Volume to density relation in adult human bone tissue Simone Tassani a , Caroline O ¨ hman a,b , Fabio Baruffaldi a,n , Massimiliano Baleani a , Marco Viceconti a a Laboratorio di Tecnologia Medica, Istituto Ortopedico Rizzoli, Bologna, Via di Barbiano, 1/10, 40136 Bologna, Italy b Engineering Faculty, University of Bologna, Bologna, Italy article info Article history: Accepted 24 August 2010 Keywords: Cortical bone Trabecular bone Tissue mineral density Bone fraction Ash density abstract Uniformity of tissue mineralisation is a strongly debated issue, due to its relation with bone mechanical behaviour. Bone mineral density (BMD) is measured in the clinical practice and is applied in computational application to derive material proprieties of bone tissue. However, BMD cannot identify if the variation in bone density is related to a modification of tissue mineral density (TMD), a change in bone volume or a combination of the two. This study was aimed to investigate whether TMD can be assumed as a constant in adult human bone (trabecular and cortical). A total number of 115 cylindrical bone specimens were collected. An inter-site analysis (96 specimens, 2 donors) was performed on cortical and trabecular specimens extracted from different anatomical sites. An intra-site study (19 specimens, 19 donors) was performed on specimens extracted from femoral heads. Bone volume fraction (BV/TV) was computed by means of a micro-computed tomography. Furthermore, ash density (r ash ) was measured. TMD was computed as the ratio between r ash and BV/TV. It was found that the TMD of trabecular (1.2470.16 g/cm 3 ) and cortical (1.1970.06 g/cm 3 ) bone were not statistically different (p ¼0.31). Furthermore, the linear regression between r ash and BV/TV was statistically significant (r 2 ¼0.99, p o0.001). Intra- and inter-site analyses demonstrated that the mineral distribution was independent of the extraction site. The present study suggests that TMD can be assumed reasonably constant in non-pathological adult bone tissue. Consequently, it is suggested that TMD can be managed as a constant in computational models, varying only BV in relation to clinical densitometric analysis. & 2010 Elsevier Ltd. All rights reserved. 1. Introduction Analysis of bone composition is an important issue for the mechanical characterization of the tissue. Furthermore, a correct description of the material properties of cortical and trabecular bone is a critical point for the implementation of computational models. Several studies (Keller, 1994; Li and Aspden, 1997; Hernandez et al., 2001; Zioupos et al., 2008; Busse et al., 2009) have investigated the influence of bone composition and tissue properties on its mechanical behaviour. Although the attention has mainly been focused on the structure and tissue composition of trabecular bone (Turner, 1992; Goulet et al., 1994; Li and Aspden, 1997; Majumdar et al., 1998; Ding et al., 2002; Matsuura et al., 2007; Burghardt et al., 2008; Kazakia et al., 2008; Busse et al., 2009) some studies have also investigated cortical bone (Cowin, 1999; Wachter et al., 2001a,b; Cooper et al., 2003; Basillais et al., 2007). Trabecular bone can be distinguished from cortical bone based on clear differences in their microstructures, i.e. trabecular framework (Odgaard, 1997) and haversian channels (Cowin, 1999). It is still debated whether there is a difference between trabecular and cortical bones in terms of tissue mineralisation, i.e. tissue mineral density (TMD). TMD is defined as the mineral quantity in the actual volume of the bone tissue, and it can be computed as the bone mineral content over the bone volume. Some studies have indicated the degree of porosity as the only macroscopic difference between cortical and trabecular bone tissues (Gibson, 1985, 2005). This suggestion would simplify the predictions of mechanical properties of bone tissue since it would become a function of only bone quantity and structure, and hence, not a function of TMD. Different assumptions of the relation between trabecular and cortical bones can be found. One study suggested that no correlation between ash fraction (i.e. ash mass over dry bone mass) and bone volume fraction (i.e. BV/TV; bone volume over total specimen volume) of trabecular and cortical bone exists (Hernandez et al., 2001) However, a wide distribution of ash fraction was reported for BV/TV values in the transition between trabecular and cortical specimens. Another work (Zioupos et al., 2008) suggested that a ‘‘boomerang’’-like distribution could describe the data reported by Hernandez et al. (2001). In fact, such a distribution of tissue mineral Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com Journal of Biomechanics 0021-9290/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jbiomech.2010.08.032 n Corresponding author. Tel.: + 39 051 6366864; fax: + 39 051 6366863. E-mail address: baruffaldi@tecno.ior.it (F. Baruffaldi). Please cite this article as: Tassani, S., et al., Volume to density relation in adult human bone tissue. Journal of Biomechanics (2010), doi:10.1016/j.jbiomech.2010.08.032 Journal of Biomechanics ] (]]]]) ]]]–]]]