MECHANICAL AND GEOMETRIC CHARACTERISTICS OF BONE IN THROMBOSPONDIN2 DEFICIENT MICE

Participants: E.A. Smith, S.A. Goldstein, J. Bonadio

Keywords: bone mechanics, extracellular matrix, transgenic animal

Introduction

Thrombospondin-2 exists in the extracellular matrix of bone, and, while its precise function in bone is unclear, it has been postulated to play a role in cell-matrix interactions. It is capable of binding both Type I collagen and bone mineral, and contains an RGD-sequence and can therefore bind to bone cells via their integrin receptors. A thrombospondin-2 deficient mouse (thbs2 -/-) has been created at the University of Washington, and its phenotype includes a wide variety of abnormalities, such as increased skin fragility and abnormalities in collagen fibers and fibrils. The goal of this study is to characterize the mechanical and geometrical properties of bone from thbs2 -/- mice in an attempt to elucidate the role of this protein in bone.

Materials and Methods

The right femur from five thbs2 -/- and five wild type mice were acquired from collaborators at the University of Washington. The femora were scanned on a µCT system at a resolution of 20 µm, and cross-sectional area, cortical thickness and moments of inertia of the mid-diaphyseal region were determined. This cortical thickness and inner and outer fiber lengths were further examined at 10 degree intervals around the circumference of the diaphysis. The femora were then tested to failure in four-point bending on an MTS servohydraulic testing machine at a constant displacement rate of 1.0 mm/sec. Load-displacement data were acquired, and used to determine stiffness, and loads and displacements to both yield and failure. Statistical analysis was performed using one-way Analysis of Variance (ANOVA) and Tukey's post-hoc test was used for comparisons between groups, with p<0.05 considered statistically significant.

Progress

All mechanical testing and µCT analysis has been completed. A manuscript has been recently published in the Journal of Bone and Mineral Research.

Results

The results of mechanical testing showed no differences between the knockout and wild type groups. The geometrical properties of the thbs2 -/- mice did, however, exhibited significant differences when compared to the wild type group. On average, the knockout showed a slight trend towards a decreased cross-sectional area and significantly decreased moments of inertia (Ixx, Iyy and J) with no changes in cortical thickness (see Figure 1). Regional analysis showed that both the inner and outer fiber lengths were significantly decreased in the posteromedial through anterolateral regions, with minor increases in cortical thickness anteriorly and posteromedially. This data seems to indicate that the thbs2 -/- mice had significantly smaller bones and with increased endosteal bone formation, which resulted in the decreases in moments of inertia.

Figure 1