关键词: 个性化根形种植体, 固位结构, 三维有限元分析, 即刻负载, 初期稳定性

Abstract: Objective    To evaluate the biomechanical influence of the different placements and base widths of retentions on custom-made root analogue implants（RAI）under immediate loading by means of three-dimensional finite elements analysis，providing reference for the clinical design of RAI. Methods    This study was conducted in the VIP Department of Stomatology Hospital Affiliated to China Medical University. Three-dimensional finite elements models of mandible containing mandibular first premolar RAI were constructed by means of computer aided design（CAD）software. The retentions were designed in the interdental space of RAI root. All of the protruding heights were 0.2 mm. The models were divided into two groups according to different placements of the retentions. Group A：the retentions were placed at the junctions between root neck 1/3 and root middle 1/3，and between root middle 1/3 and root apex 1/3，respectively. Group B：retentions were designed in the middle line of root neck 1/3，root middle 1/3 and root apex 1/3，respectively. In each group，seven experimental models were established according to different base widths of the retentions（0.8，1.0，1.2，1.4，1.6，1.8，2.0 mm），denoted as M1-M7. The standard RAI without retention was set as a blank control and recorded as M0. An average force of 129 N was applied to the base center of RAI under immediate loading vertically and buccolingually inclined 30° with the axis of the RAI，respectively. The maximum von Mises stress and stress distribution of bone tissues surrounding RAI and the maximum displacement，resonance frequency of the implants were analysed using CAD software. Results    In all models the peak value of bone density stress appeared around the neck of RAI. Except M1 model，whose maximum stress of cancellous bone was located at the retention，the maximum stress of cancellous bone was located at the root tip of RAI in other models. Under the same loading，the maximum displacements of group A M1-M7 were higher than that of M0，and the maximum displacements of group B M2-M7 were less than that of M0. Under the same loading and with same base width，the maximum displacement of group A was greater than that of group B，and the resonance frequency of group A was much lower than that of group B. The maximum von Mises stress of cancellous bone and cortical bone as well as the maximum displacements of RAI increased significantly，while the resonance frequency of RAI decreased significantly，compared to M0. In the M1-M4 models，the maximum von Mises stress of the bone tissues around the RAI and the maximum displacement of RAI decreased as the base width increased. In M5-M7 models，the maximum displacement of RAI increased as the base width of retention increased，and the maximum von Mises stress of bone tissue around RAI was roughly the same. In terms of the resonance frequency of RAI，the axial resonance frequency of RAI was higher than that of the buccal and lingual under the same loading. In the M1-M3 models，the resonance frequency of RAI increased as the base width of the retention increased，but it decreased in the M4-M7 models. Conclusion    Adding retention to RAI interdental space at the middle line of root neck 1/3，root middle 1/3 and root apex 1/3 helps to reduce the maximum displacement of the RAI and increase the resonance frequency. It is more beneficial to improve the stress distribution of bone tissue around RAI and increase the initial stability of the RAI when the base width of retention is 1.4 mm.

Key words: custom-made root analogue implants, retentions, three-dimensional finite elements analysis, immediate loading, initial stability