STRUCTURAL ADAPTATION OF CORTICAL BONE IN AN OVINE MODEL OF OSTEOPOROSIS

Introduction: With aging and in disease, the changes in bone microstructure and geometry influence the mechanical properties of cortical bone. We examine the cross-section geometrical properties of cortical bone, area and second moment of inertia, and microstructural parameters in an ovine model of osteoporosis. Methods: Twenty seven skeletally mature sheep were randomly divided into two groups: ovariectomy (OVX; n=11) and control (CON; n=16). Animals were sacrificed at 31 months following surgery. Compact bone samples were harvested from mid-diaphysis of the left and right metatarsal,4cm proximal to the metatarsal-phalangeal joint using a low speed diamond saw (Accutom 50, Struers, Ballerup, Denmark). For histological analysis, thin sections (150–200μm) were prepared. Each section was initially examined using brightfield microscopy (Olympus 1X51, Hamburg, Germany). Cortical area was measured using an image analysis system by measuring area enclosed by the perisoteal surface and subtracting the area of the medullary canal (analySIS, Soft Imaging systems, Munster, Germany). Sections were then examined using polarised light microscopy, cortical thickness was measured in four regions: anterior, posterior, medial and lateral. These regions were defined by finding the widest diameter of the medullary cavity of the section and drawing a line perpendicular through the midpoint. At each point total cortical thickness, periosteal thickness and endosteal thickness was measured. All measurements were scaled according to animal weight. Results: The outer cortical area was significantly greater in the OVX group compared to CON (p=0.006), the inner medullary area was also greater in the OVX group, but not significant. The actual cortical area (outer cortical area – medullary area) was significantly greater in OVX (129.27mm2 vs 119.24 mm2, p=0.005). Second moment of inertia (I), was significantly greater in OVX (2.53 m4 v 2.21m4, p=0.002). In all four cortical regions OVX was thicker than CON, however this never achieved significance. Similarly, in all four regions endosteal bone was thicker in OVX, but this was not significant. Periosteal bone was thicker in CON in the medial and lateral regions, whereas OVX periosteal bone was thicker in anterior and posterior regions (NS). Conclusion: Our results demonstrate structural adaptation of cortical bone in a model of ovine osteoporosis. In theory these changes result in improved biomechanical properties of that bone; resistance to bending (second moment of inertia) and compressive strength (cross-sectional area). However in osteoporosis this biomechanical advantage is offset by diminution of bone quality.