The predicted principal strains by the validation FE model had a trend similar to those measured from ESGV among all loadings. The deviations in strain values between the ESGV and the FE simulation may be attributed to the measuring error in material properties, and the loading locations and directions between two approaches.
Besides, the orientation of the strain gauge, which does not align perfectly with the direction of principal strains, may also contribute to the deviations in strain. However, these deviations were proportionally changed in response to the alternation of biomechanical parameters in the models. For a purpose of comparing different implant designs, the FE simulation in this study was, therefore, verified.
The advantage of the present FE models was assured by the computed tomography images. In general, the simulated results of FE modeling depend considerably on the geometric structures of the models. In previous finite element studies, the mandibular model was either simplified as a rectangular configuration (Stegaroiu 1998; Wang 2002) or constructed by digital laser scanner (Ciftci 2000;
Akca 2001). The latter approach provides accurate surface topography but lacks the Fig 15 The von-Mises stress distribution of the crestal bone of the ISPP along three placement designs under the oblique loading condition.
cortical shell information, which may over/under predict stress/strain distribution of bone around the implant. Further, by introducing the anisotropic model (O’Mahony 2001), the characteristic of bone material was better reflected.
The investigation of splinted or non-splinted crowns with standard, wide and two implants
The use of the wide implant or two implants in the molar region can provide the advantage of reducing stress in the surrounding bone as shown in this study. This is because of the increased structural capacity and the enlarged bone-implant contact area offered by these implants. Balshi et al. (1996) indicated that a molar crown supported by a standard size implant can easily introduce large bending moments to bone because the dimension of the crown is usually greater than the diameter of the implant. Therefore, the wide implant or two implants are suggested for placement at the molar region to reduce the possibility of overload, which may lead to implant failure associated with the marginal bone loss (Rangert 1995). However, whether the wide-diameter implant (D 5.0mm) or two implants (D 3.75mm) is preferred for the edentulous molar restoration is still an issue. Based on the outcomes of this study;
differences between these two treatments are not significant. Therefore, with sufficient posterior mandibular bone width (buccal-lingual direction), the wide implant is suggested to reduce the surrounding bone stress due to the simplicity of its surgical procedures. However, according to the report of the Davarpanah et al. (2001), placing the wide implants in narrow posterior ridges can lead to marginal bone loss that may raise the risk of implant failure. Therefore, in the cases of insufficient posterior mandibular bone width, two implants are preferred because the stress reduction by two implants is about the same as that induced by the wide implant.
Further, the narrow distance (2.5 mm in this study) between the two implants of two-implant treatment would not increase the bone stress. Nevertheless, it is necessary to note that, recent clinic reports (Attard 2003, Ivanoff 1999) showed that using wide implants could result in higher failure rates than that of the standard implant. However, the authors pointed out that the failure might be associated with the surface treatment and shape of the implant and patients’ bone quality rather than the usage of wide implants.
Some scientific data suggested that prosthetic crown splinting had biomechanical advantage and could raise the success rate because occlusal force could be shared through splinted crowns, thus, decreasing the peak-stresses (Guichet 2002; Wang 2002). However, there is insufficient quantitative evidence to support this hypothesis.
Wang et al. (2002) had developed simplified FE models to evaluate this splinting effect and demonstrated that splinting the prosthetic crowns could reduce stresses in bone. Similar observations were presented in the study of Guichet et al. (2002) using photoelastic models. However, in their simulations the implant structures were loaded on the premolar only. When loading is applied to a single crown and, by splinting the crowns, the loading would redistribute itself through the implant under the unloaded crown, and then the peak stress of bone is decreased certainly. In the present research, the bite forces were exerted at both functional cusps of two crowns to mimic full contacts of normal occlusion. The results of the present study showed that there is no significant difference between Model Spl-S and Model nSpl-S; that is, with standard implants for both premolar and molar, the splinting effect is minimal. Our result appears to support clinical observations of Herbst et al. (2000), who showed a similar survival rate for the splinted prosthetic crown and non-splinted prosthetic crowns.
However, we cannot explain why some clinicians observed higher implant failure in the splinted cases than in non-splinted cases (Naert 2002). Further investigations such as crown misfit as proposed by Jemt et al. (1996) may provide some clues and help answer this question.
However, the combination of one wide implant or two implants at the molar region and one standard implant at premolar region provided a notable stress sharing effect when prosthetic crowns were splinted. By sharing, stresses in premolar regions decreased more than 25%, which may protect overload damage if patients have an inadequate bone quality in their premolar ridge. Therefore, the splinted prostheses should only be considered when two crowns are held by different implant supports.
The investigation of inline, buccal offset and lingual offset placements
Higher implant stresses occurred in oblique loading mode was found in this study.
This outcome was similar to other studies(Stegaroiu 1998, Holmgren 1998), which indicated that large bending moments created by oblique loads elevate the force/stress of implants. Further, the use of the offset placement could decrease the forces/moments on the implants as demonstrated in this and other studies(Rangert 1995, Rangert 1997, Daellenbach 1996). However, some of the researches found the offset placement could result in higher force or torque in the implants(Weinberg 1996, Sato 2000). Nevertheless, the quantitative evidences of these studies were insufficient to support this hypothesis. In Sato et al.’s study (2000), only a single force was applied on the first molar cusp instead of multiple forces as the loading condition;
Also, Weinberg et al. (1996) ignored the loading interaction of adjacent crowns due to the missing of splinted factor of the crowns in their analysis. Our study consists of similar observation with Rangert et al.(1995), which illustrated that the loading in
implants could be decreased in offset placements.
Regarding the maximum stress and stress distribution of bone, under both vertical and oblique loading modes, high stress regions were located at the alveolar crest around the implants. These stress-concentrated areas coincided with the regions of alveolar bone loss(Rangert 1995, Donath 1995). According to previous studies (Itoh 2004, Akca 2001), the offset placement would increase the stress value at the surrounding bone. In our analyses, which included the anatomic cortex shell, anisotropic material properties of mandible and p-method convergent test, In our analyses, which included the anatomic cortex shell, anisotropic material properties of mandible and p-method convergent test, the offset placements showed the bone stress decreasing around some implant as comparing with the in-line implant, however, the highest bone stress was also revealed around some implants in offset placement.
These supreme stresses might suffer the bone and raised the risk of implant’s instability. Therefore, based on the outcomes of bone stress, the offset placements provide no obvious mechanical advantage over the in-line placement, and further, it might endanger the bone around some implants.