The implant neck: smooth or provided with retention elements. A biomechanical approach.

Abstract

A combined three‐dimensional and axisymmetric finite element analysis was made of the effect upon the peak interfacial shear stress of providing an axially loaded mandibular dental implant with retention elements all the way up to the crest of the implant as opposed to a smooth neck. The effect of increased wall thickness of the implant and of using bi‐cortical fixation as opposed to unicortical fixation was also studied. Retention elements at the implant neck were found to bring about a major decrease in the peak interfacial shear stress. Increased wall thickness and bi‐cortical fixation also resulted in decreased peak interfacial shear stress but this effect was minor. The interpretation of this was that all these three measures increase the capacity of the implant to carry axial loads. Thus from a biomechanical viewpoint it appears to be advantageous to provide the neck of screw‐shaped implants with retention elements, for example a rough surface of suitable micro‐architecture and/or a microbhyphen;thread. It is furthermore suggested that retention elements at the implant neck will counteract marginal bone resorption in accordance with Wolff's law. This paper is a revision of: Hansson, S. (1997) Some steps to improve the capacity of dental implants to resist axial loads. In: Hansson, S., ed. Towards an optimized dental implant and implant bridge design: A bio‐mechanical approach. Thesis. Göteborg; Chalmers University of Technology.

Influence of microthreads and platform switching on stress distribution in bone using angled abutments

“…In the present study, higher stress/strain concentration was observed for the microthread implants, which may be explained by the horizontal profile of the microthread [30]. However, in this study the thread design of both modeled implants with or without microthreads had the upper region of the implant with a smooth surface.…”

“…In this study, the difference in the concentration observed between the microthread and smooth models may act as a bone stimulus [30], since the bone strength acts as a physiological limit. Local overloading in cortical bone occurs in compression when the maximum compressive principal stress exceeds 170-190 MPa in modulus, and in tension when the maximum tensile principal stress exceeds 100-130 MPa [28,32].…”

“…Hansson et al [30] showed that retention elements in the implant neck increase an implant's ability to resist an axial load 2.64-4.79 times higher than the load that can be resisted by an implant with a smooth neck [30].…”

“…Some authors [29] have suggested that the magnitudes of the concentrations should be presented in the s max for evaluation of stress [24] 1.37 0.3 Titanium [24] 110 0.35 Ceramic [24] 68.9 0.28 Gold alloy [25] 90 0.3 Cement layer [26] 18.6 0.28 distribution in a non-ductile structure such as bone. Other authors have claimed that shear stress is the best parameter because it shows how the load diffuses into bone, and because the fixation strength of bone implants commonly is measured by interfacial shear strength by push-out and removal torque laboratory tests [30].…”

Application of the finite element method in dental implant research

“…82 These surfaces have in general been compared to the socalled machined or turned surfaces, with little emphasis being paid to the range of surface textures that might exist within a machined screw implant. For crestal bone, Hansson 83 compared implants with smooth necks to those with screw thread along the entire implant to bone contact surface. It was found that a major decrease in peak interfacial shear stresses occurred if the entire implant to bone contact had a screw thread.…”