Objective To investigate the effects of three types of twin-block(TB)appliances on the stress and displacement of anterior teeth,periodontal ligaments,and alveolar bone.Methods A three-dimensional(3D)finite element model was constructed,including maxillofacial bones,articular discs,teeth,and periodontal ligaments.Three types of twin-block appliances were designed:classic twin-block(classic-TB),twin-block with acrylic capping(capping-TB),and clear twin-block aligner(CTBA).All appliances had an inclination angle of 70°,and a masticatory force of 200 N was applied to their inclined planes.The finite element method was used to analyze the stress distribution and displacement differences of anterior teeth.Results All three types of TB appliances induced lingual tilting of maxillary anterior teeth and labial tilting of mandibular anterior teeth.The CTBA group showed the greatest lingual displacement and stress of maxillary anterior teeth,with a maximum stress of 30.6 MPa,while the mandibular anterior teeth in this group exhibited the smallest labial displacement(approximately 0.02 mm)and stress.Additionally,the CTBA group had the lowest compressive stress in mandibular anterior teeth,periodontal ligaments,and alveolar bone,whereas the classic-TB group had the highest.Conclusions In the treatment of Angle Class Ⅱ malocclusion,classic-TB(with or without acrylic capping)causes labial inclination of mandibular anterior teeth.Compared with classic-TB,CTBA effectively reduces the compressive stress and displacement of mandibular anterior teeth,potentially minimizing adverse periodontal risks.However,attention should be paid to the lingual displacement of maxillary anterior teeth.

Objective To compare the differences in thickness and fitting accuracy between direct-printed aligners(DPA)and conventional thermoformed aligners(TFA),and to provide experimental evidence for the clinical application of clear aligners(CAs).Methods Sixteen adult subjects with mild dental crowding and no significant caries or restorations were recruited.For each subject,CAs were fabricated using direct three-dimensional(3D)printing and conventional thermoforming methods.The CA thickness was measured at the labial/buccal and lingual surfaces of incisors,canines,and first molars using a high-precision electronic thickness gauge.Micro-CT scanning was employed to analyze the gap between the CAs and dental models,followed by statistical analyses.Results The overall mean thickness of the DPA group was(0.60±0.04)mm,significantly higher than that of the TFA group(0.48±0.06)mm(P＜0.000 1),with superior thickness uniformity.The average gap between CAs and dental models in the DPA group was(0.29±0.08)mm,significantly smaller than that in the TFA group(0.31±0.16)mm(P＜0.05),particularly at the incisal edges of incisors,buccal surfaces of canines,and occlusal surfaces of first molars.Conclusions Compared to conventional TFA,DPA demonstrates significant advantages in thickness uniformity and fitting accuracy,indicating that DPA has greater application potential in orthodontic clinical treatment.

Objective To investigate the damage characteristics of peri-implant bones with varying densities under impact and occlusal forces using numerical simulation.Methods A finite element model of the microstructure of an implant and bones with different densities was established.Impact and occlusal forces were applied sequentially to the implant.A user material subroutine was created for failure judgment using stress-based failure criteria,enabling the analysis of bone damage caused by impact and occlusal forces.Results No cortical bone damage was observed in bones of varying densities under impact force.Damage primarily occurred in the trabecular bone at the base of the implant,with the extent of damage worsening as bone density decreased.Additionally,the number of failed bone elements generated by the damage increased with reduced bone density.Bone tissues with pre-existing impact damage sustained secondary damage when subjected to occlusal force:the bonding interface between the implant and cortical bone was damaged,leading to implant displacement and fracture of peri-implant trabecular bone.The damage caused by occlusal force also worsened as bone density decreased.Conclusions The degree of damage from impact and occlusal forces is correlated with bone density,with damage worsening as bone density decreases.This underscores the protective role of cortical bone.The application of occlusal force exacerbates bone tissue damage,leading to implant displacement when the cortical bone is damaged.In clinical practice,patients with a history of impact damage should undergo thorough examination and evaluation.The occlusal force borne by damaged bones should be reduced;if necessary,the implant should be removed and reinserted after re-establishment of osseointegration.

Objective To conduct preoperative simulations of three different osteotomy line design schemes under centric occlusion based on two distinct material assignment methods,evaluate biomechanical properties of the models,and explore which osteotomy line design schemes are more suitable for different types of mandibles.Methods Three types of mandibles were selected,and CT images were obtained for three-dimensional(3D)reconstruction.Material assignment was completed using the cortical/cancellous bone assignment method and the gray value assignment method.Osteotomy was simulated according to the three osteotomy line design schemes,followed by finite element analysis.Results In all simulation results of the mandibles,the maximum stress was 81.10 MPa,the maximum strain was 0.035 52,and the maximum displacement was 432.4 μm.The stress distributions obtained by the cortical/cancellous bone assignment method showed a larger stress distribution range than that that by the gray value assignment methods,but the maximum stress,strain,and displacement were generally lower.For the outflare type and common type mandibles,Scheme 1 showed lower maximum stress,strain,and displacement under both material assignment methods,but no clearly suitable scheme was found for the retracted type.Conclusions The outflare type and common type mandibles are more suitable for adopting the osteotomy line design scheme of Scheme 1.For the retracted type,other mandibular angle osteotomy plastic surgery methods may be considered to ensure better biomechanical characteristics.Whether choosing the osteotomy line design scheme or the modeling material assignment method,it is necessary to make the final decision based on the specific analysis objective and resource conditions.

Objective To address the personalized differences in motion gait between stroke patients and healthy older adults,as well as the issue of abnormal gait recognition,a deep learning fusion-based approach is proposed to effectively improve the accuracy of abnormal gait recognition.Methods A model fusing convolutional neural networks(CNN)and bidirectional long short-term memory networks(BiLSTM)was adopted,with the introduction of a residual network(ResNet).Unilateral ankle joint movement data at different walking speeds within a comfortable range were collected from healthy older adults and stroke patients.Signals from inertial sensors and electromyography sensors were used as inputs,while gait features were analyzed and gait differences between the two groups were compared.The effectiveness of the model was validated by comparing the classification performance of traditional deep learning models and CNN-ResNet-BiLSTM models with different layer combinations in terms of abnormal gait recognition accuracy.Results The CNN-ResNet-BiLSTM model,which introduced residual connectivity,performed excellently in abnormal gait recognition.Compared with traditional deep learning models such as the gated recurrent unit(GRU)and long short-term memory network(LSTM),its prediction accuracy was improved by 13.6％and 8.36％,respectively.Additionally,compared with other model combinations,this model achieved an overall accuracy of 97.78％.Conclusions The algorithm proposed in this study can be applied to stroke-related abnormal gait detection,providing technique support for the early diagnosis and precise monitoring of such diseases.

Objective To explore the peak moment characteristics of lower limb joints,as well as the contribution rate and compensation mechanism of lower limb joints when older adults with a history of falls cross obstacles at different paces.Methods Thirty healthy older adults and 30 eldely fallers were recruited.The Qualisys infrared high-speed motion capture system and the Kistler three-dimensional force platform were used to collect the biomechanical data of the older adults when they crossed an obstacle with a height of 15％of their height at three walking speeds(1.05,1.41,1.74 m/s).The data were then modeled and analyzed using Visual 3D software.Results As walking speed increased,the peak knee extension moment,peak ankle plantar flexion moment,and double peak value of hip flexion moment in healthy group all increased significantly(P＜0.05).Compared with healthy group,the double peak values of hip flexion moment in faller group were significantly smaller than those in healthy group(P＜0.05).During walking at moderate speed,the contribution rate of the left hip joint in faller group was significantly higher than that of healthy group(P=0.025),while the contribution rate of the ankle joint was significantly lower(P=0.044).The margin of stability in the anterior-posterior direction at the moment of ground contact of the stance leg and the stride leg increased with walking speed(P=0.007,P=0.002).Conclusions Compared with healthy older adults,the elderly fallers have lower peak torque,peak ground reaction force,and dynamic stability in the anterior-posterior direction.As the walking speed increases,the mechanical parameters and the margin of stability of older adults increase significantly,and walking stability is improved.Compared with healthy older adults,elderly fallers usually rely more on the contribution of hip joint movements and reduce the involvement of ankle joints.It is recommended to incorporate fast walking exercises into the daily fall prevention exercise program for older adults,with combination of coordinated training of the hip,knee,and ankle joints.

With the advancement of economic development and clear aligner technology,invisible orthodontics has become an important treatment approach for malocclusion.However,in extraction cases involving anterior tooth retraction,the limited three-dimensional force control of aligners often leads to lingual tipping of incisors,deepened overbite,and anchorage loss of posterior teeth,thereby increasing the complexity and uncertainty of orthodontic treatment.As an effective tool for simulating complex biomechanical behaviors,finite element analysis(FEA)enables quantitative assessment of tooth movement and stress distribution in the periodontium,providing theoretical support for optimizing aligner design.This review focuses on FEA studies of anterior tooth retraction using clear aligners in extraction cases,summarizes tooth movement characteristics,and discusses key influencing factors including aligner parameters,movement patterns,attachment design,and auxiliary appliances.Finally,the future directions of FEA in dynamic simulation and personalized modeling are explored to support more predictable and effective clinical outcomes.

Objective This study investigates the force distributions and movement patterns of the maxillary dentition during molar intrusion with clear aligners,aiming to provide a theoretical basis for optimizing clinical orthodontic treatment strategies.Methods A three-dimensional(3D)finite element model of the periodontal ligament-teeth-clear aligners complex was established to simulate different intrusion modes,including bilateral first molar intrusion,bilateral second molar intrusion,and simultaneous intrusion of bilateral first and second molars.The von Mises stress distribution characteristics and displacement patterns of each tooth under different intrusion conditions were systematically analyzed.Results Compared with simultaneous molar intrusion,the individual intrusion design resulted in greater intrusive movement(4.260-10.500 μm)accompanied by distal-lingual crown inclination(distal displacement:-7.690--5.100 μm;buccal displacement:-20.500--6.750 μm).Anchorage teeth displayed a displacement trend opposite to that of the intruded molars.The anterior teeth demonstrated minimal displacement and low stress levels.During maxillary molar intrusion with clear aligners,the maximum equivalent stress in the periodontal ligaments occurred at the anchorage teeth mesial to the intruded molars,primarily concentrated in the apical region and the mesial aspect of the buccal cervical area.Conclusions A sequential intrusion strategy enhances vertical control efficiency compared to simultaneous intrusion.Unanticipated mesiodistal and buccolingual displacements in the posterior region necessitate the implementation of counteracting mechanisms in aligner design.In clinical practice,priority should be given to monitoring the risks of root resorption and bone remodeling effects in stress-concentrated zones(apical and buccal cervical regions)of anchorage teeth.

Objective To investigate the biomechanical characteristics of the mandibular anterior teeth intrusion assisted by buccal mini-implants,so as to provide theoretical guidance for the clinical treatment of deep overbite and other conditions requiring intrusion of the mandibular anterior teeth.Methods A finite element model of implant screws,clear aligners(CAs)and mandibular complex including the mandibular dentition,periodontal ligament(PDL)and alveolar bone was constructed.The model was designed with 0.2 mm intrusion in the mandibular anterior teeth.Seven groups were set:without traction,30 g,50 g,100 g,150 g,200 g,and 250 g force groups.The stress and displacement of the teeth as well as the related stress distributions of PDL and CAs in each group after loading were analyzed.Results Without traction and under different traction forces,the mandibular anterior teeth tended to be tipped and intruded.With the increase of traction force,the increasing trend was manifested in the vertical displacement and labial displacement of the mandibular anterior teeth.When the traction force reached 200 g,the vertical displacement showed a significant increase,the subsequent increase then showed a tendency to flatten.The ratio of the labial tipping displacement to the intrusive displacement of the mandibular anterior teeth showed a decreasing trend with the increase of traction force.Within a reasonable force range,increasing traction force led to greater bodily intrusion of the anterior teeth.When the traction force was less than 150 g,the mandibular posterior teeth exhibited a tendency of extrusion.When the traction force of more than 150 g was applied,it showed a tendency of intrusion.The PDL stress was concentrated on the labial and cervical surface of the anterior teeth.Conclusions Intrusion efficiency of the mandibular anterior teeth has been effectively improved with CAs assisted by buccal mini-implants.Moreover,the traction force of the mini-implants has a significant impact on the movement of the mandibular anterior teeth.Based on a comprehensive analysis of factors such as the displacement trend of the mandibular dentition and the PDL stress,the optimal force value for buccal traction of the mini-implants to intrude the mandibular anterior teeth can be selected as 150-200 g force.

Objective To investigate the biomechanical characteristics of clear aligners combined with Class Ⅱelastics during retraction of upper anterior teeth,and compare the differences between two traction methods.Methods A case with a molar distal relationship and extraction of four first premolars was selected.The finite element method was applied to analyze tooth displacement,force distribution,and periodontal ligament(PDL)stress during 0.2 mm en-masse retraction of the anterior teeth.Three working conditions were defined:en-masse retraction without elastics(Condition 1),120 g Class Ⅱ elastics with aligner-cut hooks on upper canines and lower first molars(Condition 2),120 g Class Ⅱ elastics with aligner windows and bonded buttons on upper canines and lower first molars(Condition 3).Results Class Ⅱ elastics significantly enhanced lingual movement of the upper anterior teeth and mesial movement of the lower posterior teeth,while reducing mesial movement of the upper posterior teeth and lingual movement of the lower anterior teeth.In the transverse direction,the forces exerted on the teeth in all three conditions were minimal.In the sagittal direction,in Condition 2,the mesial force of the upper posterior teeth was effectively reduced by an average of 0.13 N,and the mesial force of the lower posterior teeth was increased by an average of 0.31 N.In Condition 3,the distal force of the upper canine teeth and the mesial force of the lower first molar were significantly increased by 0.40 N and 1.14 N,respectively.In the vertical direction,In condition 2,the average extrusive force of the upper teeth and the extrusive force of the lower molars were increased by 0.22 N and 0.20 N,respectively.In Condition 3,the upper canine extrusive force was increased by 0.91 N,while the lower molar intrusive force and the second molar extrusive force were reduced by 0.27 N and 0.25 N,respectively.The PDL stress distribution in the three conditions was generally similar.In Condition 3,the maximum principal stress distribution area on the lower first molars expanded slightly,but the magnitude did not increase significantly.Conclusions Condition 2 optimized the lower posterior teeth mesialization through balanced force distribution and protected the upper posterior teeth anchorage.Condition 3 significantly increased extrusive and distal forces on the upper canines and mesial forces on the lower first molars but did not substantially elevate periodontal risks for these teeth.