Biomechanical analysis of alveolar bones with compromised quality supporting a 4-unit implant bridge; a possible association with implant-related sequestration (IRS).

OBJECTIVES: This study aimed to investigate the strain in the bone surrounding dental implants supporting a 4-unit bridge and assess the role of excessive strain as a possible risk factor for implant related sequestration (IRS) or peri-implant medication-related osteonecrosis of the jaw (PI-MRONJ). MATERIALS AND METHODS: A 3D-mandibular model was constructed using computed tomography and segmented it into cortical and cancellous bones. The 4-unit implant-supported bridges replacing the mandibular posteriors were constructed, and each featuring two, three, and four implants, respectively. The Young's modulus was assigned based on the quality of the bone. A maximum occlusal force of 200 N was applied to each implant in the axial and in a 30-degree oblique direction. RESULTS: The maximum principal strain of the fatigue failure range (> 3000 µÎµ) in the bone was analyzed. The volume fraction of fatigue failure was higher in poor-quality bone compared to normal bone and oblique load than in axial load. An increasing number of implants may dissipate excessive strain in poor-quality bones. CONCLUSIONS: Occlusal force applied to poor-quality bone can result in microdamage. Given that unrepaired microdamage may initiate medication-related osteonecrosis of the jaw, long-term occlusal force on fragile bones might be a risk factor. CLINICAL RELEVANCE: When planning implant treatment for patients with compromised bone status, clinical modifications such as strategic placement of implants and optimization of restoration morphology should be considered to reduce excessive strain which might be associated with IRS or PI-MRONJ.

Automated vector analysis to design implant-supported prostheses: A dental technique.

The prosthesis loading force is an important factor for dental implant survival. Even if adequate osseointegration of the dental implant has been achieved, if the occlusal forces are not correctly distributed, lateral torque can be generated causing high stress on surrounding tissues. The stress value of implant prostheses could be different whether the direction of load is vertical or oblique, affected by the shape of the occlusal surface. When an implant-supported prosthesis is designed with a dental computer-aided design software program, the average vectors from each occlusal contact point can be visualized. If the visualized vector generates lateral torque, the occlusal surface design can be modified before finalizing the design. The described technique uses automated vector analysis to enable visualization of the occlusal vector to improve prosthesis design, optimizing occlusal forces.

Deep Neural Network for Navigation of a Single-Element Transducer During Transcranial Focused Ultrasound Therapy: Proof of Concept.

Transcranial focused ultrasound (tFUS) has gained attention in the field of brain stimulation owing to its non-invasive neurotherapeutic potentials. However, complex interactions between acoustic waves and the cranium may introduce misalignment of the acoustic focus from a geometric target location, thus, necessitate on-site feedback of real-time navigational information of the transducer (spatial coordinates and angular orientation) for the operators to accurately place the acoustic focus to the desired brain area. In this study, we propose a deep-learning-based network model that can provide spatial navigational information of a single-element FUS transducer with respect to the targeted brain region. The training dataset were acquired through forward simulations that reflect the different tFUS transmissions for each skull structure using cranial computed tomography (CT) image data. The performance of the network was evaluated through three ex vivo calvaria. As a result show that the presented neural network-based method can an accurately navigate the FUS transducer with the conformity of ∼99.59% in placement of the transducer and ∼74.49% in the focal volume and an average difference of ∼0.96 mm in the focal point, all capable of real-time operation (∼10 ms).

Effects of assessing the bone remodeling process in biomechanical finite element stability evaluations of dental implants.

BACKGROUND AND OBJECTIVE: While an accurate assessment of the biomechanical stability of implants is essential in dental prosthesis planning and associated treatment assurance, the bone remodeling process is often ignored in biomechanical studies using finite element (FE) analysis. In this study, we aimed to analyze the significance of assessing the bone remodeling process in FE analysis for evaluating the biomechanical stability of dental implants. We compared the FE results considering the bone remodeling process with FE results simulated using commonly used conditions, with no considerations of the bone remodeling process. METHODS: The mathematical model proposed by Komarova et al. was used to calculate cell population dynamics and changes in bone density at a discrete site. The model was implemented in the FE software ABAQUS, using the UMAT subroutine. Three-dimensional FE models were constructed for two types of bone (III and IV) and three values of implant diameter (4.0, 4.5, and 5.0 mm). An average biting force of 50 N in the vertical direction was applied during the bone remodeling process for 150 days. Afterwards, the maximum biting force of 200 N in the 30° oblique direction was applied to evaluate the stability of the implant systems. RESULTS: To understand the impact of bone remodeling on the resultant mechanical responses, we focused on peri-implant cancellous bone based on two parameters: apparent density change and microstrain distribution. The bone density decreased by an average of 5.3 % after implantation, and it was the lowest on the 6th day. The average density increases of the peri-implant cancellous bone were 264.4 kgm3 (bone type III) and 220.0 kgm3 (bone type IV) over 150 days. For the bone stability analysis, the maximum principal strain in the peri-implant bone was used to evaluate the bone stability. If the bone remodeling process is ignored, then the bone volume within the fatigue failure range of the microstrain differs significantly from that if the bone remodeling process is considered, i.e., 60 % higher for bone type III and 33.4 % lower for bone type IV than when the bone remodeling process is considered. CONCLUSIONS: The FE result without considering the bone remodeling process could be considered a conservative criterion for bone type III. However, in bone type IV, the FE result without considering the bone remodeling process tends to underestimate the risks. The bone remodeling process is more affected by the initial bone quality than the implant diameter.

Effects of implant diameter, implant-abutment connection type, and bone density on the biomechanical stability of implant components and bone: A finite element analysis study.

STATEMENT OF PROBLEM: Various kinds of implants of different diameters and connection types are used for patients with a range of bone densities and tooth sizes. However, comprehensive studies simultaneously analyzing the biomechanical effects of different diameters, connection types, and bone densities are scarce. PURPOSE: The purpose of this 3-dimensional finite element analysis study was to evaluate the stress and strain distribution on implants, abutments, and surrounding bones depending on different diameters, connection types, and bone densities. MATERIAL AND METHODS: Twelve 3-dimensional models of the implant, restoration, and surrounding bone were simulated in the mandibular first molar region, including 2 bone densities (low, high), 2 implant-abutment connection types (internal tissue level, internal bone level), and 3 implant diameters (3.5 mm, 4.0 mm, and 4.5 mm). The occlusal force was 200 N axially and 100 N obliquely. Statistical analysis was performed using the general linear model univariate procedure with partial eta squared (ηp2) (α=.05). RESULTS: For bone tissue, low-density bone induced a larger maximum and minimum principal strain (in magnitude) than high-density bone (P<.001). As the implant diameter increased, the volume of the cancellous bone in low-density bone at the atrophy region (strain<200 µÎµ) increased (P<.001). For implant and abutment, the internal bone-level connection type was associated with increased peak stress as compared with the tissue-level connection type (P<.001). For all models, the stress distribution on the implant complex was influenced by implant diameter (P<.001): a decrease in implant diameter increased the stress concentration. CONCLUSIONS: The implant connection type had a greater impact on the stress of the implant and abutment than the diameter. A tissue-level connection was more advantageous than a bone-level connection in terms of stress distribution of the implant and abutment. Bone density was the most influential factor on bone strain. The selection of dental implants should be made considering these factors and other important factors including tooth size.

Biomechanical effects of dental implant diameter, connection type, and bone density on microgap formation and fatigue failure: A finite element analysis.

BACKGROUND AND OBJECTIVE: Understanding fatigue failure and microgap formation in dental implants, abutments, and screws under various clinical circumstances is clinically meaningful. In this study, these aspects were evaluated based on implant diameter, connection type, and bone density. METHODS: Twelve three-dimensional finite element models were constructed by combining two bone densities (low and high), two connection types (bone and tissue levels), and three implant diameters (3.5, 4.0, and 4.5 mm). Each model was composed of cortical and cancellous bone tissues, the nerve canal, and the implant complex. After the screw was preloaded, vertical (100 N) and oblique (200 N) loadings were applied. The relative displacements at the interfaces between implant, abutment, and screw were analyzed. The fatigue lives of the titanium alloy (Ti-6Al-4V) components were calculated through repetitive mastication simulations. Mann-Whitney U and Kruskal-Wallis one-way tests were performed on the 50 highest displacement values of each model. RESULTS: At the implant/abutment interface, large microgaps were observed under oblique loading in the buccal direction. At the abutment/screw interface, microgap formation increased along the implant diameter under vertical loading but decreased under oblique loading (p < 0.001); the largest microgap formation occurred in the lingual direction. In all cases, the bone-level connection induced larger microgap formation than the tissue-level connections. Moreover, only the bone-level connection models showed fatigue failure, and the minimum fatigue life was observed for the implant diameter of 3.5 mm. CONCLUSIONS: Tissue-level implants possess biomechanical advantages compared to bone-level ones. Two-piece implants with diameters below 3.5 mm should be avoided in the posterior mandibular area.

Biomechanical stress and microgap analysis of bone-level and tissue-level implant abutment structure according to the five different directions of occlusal loads.

PURPOSE: The stress distribution and microgap formation on an implant abutment structure was evaluated to determine the relationship between the direction of the load and the stress value. MATERIALS AND METHODS: Two types of three-dimensional models for the mandibular first molar were designed: bone-level implant and tissue-level implant. Each group consisted of an implant, surrounding bone, abutment, screw, and crown. Static finite element analysis was simulated through 200 N of occlusal load and preload at five different load directions: 0, 15, 30, 45, and 60°. The von Mises stress of the abutment and implant was evaluated. Microgap formation on the implant-abutment interface was also analyzed. RESULTS: The stress values in the implant were as follows: 525, 322, 561, 778, and 1150 MPa in a bone level implant, and 254, 182, 259, 364, and 436 MPa in a tissue level implant at a load direction of 0, 15, 30, 45, and 60°, respectively. For microgap formation between the implant and abutment interface, three to seven-micron gaps were observed in the bone level implant under a load at 45 and 60°. In contrast, a three-micron gap was observed in the tissue level implant under a load at only 60°. CONCLUSION: The mean stress of bone-level implant showed 2.2 times higher than that of tissue-level implant. When considering the loading point of occlusal surface and the direction of load, higher stress was noted when the vector was from the center of rotation in the implant prostheses.

Three-Dimensional Evaluation of Peri-implant Soft Tissue When Tapered Implants Are Placed: Pilot Study with Implants Placed Immediately or Early Following Tooth Extraction.

PURPOSE: This study examined a new 3D volumetric analysis method for the assessment of baseline-to-12-month changes of the soft tissue volume at early and immediately placed tapered implants after loading with ceramic single crowns. MATERIALS AND METHODS: Eligible patients with one incisor, canine, or premolar to be extracted were included. The patients were divided randomly into early-placement or immediate-placement groups. Tapered implants (BLT, Institut Straumann) were placed after the extractions. In the early-placement group, the implants were placed 8 weeks after extraction. In the immediate-placement group, the implants were placed immediately after the extraction. All implants healed transmucosally, and the final crowns were inserted after healing (baseline). Impressions were made at screening, baseline, and 12 months after crown insertion (Permadyne, 3M). The casts were scanned (Imetric 4D) and aligned, and a superimposed area of interest (AOI) (labial/buccal aspects) was defined to assess the volumetric changes (GOM Inspect). Specific software (3Matic, Materialise NV) was used for volumetric analysis. The vertical mucosal recession was measured at each time point. Repeated-measures one-way analysis of variance and the Tukey method were used for statistical analysis (SPSS 22, IBM). RESULTS: Twenty tapered implants (16 regular and four narrow) were placed in 20 patients (12 men and 8 women) in the early-placement (EP; n = 10) and immediate-placement (IP; n = 10) groups, respectively. Threedimensional volumetric analysis revealed soft tissue volume loss in both groups of 10.0 ± 16.5 mm3 (EP) and 24.3 ± 21.3 mm3 (IP) between baseline and 12 months (P = .6). The analysis also revealed local differences in the changes, displaying both localized gain and loss in both groups. CONCLUSION: With this novel 3D analysis method, true volumetric soft tissue differences, ie, both localized gain and loss, were specified between the treatment groups.

Designing a mandibular advancement device with topology optimization for a partially edentulous patient.

STATEMENT OF PROBLEM: Patients with partial tooth loss treated with implant-supported fixed partial dentures (FPDs) have difficulty using conventional mandibular advancement devices (MADs) because of the risk of side effects. Also, which design factors affect biomechanical stability when designing MADs with better stability is unclear. PURPOSE: The purpose of this finite element (FE) analysis study was to analyze the effect of the MAD design on biomechanical behavior and to propose a new design process for improving the stability of MADs. MATERIAL AND METHODS: Each 3D model consisted of the maxillofacial bones, teeth, and implant-supported FPDs located in the left tooth loss area from the first premolar to the second molar and a MAD. Three types of custom-made MADs were considered: a complete-coverage MAD covering natural tooth-like conventional MADs, a shortened MAD excluding the coverage on the implant-supported FPD, and a newly designed MAD without anterior coverage. For the new MAD design, topology optimization was conducted to reduce the stress exerted on the teeth and to improve retention of the MAD. The new MAD design was finished by excluding the coverage of the maxillary and mandibular central incisors based on the results of the topology optimization. A mandibular posterior restorative force for a protrusion amount of 40% was used as the loading condition. The principal stress and pressure of the cancellous bone and periodontal ligaments (PDLs) were identified. RESULTS: Considering the load concentration induced by the complete-coverage MAD, bone resorption risk and root resorption risk were observed at both ends of the mandibular teeth. The shortened MAD resulted in the highest stress concentration and pressure with the worst stability. However, in the case of the complete-coverage MAD, the pressure in the PDLs was reduced to the normal range, and the risk of root resorption was reduced. CONCLUSIONS: For patients with implant-supported FPDs, MAD designs with different extents of coverage had an influence on biomechanical behavior in terms of stress distribution in cancellous bone and PDLs. A MAD design without anterior coverage provided improved stability compared with complete-coverage or shortened designs. The presented method for MAD design, which combined FE analysis and topology optimization, could be effectively applied in the design of such improved MADs.

Biomechanical analysis of 4 types of short dental implants in a resorbed mandible.

STATEMENT OF PROBLEM: Short implants have been increasingly used in the aging society. However, studies which explain the difference of stress distribution according to different connections in short implant treatment are scarce. PURPOSE: The purpose of this finite element (FE) analysis was to evaluate the stress and strain distribution of short implants and surrounding bone under static and cyclic loading conditions with 4 different connections. MATERIAL AND METHODS: Three-dimensional models of 4 types of implant systems were considered: internal tissue level, internal tissue level wide, internal bone level (IB), and external bone level. Each system had different types of abutment, implant, and screw with the resorbed mandibular segment of the bone block. Static FE analysis was performed under external loads of 200 N (vertical or 30-degree oblique) to each cusp tip. The strain distributions of the peri-implant bone and von Mises stress fields in the abutment, implant, and screw were evaluated. Based on the static FE results, a computational fatigue analysis was performed to predict the risk of fracture caused by fatigue accumulation of repetitive mastication. RESULTS: Bone tissues in fatigue failure level (greater than 4000 µÎµ) were observed in the alveolar ridge and the plateaus close to the implant apex in all situations. Under the oblique loading condition, the total volume of the bone tissue in hypertrophy and fatigue failure levels (greater than 2500 µÎµ) was the largest at IB and the smallest at external bone level. Among the 4 situations, the highest stress occurred in the abutment (506.9 MPa) and implant (311 MPa) of IB. In fatigue analysis, fracture was only predicted in the IB abutment model (588 301 cycles), and cracking occurred in the lingual direction, where stress concentration occurred when the oblique load was applied. CONCLUSIONS: The abutment of IB showed the highest stress of the implant component, and internal tissue level model showed the highest strain of bone. In all groups, the bone strain values mostly appeared within physiologic capacity (under 4000 µÎµ). Various mechanical situations should be considered when using internal bone-level connections in short implants for replacing posterior teeth.

Effects of cementless fixation of implant prosthesis: A finite element study.

PURPOSE: A novel retentive type of implant prosthesis that does not require the use of cement or screw holes has been introduced; however, there are few reports examining the biomechanical aspects of this novel implant. This study aimed to evaluate the biomechanical features of cementless fixation (CLF) implant prostheses. MATERIALS AND METHODS: The test groups of three variations of CLF implant prostheses and a control group of conventional cement-retained (CR) prosthesis were designed three-dimensionally for finite element analysis. The test groups were divided according to the abutment shape and the relining strategy on the inner surface of the implant crown as follows; resin-air hole-full (RAF), resin-air hole (RA), and resin-no air hole (RNA). The von Mises stress and principal stress were used to evaluate the stress values and distributions of the implant components. Contact open values were calculated to analyze the gap formation of the contact surfaces at the abutment-resin and abutment-implant interfaces. The micro-strain values were evaluated for the surrounding bone. RESULTS: Values reflecting the maximum stress on the abutment were as follows (in MPa): RAF, 25.6; RA, 23.4; RNA, 20.0; and CR, 15.8. The value of gap formation was measured from 0.88 to 1.19 µm at the abutmentresin interface and 24.4 to 24.7 µm at the abutment-implant interface. The strain distribution was similar in all cases. CONCLUSION: CLF had no disadvantages in terms of the biomechanical features compared with conventional CR implant prosthesis and could be successfully applied for implant prosthesis.

Biomechanical effect of mandibular advancement device with different protrusion positions for treatment of obstructive sleep apnoea on tooth and facial bone: A finite element study.

BACKGROUND: The mandibular advancement device (MAD) is widely used for obstructive sleep apnoea (OSA) treatment, and several studies have demonstrated its effectiveness. However, no comprehensive studies have yet examined the biomechanical safety of the MAD. OBJECTIVES: The objective of this study was to analyse the biomechanical effect of different protrusion positions of a MAD on the teeth and facial bones. METHODS: The posterior restorative forces due to the stretched mandibular muscles were measured by pressure sensors attached to the experimental mandibular advancement device for mandibular protrusions of 10-70% of the maximum protrusion of the subject. A detailed three-dimensional biomechanical model of the study subject, constructed from computed tomography scans, was used in finite element analysis, with loading conditions calculated from the measured posterior restorative forces. The outcome measures were the principal stresses on the periodontal ligaments (PDL) and cancellous bone, and the pressure at the PDL surfaces. The measurements were used to analyse the risk of the tooth movement, tooth root resorption, and bone resorption. RESULTS: The lowest and highest restorative forces occurred at 40% and 70% of maximum protrusion, respectively. The highest risk of tooth movement occurred at the mandibular molar teeth. The mandibular second molar teeth had the highest risks of root and bone resorption. CONCLUSIONS: Mandibular advancement at 70% of maximum protrusion induces risks of tooth root resorption and bone resorption. The mandibular second molars were subjected to the highest stresses. Stress on the teeth and facial bones was the lowest at 40% of maximum mandibular advancement.

Which plate results in better stability after segmental mandibular resection and fibula free flap reconstruction? Biomechanical analysis.

OBJECTIVE: This study investigated the biomechanical stability of 2 plate systems-mini-plates and reconstruction plates-in reconstruction with fibular free flaps. STUDY DESIGN: The reconstruction models were constructed by using 2 types of plates in representative cases with segmental mandibular defect (C, L, LC1, LC2). In each model, a masticatory simulation approximating 3 clenching tasks was conducted, using the muscle forces adjusted to the mandible structure used in this study. In addition, to evaluate the sensitivity of the 2 plate systems for masticatory load changes, a sensitivity analysis was also performed by using finite element analysis. RESULTS: The risks of plate fracture and screw loosening measured by stress concentrations were higher in the cases using mini-plates compared with those using reconstruction plates. Moreover, the mini-plate was more sensitive to varied loads compared with the reconstruction plate and was observed to have less flexibility to absorb external forces. Mini-plates also caused high strain values, indicating the risk of hypertrophy of bone around the screw holes. CONCLUSIONS: The use of a reconstruction plate should result in more stable surgical outcomes in most cases, but we noted that the risk of atrophy may increase with the use of reconstruction plates because of lack of bone stimulation.

Effects of moderate intensity static magnetic fields on osteoclastic differentiation in mouse bone marrow cells.

Although we recently demonstrated that static magnetic fields (SMFs) of 3, 15, and 50 mT stimulate osteoblastic differentiation, the effects of SMFs on osteoclastogenesis are still poorly understood. This study focused on the suppressive effects of SMFs on receptor activator of nuclear factor κB ligand (RANKL)-induced osteoclastogenesis and bone resorption. Direct SMFs inhibit RANKL-induced multinucleated osteoclast formation, tartrate-resistant acid phosphatase activity, and bone resorption in mouse bone marrow-derived macrophage cells. The conditioned medium from osteoblasts treated with SMFs also resulted in the inhibition of osteoclast differentiation as well as resorption. The RANKL-induced expression of osteoclast-specific transcription factors, such as c-Fos and NFATc1, was remarkably downregulated by SMF at 15 mT. In addition, SMF inhibited RANKL-activated Akt, glycogen synthase kinase 3ß (GSK3ß), extracellular signal-regulated kinase, c-jun N-terminal protein kinase, mitogen-activated protein kinase (MAPK), and nuclear factor-κB (NF-κB) formation. These findings indicate that SMF-mediated attenuation of RANKL-induced Akt, GSK3ß, MAPK, and NF-κB pathways could contribute to the direct and indirect inhibition of osteoclast formation and bone resorption. Therefore, SMFs could be developed as a therapeutic agent against periprosthetic or peri-implant osteolysis. Additionally, these could be used against osteolytic diseases such as osteoporosis and rheumatoid arthritis. Bioelectromagnetics. 39:394-404, 2018. © 2018 Wiley Periodicals, Inc.