Objective: The aim of this study was to evaluate strain distribution in peri-implant bone, stress in the abutments and denture stability of mandibular overdentures anchored by different numbers of implants under different loading conditions, through three-dimen- sional finite element analysis (3D FEA)。76167

Methods: Four 3D finite element models of mandibular overdentures were established, using between one and four Straumann implants with Locator attachments。 Three types of load were applied to the overdenture in each model: 100 N vertical and inclined loads on the left first molar and a 100 N vertical load on the lower incisors。 The biomechanical behaviours of peri-implant bone, implants, abutments and overdentures were   recorded。

Results: Under vertical load on the lower incisors, the single-implant overdenture rotated over the implant from side to side, and no obvious increase of strain was found in peri- implant bone。 Under the same loading conditions, the two-implant-retained overdenture showed more apparent rotation around the fulcrum line passing through the two implants, and the maximum equivalent stress in the abutments was higher than in the other models。 In the three-implant-supported overdenture, no strain concentration was found in cortical bone around the middle implant under three loading  conditions。

Conclusions and clinical significance: Single-implant-retained mandibular overdentures do not show damaging strain concentration in the bone around the only implant and may be a cost- effective treatment option for edentulous patients。 A third implant can be placed between the original two when patients rehabilitated by two-implant overdentures report constant and obvious denture rotation around the fulcrum  line。

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1。 Introduction

In recent years, the treatment of a fully edentulous mandible by  means  of  an  implant overdenture has  become  a routine

strategy。1–3 Completely edentulous patients who have persis- tent problems using conventional mandibular prostheses can benefit significantly from implant overdentures。4

In 2002, the McGill consensus statement suggested that an overdenture  retained  by  two  implants  should  be  the  first

choice of treatment for the edentulous mandible。5 Recently, there have been reports suggesting that a single implant is adequate for overdenture retention and can result in a high success rate comparable to overdentures supported by multiple implants。6–11 Walton et al。 compared patient satis- faction and prosthetic outcome for mandibular overdentures retained by one or two implants in 86 participants for one year。 In  this  randomized  clinical  trial,  researchers  found   lower

research comparing the stress or strain in peri-implant bone using mandibular overdentures retained by different numbers of implants。 Therefore, the main goal of this study was to compare through 3D FEA the strain distributions in peri- implant bone, stress in the abutments and denture stability of mandibular overdentures retained by one, two, three, or four implants。

component   costs   and   treatment   times,   with comparable

satisfaction and maintenance time, for overdentures retained by a single midline implant。8 However, some authors have reported unexpectedly high failure rates for single-implant retained mandibular overdentures using an immediate load- ing  protocol。12,13

Other researchers have investigated mandibular over- dentures using three or four implants。 Theoretically, the application of three or four implants creates an angular relationship between the implants instead of a straight-line relationship。 In the three-implant-supported overdenture, the most anteriorly positioned implant may provide indirect retention for the denture by preventing the intrusion of the anterior portion of the denture towards the tissues。14 Therefore, it has been recommended by some authors that clinicians should use three or four implants in situations that require increased retention, such as high muscle attachment or prominent mylohyoid ridges。15 However, in general, the use of mandibular overdentures supported by more than two implants does not lead to greater patient satisfaction in terms of denture and social function。 Meijer et al。 found no clear difference in either clinical or radiographic outcomes between two-implant-retained and four-implant-supported mandibu- lar overdentures over a 10-year evaluation period。16 Mericske- Stern compared the clinical results between two-, three- and four-implant mandibular overdentures and proposed that two implants can adequately serve as retention for a complete mandibular denture。17 For reasons of cost-effectiveness, Meijer et al。 proposed that a two-implant overdenture is advisable for patients with Cawood classes IV–VI resorption of the mandible and complaints concerning retention and stability of the lower  denture。16