Response of the Osteoporotic Skeleton to In Vivo Loading: We are interested in determining if the skeleton has diminished responsiveness to mechanical loading with age. We are subjecting young-adult and aged mice to two types of loading: 1) high-amplitude, low-frequency tibial compression, and 2) low-amplitude, high-frequency whole-body vibration (WBV). These studies will clarify whether or not there is reduced mechanoresponsiveness in the aged/osteoporotic skeleton.
Figure. Whole-body vibration setup. Mice are placed in the plastic housing and the plate is accelerated in the vertical direction at low-amplitude (0.1 – 1.0 g), high frequency (30-90 Hz) for 15 min/day.
Figure. MicroCT images of the tibia from 7 and 22 month old BALB/C mice illustrating differences in bone structure. At 22-months of age (~75 years in human) there is less trabecular bone in the proximal tibia compared to 7-months (~25 years in human). In addition, the medullary cavity is expanded and the cortex is thinner (arrows) in the diaphysis of the 22-mo mice compared to 7-mo. These age-related changes mimic what is seen in humans.
Figure. Sections of the tibial diaphysis from control (non-loaded) and loaded legs of 7- and 22-mo old mice after 5 days of tibial compression loading. Sites of bone formation are labeled with calcein green and alizarin red. There is little bone formation in control tibias. In contrast, the loaded tibias of both 7- and 22-mo mice have abundant new bone formation both on the endocortex (Ec) and the periosteum (Ps). We found no deficit in the ability of aged mice to respond to high-amplitude tibial compression. (B-bone; Ma-marrow)
Osteogenic and Angiogenic Responses to Loading: We are characterizing the skeletal response to damaging loading, with a focus on woven bone formation and associated changes in vascularity and gene expression. We refined the rat forelimb compression model to produce several levels of sub-fracture bone damage using either fatigue or creep displacement. We discovered that the osteogenic response to fatigue loading is proportional to the level of damage, with increasing amounts of woven bone formed with increasing damage (dose-response). In the first week after loading, woven bone area increases, leading to a partial recovery of bone strength; in the second week, the woven bone area is unchanged but it becomes denser and more mineralized, leading to full recovery of strength. We then determined that static creep loading that generates damage also induces a woven bone dose-response, indicating that damage is the predominant stimulus for the woven bone response seen in skeletal fatigue. In terms of vascular changes, both vessel area and number increase prior to increases in bone area, and their spatial distribution matched the subsequent pattern of bone formation. Angiogenic genes (e.g., VEGF) are upregulated 1 hr after fatigue loading, and BMP-2 is localized to vascular cells at this early time. Other known osteogenic genes (e.g., BSP, Osx) are then upregulated starting on day 1. Thus, vascular responses occurred in the immediate stages after fatigue loading, followed by osteogenesis.
Figure: MicroCT scans of ulnae loaded cyclically to different levels of sub-fracture displacement (expressed as % of fracture displacement) illustrate progressive increases in crack severity at the midshaft that corresponded to progressive decreases in ulnar strength.
Figure: Combined PET/CT image from cover of Bone (August 2006) illustrating increased uptake of fluoride after creation of a stress fracture.
Figure. Ulna 7 days after fatigue loading, illustrating stress fracture (arrowheads) which forms where the strain magnitude is greatest, on the medial aspect of the ulna near its midpoint. The pattern of woven bone corresponds to the location of the stress fracture, with the greatest amount of new bone near the midpoint on the medial side.
Figure. Photomicrographs illustrating dramatic increase in periosteal vasculature (perfused vessels are black) 3 days after fatigue loading. (B-bone; P-periosteum; M-muscle)
Figure. Photomicrographs upregulation of BMP-2, Osx, PCNA and BSP expression in the periosteum of loaded ulnae after fatigue loading. There is early activation of surface cells in the first day, and by day 3 the nascent woven bone is forming. (B-original cortical bone; P-periosteum; WB-nascent woven bone)