Internal Hexagon Research Articles

The Fracture Resistance Comparison between Titanium and Zirconia Implant Abutments with and without Ageing: Systematic Review and Meta-Analysis.

Implant abutments are essential components of implant prosthetic restorations. The golden standard for abutment material is titanium; however, due to its properties, the esthetic result can be compromised. The most popular esthetic material alternatives are one- and two-piece zirconia. The study aimed to answer the questions of whether zirconia abutments can be used interchangeably with titanium in both anterior and posterior regions and how aging of the abutment affects durability. For this study, an electronic search of MEDLINE (PubMed) and Scopus (Embase) was conducted. The PRISMA guidelines were followed, and a systematic review was registered with PROSPERO. The search revealed 4031 results, of which 17 studies were selected. The strongest material for abutments is titanium, closely followed by two-piece zirconia. One-piece zirconia abutments were the weakest. The cyclic loading above 1,000,000 cycles decreased the fracture resistance of the abutments. Differences in implant diameter, angulation, and restoration affected the fracture strength of all compared materials. The main mode of failure for titanium abutments was screw bending or screw fracture. One-piece zirconia most often presented catastrophic failure with internal hexagon fracture below the implant neck. Two-piece zirconia exhibits a combination of failure modes. Two-piece zirconia abutments may be suitable for use in the posterior region, given their comparable fracture resistance to titanium abutments. Despite the fact that one-piece zirconia is capable of withstanding forces that exceed those exerted during mastication, it is recommended that it be employed primarily in the anterior dentition due to its propensity for unfavorable failure modes.

Resistance to Torsion of Cement vs Screw-Retained Abutments Under a Tangential Load: A Pilot Study.

The aim is to compare the resistance to torsion between 2 implant systems with internal hexagon connection: one using screw-retained abutments (Titanium Fix) and the other using cementable abutments (ITEC) under a tangential load. An in vitro experimental study was carried out. Fourteen implants, 7 implants from each system, were included in this study. The implants were placed at a 45° angle into metal blocks to simulate their position in the maxilla. Then, implants were subjected to a resistance test on a CMT5L universal testing machine, and the maximum load was applied to each sample. The maximum force with which the torsion was achieved in each sample was analyzed. The data were tested using the Shapiro-Wilk test and showed normal distribution. Student t test was used to examine statistical significance between the two groups, and the P value was set at P < .05. There was a statistically significant difference between the two groups (P = .001). ITEC implants with a cementable abutment showed greater flexural strength compared to the Titanium Fix with a screwed abutment implant system. The cemented abutment showed more resistance to torsion against a tangential load compared with the screwed abutment.

OVERVIEW AND ANALYSIS OF EQUIPMENT FOR OIL PRODUCTION BY PRESSING AND GRANULATION

The article is devoted to the development of a feed granulator. Granular feed is considered as a promising product for animal husbandry due to their high nutritional value. The description of the feed granulator as the basis for the production of pellets from fodder crops is given. One of the key elements of the granulator is a matrix, which is a steel part with many holes through which the nutrient mass is passed under pressure during granulation. The matrices are divided into planar and annular. Granulators by design are also divided into roller, drum, disc, blade, vibrating and rotarycentrifugal. Granulators can also be domestic and industrial, which differ from each other in different overall dimensions and performance. To date, the production of granulators is carried out at some plants in Russia. Based on the message of the President of the Republic of Kazakhstan, it follows that Kazakhstan needs to establish its own production of equipment for processing raw materials of the agro-industrial complex, including feed pellets. In the course of our research, we have developed a feed granulator. The design of the developed granulator will reduce peak loads on the working bodies, increase reliability, and improve the feed mixture supply to the area of operation of the working bodies. The granulator consists of a base (housing), shafts, an unloading blade, a die, a washer, a screw with an internal hexagon, a spring lock washer, a nut, a screw with an internal hexagon, bearings, a lock washer, a prismatic key, a ring, a screw, a roller and a distribution blade. The developed feed granulator has a simple design and compact appearance. Its design will allow the production of granular feed of good quality and optimal consistency.

Престелген түйіршіктерді өндіру құрылғысын жетілдіру

The article is devoted to the development of a device for the production of pressed granules. Granular feed is considered as a promising product for animal husbandry due to their high nutritional value. The description of the feed granulator as the basis for the production of pellets from fodder crops is given. One of the key elements of the granulator is a matrix, which is a steel part with many holes through which the nutrient mass is passed under pressure during granulation. The matrices are divided into planar and annular. Granulators by design are also divided into roller, drum, disc, blade, vibrating and rotary-centrifugal. Granulators can also be domestic and industrial, which differ from each other in different overall dimensions and performance. To date, the production of granulators is carried out at some plants in Russia. Based on the message of the President of the Republic of Kazakhstan, it follows that Kazakhstan needs to establish its own production of equipment for processing raw materials of the agro-industrial complex, including feed pellets. In the course of our research, we have developed a feed granulator. The design of the developed granulator will reduce peak loads on the working bodies, increase reliability, and improve the feed mixture supply to the area of operation of the working bodies. The granulator consists of a base (housing), shafts, an unloading blade, a die, a washer, a screw with an internal hexagon, a spring lock washer, a nut, a screw with an internal hexagon, bearings, a lock washer, a prismatic key, a ring, a screw, a roller and a distribution blade. The developed feed granulator has a simple design and compact appearance. Its design will allow the production of granular feed of good quality and optimal consistency.

Método in vitro factible para evaluar la infiltración bacteriana en tres sistemas de conexión implante-pilar

Introduction: Microorganism infiltration through the im-plant-abutment interface causes oral health problems such as periimplantitis, leading to implant loss. Materials and Methods: A feasible new method to quantify the Streptococcus mutans (S. mutans) infiltration through the implant-abutment interface gap is introduced in the present work. Internal hexagon (IH; n = 10), external hexagon (EH; n = 10), Morse taper (MT; n = 10), and a control for each group (n = 1) were tested. Bacteria suspension was prepared at 1.5x108 CFU/mL (CFU: colony forming units), and the implants were individually submerged up to the connection level, allowing the bacteria to contact it. The abutment was removed, and bacteria count was performed. Results: The implant sets were tested under normal bacterial growth and early and late biofilm growth conditions. Colony-forming units per mL were obtained, and the results were compared among groups. Differences in bacterial count between the MT and EH (p&lt;0.001) and the MT and IH (p&lt;0.001) groups were significantly higher in the MT-type implant. There was a significant increment of bacterial infiltration in the MTs submitted to late biofilm growth conditions. EH and IH connections are more effective in preventing bacterial infiltration independent of the growth condition. Conclusions: The proposed methodology is feasible to evaluate the infiltration of microorganisms through the implant-abutment interface.

Patient-, implant- and prosthetic-related factors on peri-implant mucositis and bone loss.

Peri-implant diseases, including peri-implant mucositis (PIM) and peri-implantitis, are a chronic inflammatory disorder triggered by bacterial biofilm in susceptible hosts. Potential risk factors for peri-implant diseases include smoking, dental plaque accumulation, poor oral hygiene, genetics, and absence of peri-implant keratinized mucosa. This cohort study aimed to evaluate the influence of patient-, implant-, and prosthetic-related factors on PIM and peri-implant bone loss (PBL) around dental implants after 1 year of loading. A total of 54 subjects (22 males and 32 females) were included in the study. Peri-implant clinical parameters were assessed and standardized periapical radiographs of each dental implant were obtained 15 days after the definitive prosthesis installation (baseline) and at 3, 6, and 12 months of follow-up. A total of 173 implants were evaluated. PIM affected 44.8% of the implants and no significant association was found between the investigated parameters and PIM incidence, except for type of implant connection. A significantly higher incidence of PIM (80.0%) was observed for implants with internal hexagon connection type after 1 year of follow-up (p = 0.015). Moreover, a mean PBL of 0.35 ± 1.89 mm was observed and no dental implant was affected by peri-implantitis after 1 year of function. No specific influence of patient, implant, or prosthetic factors on PBL was observed. No association was found between the occurrence of PIM/PBL and the patient-, implant-, and prosthetic-related factors investigated in this cohort study, except for the type of dental-implant connection.

Biomechanical Analysis of Titanium Dental Implants in the All-on-4 Treatment with Different Implant-Abutment Connections: A Three-Dimensional Finite Element Study.

The type of implant-abutment connection is one of the factors influencing the distribution of occlusal forces. This study aims to investigate the biomechanical performance of the mandibular all-on-4 treatment with different implant-abutment connections. Two connection types with 30° abutments and 18-mm implant fixtures were chosen for the posterior implants of the all-on-4 assembly. For the external hexagon connection (EHC) group, the implants with 4 mm in diameter were used. For the internal hexagon connection (IHC) group, we selected implants with 4.3 mm in diameter. A vertical force of 190 N was applied to the cantilever region. The FEA results indicated that the most stressed region in the two groups was prosthetic screws, followed by multi-unit abutments (MUAs). The lowest values of von Mises stress were both observed on the bone. The peak stress value of the implant screw and implant fixture in the EHC group were 37.75% and 33.03% lower than the IHC group, respectively. For stress distribution patterns, the load force tended to be concentrated at locations where components were interconnected. The EHC and IHC are clinically durable under the tested loading conditions, but the prosthetic screws and MUAs can be the weak point on the posterior implant within the mandibular all-on-four assembly. The peak stress values of implant screw and implant fixture in the EHC groups were lower than the IHC group.

Effect of gingival height of a titanium base on the biomechanical behavior of 2-piece custom implant abutments: A 3-dimensional nonlinear finite element study

Statement of problemTitanium base (TiBase) abutments to restore an implant-supported single crown are available in different gingival heights, but information on the biomechanical effects of the gingival heights is lacking. PurposeThe purpose of this nonlinear finite element analysis study was to evaluate the effects of TiBase gingival heights on the biomechanical behavior of custom zirconia (CustomZir) abutments and TiBase, including von Mises stress and maximum and minimum principal stress. Material and methodsTiBases with different gingival heights (0.5 mm, 1 mm, 1.5 mm, and 2 mm) with internal hexagon Morse taper connections were simulated in 3-dimensional models. The simulations (ANSYS Workbench 2020; ANSYS Inc) included the OsseoSpeed EV implant (Ø5.4 mm) (AstraTech; Dentsply Sirona), restoration, and surrounding bone in the mandibular first molar region. An occlusal force of 200 N was applied with a 2-mm horizontal offset toward the buccal side and a 30-degree inclination from the vertical axis. ResultsHigh-stress concentration was observed in the uppermost internal connection area on the buccal side and the antirotational part of the titanium abutment on the lingual side in all models. CustomZir abutments with a shorter gingival height exhibited larger concentrated areas of volume average stress von Mises stress and higher magnitude of maximum and minimum principal stress compared with a taller gingival height. ConclusionsA TiBase abutment with a taller gingival height reduced the fracture risk of a CustomZir abutment without increasing any mechanical risk.

Marginal Bone Loss Compared in Internal and External Implant Connections: Retrospective Clinical Study at 6-Years Follow-Up.

The aim of this study was to assess and compare the marginal bone loss between two different categories of implants (Winsix, Biosafin, Ancona, Italy) having the same diameter and belonging to the Torque Type® (TT®) line, in the internal hexagon version (TTi, Group A) and in the external hexagon configuration (TTx, Group B). Patients with one or more straight implants (insertion parallel to the occlusal plane) in the molar and premolar regions in association with tooth extraction at least 4 months prior to implant insertion, who have a fixture diameter of 3.8 mm, who followed up for at least 6 years, and whose radiographic records were available were enrolled in this study. Depending on implant connections (externally or internally), the sample was divided into groups A and B. For externally connected implants (66), the marginal resorption was 1.1 ± 0.17 mm. The subgroup of single and bridge implants showed no statistically significant differences with a marginal bone resorption of 1.07 ± 0.15 mm and 1.1 ± 0.17 mm, respectively. Internally connected implants (69) showed an overall marginal resorption of 0.91 ± 0.17 mm, while the subgroup of single and bridge implants showed resorption of 0.90 ± 0.19 mm and 0.90 ± 0.17 mm, respectively, with no statistically significant differences. According to the obtained results, internally connected implants showed less marginal bone resorption than externally connected implants.

Microleakage and mechanical behavior of conical vs. internal hexagon implant-abutment connection under a cyclic load fatigue test.

The aim of the present investigation was to evaluate, by an in vitro simulation, the mechanical behavior of the conical vs. internal hexagon under cyclic load and the microleakage of the prosthetic connection of the fixture. A standardized cyclic loading was performed considering the implant with conical connection (diameter 4 mm - length 10 mm) (CS) and internal hexagon connection (diameter 4 mm - length 10 mm) (IH). The toluidine blue infiltration has been evaluated with the paper cone test. After a total of 5x104 loads, the screw has been removed and the abutment appears solid and stable to the implant fixture for CS, while the IH was unstable. There was no infiltration of the toluidine marker in the connection interfaces of CS implants, while the IH was positive to the paper cone test. The study data showed that the conical connection showed higher stability compared to the internal hexagon connection under the loading and it is able to prevent bacterial microleakage. This effectiveness should be considered for the long-term maintenance of the peri-implant soft and hard tissues around the fixture.

In Vitro Trueness and Precision of Intraoral Scanners in a Four-Implant Complete-Arch Model

(1) Background: New intraoral (IOS) and laboratory scanners appear in the market and their trueness and precision have not been compared. (2) Methods: Seven IOS and two laboratory scanners were used to scan a mandibular edentulous model with four parallel internal hexagon implant analogues and PEEK scan bodies. Digital models in Standard Tessellation Language (STL) were created. The master model with the scan bodies was scanned (×10) with a computerized numerical control 3D Coordinate Measuring Machine (CMM). The short (distances of adjacent scan posts) and long distances (distances of the scan posts with non-adjacent sites in the arch) among the centroids of the four analogues were calculated using CMM special software. Trueness (comparisons with the master model) and precision (intragroup comparisons) were statistically compared with ANOVA, chi-square and Tukey tests. (3) Results: Laboratory scanners had the best trueness and precision compared to all IOSs for long distances. Only iTero (Align Technologies Inc., Milpitas, CA, USA) had comparable trueness with one laboratory scanner in short and long distances. For short distances, CS3600 (Carestream Health, Inc., Rochester, NY, USA), Omnicam, Primescan (Sirona Dental Sys-tems GmbH, Bens-heim, Germany) and TRIOS 4 (3Shape A/S, Copen-hagen, Denmark) had similar trueness to one laboratory scanner. From those, only Omnicam and Primescan had similar precision as the same laboratory scanner. Most IOSs seem to work better for smaller distances and are less precise in cross-arch distances. (4) Conclusions: The laboratory scanners showed significantly higher trueness and precision than all IOSs tested for the long-distance group; for the short distance, some IOSs were not different in trueness and precision than the laboratory scanners.

Mechanical Strength and Energy Absorption Optimization of Biomimetic Honeycomb Anti-Collision Pier

The anti-collision pier plays an irreplaceable role in road traffic protection due to its significance. In this research, the biomimetic honeycomb structure was applied to internal anti-collision pier interior structures. The enhancement of mechanical strength and energy absorption characteristics was explored and optimized by five anti-collision pier honeycomb structures. Finite elements of the piers are designated as 650 mm in diameter and 850 mm in height. Polypropylene Acetate (PLA) material is utilized in this research due to its environment-friendly characteristics. Displacement loading in finite element simulation is 50 mm to the middle region of the model at YOZ direction. The energy-absorbing properties of five optimized honeycomb anti-collision piers at the same force position will be carefully compared. Moreover, the influence of internal hexagon direction-quantity configuration upon loading resistance under displacement loading is outlined. The results determined the best biomimetic structure to be three honeycomb shapes with a central triangle area, with maximum stress of 503.8 MPa and fracture displacement of 58.02 mm. Furthermore, the numerical simulation shows that the number of nest increases has a negative relationship with the effect upon force and deformation of the model. Moreover, the triangular central area is superior to the Y-shape central area in both mechanical strength and energy absorption performance.

Machine learning based screw drive state detection for unfastening screw connections

The electrification of the transport sector, limited primary materials, and the resulting need for a circular economy drives the automated disassembly of End of Life battery systems. Environmental influences and improper maintenance may cause stripped screw drives. Industrial screw drivers and nut runners evaluate unfastening operations in binary form as OK and NOK (Not OK). However, deeper evaluations such as the detection of stripped screw drives are necessary to decide between destructive and non-destructive disassembly and the resulting tool selection. This paper presents a novel supervised learning based approach for detecting stripped screw drives during the positive locking phase of unfastening operations by using intrinsic data (torque of the screw driver), where the bit or nut are inserted inside the screw drive. The basis of this work is an experimental data set with more than 1,000 unfastening operations from a manual and robot-based test bench, which includes four screw drive types: Torx, Phillips, external hexagon, and internal hexagon. A comparison between multilayer perceptrons (MLPs) and convolutional neural networks (CNNs) have been examined using the experimental data set. Additionally, the reliability and accuracy of the proposed approach have been examined. The findings from this paper demonstrate the feasibility of CNN-based detection for a revolution number lower than one, which is one revolution better than the state of the art techniques, a screw size and screw drive independent detection of stripped screw drives. The CNN model gives an accuracy of 0.98 to predict the screw drive state.

Stress analysis and factor of safety in three dental implant systems by finite element analysis

ObjectiveThe purpose of this study was to compare the stress distribution and the factor of safety of three dental implant systems using the finite element method. Materials and methodsThree commercial dental implant systems were designed using Solid Works 2020 software: Model A with an internal octagonal connection and matching platform, Model B with an internal hexagon connection and switching platform, and Model C with an internal 15° conical-cylindrical connection and switching platform. A 200 N load was applied to each design in both axial and 30° oblique directions using the finite element method. ResultsIn the three dental implant systems, the maximum von Mises stress was concentrated at the cervical level of the bone-implant interface in all models. Model C showed lower maximum stress values in both axial and 30° oblique loads. The highest maximum stress value was observed with the application of the oblique load in all the study models, and the factor of safety was less than one in Model A when subjected to a 200 N oblique load. ConclusionThe switching platform models generated lower maximum stress values and a factor of safety higher than one which is considered an acceptable value.Clinical relevance: A dental implant system with an internal hexagon or conical connection and a switching platform generates lower maximum von Mises stress values both on the implant components and on the peri-implant tissues.

Effects of Marginal Bone Loss Progression on Stress Distribution in Different Implant–Abutment Connections and Abutment Materials: A 3D Finite Element Analysis Study

Peri-implantitis is a common implant-supported prosthesis complication, and marginal bone loss affects the stress distribution in implant systems. This three-dimensional finite element analysis study investigated how bone loss affects the implant assembly; in particular, models including two implant systems with different connection systems (external or internal hexagon), abutment materials (titanium or zirconia), and bone loss levels (0, 1.5, 3, or 5 mm) were created. We observed that the maximum von Mises stress distinctly increased in the groups with bone loss over 1.5 mm compared to the group without bone loss, regardless of the connection system or abutment material used. Moreover, the screw stress patterns with bone loss progression were determined more by the connection systems than by the abutment materials, and the magnitude of the stress on the fixture was affected by the connection systems with a similar pattern. The highest stress on the screw with the external hexagon connection system increased over 25% when bone loss increased from 3 to 5 mm, exceeding the yield strength of the titanium alloy (Ti–6Al–4V) when 5 mm bone loss exists; clinically, this situation may result in screw loosening or fracture. The highest stress on the fixture, exceeding the yield strength of pure titanium, was noted with the internal hexagon connection system and 1.5 mm bone loss. Titanium and zirconia abutments—both of which are clinically durable—presented similar screw and fixture stress patterns. Therefore, clinicians should pay more attention to maintaining the peri-implant bone to achieve the long-term stability of the implant-supported prosthesis.

Settling effect of abutment and torque loss in different types of abutment after cyclic loading.

This study aimed to evaluate and compare the settling effect of implant-abutment assembly and the torque loss before and after cyclic loading in three types of abutments. Thirty internal hexagon fixtures were randomly divided into three groups (n=10). Group A used original abutments, group B used pre-machined cast abutments, and group C used compatible abutments. In addition, the abutment morse taper angle was measured using an image measuring instrument. Removal torque values (RTVs) were recorded using a digital torque meter before and after cyclic loading. All samples were tested in a universal testing machine with a vertical load between 0 and 250 N for 100 000 cycles of 10 Hz. The settling effect was measured after cyclic loading. Paired t test was performed for intragroup analysis of removal torque loss before and after cycling and one-way ANOVA. Subsequently, Tukey's honestly significant difference test was used for intergroup comparison (α=0.05). The paired t-test showed signi⁃ficant differences in the intragroup RTVs before and after cycling (P<0.001). ANOVA showed significant differences in the mean of removal torque loss after cycling (P=0.009), the abutment morse taper angle (P<0.001), and the settling values (P=0.01) among different groups. However, no significant difference was found between compatible abutments and pre-machined cast abutments. The screw removal torque was significantly reduced for all groups in this study after cyclic loading. Differences could be found in the internal accuracy of implant-abutment assembly among different groups. Within the limitations of this study, the results showed the original abutments exhibited lower percentages of torque reduction after cyclic loading than the casting abutments and the compatible abutments.

Bacterial Microleakage at the Implant-Abutment Interface: An In Vitro Study.

The objective of this study is to evaluate, in vitro, the microleakage of bacteria of 3 different implant connections for a period of 14 days. 60 dental implants (AoN) (n = 20) were distinguished into three groups, accordingly to the type of connection: External Hexagon (EH), Internal Hexagon (IH), and Cone Morse (CM) connection. All implants were inserted and fixed on sterile special vinyl support. Ten fixtures for each group were inoculated in the internal platform with 1.0 μL of Streptococcus oralis (SO) and the other ten with the same amount of Pseudomonas aeruginosa (PA). The penetration of bacterial suspension into the surrounding solution was determined by the observation of the turbidity of the broth. Five implants for each sub-group were randomly observed at SEM, to verify the correct fitting of the abutments. Considering the total of the samples analyzed, CM showed significantly lower bacterial contamination, with respect to IH. In particular, bacterial contamination was found in 45%, 55%, and 20% of EH, IH, and CM, respectively. Analyzing results for the type of inoculated bacteria, P. aeruginosa showed a higher ability to contaminate all the connections, with respect to S. oralis.

Evaluation of Implant Stability and Trephination Depth for Implant Removal-An In Vitro Study.

Malpositioned and broken implants are usually fully osseointegrated; hence, their removal, especially from the lower arch, can be very challenging. Implant removal techniques include reverse torque and trephination. Trephination is an invasive technique that can jeopardize vital structures, cause mandibular fatigue fractures, or lead to osteomyelitis. In this study, we aimed to assess the relationship between trephination depth and implant stability by recording implant stability quotient (ISQ) readings at varying trephination depths in vitro. Materials and methods: Forty-eight implants were inserted into dense synthetic polyurethane foam blocks as artificial bone. Primary implant stability was measured with a Penguin resonance frequency analysis (RFA) device. Implants of two designs with a diameter of 3.75 mm and a length of 13 or 8 mm were inserted. Twenty-four internal hexagon (IH) (Seven®) and twenty-four conical connection (CC) implants (C1®; MIS® Implants, Ltd., Misgav, Israel) were used. The primary implant stability was measured with the RFA device. Trephination was performed, and implant stability was recorded at depths of 0, 3, and 6 mm for the 8 mm implants and 0, 3, 6, 8, 10, and 11.5 mm for the 13 mm implants. Results: Linear regression revealed a significant relation between the trephination depth and the ISQ (F (1, 213) = 1113.192, p < 0.001, adjusted r2 = 0.839). The trephination depth significantly predicted the ISQ (β = −5.337, p < 0.001), and the ISQ decreased by −5.33 as the trephination depth increased by 1 mm. Conclusion: Implant stability reduction as measured using an RFA device during trephination may be a valuable guide to achieving safe reverse torque for implant removal. Further studies are needed to evaluate these data in clinical settings.

Survival and Mechanical Complications of Posterior Single Implant-Supported Restorations Using Prefabricated Titanium Abutments: A Medium- and Long-Term Retrospective Analysis with up to 10 Years Follow-up.

To evaluate the survival of implants and abutments and the incidence of mechanical complications of single posterior implant-supported restorations using prefabricated titanium abutments. This retrospective clinical study analyzed 172 Astra Tech OsseoSpeed internal hexagon implants (Dentsply Sirona) placed in 85 patients with a follow-up between January 2009 and January 2019. All implants were restored with prefabricated titanium abutments and cement-retained metal-ceramic crowns. The clinical outcomes recorded were implant and abutment survival rates and mechanical complications (abutment/implant fractures, screw loosening/fracture, decementation of the superstructure, veneer chipping/fractures) and were analyzed according to age, sex, implant length/diameter, bone graft, arch, implant position, parafunctional habit or dental status, and opposite arch. Kaplan-Meier survival analysis was used to determine whether the distribution of time to event/failure differed based on implant position (premolar or molar), implant diameter, or abutment angulation. During the observation period (mean: 108 months), implant and abutment cumulative survival rates were 97.7% and 98.3%, respectively, with no statistically significant differences between implant positions (molar/premolar), implant diameters (3.5 vs 4 mm), or abutment angles (straight vs 15 degrees). Of the 172 single posterior implant-supported restorations, 14 mechanical complications (8.2%) were recorded. In particular, 3 abutment fractures (1.7%), 2 screw loosenings (1.2%), 2 screw fractures (1.2%), 1 implant fracture (0.6%), 2 chipping/fractures of veneering materials (1.2%), and 4 decementations of the superstructure (2.3%) occurred. The single posterior implant-supported restorations using prefabricated titanium abutments remain a clinically acceptable treatment in terms of prosthetic procedure and cost-effectiveness.

A Hybrid Model Predictive Control for a Seven-Level Hybrid Multilevel Converter With Independent Low-Frequency and High-Frequency Stages

This article puts forward a hybrid model predictive control (H-MPC) that enables the decoupling of low- and high-frequency stages in a seven-level hybrid multilevel converter (7L-HMC). It first uses the sign pattern of the reference voltage vector to determine switching states of the low-frequency stage. Next, the reference vector is shifted to the internal hexagon of the original space vector diagram, which is thereby converted to a 120&#x00B0; oblique coordinate system, where the adjacent vectors can be rapidly selected. Then, the optimal current tracking is primarily safeguarded by the duty cycle optimization of the three adjacent vectors. Finally, an optimized symmetric switching sequence minimizing the dc voltage deviation over one control iteration is selected through the assessment of possible switching sequences that are subject to the chosen vectors with optimal duty cycles. The proposed H-MPC method can reduce both the current ripple and computational burden while achieving a constant switching frequency. Comprehensive simulation and experimental comparisons on an all-silicon-carbide 7L-HMC prototype verify the effectiveness of the proposed H-MPC.