Prosthesis Pull-through Test
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Prosthesis pull-trough test
Fundamentals of the technological testing method
From the perspective of polymer testing and diagnostics, the prosthesis pull-through test is one of the technological test methods used in implant testing and serves to characterise stiffness and dimensional stability [1]. The method was developed in the 1990s [2–3] and is used to evaluate the material and deformation behaviour of voice prostheses [4].
Figure 1 shows a schematic representation of the pull-through test, as well as a typical load–deformation diagram.
| Fig. 1: | Prosthesis pull-through test (a) with a schematic load–deformation diagram (b) |
In the prosthesis pull-through test, which simulates the patient removing the prosthesis, the resistance offered by the side opposite the valve during the pull-through is measured. The characteristic values here are identical to those of the lid-opening test. In contrast to dynamic tests, the loading speed (see: test speed) in these test procedures are also set relatively low, at a maximum of 10 mm min-1, in order to obtain the best possible information on the strength, stiffness and deformation behaviour.
Technological test procedures that simulate the actual in-service behaviour of the prosthesis allow conclusions to be drawn about the dimensional stability and service life of the implants.
Requirements for the in-use performance of voice prostheses
Pharyngotracheal shunt valves, commonly referred to as voice prostheses, are used for vocal rehabilitation in patients who have had their larynx completely removed. Such valve prostheses, which are manufactured from biocompatible materials, usually consist of a tubular body of varying length with flanges on both ends, which ensure that the valve sits securely in the shunt area. A valve is incorporated into the oesophageal flange, which, once the prosthesis has been fitted, allows air to pass from the trachea to the oesophagus but is impermeable to food in the opposite direction. Commercially available voice prostheses of the ‘Provox®’ or ‘ESKA-Herrmann’ types are usually made from elastomers, such as silicone rubber, special silicone rubber compounds or polyurethanes. The advantage of these materials lies in their excellent chemical and physical resistance, biocompatibility, high elasticity and sufficient durability. The main disadvantage of such valve prostheses is their limited service life in the moist, enzymatically active and non-sterile environment of the shunt, combined with high mechanical stress. This is caused in particular by the action of microflora – that is, colonisation by fungi and bacteria – which, after a relatively short time, significantly reduces the prosthesis’s ability to self-clean, thereby compromising valve functionality in particular [5]. There are various ways of minimising the effects of biocorrosion and biodegradation. These include, on the one hand, the targeted chemical and physical modification of the prosthesis surface using bio-inert or nanostructured (lotus effect) coatings and, on the other hand, frequent cleaning of the prosthesis by the patient to remove adhering biofilms at an early stage. This does not fundamentally solve the problem of biocorrosion, but it does result in a longer functional life for the valve or a longer service life for the voice prosthesis [6].
This more frequent replacement of the voice prosthesis, which must be carried out by the patient, should be straightforward to perform; in other words, the pharyngo-tracheal prosthesis should be replaceable without significant resistance. This gives rise to a general problem: if the prosthesis is easy to replace – i.e. has low structural rigidity – the valve will function well due to its smooth operation, but a secure fit within the shunt cannot be guaranteed. If, on the other hand, the aim is to achieve an optimal connection between the shunt and the voice prosthesis, the valve may operate too stiffly and be more prone to leaks once colonised by microorganisms.
Characterisation of selected biocompatible plastics for voice prostheses
Consequently, key criteria for the selection of biocompatible materials for this application include, in addition to their chemical properties, the material-specific stiffness – expressed by the modulus of elasticity – the hardness, particularly the surface hardness of the material, the aerodynamic drag of the prosthesis, especially the valve lid or clap, and the long-term behaviour (see, for example: creep tensile test). In addition, such plastics generally exhibit pronounced viscoelastic material behaviour, meaning that creep and stress relaxation under long-term static or dynamic loading (see: fatigue) are also of great importance.
Various polymer testing methods are available for characterising these material properties, such as the lid-opening test and the prosthesis pull-through test.
The prosthesis pull-through test has proven to be a sensitive testing method that can be usefully employed to determine material values.
The characteristic values of interest here are the maximum force achieved and the rise of the curve in the initial region, which serves as a measure of stiffness in N mm-1.
Figure 2 shows the maximum load for the selected prosthetic materials: polyurethane (1), silicone rubber (2) and a silicone rubber blend (3) [1].
| Fig. 2: | Prosthesis pull-trough test for selected materials [3] |
From Figure 2, it can be seen from the maximum loads that the three implant materials examined exhibit significantly different behaviour in these technical tests. In the prosthesis pull-trough test, the lowest loads are recorded for the PUR elastomer, which is certainly positive in terms of valve deflection but does not guarantee a secure fit of the prosthesis.
In the prosthesis pull-trough test, a very high resistance was measured for the pure silicone rubber, which could well pose a problem for the patient. As the lid-opening test also reveals higher values for the silicone rubber blend compared with pure silicone rubber, the silicone rubber blend therefore represents the best compromise solution for practical clinical application.
See also
Preferences
| [1] | Bierögel, C.: Implant testing. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022), 3rd Edition, pp. 629–632, (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22) |
| [2] | Bierögel, C., Bethge, I., Grellmann, W., Haberland, E.-J.: Deformation behaviour of voice prostheses – Sensitivity of mechanical test methods. In: Grellmann, W., Seidler, S. (Eds.): Deformation and Fracture Behaviour of Polymers. Springer, Berlin Heidelberg (2001) pp. 471–476 (ISBN 978-3-540-41247-2; E-Book: ISBN 978-3-662-04556-5; see AMK-Library under A 7) |
| [3] | Zwanzig, I., Haberland, E.-J., Bierögel, C., Grellmann, W.: Werkstoff- und Deformationsverhalten von funktionellen Prothesen im pharyngo-trachealen Bereich. Tagung Polymerwerkstoffe (1996), September 18–20, 1996, Merseburg, Proceedings, pp. 494–497 |
| [4] | Haberland, E.-J., Neumann, G., Löbe, L. P., Voigt, K. (1990): Stimmprothese. Patentschrift Nr. DD 275183 A1, Aktenzeichen WP A 61F, 17.01.1990 |
| [5] | Šebova, I., Haberland, E.-J., Stiefel, A.: Microbial corrosion of pharyngo-tracheal shunt valves (`voice prostheses`). In: Grellmann, W., Seidler, S. (Eds.): Deformation and Fracture Behaviour of Polymers. Springer, Berlin Heidelberg (2001) pp. 461–470 (ISBN 978-3-540-41247-2; E-Book: ISBN 978-3-662-04556-5; see AMK-Library under A 7) |
| [6] | Haberland, E.-J., Berghaus, A., Füting, M., Bethge, I., Grellmann, W.: Material parameters and ESEM characterization of functional ENT prostheses during ongoing degradation. In: Grellmann, W., Seidler, S.: (Eds.): Deformation and Fracture Behaviour of polymers. Springer, Berlin Heidelberg (2001) pp. 451–460 (ISBN 978-3-540-41247-2; E-Book: ISBN 978-3-662-04556-5; see AMK-Library under A 7) |