Some basic relationships between density values in cancellous and cortical bone.

Density is a salient property of bone and plays a crucial role in determining the mechanical properties of both its cancellous and cortical structural forms. Density is defined in a number of ways at either the bone tissue (D(app), apparent) or the bone material level (D(mat), material). The concept of density is relatively simple, but measuring it in the context of bone is a complex issue. The third dimension of the problem is the concept of porosity, or BV/TV (ratio of bone material volume over tissue volume). Recent investigations from our laboratory have revealed an interdependence of D(app) and D(mat) in the cancellous bone of at least four different cohorts of human patients. To clarify the underlying causes of this behaviour, we produced here equivalent relationships from specimens originating from cortical and cancellous areas of the same bone. Plots of D(app) vs. D(mat) showed that D(mat) was not a monotonic function of increasing D(app), but instead showed a 'boomerang'-like pattern. By empirically dissecting the data in two regions for D(app) above and below a value equal to 1.3gcm(-3), we were able to objectively isolate the bone in trabecular and compact forms. Our findings may have implications not only for the segregation of bone in these two structural forms, but also for the mechanobiological and physiological processes that govern the regulation of compact and trabecular bone areas.