<title>Three-dimensional analysis of trabecular bone structure: the need for spongiosa standard models</title>

Abstract

Thin slice spiral computed tomography techniques open up new avenues in assessing human bone architecture in vivo. A slice images do not yield accurate and reproducible results for structural parameters. The use of standard 2D histomorphometric measures is problematic because even a slice thickness of 1 mm is large compared to a trabecular thickness of less than 200 micrometers . In this study we investigated the use of topological parameters an in particular their dependence on spatial resolution. The topology was derived from a 3D skeleton of a (mu) CT dataset of a real trabecular bovine bone sample. The 3D thinning algorithm from which the skeleton was computed will be descried in detail. Decreasing the spatial resolution of the (mu) CT resulted in the following percent changes: number of branches -46 percent; number of noes -50 percent; avg. branch length +28 percent. These results indicate that it is not possible to determine topological parameters accurately in thin slice CT images. However, diagnostically relevant changes over time may still be quantifiable. In order to better analyze these problems we developed realistic spongiosa models using the rapid prototyping technique of stereolithography. Plastic models of a real trabecular bone network were built. So far these spongiosa models are slightly enlarged compared to the original data. Stereolithographic models of artificial geometries showed that a spatial resolution of 100micrometers and variations of +/- 50 micrometers are technically achievable. (mu) CT imaging of the stereolithographic spongiosa models revealed an excellent agreement between the model and the original dataset. Although the absorption characteristics of plastic and bone are different the CT contrast of the stereolithographic model imaged in air is comparable to the contrast of bone imaged in a marrow matrix. Thus for clinical purposes the plastic models can serve as a standard of trabecular bone which can, for example, be used to compare 2D and 3D structural analysis methods, the impact of spatial resolution, and the influence of segmentation techniques.