduced peri-implant bone stress and produced a more appropriate stress transfer model at the implant-bone interface when the MDL of the single missing molar was ≥14 mm.

Effects of the internal contact surfaces of dental implants on screw loosening: A 3-dimensional finite element analysis

STATEMENT OF PROBLEM

The effects of the internal contact surfaces of dental implants on screw loosening have yet to be investigated.

PURPOSE

The purpose of this 3-dimensional finite element analysis (FEA) study was to evaluate and compare the mechanical effects of the abutment implant angle (θ), the abutment screw head diameter (D), and the abutment screw length (L) on screw loosening.

MATERIAL AND METHODS

A total of 27 models presenting various mechanical scenarios were built by using combinations of 3 different θ (30 degrees, 45 degrees, and 60 degrees), D (2.65 mm, 2.75 mm, and 2.85 mm), and L (4 mm, 5 mm, and 6 mm). In FEA, a static test with a 200-N force inclined 30 degrees in the implant axial direction was applied to the upper surface of the abutment to evaluate and compare the maximum von Mises stresses of the implant components and the maximum total deformation in all models. In addition, modal analysis was applied to identify the natural frequencies in all models under free (unforced) vibration, and a Kruskal-Wallis statistical test (α=.05) was performed, followed by multiple pairwise comparisons by using the Dunn test.

RESULTS

The Kruskal-Wallis test found a significant influence of the θ on implant stress, total deformation, and natural frequency (P<.001). For example, increasing the θ from 30 degrees to 45 degrees and 60 degrees can considerably reduce the model's natural frequencies to 18% and 26%, respectively. Similarly, the test underscored the significant impact of the D on both abutment screw stress and abutment stress (P=.010 and P=.002, respectively). However, the L appeared to have no significant effect on any of the dependent variables (P>.05).

CONCLUSIONS

The θ and the D significantly influenced the stresses of dental implant components, total deformation, and natural frequency of the model, which may impact the mechanical stability of the screw joint. However, the L does not appear to affect these values significantly.

Effect of different implant angulations on the biomechanical performance of prosthetic screws in two implant-supported, screw-retained prostheses: A numerical and experimental study

STATEMENT OF PROBLEM: A mesiodistal angle frequently forms between 2 splinted implant-supported, screw-retained fixed dental prostheses (TIS-FDPs). Mechanical complications commonly occur in prosthetic screws. Studies regarding the effect of the degree of implant angulation on the biomechanical performance of prosthetic screws in TIS-FDPs are sparse.

PURPOSE

The purpose of this numerical and experimental study was to investigate the effects of different implant angulations on the biomechanical performance, including stress distribution, stability of the screw joint, and surface morphology change of the prosthetic screws in TIS-FDPs.

MATERIAL AND METHODS

TIS-FDPs were classified into 4 groups: 0, 10, 20, and 30 degrees based on the degree of mesiodistal angle between the long axes of the 2 implants. In the finite element analysis (FEA), 4 series of 3D models were constructed and loaded with simulated occlusal forces. The von Mises stresses and rotational angles of the prosthetic screws were then calculated. In the mechanical test, each group of 5 TIS-FDPs with 10 prosthetic screws was tested under 1 million loading cycles by using a universal testing machine. The removal torque values (RTVs) and the surface roughness of the prosthetic screws were measured after cyclic loading. The normality of the outcome variables was assessed by the Shapiro-Wilk test. Analysis of variance and the Kruskal-Wallis test were used for further analysis (α=.05).

RESULTS

The FEA results showed that the von Mises stresses of the prosthetic screws were concentrated in the first screw thread crest engaged with the abutment, and the maximum values of the threads and the rotation angles of the prosthetic screws increased in the 2-implant mesiodistal angulation from 0 to 30 degrees. The mechanical tests showed that the RTVs of the prosthetic screws in each group were not significantly different after 1 million loading cycles (P=.107). The surface roughness of the crest of the first 2 threads of the prosthetic screws in the 30-degree group changed significantly compared with those in the other groups.

CONCLUSIONS

When TIS-FDPs were delivered, larger angulations of the 2 splinted implants seemed to increase the stress concentrated on the crest of the first engaged thread and the rotation angles of the prosthetic screws. After 1 million loading cycles, significant surface adhesive wear was identified on the crest of the first 2 threads of the prosthetic screws in the 30-degree group compared with groups with a smaller angulation.

In Vitro Measurements of Shear-Mediated Platelet Adhesion Kinematics as Analyzed through Machine Learning

Platelet adhesion to blood vessel walls in shear flow is essential to initiating the blood coagulation cascade and prompting clot formation in vascular disease processes and prosthetic cardiovascular devices. Validation of predictive adhesion kinematics models at the single platelet level is difficult due to gaps in high resolution, dynamic morphological data or a mismatch between simulation and experimental parameters. Gel-filtered platelets were perfused at 30 dyne/cm² in von Willebrand Factor (vWF)-coated microchannels, with flipping platelets imaged at high spatial and temporal resolution. A semi-unsupervised learning system (SULS), consisting of a series of convolutional neural networks, was used to segment platelet geometry, which was compared with expert-analyzed images. Resulting time-dependent rotational angles were smoothed with wavelet-denoising and shifting techniques to characterize the rotational period and quantify flipping kinematics. We observed that flipping platelets do not follow the previously-modeled modified Jefferey orbit, but are characterized by a longer lift-off and shorter reattachment period. At the juncture of the two periods, rotational velocity approached 257.48 ± 13.31 rad/s. Our SULS approach accurately segmented large numbers of moving platelet images to identify distinct adhesive kinematic characteristics which may further validate the physical accuracy of individual platelet motion in multiscale models of shear-mediated thrombosis.