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

Influence of toothbrushing methods on tightening torque with healing abutments of different lengths: An in vitro study

STATEMENT OF PROBLEM

Early tightening torque at the implant-healing abutment interface is a recognized concern in implant treatment. However, little is known regarding the effects of toothbrushing methods on the interface established between the implant and healing abutments of different heights.

PURPOSE

The purpose of this in vitro study was to evaluate the effect of different toothbrushing methods on tightening torque for healing abutments of different heights.

MATERIAL AND METHODS

A total of 60 implants (Bilimplant; Proimtech) were embedded in epoxy resin blocks. The following 6 test groups were established: healing abutments with manual toothbrush group (Oral B Pro Expert All in one; Oral B) and 4 mm height (MTB-4, (which served as the control); 6 mm height (MTB-6, which served as the control); sonic toothbrush group (Philips Sonicare 3100 Series; Philips Oral Healthcare) and 4 mm height (S-4); 6 mm height (S-6); oscillating/rotating toothbrush group (Oral B Smart 6 6000N; Oral B) and 4 mm height (OR-4) and 6 mm height (OR-6). Each specimen was tightened to 15 Ncm and brushed. The digital torque meter (Cap Torque Tester Series TT01; Mark10) was used to calculate the reverse torque values.

RESULTS

The highest torque loss value in the average torque losses in the test groups was 1.3 (OR-6); the lowest was 0.3 (S-4). While a significant difference was found between groups S-4 and S-6 (P=.018), no statistically significant difference was found among the other groups (P>.05).

CONCLUSIONS

Torque loss was greater in the healing abutment with a height of 6 mm and with the oscillatory and rotational brushing method.

Biomechanical Effects of Ti-Base Abutment Height on the Dental Implant System: A Finite Element Analysis

This study aims to analyse, using a finite element analysis, the effects of Ti-base abutment height on the distribution and magnitude of transferred load and the resulting bone microstrain in the bone-implant system. A three-dimensional bone model of the mandibular premolar section was created with an implant placed in a juxta-osseous position. Three prosthetic models were designed: a 1 mm-high titanium-base (Ti-base) abutment with an 8 mm-high cemented monolithic zirconia crown was designed for model A, a 2 mm-high Ti-base abutment with a 7 mm-high crown for model B, and a 3 mm-high abutment with a 6 mm-high crown for model C. A static load of 150 N was applied to the central fossa at a six-degree angle with respect to the axial axis of the implant to evaluate the magnitude and distribution of load transfer and microstrain. The results showed a trend towards a direct linear association between the increase in the height of the Ti-base abutments and the increase in the transferred stress and the resulting microstrain to both the prosthetic elements and the bone/implant system. An increase in transferred stress and deformation of all elements of the system, within physiological ranges, was observed as the size of the Ti-base abutment increased.

Dental Implant Abutment Screw Loss: Presentation of 10 Cases

Re-tightening the loosened dental implant abutment screw is an accepted procedure, however the evidence that such screw will hold sufficiently is weak. The purpose of this study was material analysis of lost dental implant abutment screws made of the TiAlV alloy from various manufacturers, which became lost due to unscrewing or damaged when checking if unscrewed; undamaged screws could be safely re-tightened. Among 13 failed screws retrieved from 10 cases, 10 screws were removed due to untightening and 3 were broken but without mechanical damage at the threads. Advanced corrosion was found on nine screws after 2 years of working time on all surfaces, also not mechanically loaded. Sediments observed especially in the thread area did not affect the corrosion process because of no pit densification around sediments. Pitting corrosion visible in all long-used screws raises the question of whether the screws should be replaced after a certain period during service, even if they are well-tightened. This requires further research on the influence of the degree of corrosion on the loss of the load-bearing ability of the screw.

An in vitro study using confocal laser scanning microscopy to evaluate the marginal misfits of different implant-supported frameworks

PURPOSE

This study aimed to develop and evaluate a simple, non-destructive method for assessing the misfit and passivity of implant-retained prostheses frameworks.

MATERIALS AND METHODS

To simulate the rehabilitation of a mandible posterior partially edentulous area using 3-unit screw-retained frameworks supported by two implants were fabricated and divided into the following five groups (n = 10 in each group): OP = one-piece framework cast in Co-Cr with the conventional method (control-group); Co-Cr frameworks sectioned and welded by laser (=LAS) or tungsten inert gas (=TIG); Co-Cr CAD-CAM = milled Co-Cr framework; Zir CAD-CAM = milled zirconia framework. The horizontal |X| and vertical |Y| misfits were measured using confocal laser scanning microscopy with one or both screws tightened. Data were analyzed by a two-way ANOVA with repeated measures and Bonferroni correction (α = 0.05).

RESULTS

The greatest |X| misfit was observed in the OP group with both screws tightened (290 µm) and one screw tightened (388 and 340 µm). The conventional casting groups sectioned and welded by laser or TIG had lower mean values (235.35 µm, both screws tightened; and 275 µm, one screw tightened) than the OP framework. However, these values still exceeded those of the milled Co-Cr and zirconia frameworks (190 and 216 µm with both screws tightened). Across all reading conditions, every framework subjected to testing consistently maintained vertical |Y| misfit levels below the threshold of 53 µm; however, the milled frameworks exhibited higher vertical misfits than the frameworks obtained by the conventional cast method.

CONCLUSIONS

The frameworks, whether cast and sectioned with laser welding or milled from Co-Cr, exhibit improved marginal misfit and enhanced passive fit when compared to other fabrication methods. Additionally, the use of confocal laser scanning microscopy is highly effective for passivity and misfit analysis.

The effect of abutment material stiffness on the mechanical behavior of dental implant assemblies: A 3D finite element study

PURPOSE

This study aimed to evaluate the stress distribution and microgap formation in implant assemblies with conical abutments made of different materials under an oblique load.

MATERIALS AND METHODS

The mechanical behavior of an implant assembly with a titanium abutment was analyzed and compared with that of an assembly with a Y-TZP abutment using finite element analysis (FEA). A torque of 20 Ncm was first applied to the abutment screw, followed by oblique loads of 10 N-280 N applied to the prosthesis placed on the implant. The maximum stress in the abutment screw, the microgap formation process, and the critical load for bridging the internal implant space were evaluated.

RESULTS

No significant difference in stress distribution between the two cases was observed, with the stresses being mainly concentrated at the top half of the screw (the predicted maximum von Mises stress was approximately 1200 MPa at 280 N). The area in contact at the implant-to-abutment interface decreased with increasing load for both abutments, with the critical load for bridging the internal implant space being roughly 140 N. The maximum gap size being was approximately 470 μm with either abutment.

CONCLUSION

There was no significant difference in the stress distribution or microgap formed between implant assemblies with titanium and Y-TZP abutments having an internal conical connection.

Mechanical comparison of milled fiber-reinforced resin composite and Co-Cr frameworks with different connector cross-sectional geometries: An in vitro study

This study compared the effect of using milled fiber-reinforced resin composite and Co-Cr (milled wax and lost-wax technique) frameworks for 4-unit implant-supported partial fixed dental prostheses; and also, evaluated the influence of the connector's cross-sectional geometries on the mechanical behavior. Three groups of milled fiber-reinforced resin composite (TRINIA) for 4-unit implant-supported frameworks (n = 10) with three connectors geometries (round, square, or trapezoid), and three groups of Co-Cr alloy frameworks manufactured by milled wax/lost wax and casting technique, were analyzed. The marginal adaptation was measured before cementation using an optical microscope. Then, the samples were cemented, thermomechanical cycled (load of 100 N/2 Hz, 10⁶ cycles; 5, 37, and 55 ᵒC, a total of 926 cycles at each one), and cementation and flexure strength (maximum force) analyzed. Analysis of stress distribution in framework veneered considering resin and ceramic properties for fiber-reinforced and Co-Cr frameworks, respectively, implant, and bone was by finite element analysis under three contact points (100 N) on the central region. ANOVA and Multiple paired test-t with Bonferroni adjustment (α = 0.05) were used for data analysis. Fiber-reinforced frameworks showed better vertical adaptation (mean ranged from 26.24 to 81.48 μm) compared to the Co-Cr frameworks (mean ranged from 64.11 to 98.12 μm), contrary to horizontal adaptation (respectively, means ranged from 281.94 to 305.38 μm; and from 150.70 to 174.82 μm). There were no failures during the thermomechanical test. Cementation strength showed three times higher for Co-Cr compared to fiber-reinforced framework, as well as flexural strength (P < .001). Regarding stress distribution, fiber-reinforced had a pattern of concentration in the implant-abutment complex. There were no significant differences in stress values or changes observed among the different connector geometries or framework materials. Trapezoid connector geometry had a worse performance for marginal adaptation, cementation (fiber-reinforced 132.41 N; Co-Cr 255.68 N) and flexural strength (fiber-reinforced 222.57 N; Co-Cr 614.27 N). Although the fiber-reinforced framework showed lower cementation and flexural strength, considering the stress distribution values and absence of failures in the thermomechanical cycling test, it can be considered for use as a framework for 4-unit implant-supported partial fixed dental prostheses in the posterior mandible. Besides, results suggest that trapezoid connectors mechanical behavior did not perform well compared to round or square geometries.

Effect of crown stiffness and prosthetic screw absence on the stress distribution in implant-supported restoration: A 3D finite element analysis

This in-silico investigation evaluated the mechanical impact of Morse tape implant-abutment interface and retention system (with and without screw) and restorative materials (composite block and monolithic zirconia) by means of a three-dimensional finite element analysis (3D-FEA). Four 3D models were designed for the lower first molar. A dental implant (4.5 × 10 mm B&B Dental Implant Company) was digitized (micro CT) and exported to computer-aided design (CAD) software. Non-uniform rational B-spline surfaces were reconstructed, generating a 3D volumetric model. Four different models were generated with the same Morse-type connection, but with a different locking system (with and without active screw) and a different crown material made of composite block and zirconia. The D2 bone type, which contains cortical and trabecular tissues, was designed using data from the database. The implants were juxtaposed inside the model after Boolean subtraction. Implant placement depth was simulated for the implant model precisely at crestal bone level. Each acquired model was then imported into the finite element analysis (FEA) software as STEP files. The Von Mises equivalent strains were calculated for the peri-implant bone and the Von Mises stress for the prosthetic structures. The highest strain values in bone tissue occurred in the peri-implant bone interface and were comparable in the four implant models (8.2918e-004-8.6622e-004 mm/mm). The stress peak in the zirconia crown (64.4 MPa) was higher than in the composite crown (52.2 MPa) regardless of the presence of the prosthetic screw. The abutment showed the lowest stress peaks (99.71-92.28 MPa) when the screw was present (126.63-114.25 MPa). Based on this linear analysis, it is suggested that the absence of prosthetic screw increases the stress inside the abutment and implant, without effect on the crown and around the bone tissue. Stiffer crowns concentrate more stress on its structure, reducing the amount of stress on the abutment.

Accuracy of mechanical torque-limiting devices for implant screw tightening: A systematic review and meta-analysis

STATEMENT OF PROBLEM

A consensus is lacking on the accuracy of torque value on different types of mechanical torque-limiting devices.

PURPOSE

The purpose of this systematic review and meta-analysis was to determine the accuracy of unused mechanical torque-limiting devices.

MATERIAL AND METHODS

Electronic searches were conducted until October 2021 in 6 electronic databases. Relevant articles were manually screened in 5 journals from January 2000 to October 2021. Two reviewers screened titles, abstracts, and full texts and extracted the data independently. A meta-analysis was conducted to evaluate the weighted mean difference in torque value deviation from target torque between spring-style and friction-style devices as a primary outcome. Deviations of output torque value from target value in spring-style or friction-style devices were also analyzed as a secondary outcome.

RESULTS

A total of 11 595 articles were identified, and 16 articles were included for final statistical analysis. Meta-analysis of the included articles showed that torque value deviation in the spring-style was significantly lower than in the friction-style devices (-0.99 Ncm, 95% confidence interval [CI]: [-1.89, -0.09], P=.030). Deviations of output torque value from target value was -0.54 Ncm in the spring-style group (CI: [-1.23, 0.15], P=.122) and -0.18 Ncm in the friction-style group (95% CI: [-1.40, 1.04], P=.770). Meta-regression analysis indicated that target value was significantly associated with the mean deviation from target value only in the spring-style group. High heterogeneity was found, suggesting more studies with standardized research design are required.

CONCLUSIONS

Both spring- and friction-style mechanical torque-limiting devices can produce relatively accurate torque values; however, unused spring-type devices tend to have lower deviation from target torque value than unused friction-type devices.

The role of cortical zone level and prosthetic platform angle in dental implant mechanical response: A 3D finite element analysis

OBJECTIVE

The aim of this study was to evaluate the influence of three different dental implant neck geometries, under a combined compressive/shear load using finite element analysis (FEA). The implant neck was positioned in D2 quality bone at the crestal level or 2 mm below.

METHODS

One dental implant (4.2 × 9 mm) was digitized by reverse engineering techniques using micro CT and imported into Computer Aided Design (CAD) software. Non-uniform rational B-spline surfaces were reconstructed, generating a 3D volumetric model similar to the digitized implant. Three different models were generated with different implant neck configurations, namely 0°, 10° and 20°. D2 quality bone, composed of cortical and trabecular structure, was modeled using data from CT scans. The implants were included in the bone model using a Boolean operation. Two different fixture insertion depths were simulated for each implant: 2 mm below the crestal bone and exactly at the level of the crestal bone. The obtained models were imported to FEA software in STEP format. Von Mises equivalent strains were analyzed for the peri-implant D2 bone type, considering the magnitude and volume of the affected surrounding cortical and trabecular bone. The highest strain values in both cortical and trabecular tissue at the peri-implant bone interface were extracted and compared.

RESULTS

All implant models were able to distribute the load at the bone-implant contact (BIC) with a similar strain pattern between the models. At the cervical region, however, differences were observed: the models with 10° and 20° implant neck configurations (Model B and C), showed a lower strain magnitude when compared to the straight neck (Model A). These values were significantly lower when the implants were situated at crestal bone levels. In the apical area, no differences in strain values were observed.

SIGNIFICANCE

The implant neck configuration influenced the strain distribution and magnitude in the cortical bone and cancellous bone tissues. To reduce the strain values and improve the load dissipation in the bone tissue, implants with 10° and 20 neck configuration should be preferred instead of straight implant platforms.