Geometry effects of osteoporosis on cancellous bone

Normal cancellous bone, such as that in the vertebral body, is composed of both horizontal and vertical trabec-ulae. These trabecular struts are interconnected, much like the scaffolding used to surround buildings during construction. While the individual vertical and horizontal members, are, by themselves, important in resisting loads in particular directions (i.e., anisotropic properties), it is their interaction that gives cancellous bone its great compres-sive strength.

Osteoporosis is a disorder in which total bone mass is reduced yet the quality of the bone is normal. If a microsection of bone were to be biochemically analyzed, it would demonstrate a normal ratio of osteoid to mineral. Though total bone mass is affected, there is a predisposition to loss of the horizontal trabeculae [4]. This leads to decreased interconnectivity of the internal scaffolding of the vertebral body (Fig. 1b). Without the support of crossing horizontal members, unsupported vertical beams of bone easily succumb to minor, normally subcatastrophic, loads. Clinically, this leads to crush of the cancellous bone within the vertebral body, recognizable as an osteoporotic compression fracture, which may occur with low-energy maneuvers such as picking up a bag of groceries.

Fig. 4 The importance of interconnectivity of bone is shown by the analogy to a brick wall. Normal bone has interconnectivity, like the overlapping of the brick wall on the left. It can sustain heavy loads. Osteoporotic bone has lost its interconnectivity, like the brick wall on the right. Its walls can sustain only light loads, as they will collapse and buckle under heavier loads

Fig. 4 The importance of interconnectivity of bone is shown by the analogy to a brick wall. Normal bone has interconnectivity, like the overlapping of the brick wall on the left. It can sustain heavy loads. Osteoporotic bone has lost its interconnectivity, like the brick wall on the right. Its walls can sustain only light loads, as they will collapse and buckle under heavier loads

Using the analogy of the column of bricks detailed above, imagine two different brick buildings. The first is built in the usual manner: the bricks are overlapped with each other in a staggered pattern, representing interconnectivity of the trabeculae. The second building is built with columns of bricks stacked on top of each other with no overlapping, representing loss of interconnectivity (Fig. 4). While both houses might support some weight of objects placed on the roof, the first house would be able to support much greater loads. The walls of the second house would only be able to support much lighter loads. With heavier loads, the walls of the second house will have a tendency to buckle and topple, like an osteoporotic vertebral fracture. Taking the example one step further, consider the first house to be built with bricks made of granite and the second house made of bricks of porous sandstone. The sandstone bricks would have a greater tendency to crumble with loads, as would the osteoporotic vertebral body.

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