Study design: In vitro biomechanical study.
Objective: To measure the effect that off-axis vertebral loading has on the stiffness and failure load of vertebrae.
Summary of background data: Adjacent level vertebral fractures not only are common in patients who received a vertebroplasty treatment but also occur in patients with conservatively treated wedge fractures. The wedge-like deformity, which is present in both groups, changes the spinal alignment. The load of vertebrae adjacent to the fractured vertebra will change from perpendicular to the endplate to a more shearing, off-axis, load. This change may induce a higher fracture risk for vertebrae adjacent to wedge-like deformed vertebrae.
Methods: Twenty vertebrae, harvested from one osteopenic cadaver spine and three osteoporotic cadaver spines, were loaded until failure. The vertebrae were loaded either perpendicular to the upper endplate, representing vertebrae in a spine without wedge fractures (0° group, n = 10), or at an angle of 20°, representing vertebrae adjacent to a wedge fracture (20° groups, n = 10). Vertebral failure load and stiffness were the most important outcome measures.
Results: The failure load was significantly higher (P 5 0.028) when tested at 0° (2854 N, SD 5 622 N), compared with vertebrae tested at 20° (2162 N, SD 5 670 N). Vertebrae were also significantly stiffer (P, 0.001) when tested at 0° (4017 N/mm, SD 5 970 N/mm) than those tested at 20° (2478 N/mm, SD 5 453 N/mm).
Conclusion: The failure load of osteopenic/osteoporotic vertebrae was 24% lower under off-axis loads (20°) than under axial loads (0°). This study may lead to a better understanding of the etiology of adjacent vertebral fractures after wedge-like deformities and demonstrates the importance of height reconstruction of wedge fractures in order to normalize the loading conditions on adjacent vertebrae.