Introduction: Considering maintenance and enhancement of biological functions of the targeted organs is a primary importance in development of biomaterials. When it comes to bone, mechanical function is one of the most important aspects to sustain load. Thus, we have been investigating the bone mechanical properties and the contributory factors of them focusing on the bone’s anisotropic microstructure that is composed of preferentially oriented apatite and collagen (extra cellular matrix; ECM)[1][2]. The aims of the present study are to clarify the contribution of the ECM orientation on the bone mechanical function comparing to that of bone mineral density (BMD) which is typically used in bone diagnosis, and to propose a new implant biomaterials that was designed to enhance the induction of bone tissue with well-oriented ECM.
Materials and Methods: The degree of the preferential apatite orientation in bones was analyzed conducting a microbeam X-ray diffractometer with an incident X-ray collimated into ~100 μmφ in diameter. Since apatite crystallizes on the collagen template in an epitaxial manner, the orientation of apatite reflects that of collagen. Young’s modulus was measured as a bone mechanical property utilizing nanoindentation. These analyses were done on the several types of bones: normal bones, healing bones and artificially induced bones by exposing bones to elevated stresses.
Results and Discussion: In all cases mentioned above, the degree of apatite orientation positively and significantly correlated with Young’s modulus rather than BMD. Multiple regression analysis revealed the higher impact of apatite orientation on the bone mechanical property[3]. It is important to induce apatite orientation to guarantee bone mechanical integrity. Moreover, the animal models proved that the primary controlling factor is the in vivo stress applied onto bones[3][4]. The bone implant that enhances the formation of oriented microstructure might be achieved considering the application of an optimal stress on the bone surrounding the implant. Under this concept a hip implant stem with anisotropic grooves were designed to control stress environment in the grooves. After insertion into beagle femur the formation of new bone with anisotropic ECM arrangement was demonstrated[5].
Conclusion: The orientation of ECM is a significant contributory factor of bone mechanical function and the bone implant needs to be developed under a careful consideration of establishment of oriented ECM microstructure.
This work was supported by the Grants-in-Aid for Scientific Research (S) from the Japan Society for the Promotion of Science (JSPS).
References:
[1] Nakano T. et al., Bone 31: 479–487, 2002
[2] Nakano T. et al., Bone 51:741–747, 2012
[3] Ishimoto T., Nakano T. et al., JBMR 28:1170–1179, 2013
[4] Wang J., Ishimoto T., Nakano T.: Mater Trans 54:1257–1261, 2013
[5] Noyama Y., Nakano T., Ishimoto T., et al.: Bone 52:659–667, 2013