 Astra Tech
BioManagement Complex
|
 Conical Seal Design
Conical Seal Design™ is the original and scientifically documented conical connection of the Astra Tech Implant System™, creating a strong and stable fit between implant and abutment. Below you will find many references that primarily address the technical questions related to Conical Seal Design.
 A conical implant-abutment interface at the level of the marginal bone improves the distribution of stresses in the supporting bone. An axisymmetric finite element analysis
Purpose: Previous studies have indicated that a
concentration of peak stresses in the crestal bone is responsible for marginal bone loss. Early assumptions that a smooth neck would help to reduce these
stresses has proved incorrect since clinical evidence demonstrates consistent bone loss at smooth necks. In contrast the application of retention elements at the neck has been shown to aid stress distribution. In addition it has been suggested that an internal conical implant-to-abutment joint allows for a more apically placed concentration of stresses away from the marginal bone, when compared to a butt joint interface.
The current study utilized a finite element analysis (FEA) calculation to measure and compare peak stresses for 1- and 2-piece implants (1-P, 2-P) modeled with a conical joint configuration and a microthread, with varying wall thicknesses and moduli of elasticity.
Material and Methods: An axisymmetric model consisting of four-node elements modeled the upper cortical bone presumed to be in contact with the surface of a load carrying implant. The thickness of the cortex was modeled to be 2.8 mm, with a modulus of elasticity of 15 GPa. The 2-P implant was modeled such that the joint was effectively intraosseous, while the equivalent conical portion modeled in the 1-P implant was placed above the crest of bone, 2 mm coronally. Each set of calculations was repeated for implant wall thicknesses of 0.3 mm, 0.6 mm, and 0.9 mm. An axial load of 100 N was applied. The material (titanium) for the implant was given a modulus of elasticity of 107 GPa. Furthermore at a wall thickness of 0.6 mm, the modulus of elasticity was compared with one of 53.5 GPa, 214 GPa, and one presumed to be infinitely stiff 1.07 xio17 Pa. The bone-to-implant interface was assumed to resist compression but not tension or shear.
Results: When comparing the magnitude of compressive, tensile and von Mises stresses within the 1-P or 2-P groups for differing wall thicknesses, the data indicated that the only notable differences occurred at the most superior thread in the 2-P implant where increased wall thickness led to increased stress. Elsewhere, differences in wall thickness made little impact on the peak stresses. For differing moduli of elasticity the calculated stresses were also similar within the groups. Again such differences were not-able at the level of the first thread for the 2-P implants.
When comparing the two groups, there was a stark difference regardless of wall thickness or modulus of elasticity, such that the 2-P implants recorded their peak stress deep within bone at the level of the 5th to 9th thread compared to 1-P implants whose peak stress was always concentrated at the first two threads and with a magnitude far greater than any peak stresses recorded in the 2-P group.
Discussion and Conclusions: The current model does not reflect reality in so far as it is not 3-dimen-sional and it idealizes the characteristics of the interface and the bone, which in reality is viscoelastic, heterogeneous, and anisotropic. Nonetheless, the relative comparison of the 1-P and 2-P systems is valid and such relativity can be extrapolated to the clinical reality. Furthermore, the locations of the peak stresses calculated in the current study are in accordance with previously published data.
Certainly the results of this study support the findings of an earlier study that the use of an internal conical joint displaces the location of the peak stresses to a more apical location. This was not altered by the presence of an external microthread. In contrast the placement of the conus supracrestally in a 1-P implant design had a negative effect giving rise to much higher peak stresses in both magnitude and in location at the most crestal threads. However an increase in either wall thickness or modulus of elasticity had the effect of increasing the stresses located more coronally in the 2-P system due to increased implant stiffness. This resulted in higher peak stresses at the level of the first thread, but significantly lower than that calculated for the 1-P implant design.
It can be concluded that an implant which benefits from an internal conical implant-abutment joint, placed within the marginal bone, performs in a superior manner to a 1-P implant where the joint is located supracrestally. Furthermore, the application of microthreads, and careful consideration of implant wall thickness and stiffness will help to yield an implant optimally suited to reduce peak stresses in the crestal bone. Such results have been seen in clinical practice with superlative bone maintenance data reported in the literature for an implant based upon these design features.
|
|
|
