Mechanics in skeletal development, adaptation and disease

Marjolein C.H. van der Meule

Sibley School of Mechanical & Aerospace Engineering Cornell University 219 Upson Hall Ithaca, NY 14853-7501

Assistant Scientist, Department of Biomechanics & Biomaterials The Hospital for Special Surgery 535 East 70 Street New York, NY 10021-4892

The growth and development of organisms living within Earth's gravitational field is intricately linked to mechanical demands. This is particularly true for the skeleton and the bone tissue of which it is made up. Not only does the skeleton play an important structural role in all vertebrates, but skeletal form is a direct reflection of the mechanical function. Similar to conventional engineering structures, the skeleton bears loads, but is distinguished by the ability to continually adapt to the mechanical forces experienced. Bone tissue consists of living cells surrounded by a matrix produced by the cells themselves. The matrix production, material constituents and organization are regulated by a variety of factors including mechanical loads. The material produced is exquisitely suited to its mechanical demands, demonstrating several important features: a polymer-ceramic composite structure providing high strength and toughness; anisotropic material properties resulting in greatest strength and stiffness in the primary weightbearing directions; and, complex tissue structures further affecting anisotropy and strength. These properties change with tissue maturity, individual age and disease. Continuous maintenance and repair processes help maintain healthy tissue and prevent damage accumulation. The mechanical regulation of bone structure initiates during early development and continues throughout life. The amount of bone tissue and its distribution is coupled to the mechanical environment. In the absence of mechanical forces during early development, disorganized and distorted bones form. Postnatally changes in loading primarily produce altered geometry, but our understanding of subtle material changes is improving with our ability to characterize the material structure.