The effect of cyclic loading on adjacent-level pressure changes after vertebroplasty

Objectives Vertebroplasty has been used in the treatment of painful vertebral compression fractures showing significant and rapid pain relief in these patients. On the other hand, previous studies have shown that this procedure greatly increases the failure strength of augmented vertebrae, thereby inducing new fractures in the adjacent levels. The purpose of this study was to evaluate the load transfer to the adjacent-level vertebrae after vertebroplasty, using direct pressure measurements during cyclic loading with increasing force, and assess the effect of cement injection on failure strength.

Methods Ten specimens consisting each of a central vertebral body with its attached intervertebral discs and upper and lower adjacent vertebrae were harvested from eight human cadavers, prepared and screened for bone mineral density. Each specimen was placed in a custom-made fixation apparatus (Instron, Canton, MA) and subjected to cyclic sinusoidal dynamic compression (500-1000 N, 1 Hz, 1800 cycles) before and after vertebroplasty. The material used for augmentation was commercial bone cement (Biomet, Warsaw, IN) which injection was controlled by fluoroscopy in order to achieve endplate-to-endplate filling. The augmented specimens 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 of the adjacent vertebrae for all phases of cyclic loading using a pressure mapping system (Tekscan, Boston, MA) and related to four anatomical regions in the adjacent vertebrae. The peak and mean pressures registered in each region were analysed in MATLAB (MathWorks Inc, MA) and the differences in between tests and specimens were analysed using the Wilcoxon rank-sum test.

Results and Conclusion Increasing gradually the load applied cyclically to the augmented specimens resulted in an average increase in pressure of 48.89 ± 6.92 % for the upper endplate and 37.49 ± 15.91 % for the lower endplate. Specimen stiffness was either decreased or maintained after augmentation compared with the intact vertebrae. After 1800 cycles we could observe a restoration of the overall stiffness in 8 of the 10 specimens. On average, cyclic loading increased the stiffness of the augmented specimens by 73.34 ± 14.46 %. The mean failure load was 5472 ± 1547 N. This in vitro study aimed at investigating the effect of cyclic loading at increasing steps of load magnitude on the adjacent-level pressures and specimen stiffness. As expected, it resulted in an increase of pressure in both endplates. The multi-step cyclic loading also resulted in a significant increase of specimen stiffness. The fact that the overall stiffness of the specimens was either decreased or maintained directly after augmentation may be due to the properties of the intervertebral disc and needs to be further investigated.

Stichwörter Vertebroplasty, Adjacent-level fractures

Publication History

Article published online:
15 October 2020