MECHANOBIOLOGY OF SKELETAL REGENERATION

Skeletal regeneration is accomplished by a cascade of biologic process es that may include differentiation of pluripotential tissue, endochon dral ossification, and bone remodeling. It has been shown that all the se processes are influenced strongly by the local tissue mechanical lo ading history. This article reviews some of the mechanobiologic princi ples that are thought to guide the differentiation of mesenchymal tiss ue into bone, cartilage, or fibrous tissue during the initial phase of regeneration. Cyclic motion and the associated shear stresses cause c ell proliferation and the production of a large callus in the early ph ases of fracture healing. For intermittently imposed loading in the re generating tissue: (1) direct intramembranous bone formation is permit ted in areas of low stress and strain; (2) low to moderate magnitudes of tensile strain and hydrostatic tensile stress may stimulate intrame mbranous ossification; (3) poor vascularity can promote chondrogenesis in an otherwise osteogenic environment; (4) hydrostatic compressive s tress is a stimulus for chondrogenesis; (5) high tensile strain is a s timulus for the net production of fibrous tissue; and (6) tensile stra in with a superimposed hydrostatic compressive stress will stimulate t he development of fibrocartilage, Finite element models are used to sh ow that the patterns of tissue differentiation observed in fracture he aling and distraction osteogenesis can be predicted from these fundame ntal mechanobiologic concepts. In areas of cartilage formation, subseq uent endochondral ossification normally will proceed, but it can be in hibited by intermittent hydrostatic compressive stress and accelerated by octahedral shear stress (or strain). Later, bone remodeling at the se sites can be expected to follow the same mechanobiologic adaptation rules as normal bone.