Relations between Cancellous Bone Mechanical Properties and Bone Mineral Density in the Proximal Tibia of the Rat

INTRODUCTION: Non-invasive estimation of bone mechanical condition is a critical element in ongoing efforts aimed at reducing fractures in at-risk populations, such as those with osteoporosis, spinal cord injury, metabolic bone disease, or osteopenia due to long-term disuse or weightlessness. Considerable emphasis is placed upon bone mineral density (BMD) in the clinical setting, owing largely to the increasing availablity of dual-energy Xray absorptiometry (DEXA). Although BMD has shortcomings and limitations, particularly from DEXA, considerable effort has been put into establishing relations between BMD and mechanical properties in cancellous bone [1-4]. The underlying motivation for the current study is the question of how similar or different these relationships might be for cancellous bone from the rat. Rodent animal models have gained acceptance and widespread usage in skeletal research, but the direct applicability of specific findings from these models to the human case is not always clear. Non-invasive assessment of the rat skeleton is now possible with "small-animal" DEXA, peripheral quantitative computed tomography (pQCT), and microCT, which makes possible longitudinal evaluation of BMD in these animals in a manner much like the clinical case. The objective of the current study was to examine the relations between mechanical properties of cancellous bone in the rat and BMD as measured by pQCT. No formal or even "quasi" standards exist for measuring cancellous bone mechanical properties in the rat, particularly in the appendicular skeleton, which is most accessible to pQCT. Completely isolated specimens for testing cancellous bone are extremely problematic due to the size limitations of the rat skeleton, but a method has been developed recently for estimating mechanical properties in the proximal tibia metaphysis [5].