Introduction: Endosseous cementless implants are widely used in orthopaedic, maxillofacial and oral surgery. However, failures are still observed and the assessment of the biomechanical properties of the bone–implant interface may improve the understanding of the osseointegration process [6],[8].
This work deals with the development of experimental multimodality approaches for the characterization of the biomechanical properties of the bone-implant interface. A multiphysical approach was carried out using a dedicated animal model consisting in using coin-shaped titanium implants inserted in vivo on the proximal part of the tibia of rabbits (see Fig. 1 and 2) with different healing durations.
Materials and Methods: The first objective of this study was to investigate the evolution of elastic properties of newly formed bone tissue as a function of healing time [7]. To do so, nanoindentation [5][2] and micro-Brillouin scattering [1] techniques are coupled following a multimodality approach using histological analysis, allowing to retrieve complementary information at the micrometer scales. The biomechanical properties of newly formed bone tissue were compared with that of mature bone tissue.
The second aim was to investigate the effect of bone healing on the ultrasonic response of the bone-implant interface[4]. The ultrasound response of the interface was measured in vitro at 15 MHz after 7 and 13 weeks of healing time. The average value of the ratio r between the amplitudes of the echo of the bone-implant interface and of the water-implant interface was determined.
The third objective was to retrieve the different quantities related to the bone-implant interface such as: torsional stiffness, shear modulus, maximal torsional loading, mode III fracture energy and stress intensity factor. To do so, mode III cleavage experiments were performed [3].
Results: The bone mechanical properties were measured in mature and newly formed bone tissue. Analysis of variance and Tukey–Kramer tests reveals a significant effect of healing time on the indentation modulus and ultrasonic velocities of bone tissue. The results show that bone mass density increases by 12.2% (2.2% respectively) between newly formed bone at 7 weeks (13 weeks respectively) and mature bone.
The significant decrease of the ultrasonic quantitative indicator r (p=2.10-4) as a function of healing time (from r=0.53 to r=0.49) can be explained by the increase of the BIC (from 27% to 69%).
The approach allows to estimate torsional stiffness (around 20.5 N.m.rad−1), shear modulus (around 240 MPa), maximal torsional loading (around 0.056 N.m), mode III fracture energy (around 77.5 N.m−1) and stress intensity factor (0.27 MPa m1/2).
Discussion: The dependence of bone properties on healing time may be explained by the evolution of bone microstructure and mineralization. The significant decrease of the ultrasonic reflective properties of the bone-implant interface versus healing time can be explained by the decrease of the gap of acoustical impedance as a function of healing time, as confirmed by the biomechanical measurements realized in newly formed bone tissue.
Conclusion: This study paves the way for the development of a multimodality approach to extract quantitative information from the bone-implant interface, which might help for the development and optimization of implant biomaterial, surface functionnalization and medical treatment investigations.
This work has been supported by French National Research Agency (ANR) through the PRTS program (project OsseoWave n°ANR-13-PRTS-0015-02).
References:
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[3] Mathieu, V., Vayron, R., Barthel, E., Dalmas, D., Soffer, J.E., Anagnostou, F., and Haïat, G., "Mode III cleavage of a coin-shaped titanium implant in bone: effect of friction and crack propagation", J Mech Behav Biomed Mater 8 (2012) pp 194-203.
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