Assessment of preload, remaining torque, and removal torque in abutment screws under different frictional conditions: A finite element analysis

Adding mechanobiological cell features to finite element analysis of an immediately loaded dental implant

Finite element analysis (FEA) has been used to analyze the behavior of dental materials, mainly in implantology. However, FEA is a mechanical analysis and few studies have tried to simulate the biological characteristics of the healing process of loaded implants. This study used the rule of mixtures to simulate the biological healing process of immediate implants in an alveolus socket and bone-implant junction interface through FEA. Three-dimensional geometric models of the structures were obtained, and material properties were derived from the literature. The rule of mixtures was used to simulate the healing periods-immediate and early loading, in which the concentration of each cell type, based on in vivo studies, influenced the final elastic moduli. A 100 N occlusal load was simulated in axial and oblique directions. The models were evaluated for maximum and minimum principal strains, and the bone overload was assessed through Frost's mechanostat. There was a higher strain concentration in the healing regions and cortical bone tissue near the cervical portion. The bone overload was higher in the immediate load condition. The method used in this study may help to simulate the biological healing process and could be useful to relate FEA results to clinical practice.

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.

Resistance to loosening of intentionally shortened screws used to solve the unsuccessful removal of fractured prosthetic screws

STATEMENT OF PROBLEM

Fractured prosthetic implant screws cannot be removed in all patients, ultimately leading to the removal of the implant. Whether an intentionally shortened prosthetic implant screw (SPIS) can provide adequate retention is unclear.

PURPOSE

The purpose of this in vitro study was to evaluate the resistance to loosening of SPISs engaging the remaining coronal internal threads as a possible solution to maintaining both implant and restoration.

MATERIAL AND METHODS

Fifty grade V titanium SPISs were used to tighten 50 titanium transepithelial abutments on implants to 30 Ncm. The specimens were distributed into 5 groups (n=10) according to the conditions under which the screws were secured to manufacturer-recommended preload: dry (D), moistened in saliva (AS), moistened in chlorhexidine (CLHX), wrapped in polytetrafluoroethylene tape (PTFE), and resin cemented (RE). All groups were subjected to a cyclic loading test (240 000 cycles). The reverse torque value (RTV) of the SPIS was registered twice: 24 hours after initial tightening (T1); and after retightening and the cyclic loading test (T2). The resultant RTV was compared with the 30-Ncm tightening torque to assess torque loosening. The Kruskal-Wallis and Mann-Whitney tests were used for the comparisons between groups and the Wilcoxon test for the intragroup comparisons (α=.05 with Bonferroni correction).

RESULTS

At T1, all groups found lower mean±standard deviation RTVs than the reference tightening torque (30 Ncm) (D 24.82±2.34 Ncm, AS 25.56±2.89 Ncm, PTFE 26.02±2.26 Ncm, CLHX 26.26±1.82 Ncm), except the resin-cemented group, which increased its RTV (RE 44.01±19.94 Ncm). At T2, all the groups found lower RTVs than the reference tightening torque, and the torque values at T1 (D 19.81±6.59 Ncm, CLHX 18.98±6.36 Ncm, AS 21.28±7.32 Ncm), with the exception of PTFE (24.07±3.41 Ncm) and RE (41.47±21.68 Ncm), where RTV was similar to that recorded at T1. At T1, significant differences were found among the groups (P=.024). At T2, after cyclic loading, the RE group found the highest RTV, reporting significant differences with the D and CHLX groups (P<.05) and statistically similar to the AS group (P=.068).

CONCLUSIONS

PTFE-wrapped screws found similar RTVs after the fatigue test than dry, moistened with saliva, and moistened with chlorhexidine screws. Resin-cemented shortened prosthetic implant screws were found to be the most resistant to loosening after cyclic loading.

Bacterial Detection, Deformation, and Torque Loss on Dental Implants with Different Tapered Connections Compared with External Hexagon Connection after Thermomechanical Cycling

The relationship between bacterial infiltration and internal conical Implant-Abutment Interfaces (IAIs) with different conicities still requires investigations that can offer valuable information in the clinical understanding of peri-implant health. The present study aimed to verify the bacterial infiltration of two internal conical connections with an angulation of 11.5° and 16° with the external hexagonal connection as a comparative after thermomechanical cycling using saliva as a contaminant. Test (n = 10) and control (n = 3) groups were set up. Evaluations were made on torque loss, Scanning Electron Microscopy (SEM), and Micro Computerized Tomography (MicroCT) after performing 2 × 10⁶ mechanical cycles (120 N) and 600 thermal cycles (5°-55° C) with 2 mm lateral displacement. The contents of the IAI were collected for microbiological analysis. There was a difference (p < 0.05) in torque loss of the groups tested; groups from the 16° IAI obtained a lower percentage of torque loss. All groups presented contamination and the analysis of the results shows that the microbiological profile of the IAI differs qualitatively from the profile found in the saliva used for contamination. The mechanical loading affects the microbiological profile found in the IAIs (p < 0.05). In conclusion, the IAI environment may favor a microbiological profile different from that of saliva and the thermocycling condition may alter the microbial profile found in the IAI.

An In Vitro Study of Fluid Contaminations Influences on Reverse Torque Values of Implant-Abutment Connections

Purpose

To examine the effects of fluid contamination on the reverse torque value (RTV) of abutment screws. 484 titanium fixtures were mounted into the stainless-steel holders.

Methods

11 groups (44 specimens in each group) of implants were mounted in acrylic resin. Ten groups of fixture screw holes were contaminated with chlorhexidine, saliva, blood, fluoride, or combination groups, and one group served as a control without contamination. To simulate the oral environment, samples were subjected to thermal cycling and cyclic loading.

Results

The RTV means were less than the initial torque in both control and contamination groups. The maximum RTV mean was observed in the fluoride group (26.00 ± 1.02 Ncm). In other groups, this rate for control, blood, saliva, and chlorhexidine groups were 18.00 ± 1.78 Ncm, 22.12 ± 1.56 Ncm, 21.56 ± 1.43 Ncm, and 21.89 ± 1.02 Ncm, respectively. In combination groups, the maximum RTV mean was observed in the saliva+CHX group (23.89 ± 1.92 Ncm). In other combination groups, this rate for the blood+CHX, blood+saliva, saliva+fluoride, fluoride+CHX, and fluoride+blood groups were 22.56 ± 1.73 Ncm, 22.00 ± 1.54 Ncm, 20.11 ± 1.58 Ncm, 23.51 ± 1.19 Ncm, 21.02 ± 1.38 Ncm, and 20.11 ± 1.58 Ncm, respectively. The RTV was statistically significant (p < 0.05) for the contamination groups (except saliva) and combination groups compared to the control group. There is no statistically significant difference (p > 0.05) between the reverse torque value mean of the blood and saliva groups and between that of the fluoride and chlorhexidine groups.

Conclusion

Implant-abutment specimens are suggested to be placed in a saliva environment and should be subjected to cyclic loading.

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

The study was aimed to investigate the prosthetic screw loosening of two splinted implants-supported, screw-retained (2-4-unit) fixed dental prostheses (TIS-FDPs) in posterior region and to explore the underlying mechanism. In the retrospective study, a study group of TIS-FDPs (n = 23) presenting prosthetic screw loosening and a control group of TIS-FDPs (n = 32) absent of prosthetic screw loosening during observation period were included. The prosthesis height (PH), inter-implant distance (ID) and cantilever distance (CD) of TIS-FDPs were measured and compared within two groups. In the finite element analysis (FEA) part, three serials of models presenting different clinical scenarios were constructed based on the abovementioned PH, ID and CD values respectively. In the clinical evaluation, the values of pH and CD in study group were statistically higher than those in control group, whereas the values of ID had no significant difference. In the FEA, the results indicated that there was no linear correlation between the increased ID values and the maximum von Mises stresses and the rotation angles. On the other hand, the increased PH and CD values would result in a strong linear growth of the maximum von Mises stresses and the rotation angles. Besides, it was found that the regression coefficients in PH model were all higher than those in ID and CD models. When TIS-FDPs were delivered in posterior region, the PH and the CD, rather than the ID, seemed to have a significant impact on the stress concentration of the prosthetic screws and the incident of prosthetic screws loosening.

Effects of tightening torque on screw stress and formation of implant-abutment microgaps: A finite element analysis

STATEMENT OF PROBLEM

The mechanical behavior of the conical connection implant with different torque levels requires evaluation.

PURPOSE

The purpose of the finite element analysis study was to investigate the impact of abutment screw torque on the formation of microgaps at the implant-to-abutment interface of a conical connection under oblique loading. This is important because it is thought that bacteria can invade the internal implant space through the abutment-implant microgaps, causing peri-implantitis.

MATERIAL AND METHODS

Three-dimensional finite element analyses of the conical implant-abutment connection were performed by using screw torques of 20 Ncm and 30 Ncm. Oblique loads from 10 N to 280 N were applied to the prosthesis placed on the implant. The maximum von Mises stress in the abutment screw, the microgap formation process, and the critical load for bridging the internal implant space were evaluated.

RESULTS

The stresses in the abutment screw under oblique loading had limited sensitivity to the screw torque. However, the residual stress in the screw with a 30-Ncm torque was 35% higher than that with a 20-Ncm torque in the absence of an external load. The area in contact at the implant-to-abutment interface decreased with increasing load for both torque values. The critical load for bridging the internal implant space was 160 N for a screw torque of 20 Ncm and 220 N for a screw torque of 30 Ncm. The maximum gap size was approximately 470 μm with all the loads.

CONCLUSIONS

Increasing the screw torque can reduce the formation of microgaps at the implant-to-abutment interface. However, this will result in higher mean stress in the abutment screw, which may reduce its fatigue life and consequently that of the prosthesis. Further research is needed to evaluate the relationship between the abutment screw torque and microleakage in implant-supported restorations.

A Novel Technique Using Polytetrafluoroethylene Tape to Solve Screw Loosening Complication in Implant-Supported Single Crowns

This study aimed to analyze a novel technique to make screws with greater untightening resistance and to solve screw loosening in implant-supported single crowns. Thirty grade IV titanium straight abutments were screwed onto 30 external hex implants using grade IV titanium screws (30 Ncm). They were exposed to cyclic loading (300,000 cycles, 200 N). Samples were divided into 4 groups (15 samples per group): new screws (SCREW group) (control), reused screws (rSCREW group), new screws wrapped with polytetrafluoroethylene (PTFE) tape (PTFE group), and reused screws wrapped with PTFE tape (rPTFE group). Reverse torque values (RTVs) were recorded with a digitally calibrated implant motor. Mean RTVs observed were 14.46 N (±1.10 N) for the control group, 14.42 N (±1.22 N) for the rSCREW group, 19.97 N (±1.16 N) for the PTFE group, and 19.13 N (±2.38 N) for the rPTFE group. Statistically significant differences were found between RTVs of both groups employing screws without PTFE tape (SCREW and rSCREW groups) compared with those using screws wrapped with PTFE tape (PTFE and rPTFE) (p < 0.001). These results suggest that wrapping the implant–abutment screw with PTFE tape may effectively lower the risk of loosening and even constitute a solution when this complication occurs in implant single crowns.

Does abutment preparation affect abutment screw joint stability? An in vitro study

STATEMENT OF PROBLEM

Whether preparation of an implant abutment decreases the stability of the abutment screw joint is unclear.

PURPOSE

The purpose of this in vitro study was to evaluate the effect of the abutment preparation in laboratory conditions on the reverse torque value (RTV) of the abutment screw.

MATERIAL AND METHODS

Sixty implants, implant analogs, and abutments obtained from 2 different manufacturers were used in the present study and divided into 2 groups (groups B and N) according to the manufacturer. Each group was then divided into 3 subgroups (n=10) according to the remaining wall thickness (control, 0.4 mm, 0.6 mm). All abutments were tightened to analogs with 15 Ncm, and standardized preparation was made with a custom-designed copy-milling system. All abutments were tightened to the implants with 30 Ncm, and then, the abutment screws were removed and RTVs were recorded. Subgroups were analyzed by using 1-way ANOVA (α=.05), and the independent-sample t test was used to compare groups. Relationships between categorical variables were analyzed by chi-square tests. One abutment screw from each subgroup was randomly selected and analyzed by using a scanning electron microscope.

RESULTS

A statistically significant difference was found between RTVs of different manufacturers (group B=28.2 ±1.2 Ncm, group N=25.9 ±1.4 Ncm; P<.005). In group N, there was a significant difference among subgroups, control (26.7 ±1.4 Ncm), and 0.6-mm wall thickness (24.8 ±0.8 Ncm) (P<.05). A statistically significant relationship was found between preparation and RTV for the NucleOSS only (P<.05) and between manufacturer and RTV (P<.001). Scanning electron microscopy evaluations identified different screw head and thread designs.

CONCLUSIONS

The abutment screw joint stability of the BioHorizons implant was not affected by abutment preparation.

Abutment screw loosening in implants: A literature review

This review was intended on major factors contributing to abutment screw loosening. A search of Pubmed and Google Scholar, as well as a manual search, was conducted. Publications and articles accepted for publication up to February 2020 were included. Out of 150 studies retrieved, a total of 57 were selected for this review. Dental implants are associated with a complexity of abutment screw loosening. Implantologists and prosthodontists should be aware of factors that contribute to this problem. In this review previously identified factors were collected, the consideration of which can help to reduce the frequency of abutment screw loosening.