Keywords: computer modelling, dental implant, finite element technique.

The loading transferred to the bone is significantly influenced by the design of the implant. The implant dimensions influence the distribution and magnitude of stresses within the bone, as documented by Himmlova et al. [1], Plikçiolu et al. [2] and Pierrisnard et al. [3]. Considering the increasing demand for implants and changing clinical protocols, the relationship between implant design and stress distribution at the bone/implant interface becomes an important issue. From a bioengineering perspective, an important criterion in designing an implant is to have a geometry that can minimize the peak bone stress caused by an extensive range of loading.

As part of the implantation process, the torque is applied to the abutment screw causing an equivalent preload or clamping force between the abutment and implant. To date no published research appears to have investigated the influence of abutment screw preloading on the cancellous bone for various implant thread designs. Therefore, the aim of this study is to evaluate the stress at the bone-implant interface for various implant thread designs under different abutment screw preloads.

The finite element method is used to replicate the complex geometric and material properties of the bone and implant. In addition to the various implant thread design, the abutment screw preload as a result of the applied torque is also considered as a variable. The characteristics of von Mises stresses in the cancellous bone are examined for all variables. The assumptions made in the modelling and simulation process include:

• a two-dimensional representation of the bone and implant is used because this is considered to be equally accurate and more efficient in terms of computation time, as compared to a three-dimensional approach;
• temperature sensitive elements are used to replicate the preload within the abutment screw.

This study has demonstrated that the Neoss thread design yields a more favourable stress profile within the cancellous bone than the Nobel Biocare and 3i designs. This is under the condition that the inner thread design and the crown-abutment components are identical to the Neoss system for the three different outer thread designs. Compared to the Neoss' more smoothed-out thread profile, both the Nobel Biocare and 3i designs have more abrupt changes in geometry, which results in higher stresses. This is particularly true for the Nobel Biocare design where a notch is present. Although the existence of the notch helps to promote osseointegration, it on the other hand produces undesirable stresses.

1

L. Himmlova, T. Dostalova, A. Kacovsky, S. Konvickova, "Influence of implant length and diameter on stress distribution: a finite element analysis", Journal of Prosthetic Dentistry, 91(1), 20-25, 2004. doi:10.1016/j.prosdent.2003.08.008

2

H. Plikçiolu, K. Akça, "Comparative evaluation of the effect of diameter, length and number of implants supporting three-unit fixed partial prostheses on stress distribution in the bone", Journal of Dentistry, 30(1), 41-46, 2002. doi:10.1016/S0300-5712(01)00057-4

3

L. Pierrisnard, F. Renouard, P. Renault, M. Barquins, "Influence of implant length and bicortical anchorage on implant stress distribution", Clinical Implant Dentistry and Related Research, 5(4), 254-262, 2003. doi:10.1111/j.1708-8208.2003.tb00208.x

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