Effects Of Cortical Penetration Patterns On The Stability Of Miniscrew Implants In The Maxillary Posterior Region: A Three-Dimensional Finite Element Study

Objective:To compare the biomechanical differences between adolescents and adults under different force loads with different horizontal and vertical force values under different cortical penetration patterns of miniscrew in the maxillary posterior region by threedimensional finite element method,so as to explore the most suitable miniscrew implantation and loading conditions,and provide theoretical guidance for improving the stability of miniscrew implantation in the clinic.Method:The three-dimensional model of the 1.4 × 8 mm self-drilling miniscrew and the simulative bone blocks between the first and second maxillary molars in the adult and adolescent groups with different cortical bone thicknesses were established.The positions of the miniscrews in the bone were set as follows: Pattern 1,the depth of implantation was 6 mm,penetrating the buccal cortical bone of the alveolar ridge;Pattern 2,the depth of implantation was 6 mm,penetrating the buccal cortical bone of the alveolar ridge and the cortical bone of the maxillary sinus floor;Pattern 3,the depth of implantation was 6 mm,penetrating the buccal cortical bone of the alveolar ridge and the cortical bone of the maxillary sinus floor with 1 mm maxillary sinus perforation;Pattern 3,the depth of implantation was 3 mm,penetrating the cortical bone of the infrazygomatic crest and the maxillary sinus floor;Pattern 5,the depth of implantation was 4 mm,penetrating the cortical bone of the infrazygomatic crest and the maxillary sinus floor with 1 mm maxillary sinus perforation.After material and contact setting and mesh division,0.5 N,1 N,2 N,and 5 N gradient horizontal and vertical force were respectively loaded on the head of the miniscrew.The deformation and von Mises stress of the miniscrew,and the von Mises stress and strain of the bone under various working conditions were solved and derived.The function fitting and fitting curve drawing was completed by the Origin software.Results:(1)For the miniscrew micromotion,under the same force of the same model,the adolescent group is always greater than the adult group;Under the same force of the same age group,patterns 1～3 are almost at the same low level,and pattern 4 is the largest;Under the same force value of the same pattern and the same age group,the micromotion under the vertical force is always greater than that under the horizontal force.(2)For the miniscrew stress,significant stress concentrations were observed in the cervical areas of the miniscrew where the miniscrew contacted the buccal cortical bone.The miniscrew tip entering the cortical bone would cause stress concentration on the miniscrew tip.The miniscrew peak stress in patterns 4 and 5 is greater than that in patterns 1 to 3.The miniscrew peak stress is far lower than the yield strength of titanium alloy in all working conditions.(3)For the bone stress and strain concentrations,were observed on the cortical bone of all patterns,and the cortical bone or cancellous bone around the miniscrew tip was also the main stress and strain concentration area.As long as the tip of the miniscrew is not in the cancellous bone,no obvious stress distribution can be observed in the cancellous bone,but an obvious strain distribution can be observed.(4)For the bone stress and strain peak values,patterns 4 and 5 under horizontal force and pattern 5 under vertical force would exceed the bone stress safety threshold of 67.37 MPa under certain force values.The bone strain of pattern 2 and pattern 4 in the adult group and pattern 1 in the adolescent group in the range of 0.5～5 N force load exceeded the safety threshold of 4000 microstrains.The bone strain of pattern 3under horizontal force and patterns 1 and 3 under vertical force in the adult group,and patterns 2 and 3 under vertical force in the adolescent group within the range of 0.5～5N load were below the safety threshold.Conclusions:Orthodontic force load will not lead to the failure of titanium alloy miniscrew material.The cortical bone is the main structure that bears stress.The bone around the miniscrew tip is the main stress concentration area.The miniscrew micromotion of the adolescent group was always greater than that of the adult group under the same conditions.Interroot and penetrate maxillary sinus 1 mm is probably the most stable implantation.To completely avoid the risk of maxillary sinus inflammation,monocortical interroot implantation can be chosen by adults,but the force value should not exceed 3.02 N.Adolescents should avoid monocortical implantation.Adults should avoid bicortical implantation without sinus penetration.When implanting miniscrews in the infrazygomatic crest,the maxillary sinus should be penetrated by 1 mm,but the force value should not exceed 2.24 N.