Spatial clustering of cortical remodelling and the temporal effects of reduced mechanical loading on bone micro-architecture

Bone fracture has a major impact both on the economics of farmed animals and man as well as their quality of life. We have found that in humans, hip fracture is associated with the presence of defects in the bone that occur locally and are associated with small groups of the microscopic canals that run through the bone. These canals can join up to form larger (giant) defects that markedly reduce the strength of the bone. We think that the grouping that precedes this may be reduced if normal loading is applied to the skeleton during everyday activities, while reduced loads increase the numbers of these giant canals. Other studies have shown that bone responds to load by changes in cellular activity that leads to alteration in the overall architecture of individual bones. To test this idea we shall reduce the load on the ankle joint of otherwise normal sheep with a brace (like a plaster cast for treating fracture) and investigate the resulting changes in the cells present in the bone and architectual changes in the bone structure. To do this we will use new techniques such as high resolution imaging to determine the time course of events following a reduction in load. If our ideas are proved to be correct it will open the way for new ways to prevent bone fracture in both animals and man. This will be either through new medicines or, perhaps more interestingly and cheaply, through allowing us to design physical activities that effectively strengthen bones throughout life.

A bone's internal microarchitecture is a key determinant of its strength and for more than a century we have understood that this microarchitecture adapts to mechanical useage. While the osteocyte is considered to be the major mechano-sensor in bone we understand poorly how bone modelling and remodelling is targeted in response to changes in osteocyte activity and how the balance between bone formation and resorption is adjusted to suit a bone's mechanical needs. With this application we shall study the partially immobilised ovine calcaneus because as a cantilever its two main cortices generally experience compression and tension rather than torsion (which is very complex to evaluate) and also because, as in most large mammals, they are remodelled. Using the un-immobilised calcaneus as a control, we shall test the hypothesis that the experimentally underloaded cortex experiences net bone loss through osteoclastic expansion of haversian canals in closely neighbouring osteons (super-osteons) and that by perforation of the bone between them the canals merge into composite canals that are incapable of being in-filled. We shall relate these effects to surface strain changes determined directly with strain gauges during underloading To delineate the time course of the remodelling and bone tissue loss, groups of 6 animals each will be killed at 8 different time points. Using software developed in house for analysis of the human femoral neck, we shall study the effects of this intervention on cortical bone structure and turnover using conventional histomorphometry and we shall also study osteocyte viability and apoptosis in relation to the strain environment. To probe the mechanisms responsible for targeting of osteoclasts to remodelling super-osteons, we shall study the expression of key candidate osteocyte genes in situ and evaluate the effects of remodelling on the cortical canal network with high resolution 3-D pQCT and uCT imaging. Finally the effects of these micro-architectural changes on bone strength will be evaluated in 4 point bending tests. This project will provide important information on the temporal cascade of events that lead to a reduction in bone strength with underloading. It will, for the first time, causally link changes in osteocyte activity with consequent alterations in cortical and cancellous bone remodelling, which lead to the formation of cortical defects and trabecular fenestration as well as demonstrating their effects on mechanical strength.