Recent publications have reported that bone plastic deformation properties are determined not only by mineral content, but also by the organic matrix and interactions between these two components [41], and that tissue mineral density is an incomplete surrogate for tissue elastic modulus [42]. Bone structural and material properties (including mineral density expressed CDK activation as mineral/matrix, mineral maturity/crystallinity and collagen cross-links) are important contributors to bone strength [2]. Moreover, the organic matrix is proposed to play an important role in alleviating damage to mineral crystallites, and to matrix/mineral interfaces, behaving like a soft wrap around mineral crystallites thus protecting

them from the peak stresses, and homogenizing

stresses within the bone composite [2], [43] and [44]. The importance of collagen properties in determining bone strength is emphasized by several publications in the literature reporting altered collagen properties associated with fragile bone, in both animals and humans [6], [17], [18], [22], [34], [37], [38], [39], [45], [46], [47], [48], [49], [50] and [51]. Employing FTIRI analyses, we have previously signaling pathway reported altered collagen cross-link ratio (PYD/divalent) in forming trabecular surfaces in osteoporotic patients and patients with fragility fractures [17] and [18]. The surprising finding was that these alterations compared to normal bone were restricted in forming surfaces only, thus whether these alterations were important contributors to bone fragility remained in question. To address this, an animal model was Thiamine-diphosphate kinase utilized in the present study to test the hypothesis that even anatomically confined alterations in collagen cross-links can affect

whole bone mechanical performance independent of mineral. It has been previously shown that in vivo β-APN treatment causes significant changes in the mechanical properties of rat femora (26% decrease in failure stress and a 30% decrease in elastic modulus as determined in a bending test after 30 days of treatment) [22], and that it affects the cross-linking of collagen in the dosage used in the present study [52], [53], [54], [55] and [56]. β-APN treatment, as expected, caused significant reductions in vertebral DHLNL, PYD, and DPD cross-links, as well as the calculated Pyd/divalent collagen cross-link ratio, as determined through biochemical analysis of whole bone homogenate. Interestingly, the alterations in divalent and trivalent cross-link concentrations were disproportionate; thus there were significant increases in the PYD/DHLNL ratio in the treated animals compared to corresponding controls whereas the treatment effects on HLNL were much less marked than for DHLNL. Although a relative decrease in the proportion of DHLNL with animal age may have contributed to the results, the observed changes were primarily due to the administration of β-APN.