gms | German Medical Science

Deutscher Kongress für Orthopädie und Unfallchirurgie, 75. Jahrestagung der Deutschen Gesellschaft für Unfallchirurgie, 97. Tagung der Deutschen Gesellschaft für Orthopädie und Orthopädische Chirurgie, 52. Tagung des Berufsverbandes der Fachärzte für Orthopädie und Unfallchirurgie

25. - 28.10.2011, Berlin

Nitinol plates before and after shape transformation show variable stiffness: correlation between experimental and numerical results

Meeting Abstract

  • G.D. Olender - Laboratory for Biomechanics and Biomaterials, Department of Orthopaedics, Hannover Medical School, Hannover, Germany
  • R. Pfeifer - Laser Zentrum Hannover, Hannover, Germany
  • C.W. Müller - Medizinische Hochschule Hannover, Unfallchirrugische Klinik, Hannover, Germany
  • T. Gösling - Medizinische Hochschule Hannover, Unfallchirrugische Klinik, Hannover, Germany
  • S. Barcikowski - Laser Zentrum Hannover, Hannover, Germany
  • C. Hurschler - Laboratory for Biomechanics and Biomaterials, Department of Orthopaedics, Hannover Medical School, Hannover, Germany

Deutscher Kongress für Orthopädie und Unfallchirurgie. 75. Jahrestagung der Deutschen Gesellschaft für Unfallchirurgie, 97. Tagung der Deutschen Gesellschaft für Orthopädie und Orthopädische Chirurgie, 52. Tagung des Berufsverbandes der Fachärzte für Orthopädie. Berlin, 25.-28.10.2011. Düsseldorf: German Medical Science GMS Publishing House; 2011. DocPO16-622

DOI: 10.3205/11dkou639, URN: urn:nbn:de:0183-11dkou6395

Veröffentlicht: 18. Oktober 2011

© 2011 Olender et al.
Dieser Artikel ist ein Open Access-Artikel und steht unter den Creative Commons Lizenzbedingungen (http://creativecommons.org/licenses/by-nc-nd/3.0/deed.de). Er darf vervielfältigt, verbreitet und öffentlich zugänglich gemacht werden, vorausgesetzt dass Autor und Quelle genannt werden.


Gliederung

Text

Questionnaire: Nitinol is a shape memory alloy that is a resourceful biomaterial due to its shape changing ability, biocompatibility and hyperelasticity [1]. Until now, very limited applications have been tested for use of Nitinol plates in the area of fracture fixation to improve healing conditions [2]. In this experiment, bending stiffness of dogbone deformed shaped plates was conducted by four-point bending before and after shape transformation. Results show that the stiffness of a fracture fixation plate can be altered in a consistent manner and that preliminary FEM models were useful for stiffness prediction. Applying a specific alteration of stiffness in a fracture plate at a desired time could be beneficial to osteosynthesis as well as avoiding additional invasive procedures [3].

Methods: The sample plates were created by laser welding of Nitinol plates [4]. Stack thickness was varied between groups (2.5, 3.0, 3.5 and 4.0 mm). Each group had 3 samples. The plates were tested according to the protocol ISO 9585: 1990(E). A four-point bending rig was attached to the uniaxial MTS test machine (Mini Bionix 858, MTS Systems, USA). The testing rig was surrounded by a Plexiglass chamber that held water and was cycled by a pumping device (CC-205, Huber, Germany). The heating chamber of the device controlled the temperature, as it was raised from 37º C, for testing, to 55º C, initiating shape change. The plates were cooled, back to 37º C and tested again. CAD models were designed with Solid Edge and FEM was done with NX 7, (Siemens GmbH, Germany).

Results and Conclusions: Four-point bending of a single Nitinol plate changed equivalent stiffness from 0.77 Nm2 to 3.07 Nm2 due to material phase and shape change. This was an increase of 301%. When the sample was cooled and tested again at 37º C, the increase in stiffness was 160% over the dogbone shape baseline (Figure 1 [Fig. 1]).

The repeated bending tests between the pre transformation (dogbone) and post transformation (straight) all showed significant increase in stiffness (p< 0.05) by one sided, paired Student T-tests. The plate of 4 mm thickness had the highest stiffness for the straight shape (1.45 Nm2; SD 0.06) and for the dogbone samples (0.99 Nm2; SD 0.03).

FEM results also displayed an increase in stiffness between the two shape forms of the same thickness dimension. The percent differences were similar to the experimental results, although the agreement varied.

Calculation of the area moment of inertia alone is not sufficient to predict stiffness changes because the changes in cross section affects only a relatively small central section, and are not homogeneous throughout the plate.

In this study, we found that there were significant changes in bending stiffness due to a temperature induced configuration change. Amongst the various design thicknesses were varying degrees of stiffness alteration after transformation. The shape transformation within the sample groups also displayed consistency (low SD) as seen in Fig. 2.