Misconceptions (2): turnover is always higher in cancellous than in cortical bone.

Since it became generally recognized that bone is subject to turnover, it has often been asserted without qualification that cancellous bone has higher turnover than cortical bone. For example: “Much is often made of differences in cortical and metabolically more active cancellous bone . . .”; “. . . trabecular bone . . . is eight times more metabolically active than cortical bone”; “. . . trabecular bone is metabolically more active than cortical bone, with an annual turnover of approximately 20% to 30% for the former and 3% to 10% for the latter”; and “the cancellous bone, which represents about 20% of the skeletal mass, makes up 80% of the turnover, while the cortex, which represents 80% of the bone, makes up only 20% of the turnover.” Herein, I clarify exactly what such a belief implies, trace its origins, define the circumstances in which it is true, and specify the necessary qualifications, which are invariably omitted. But first I must explain precisely what is meant by “bone turnover.” The language of science frequently takes words in common use and redefines them for a different purpose, but the meaning of “turnover” in bone physiology is exactly the same as its everyday meaning, which is replacement. The personnel manager of a business refers to the turnover of employees and the owner of a store to the turnover of items of merchandise. In both cases, the rate of turnover is expressed as a proportion of the total (usually a decimal fraction or a percentage) per unit of time, which could be a year in the former case or a week in the latter. The reciprocal of turnover is always a period of time. For example, if the turnover of checkout clerks is 50%/year, their average length of employment will be 2 years, although their individual lengths may vary over a wide range. In everyday use, turnover is most often of people or things. Bone is a continuous material rather than an assemblage of discrete entities, but otherwise its turnover is defined in exactly the same way—it refers to proportional volume replacement per unit time, usually expressed as percent/year. As before, the reciprocal of turnover is a period of time; if bone turnover is 10%/year, then the mean lifetime of each moiety of bone is 10 years, although their individual lifetimes may vary over a wide range. People are usually replaced in response to resignation, and goods in response to sales, but replacement of bone is a more complex process that occurs in the adult skeleton by remodeling, of which the instrument is the basic multicellular unit, or BMU. Why remodeling occurs at all, a neglected but important question, does not bear on the present topic and has been discussed extensively elsewhere. Most BMUs originate on one of the three subdivisions of the endosteal envelope—cancellous, endocortical, or intracortical. Periosteal bone cell activity in the adult should probably be classified as remodeling rather than as modeling (at least in women), although the evidence remains incomplete. Remodeling on the endosteal envelope is imperfectly efficient after the age of about 40 years, because each transaction replaces slightly less bone than was removed, but within the limits of experimental error, measurement of the tetracycline-based bone formation rate is a reasonable starting point for the histological estimation of bone turnover. This measurement is usually expressed per unit of bone surface (BFR/BS), which must be multiplied by the geometrical quantity surface-to-volume ratio (BS/BV) to obtain a measurement of volume replacement (BFR/BV, expressed as percent/year). The concept of surface-to-volume ratio is commonplace in the physics of such processes as heat transfer, diffusion, and evaporation, but its application to bone raises an infrequently recognized problem that derives from its complex structure— how to decide which volume should be assigned to a particular surface. Rules have been formulated for defining where cancellous bone ends at its junction with the cortex, so that measurement of its surface-to-volume ratio requires only appropriate attention to stereology; however, the situation in cortical bone is less straightforward, because it has three surfaces—periosteal, intracortical (or haversian), and endocortical. For some purposes, it is useful to combine the endocortical and cancellous surfaces because both are in contact with marrow. The bone beneath the endocortical surface forms part of the transitional zone destined for eventual transformation to cancellous bone, and one possibility is to assign this bone to a depth of 75 m (approximately one-half the mean trabecular thickness) to the endocortical surface. Similar calculations have been carried out for the periosteal surface of the ilium using a depth of 50 m, based on the distribution of cement lines. For the present purpose, it seems reasonable to calculate also a single surface-to-volume ratio for the entire cortex including all three surfaces, with a corresponding single value for bone turnover. In whatever way surface-to-volume ratio in bone is defined and measured, it is obvious from simple inspection that this quantity is greater in cancellous than in cortical bone, even if precisely how much greater is not known. The wide appreciation Address for correspondence and reprints: Dr. A. Michael Parfitt, Division of Endocrinology and Center for Osteoporosis and Metabolic Bone Diseases, University of Arkansas for Medical Sciences, 4301 West Markham, Slot 587, Little Rock, AR 72205-7199. E-mail: parfittlwynm@uams.edu Bone Vol. 30, No. 6 June 2002:807–809