Effect of attachment types and number of implants supporting mandibular overdentures on stress distribution: A computed tomography-based 3D finite element analysis
Introduction
With the rise of the number of aged people, it is estimated that the ratio of individuals over 65 years of age will reach up to 50% of the whole population in the coming decades and the number of edentulous patients even in countries with a high standard of dental health care will be significant in near future. Especially in edentulous cases, implant overdentures (IODs) help obtain better retention, thus a more comfortable function (Jemt and Staldblad, 1986).
The finite element (FE) methods are widely used in implant dentistry to predict the effects of clinical factors on implant success (DeTolla et al., 2000, Geng et al., 2001, Cruz et al., 2009). Validation of biomechanical stress and strain measurements by using strain gauges is important to have reliable results (Baiamonte et al., 1996). Nonetheless, strain gauge measurements are limited to the area where the gauge is bonded, whereas the FE methods can be used to calculate stress and strain in any location of the model (Baiamonte et al., 1996).
In literature, comparison of stress distribution between complete dentures and IODs, effects of mucosa thicknesses and resiliency on stress distribution of IODs (Barão et al., 2008, Assunção et al., 2009), influence of the retention mechanism on biomechanical behavior of IOD with two implants (Daas et al., 2008), comparison of stress distribution of IOD with ball, O-ring and magnetic attachments (John et al., 2012), stress analyses of IODs with four different bar heights (Rismanchian et al., 2012), effect of different implant positions on pre-implant bone stress of IODs with two-implants (Hong et al., 2012) and effect of different designs of IODs and fixed full-arch implant-supported prosthesis on stress distribution in edentulous mandible (Barão et al., 2013) were investigated by using the FE methods. The influence of different attachment types (namely, bar and ball attachments) on stress distribution was also investigated in several studies (Assunção et al., 2008, Barão et al., 2009, Vafaei et al., 2011) and no significant difference between these attachments was reported (Thayer and Caputo, 1980, Skalak, 1983, Menicucci et al., 1998a, Menicucci et al., 1998b, Duyck et al., 1999, Vafaei et al., 2011). While there are studies having evaluated satisfaction and reporting higher scores for bar attachments (Cune et al., 2005, Timmerman et al., 2004), other studies contend equal (Gotfredsen and Holm, 2000, MacEntee et al., 2005) or less satisfaction of patients (Naert et al., 2004) with bar attachments. To date, there is no consensus on the influence of implant number on stress distribution (Meijer et al., 1996; Klemetti, 2008; Liu et al., 2013). Usually, two implants are considered sufficient to support an IOD (Klemetti, 2008) and there is no concrete evidence that such IODs have a better success and satisfaction rate.
A few validated comprehensive comparative FE studies exist in literature that consider the influence of different attachment types, implant numbers and loading conditions on biomechanical behavior of IODs. The aim of this study is to calculate stresses occurring during occlusal loadings of mandibular IODs with two different attachment types and varying numbers of supporting implants and to give clinicians, from force distribution point of view, a comparative insight on the influence of implant number as well as IOD attachment type.
Section snippets
Materials and methods
A human adult edentulous mandible retrieved from a formalin fixed cadaver, that is judged by visual inspection, is used to define the geometry of the FE model. Following, a 3D advanced computer-aided design (CAD) model is created by using a commercially available CAD software (CATIA, Dassault Systemes, Vélizy-Villacoublay Cedex, France) from dental volumetric cone beam tomography scan data (Newtom, Elmsford, NY, USA) of the mandible. Overdentures and the supporting soft tissue are included in
FE model validations
To be able to compare the numerical results with the strain gauge measurements reported in the study (Arat Bilhan et al., 2013), the same gauge locations are chosen to be evaluated (Fig. 3) as follows: R1V (right first premolar vestibular), R2V (right second incisor vestibular), SV (symphysis midline vestibular), L1V (left second incisor vestibular), L2V (left first premolar vestibular), R1L (right first premolar lingual), R2L (right second incisor lingual), SL (symphysis midline lingual), L1L
Discussion
Since the knowledge about functional loads on implants is essential to achieve long-term implant success, correct qualification and quantification of forces on implants are crucial to understand their biomechanical characterization. Precise measurement and evaluation of these forces is impossible with today’s technology, because the contribution of several biomechanical factors, such as bone density, number of supporting implants, angulations of implants in bone, direction and amplitude of
Conclusions
Within the limitations of an FEA study, it is concluded that the increase in number of implants and use of a splinted attachment can be preferred in order to reduce forces emerging around the implants. The slightly lower stress values for the bar attachment could be particularly important in cases with reduced implant size and/or low bone quality. The use of 2 single attachments in cases with good bone quality and ideal sized implants seems to be a safe and sufficient solution for the treatment
Conflict of interest statement
None of the authors have a conflict of interest with respect to the work reported here.
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