Biomedical subjects
Publications and source records attributed to H Zitter.
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The torsion test for cortical and cancellous screws introduced by DIN and adopted by ISO is applied to control the tolerances defined in these standards. Both standards contain wants of clearness and the essential fault to tolerate products which achieve the claimed torsional moment only by plastic deformation during the test. The torsional moment determined in this way simulates a higher torsional resistance as it is characteristic for a rigid junction. This defect is demonstrated with screws of different tensile properties and compared with a lot of samples. In order to be able to recognize and eliminate insufficient products the torsion moment at a small angle of torsion is proposed as characteristic value instead of the maximal torsion moment at an undefined angle of torsion.
According to two agreeing standards, the mechanical requirements for bone screws consist of a minimum breaking torque and of the minimum angle of rotation to failure of 180 degrees. Apparently it has been overlooked that these requirements admit austenitic steels with a very low yield strength, which reach the required torque at fracture only by the work-hardening brought about during the torsion test. As a high yield strength is a prerequisite of a stable fixation, a formulation is proposed which guarantees not only a high elastic torque but also an adequate work of deformation in the plastic state as an insurance against embrittlement.
This study introduces a simple in vitro arrangement to measure current densities of implant metals. The in vivo condition of a metallic implant lying in tissues exhibiting different redox potentials is simulated in so-called straddle tests by applying a constant potential difference of 250 mV in saline containing the stable, fast-reacting redox system K4Fe(CN)6/K3Fe(CN)6. From a variety of corrosion-resistant implant metals and alloys, gold showed the highest current densities, followed by the stainless steel, the cobalt-based alloy, and the TiAIV-alloy. The pure metals titanium, niobium, and tantalum showed the lowest values. This can be explained by the stable oxide layer on these base metals, preventing an exchange of electrons and thus any redox reaction. This rating of metallic implant materials based on in vitro measurements of current densities is in good accordance with their biocompatibility rating reported from in vivo experiences. It seems that simple and cheap electrochemical tests allow an even more precise differentiation of the suitability of metallic materials for implant purposes than most of the conventional implantation tests, considering that biocompatibility is not only determined by corrosion products, but also by exchange currents and reaction products of redox processes involving tissue compounds.
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Failure analysis reveals, that using different materials for expansion screws and fixing elements may cause severe galvanic corrosion or pitting corrosion. Such failures can easily be avoided by using only one alloy with good passivation characteristics instead of different alloys.
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