MicroThread™ is the minute thread design on the neck of the Astra Tech implants, introduced as early as 1991. Scientific articles present the ability of MicroThread to ensure positive biomechanical bone stimulation and to maintain marginal bone levels in the long term. Summarized on the following pages, you will find articles about the continuous follow-up of the MicroThread.
The implant neck: smooth or provided with retention elements. A biomechanical approach
Purpose: This study set out to establish the influence
on peak bone stress at the bone-to-implant interface ky providing retention elements along the entire
length of the implant neck, and to also evaluate the
impact of bi-cortical fixation and implant axial stiffness, which have also been shown to help in reducing peak bone stresses.
Materials and Methods: Calculations were made using finite element analysis. In order to obtain sufficient accuracy, initial data was calculated from a 3-dimensional (3-d) model of a 72 mm long uniform section of a mandible, built up with 8 node cubic elements, in order to evaluate its elastic behaviour. Data from this model was then transferred into a simpler axisymmetric model built up with 4 node square elements. This axisymmetric model was then adjusted to ensure that relative displacement of upper and lower cortices under a 200 N centrally located load was similar to that obtained in the 3-d model. Certain assumptions were incorporated into the modelling of the bone to allow for its viscoelastic behaviour and to avoid the formation of high peak stress artifacts which were seen to occur at singular points. It was also assumed that for a smooth implant surface only compressive stresses would be resisted, compared to compressive and shear stresses for a surface provided with retention elements.
Into this axisymmetric model a 3.5 mm diameter titanium implant was inserted, built up with 4 node elements and with appropriate information on the modulus of elasticity and Poisson's ratio for titanium. The width of the central bore was altered so that the wall thickness varied between 0.3-0.8 mm which in turn would affect axial stiffness of the implant and its surface was either modelled to be smooth or rough by the incorporation of retention elements, all the way to the top of the implant. Additionally, variations in thickness of the cortical bone were modelled and the implant length was varied to allow for uni-and bi-cortical fixation.
A 1000 N vertical load was applied evenly to all upper implant nodes and the influence of surface character, wall thickness and the presence of uni- or bi-cortical fixation was calculated with regards to the peak interfacial bone stress.
Results: When considering the influence of surface characteristic for implants with bi-cortical fixation, and a wall thickness of 0.6 mm, peak interfacial shear stress reduced from 80.6 MPa to 29.6 MPa when the neck was characterized with retention elements. Indeed in all calculations there was always an approximate 60 to 80% decrease in peak stress when the implant neck was characterized with retention elements. An increase of wall thickness from 0.3 to 0.8 mm decreased peak stresses by only 10 to 20%. The influence of uni- or bi-cortical fixation was similar.
Discussion and conclusion: The majority of implants have a smooth cervical portion around which significant bone loss has been reported, particularly for those with a long conical shaped neck. However Palmer et al., have reported remarkable maintenance of marginal bone around similarly shaped implants provided with retention elements. It has been postulated that with a smooth neck the bone does not partake in distributing axial load and suffers from atrophy according to Wolff's law. By contrast the interlock afforded by retention elements allows for the axial load to be dissipated via interfacial shear which has been shown to be a critical stressor.
It can be concluded that the provision of retention elements (micro-architecture and/or micro-thread), an increase in axial stiffness of the implant and bi-cortical fixation will all enhance the performance of an implant to resist higher axial loads.
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