CONFINED AND UNCONFINED COMPRESSION TESTS OF HUMAN TRABECULAR BONE UP TO VERY LARGE STRAINS

Presentation O-45

S48

Trabecular Bone

CONFINED AND UNCONFINED COMPRESSION TESTS OF HUMAN TRABECULAR BONE UP TO VERY LARGE STRAINS Mathieu Charlebois, Christian Haunschmid, Philippe K. Zysset

Institute of Lightweight Design and Structural Biomechanics, Vienna University of Technology, Austria

Introduction Osteoporotic fractures often involve large and permanent deformation as well as compaction of trabecular bone. Efforts are currently made to perform nonlinear simulations of bone and boneimplants system that include the related post-yield behaviour. The aim of this study is to quantify the post-yield properties in confined and unconfined compression up to 60% strain of human cancellous bone from different anatomical sites.

Methods Cylindrical samples (Ø=8mm, L=12mm) from human femoral heads (n=24), calcanei (n=26), and distal radii (n=20) were extracted using a standard procedure. All samples were scanned using a CT system to obtain volume fraction ( ) and axial trabecular orientation (fabric, ma). Confined (C) and unconfined (U) uniaxial compression tests were performed with a servohydraulic actuator. The protocol consisted of 10 strain controlled cycles in the elastic regime followed by an 8mm ramp at a rate of 0.015mm/sec. The variables evaluated for each curve were elastic modulus, ultimate and minimal postyield stress and strain (blue points on Fig. 1), as well as the dissipated energy (WD) during compaction (grey area).

the tangent stiffness decreases. Then, stress reaches an ultimate point (Sult) and softens down to a minimum point (Smin) from which it increases again due to compaction. This rehardening presents first an irregular linear trend. In confined experiments, this trend was followed by an exponential increase of slope at very large strains. Statistics of the key post-yield variables are shown in Table 1.

Radius

C

=0.131 ma=1.438

U

Femur

C

=0.230 ma=1.080

U

Calcan.

C

=0.189 ma=0.758

U

Sult [MPa]

Eult [-]

Smin/Sult [-]

WD [MPa]

2.857 ±1.059 2.932 ±1.925 5.163 ±3.842 5.291 ±2.821 1.340 ±1.196 1.304 ±0.876

0.010 ±0.003 0.016 ±0.024 0.022 ±0.009 0.023 ±0.010 0.023 ±0.011 0.036 ±0.028

0.332 ±0.604 0.209 ±0.223 0.793 ±0.954 0.614 ±0.936 0.820 ±0.922 0.783 ±0.843

0.725 ±0.412 0.368 ±0.225 2.682 ±2.276 1.690 ±0.872 1.271 ±1.650 0.863 ±0.595

Table 1: Summary of the post-yield variables.

Discussion The distribution of volume and fabric follows previous observations in the same anatomical sites [Matsuura, 2006]. As expected, ultimate stress increases with volume fraction and fabric, while ultimate strain decreases slightly with fabric. The dimensionless ratio Smin/Sult seems also to be inversely related to fabric. The dissipated energy is clearly dominated by volume fraction and is, like in a former study, significantly higher for confined experiments [Hayes, 1976]. These results will allow identifying the material parameters of a novel large strain constitutive law for trabecular bone based on volume fraction and fabric.

Acknowledgement Figure 1: Typical stress-strain curve obtained from a confined compression test.

This project was supported by grant no 05-Z26 of the AO Research Fund, AO Foundation.

References Results A typical loading curve (Fig. 1) shows, at first, an elastic increase of stress up to a yield point where

Journal of Biomechanics 41(S1)

Hayes et al., J. Biomed. Mater. Res. Symposium, 7:537-544, 1976. Matsuura et al., Biomech. Model. Mechanobiol., online, 2007.

16th ESB Congress, Oral Presentations, Monday 7 July 2008