A geometric and microradiographic study of functional adaptation in the human skeleton

Abstract

<p>Continuing Periosteal Apposition (CPA) refers to recent lamellar bone formation on the external surface of the mature skeleton. CPA is most often viewed as mechanical compensation, a form of functional adaptation in the event of overstrain pursuant either to high levels of physical activity, or to prior endosteal bone resorption and increased endocortical porosity. The theoretical basis for this expectation derives primarily from Frost's Mechanostat, describing the relationship of bone surfaces and bone cellular activity in response to changes in the functional strain environment. An objective of the study was to ascertain how CPA might be associated with structural (geometric) properties characterizing entire cross-sections (areas and moments of area), as well as with proximate histological variables, such as intracortical porosity and mean bone age. The latter variable is derived from a mathematical model developed in this study, which provides an index of intracortical remodelling 'history'. Left and right second metacarpals, and the left second metatarsal, obtained from medical school cadavers (n = 89 bones) and a small EuroCanadian historic cemetery (n = 21 bones) comprised the sample for this study; the majority were male, and over 50 years of age. Cross-sectional geometric properties indicative of structural strength were measured from three sections prepared from each element, one at midshaft and two from the distal diaphysis; a subsample was microradiographed for histological evaluation at four locations within each cross-section. Although not without exception, the directional asymmetries identified between the various structural and histological variables: (1) tend to reflect presumed functional strain inequities pertaining to side (e.g., right > left), gender (male > female), age (young > old). The presumed inequity for 'lifestyle' (historic > cadaver) was not supported; (2) support the conclusion that CPA constitutes a component of functional adaptation in the skeleton and may act as mechanical compensation; and (3) support the Mechanostat as an interpretive framework for skeletal biological analyses.</p>