Comment www.thelancet.com Vol 375 May 15, 2010 1673 On May 17, 2010, when representatives of the 193 Member States of WHO gather at the yearly World Health Assembly (WHA) in Geneva, a draft global code of practice on the international recruitment of health personnel will be on the agenda. Negotiation and adoption of a WHO code of practice is an unprecedented opportunity to advance global consensus to address the critical challenges of migration of health workers and its effect on health systems worldwide. Increased demand for health services in high-income countries is attracting large numbers of trained health workers from the world’s poorest nations, contributing to severe shortages in those low-income countries. 1 Pull factors, including targeted recruitment from International recruitment of health personnel trabecular surfaces at about the same time, bone density actually increases and risk of fracture falls. Although measurement of BMD with dual energy X-ray absorptiometry is considered the gold standard as a surrogate for strength and risk of fracture, interpretation is problematic when assessing the effects of treatments such as teriparatide that stimulate bone remodelling and thereby increase porosity transiently. As Zebaze and co-workers point out, 60% of specimens in the sample that had high porosity had normal BMD. Thus BMD is an imperfect measure, especially when trying to associate it with risk of fracture. Today’s article is important because it emphasises how much of the bone loss in osteoporosis is cortical. Zebaze and colleagues show that much cortical bone loss is age-related, occurring after age 65 years, and is not driven solely by the fall in oestrogen concentration; thus, both men and women are affected. Importantly, they have also noted that once a substantial portion of cortical bone has been lost, and trabecularisation of the inner cortex has occurred, bone loss from these cortical remnants is mistakenly regarded as trabecular bone loss. This process leads to underestimation of the age-related increase in porosity, and contributes to difficulties in the use of BMD to assess fracture risk. Agents that control loss of cortical bone and consequent increased porosity have important effects on reduction of fracture risk. One recommendation for treatment that stems from this work is that drugs targeting cortical bone should be used to treat age-related bone loss late in life. The recent finding that risedronate can reduce porosity substantially during 5 years of treatment could contribute to the effectiveness of this drug in treatment of low bone mass and reduction of fractures. 13 Today’s findings—namely, that most fractures occur after age 65 years and are attributable to greater loss of cortical than of trabecular bone—have important implications for both identification and treatment of osteoporosis. David B Burr Department of Anatomy and Cell Biology, Indiana University School of Medicine, Indianapolis, IN 46077, USA dburr@iupui.edu I have been a consultant for and have received grant support from Eli Lilly and Company, Amgen, and Procter and Gamble Pharmaceuticals. I serve on a scientific advisory board for Eli Lilly and Co, and have been a paid speaker for Eli Lilly and Co and Amgen. 1 Zebaze RMD, Ghasem-Zadeh A, Bohte A, et al. Intracortical remodelling and porosity in the distal radius and post-mortem femurs of women: a cross-sectional study. Lancet 2010; 375: 1729–36. 2 Rockoff SD, Sweet E, Bleustein J. The relative contribution of trabecular and cortical bone to the strength of the human lumbar vertebrae. Calcif Tissue Res 1969; 3: 163–75. 3 Holzer G, von Skrbensky G, Holzer LA, Pichl W. Hip fractures and the contribution of cortical versus trabecular bone to femoral neck strength. J Bone Miner Res 2009; 24: 468–74. 4 Schaffler MB, Burr DB. Stiffness of compact bone: effects of porosity and density. J Biomech 1988; 21: 13–16. 5 Rice JC, Cowin SC, Bowman JA. On the dependency of the elasticity and strength of cancellous bone on apparent density. J Biomech 1988; 21: 155–68. 6 Neer RM, Arnaud CD, Zanchetta JR, et al. Effect of parathyroid hormone (1-34) on fractures and bone mineral density in postmenopausal women with osteoporosis. N Engl J Med 2001; 344: 1434–41. 7 Lindsay R, Nieves J, Formica C, et al. Randomised controlled study of effect of parathyroid hormone on vertebral-bone mass and fracture incidence among postmenopausal women on oestrogen with osteoporosis. Lancet 1997; 350: 550–55. 8 Hirano T, Burr DB, Turner CH, Sato, M, Cain RL, Hock JM. Anabolic effects of human biosynthetic parathyroid hormone fragment (1-34), LY333334, on remodeling and mechanical properties of cortical bone in rabbits. J Bone Miner Res 1999; 14: 536–45. 9 Sato M, Westmore M, Ma YL, et al. Teriparatide [PTH(1-34)] strengthens the proximal femur of ovariectomized non-human primates despite increasing porosity. J Bone Miner Res 2004; 19: 623–29. 10 Hodsman AB, Kisiel M, Adachi JD, Fraher LJ, Watson PH. Histomorphometric evidence for increased bone turnover without change in cortical thickness or porosity after 2 years of cyclic hPTH (1-34) therapy in women with severe osteoporosis. Bone 2000; 27: 311–18. 11 Mashiba T, Burr DB, Turner CH, Sato M, Cain RL, Hock JM. Intermittently administered human parathyroid hormone (1-34), LY 333334, on bone mass, remodeling and mechanical properties of cortical bone during the first remodeling cycle in rabbits. Bone 2001; 28: 538–47. 12 Burr DB. Does early PTH treatment compromise bone strength? The balance between remodeling, porosity, bone mineral, and bone size. Curr Osteoporos Rep 2005; 3: 19–24. 13 Borah B, Dufresne T, Nurre J, et al. Risedronate reduces intracortical porosity in women with osteoporosis. J Bone Miner Res 2010; 25: 41–47.