Michael R. Norton

The relation between surface roughness and interfacial shear strength for bone-anchored implants. A mathematical model.

A rough bone implant surface was conceptualized as being built up of pits of different sizes and of different shapes. Hypotheses were formulated regarding the mechanical strength of the interfacial bone based upon the present knowledge of the character of the tissues adjacent to endosseous implants and the mechanical characteristics of different bone constituents. A surface roughness parameter was derived, the pit effectivity factor (fpe), which describes how effective the individual pits of the rough surface are as retention elements with regard to shear. Another surface roughness parameter was defined, the pit density factor (fpd), the value of which depends upon how densely packed the pits are. The interfacial shear strength of a rough implant surface with known microgeometry can be estimated by means of these two surface roughness parameters. The effectiveness of pits of different sizes and of different shapes was investigated using this model.

Assessment of cold welding properties of the internal conical interface of two commercially available implant systems.

STATEMENT OF PROBLEM The cone-screw abutment has been shown to diminish micromovement by reducing the burden of component loosening and fracture. However, anecdotal concern for cold welding of cone-screw joints in implant design has been identified as a potential source for lack of retrievability. PURPOSE This comparative study evaluated the loosening torque, as a percentage of tightening torque, for the ITI Straumann and Astra Tech (3.5 and 4.0 mm diameters) implant systems, which use an 8-degree and 11-degree internal cone, respectively. MATERIAL AND METHODS Implants and abutments from each system were mounted in a torque device, and a range of tightening torques was applied. Loosening torques were then measured, and the influence of conus angle, interfacial surface area, saliva contamination, and time delay to loosening were all assessed. RESULTS The loosening torque only exceeded tightening torque at the highest levels, just before component failure, when plastic deformation was expected. For all clinically relevant levels of torque, both in a dry environment and with components bathed in artificial saliva at 37 degrees C, loosening torque was always seen to be 80% to 90% of tightening torque, demonstrating that cold welding does not occur. There was a high correlation between loosening and tightening torque for all systems tested, but no statistical difference when comparing wet versus dry or comparing individual data for each system. CONCLUSIONS It can be concluded that for clinically relevant levels of tightening torque, no problems are anticipated with respect to retrievability.

In vitro evaluation of the strength of the conical implant-to-abutment joint in two commercially available implant systems☆

STATEMENT OF PROBLEM The cone-screw abutment has been shown to diminish micromovement, reducing the burden of component loosening and fracture. However, it is unclear whether the conical taper and joint design influence strength of the interface, with respect to unfavorable bending moments. PURPOSE This comparative study evaluated the resistance to bending for the ITI Straumann and Astra Tech ST implant systems using an 8- and 11-degree internal cone, respectively. MATERIAL AND METHODS Assembled units from each system were mounted in a 3-point bending apparatus. High load tests were performed, 4 mm from the joint, and bending moments necessary to induce first point of plastic deformation and ultimate failure were measured. All units were inspected to determine the critical zone of failure. RESULTS Bending moments necessary to induce first point of plastic deformation were considered well above that expected in clinical function for both systems. However, the critical zones of failure differed in that the solid Astra abutment deformed before the cone joint with its 11-degree taper and smooth transition into the neck of the screw, preventing screw fracture. By contrast, all ITI screws fractured at the head of the screw where it met the base of the 8-degree cone. It is unclear which aspects of the joint design were responsible for the difference observed in mode of failure or if it was a direct result of the experimental design. CONCLUSION For clinically relevant levels of bending moment, no problems were anticipated with respect to component failure for either system.

Letter to the Editor Re: Kim, Nowzari, and Rich (2013).

Dear Editor, We are writing this letter with regard to an article recently published in Clinical Implant Dentistry and Related Research by Yeoungsug Kim and colleagues, entitled “Risk of Prion Disease Transmission through Bovine-Derived Bone Substitutes: A Systematic Review” (2013; 15:645–653). The aim of the article was “to assess the risk of BSE transmission through anorganic bovine bone substitutes.” This review concluded that “bovinederived graft biomaterials may carry a risk of prion transmission to patients.” There are many shortcomings with the published review paper, which fails to substantiate the conclusions reached by the authors.