Push-Out Test

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Push-out test

Fundamentals of the interface behaviour between implant materials and bone

For the assessment of the suitability of implant materials (see: implant testing), the development of new bio-plastics and the targeted surface modification of implants intended to be firmly anchored in the skeletal system, the quantitative investigation of the load-bearing capacity of the interface (see: phase boundary surface) between the respective implant material and the bone is of fundamental importance. An essential prerequisite for the long-term load transfer between the implant and the bone is direct contact without any connective tissue-based intermediate layers, a process also known as osseointegration. To date, there is no unified theory regarding the nature and functionality of the interface—that is, the biomechanics—as some authors have been able to demonstrate afibrillar intermediate layers with a thickness of approximately 0.1 µm, whilst others have observed direct contact with the mineralised bone. Regardless of these findings, the achievable load transfer depends significantly on the implant material used, the geometry and surface structure of the implant, and the possible presence of bioactive surface layers.

Experimental investigation of the interface (see: phase boundary surface) using model implants in animal experiments (mostly rabbits) has proven effective for the quantitative characterisation of interface behaviour and the assessment of interface strength. The tests most commonly used in testing practice are the pull-out test (see: tensile test and sound emission analysis) and the push-out test, in which the maximum force or shear strength achieved is generally equated with the ultimate failure (see: fracture) of the implant--bone composite. Contrary to these findings, however, abrupt failure is often not observed in clinical practice; instead, there is a gradual loosening of the implant with stepwise interface failure and a subsequent sinking of the endoprosthesis. This consecutive tearing of the trabeculae or partial debonding of the interface between bone and implant leads to local stress peaks at the remaining contact points and ultimately to unstable crack propagation with macroscopically and clinically evident loosening.

The expanded push-out test

Due to the complex interfacial behaviour between implant materials and bone materials, the conventional push-out test (see Figure 1) has been supplemented with a damage-sensitive, non-destructive testing method, namely sound emission testing. This significantly increases the information content of this technological testing method within the field of polymer diagnostics. The sensing pin, designed as a waveguide, pushes the implant (shown in grey) out of the implant-bone complex. The implant is approximately 5 to 10 mm long and has a diameter of 5 mm, although interchangeable disc geometries also allow for other implant geometries.

Dig. 1:

Expanded push-out test with a connected acoustic emission sensor (a) and a schematic diagram of the puncture impact test

Performing the expanded push-out test

At the start of the test, the load–extension diagram and the emitted acoustic emission are recorded in real time using the connected AE receiver (see: sound emission analysis). As a rule, low test speeds in the range of 1 to 5 mm min^-1 are selected in order to achieve high acoustic signal resolution and to avoid an excessively high event density of the acoustic signals. The shear stress τ and the shear deformations γ can be calculated from the F–Δl diagram:

$\tau \,=\,{\frac {F}{A_{M}}}$

$\gamma \,=\,\arctan \,{\frac {2\Delta l}{D-d}}$

A_M is the outer surface area of the implant; however, this should be verified histologically at a later stage.

$A_{M}\,=\,\pi dh$

In the linear region of the shear stress–shear deformation curve, the shear modulus G can be calculated after eliminating any offset effects:

$G\,=\,{\frac {\tau }{\gamma }}$

See also

• Implant testing
• Lid-opening test
• Puncture impact test
• Instrumented puncture impact test

Referencees

• Holweg, K., Brandt, J., Bierögel, C., Grellmann, W., Altenbach, H.: Schädigungscharakterisierung der Implantat-Knochen-Grenzfläche. 4. Tagung des DVM-Arbeitskreises "Biowerkstoffe": Grenzflächen bei Implantaten – Mechanische und biologische Aspekte, March 26–27, 2004, Cologne, Proceedings pp. 9–17 Download as pdf No. 252
• Bierögel, C.: Push-out tests for implants. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 626–629 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library unter A 22)
• Brandt, J., Bierögel, C., Holweg, K., Hein, W., Grellmann, W.: Erweiterter push-out-Test zur Schädigungscharakterisierung der Implantat-Knochen-Grenzfläche. Biomedizinische Technik 50 (2005) 6, pp. 1–6 DOI: https://doi.org/10.1515/BMT.2005.028
• Brandt, J., Pfennig, M., Bierögel, C., Grellmann, W., Bernstein, A.: Hydroxyapatite coating improves bone integration and interface strength of polymer implants in bone. Key Engineering Materials Vols. 396–398 (2009) 331–335 DOI: https://doi.org/10.4028/www.scientific.net%2FKEM.396-398.331
• Holweg, K.: Bestimmung der Eigenschaften der Grenzfläche zwischen Knochen und Implantat. Dissertation, Martin-Luther-Universität Halle-Wittenberg (2010) (see AMK-Library under B 1-22)