1.1 Problems of Dental Implant-Supported Partial Restoration (ISPP)
The biomechanical behavior of dental implant is quite different from natural teeth. One of the major reasons is a lack of the function of periodontal ligament (Fig 1). Because the material of periodontal ligament is a soft tissue, it could function as an intermediate cushion element (Weinberg 1993) which absorbs the impact force and uniformly transfers the occlusal forces into the surrounding bone. However, the bio-structure of dental implant is directly connected with bone. That would cause the non-uniform stress pattern at bone and might induce the biomechanical overloading failures in implant and bone (Quirynen 1992; Rangert 1995). This overload would cause the microdamage accumulation at bone and results in primary marginal bone loss (Hansson 2003). Then the bacterial invasion might occur in the area of the bone loss and cause the serious progressive bone resorption (Spiekermann 1995). This kind of insufficient bone support is dangerous for losing implant stability and increasing the risk of the implant fracture.
The implant restoration presents a biomechanical challenge in implant-supported partial prostheses (ISPP). That is because most of the ISPP cases used in premolar and molar area, and large chewing forces in the premolar or molar region would induce the bending moments that contribute to potential bending overload on bone and implant. That could be detrimental to the prostheses and alveolar bone (Rangert 1995;
Ishigaki 2003), and generate marginal bone loss, decrease implant stability, and thus imperil the implant and its supra-structures (Brunski 1999; Miyata 2000).
The inappropriateness of the crown-implant ratio was obviously happened in the molar region. On the single, implant-supported molar restoration the dimension of the prosthetic crown are apparently larger than the size of the implant (Fig 2). When bite forces are executed on the crown, the large crown-implant ratio will easily occur the unfavorable bending moment and will increase the possibility of implant failure (Balshi 1996).
Using different implant designs and treatment protocols of implants seems to attribute the better biomechanical performance in the molar restoration. However, what kinds of implant designs and treatment protocols are effective to decrease the overload in implant and bone, especially at the molar region, it seems an issue.
Therefore, the investigation of the biomechanical effect of implant designs and treatment protocols on implant-supported partial prostheses is essential because this information could help implant designers and dentists know of how to decrease
potential overload in both implant and bone and prevent the implant failure.
Fig 1 The bio-structure of natural tooth and implant which attaches to bone. (Taylor 1990)
Fig 2 The use of the standard size of implant to support the prosthetic molar crown, the larger ratio of crown-to-root induces the various direction of the potential bending moment. (Balshi 1996)
1.2 Treatment Designs for Implant-Supported Partial Prostheses
Researches studying the treatment designs for implant-supported partial prostheses in molar area showed quite incompatible results. For single molar restoration, using wider implants (Fig 3) could decrease the percentage of failures (van Steenberghe 1995; Davarpanah 2001) and increase the removal torque (Ivanoff 1997). Likewise, using two implants (Fig) to support a single crown could reduce the rotational moment (Bahat 1996) and decrease implant mobility (Balshi 1996). Either wide implant or two implant has its own advantages. However, the biomechanical criteria for choosing one design over the others have not been defined. It has been stipulated that using large diameter of implants and two implants can increase stiffness of the implant(s) and bone-to-implant contact surfaces (Langer 1993; Balshi 1997). Nevertheless, the use of wide-diameter implants could lead to bone loss when narrow posterior ridges exist (Davarpanah 2001). Higher failure rates for wide implant have been found in clinical reports (Attard 2003, Ivanoff 1999). Furthermore, the stress states in the narrow space of bone between the two implants are unclear.
Splinting the adjacent crowns (Fig 4) is a proposition to decrease peak stresses by load sharing (Wang 2002, Guichet 2002). This is why partial prostheses usually used for implant treatment on the partial edentulous area. Nevertheless, some clinical studies didn’t share this concussion. They found no significant difference between the splinted and non-splinted prosthetic crowns (Herbst 2000) or there are worse results on the splinted prostheses (Naert 2002; Rangert 1995).
Non-straight line placement of implants, which is named the offset placement of implants (Fig 5) have been put in use clinically in partial edentulous of molar restoration for several years. Studies (Rangert 1995; Rangert 1997) indicated that the bending moment at all implants would be diminished if the implants were placed as offset placement. The research of Daellenbach et al.(1996) showed that the forces and moments of all the implants on ISPP were decreased by the offset placement.
However, some articles had apparent disagreements on the effect of offset placement.
These studies found that the offset placement of implants did not always decrease the load in all implants (Weinberg 1996; Sato 2000). Instead, it would induce high stresses in surrounding bone(Itoh 2004; Akca 2001).
Fig 3 Widening the diameter of implant can reduce the ratio of crown-to–root.
However, this value is further minimized by two implants treatment. (Balshi 1997)
Fig 4 The crowns of these implants are modeled as connected and disconnected to mimic (a) the splinted and (b) non-splinted designs.
(b) (a)
1.3 Motivation and objectives
For implant-supported partial prostheses, the implant and treatment designs are commonly used in clinic. However, their clinical outcomes and research reports were discordant. Some of these designs, such as two implants, splinting crowns and spiral thread characteristic, the mechanisms of implant and bone’s stress/strain distribution are still unclear. Besides, some finite element (FE) studies showed the limitations, such as ignoring the real pattern of cortical shell, anisotropic material properties of bone, and the convergent validation all raised the error for result prediction.
The aim of this study, therefore, is to investigate the effects of various implant and treatment designs of implant-supported partial prostheses in molar area. The results of these design parameters are evidenced by anatomic three-dimensional FE analysis and the parameters examined are listed as follows:
(1) Different supporting structures of implants. Standard, wider size of implants and two implants are selected to estimate the benefit of stress decreasing at bone.
(2) Splinted and non-splinted prosthesis. The adjacent crowns are modeled as connected and disconnected on partial molar restoration to assess the contribution of their load sharing effects.
(3) Arrangement of Implant placements. In-line, buccal offset and lingual offset Fig 5 Number of implant, implant position and arrangement of the implant affect the stress levels on the implants. (Rangert 1997)
placements of implants on partial molar restoration are analyzed for their biomechanical effects.
The experimental strain gauge verification (ESGV) is also performed to validate the finite element simulation.
2. Materials and Methods