The mechanical or metabolic function of secondary osteonal bone in the monkey Macaca fascicularis

Secondary osteonal bone is believed by many to serve a mechanical function, altering the properties and/or orientation of bone in response to fluctuating mechanical demands or in the prevention and/or repair of fatigue microdamage. Based on this belief, secondary osteons should be concentrated mainly in regions experiencing high peak-strain conditions. Others contend that secondary osteonal bone functions primarily in meeting the body's calcium needs, and should be expected to form principally in low peak-strain regions so as to avoid compromising the mechanical strength of the bone. These two hypotheses were tested by examining the distribution of secondary osteonal bone in both relatively high- and low-strain regions of the macaque face. Previous strain-gauge studies have demonstrated a steep strain gradient in the macaque face, with relatively high peak strains in the anterior portion of the zygomatic arch and in the mandibular corpus. Relatively low peak strains have been found in the posterior portion of the zygomatic arch and supraorbital bar. Results presented here show that in the mature macaques, there is no consistent relation between newly forming secondary osteons (i.e. those labelled with fluorescent dyes) and peak strain levels. From these data it is concluded that, in the non-perturbed adult, either mechanical and metabolic factors contribute equally to the observed pattern or that metabolically driven remodelling is initiated without regard to strain levels. In immature macaques, however, the relation between peak strain levels and secondary osteon density is positive, with a significantly higher density of labelled osteons in the high strain regions. From these data it is concluded that, in immature individuals, mechanical factors are predominantly responsible for the initiation of secondary osteonal remodelling.