A demineralized calf vertebra model as an alternative to classic osteoporotic vertebra models for pedicle screw pullout studies

Abstract

Screws, clamps and other spinal instrumentation materials are tested using healthy animal and healthy human vertebrae, but the application of similar tests to an osteoporotic vertebra is generally neglected because of high costs and limited availability of high quality and consistent osteoporotic vertebrae. The objective of this study is to develop an in-vitro method to decrease the mineral content of an animal vertebra utilizing decalcifying chemical agents that alters the bone mineral density and some biomechanical properties to such an extent that they biomechanically mimic the osteoporotic spine. This study was performed on 24 fresh calf lumbar vertebrae. Twelve out of these 24 vertebrae were demineralized and the others served as control. A hole was opened in the pedicles of each vertebrae and the bone mineral density was measured. Each vertebra was then placed into a beher-glass filled with hydrochloric acid decalcifier solution. The decalcifier solution was introduced through the holes in the pedicles with an infusion pump. The vertebrae were then subjected to DEXA to measure post process BMD. Pedicle screws were introduced into both pedicles of each vertebrae and pullout testing was performed at a rate of 5 mm/min. The difference of BMD measurements between pre- and post-demineralizing process were also statistically significant (p < 0.001). The difference of pullout loads between pre- and post-demineralizing process were also statistically significant (p < 0.001). The acid demineralizing process may be useful for producing a vertebra that has some biomechanical properties that are consistent with osteopenia or osteoporosis in humans.

Fig. 1

The percent change of the BMD to demineralization time graphic. Data were achieved from pilot studies. As clearly seen, the percent change of the BMD increases with demineralization time

Fig. 2

Schematic diagram showing the experiment setup used for the demineralization process

Fig. 3

Bar graph with error bands showing comparison of mean pullout loads of the control and demineralized vertebra groups

Fig. 4

Representative CT scans of vertebra. Before (a) and after (b) demineralizing process. Cortex thinning (bold arrows) and the dense subcondral bone which seen under the endplates loosening are observed (fine arrows)