Mechanobiology and joint conformity regulate endochondral ossification of sesamoids

Sesamoid bones form by the endochondral ossification of sesamoid cartilages. This ossification process is thought to be similar to that responsible for the formation of secondary ossific nuclei in long-bone epiphyses. Sesamoids ossify much later in development than do epiphyses, however, and bone formation within sesamoids often begins by way of multiple ossific nuclei. Endochondral growth and ossification in the formation of secondary ossific nuclei have previously been correlated with distributions of the octahedral shear and hydrostatic stresses generated in vivo within cartilage anlagen. In this study, we used two-dimensional finite element analysis to predict the distributions of octahedral shear and hydrostatic stresses in an idealized model of a sesamoid cartilage subjected to in vivo loading. We examined the influence of sesamoid joint conformity. The distribution of an osteogenic stimulus was calculated with an approach similar to that used to predict epiphyseal ossification. The results suggest that, compared with conforming joints, nonconformity between the sesamoid cartilage and its articulating surface, which arises during early development, produces higher contact pressures within the sesamoid and leads to a thicker articular cartilage layer. For a nonconforming joint surface, the results suggest that ossification is favored anywhere within a broad internal region of the sesamoid, whereas a layer at the articular surface will remain cartilaginous. These findings highlight the subtle differences between ossification processes in epiphyses and sesamoids, indicating that the mechanical stress environment in sesamoids produces a diffuse stimulus leading to the onset of ossification and that the degree of joint nonconformity may influence the thickness of the articular cartilage layer.