gms | German Medical Science

Objectives: While previous studies showed that vertebroplasty greatly increases the failure strength of augmented vertebrae [1], there is little consensus in whether bone cement can induce new fractures in the adjacent levels. The purpose of this study was to evaluate the load transfer during cyclic loading to the adjacent-level, non-augmented vertebrae before and after vertebroplasty using direct pressure measurements.

Methods: Ten functional spine units (FSUs) ranging from level T12 to L5 were harvested from eight human cadavers, prepared and screened for bone mineral density. For mechanical testing each FSU was placed in a custom made fixation apparatus (Figure 1a [Fig. 1]) and mounted in a material testing machine (Instron, Canton, MA). The specimens were subjected to cyclic sinusoidal dynamic compression (500–1000 N compression, 1 Hz frequency, total of 1800 cycles) before and after vertebroplasty. The augmented FSUs were further cyclically loaded at steps of 1000, 1250, 1500 and 1750 N (1 Hz frequency, total of 4800 cycles) and afterwards compressed along their central axis at a constant displacement rate of 0.5 mm/s until failure occurred. Pressure was measured in the upper and lower endplates for all phases of cyclic loading using a pressure mapping system (Tekscan, Boston, MA) and related to anatomical regions in the adjacent vertebrae (Figure 1b [Fig. 1]).

Results and conclusion: There were no significant changes in the adjacent vertebrae peak pressures before and after vertebroplasty. The anterior region of the upper endplate and the lateral right region of the lower endplate registered the highest peak pressures during all dynamic load series. The mean failure load was 5472 ± 1547 N and the mean FSU stiffness, defined as the slope of the near-linear portion of the load versus displacement curve, was 4900 ± 1866 N/mm.

This in vitro study aimed at clarifying if vertebroplasty was responsible for peak loads in the upper and lower endplates and the obtained results are in accordance with previous studies [2], [3] supporting the fact that cement injection is not primarily responsible for the occurrence of new fractures in the adjacent vertebrae.