Morphology of the Drifting Osteon

Abstract

Drifting osteons were followed longitudinally through the cortex of human and baboon long bones using serial sections. Direction of transverse drift was recorded at different cross-sectional levels of the same systems, and maximum angular change in drift direction was measured for each system. Most drifting osteons exhibit: (1) substantial (∼90°) variation in the direction of transverse drift along their longitudinal axes, (2) intermittent regions of concentric (type I) morphology, and (3) change in drift direction over time, evident at single cross-sectional levels. Additionally, 3-dimensional reconstruction reveals that the basic multicellular units (BMUs) responsible for creating drifting osteons are morphologically distinct from the cutting-cone–closing-cone model BMUs that produce other types of osteons. The stimulus involved in the activation and guidance of drifting BMUs is unclear, but it is likely that the complex strain environment experienced by long bone cortices exerts a significant influence on their morphology.

Are Distributions of Secondary Osteon Variants Useful for Interpreting Load History in Mammalian Bones?

“…Actively (AC) drifting: a secondary osteon that exhibits a history of continuous resorption on one side and continuous formation on the other; as a result the osteon becomes flattened in one plane and elongated transversely to this plane. The arrow indicates direction of drift of the osteon on the right [reproduced from Robling and Stout, 1999, with permission of the publisher, S. Karger AG, Basel, Switzerland]. D Elongated: A secondary osteon in which the length of one axis (or diameter) is at least 2 times the length of the short axis (or diameter).…”

“…The drifting osteon is a highly unusual secondary osteon variant that may play a role in enhancing bone mechanical properties, especially toughness, by creating greater interfacial surfaces (cement lines) [Robling and Stout, 1999]. Johnson [1964, p. 585] described drifting osteons as 'geographic osteons', and Knese and Titschak [1962] described them as 'schalensteons' (their lamellae resemble the grooved patterns of a mussel shell).…”

“…Epker and Frost [1965] suggested that in human ribs this osteon variant consistently drifts toward the endosteum (i.e., probably bone with greatest mean tissue age, hence highest mineralization) thereby maximizing available mineral for homeostatic purposes. According to Robling and Stout [1999], Coutelier [1976] also suggested that the primary stimulus for drifting osteons is mineral homeostasis, while mechanical influences are secondary consequences. But this interpretation is controversial [Robling and Stout, 1999].…”

“…According to Robling and Stout [1999], Coutelier [1976] also suggested that the primary stimulus for drifting osteons is mineral homeostasis, while mechanical influences are secondary consequences. But this interpretation is controversial [Robling and Stout, 1999]. Although drifting osteons are thought to primarily occur in young bone, Robling and Stout [1999, unpubl.…”

“…4 ). There are many additional examples showing how secondary osteons can course obliquely, become extensively interconnected, and/or exhibit directional changes, including studies of drifting osteons [Robling and Stout, 1999] and osteons in shear/torsion environments [Stover et al, 1992;Skedros et al, 1997;Sorenson et al, 2004] or in complex mechanical environments near implanted devices such as metal screws and tooth fixtures [Albrektsson and Johansson, 1991]. Studies of the microstructural organization of deer calcanei also suggest that the significant differences between the dorsal, plantar, and medial/lateral cortices in the prevalence of osteonal variants [Skedros et al, 1994b;Sorenson et al, 2004] may represent differential adaptations that toughen regions by minimizing microdamage formation and/or propagation in their respective habitual strain modes (i.e., tension, compression, or shear) [Anderson and Skedros, 2005] ( fig.…”