The Fracture Risk of Adjacent Vertebrae Is Increased by the Changed Loading Direction After a Wedge Fracture

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 degrees group, n = 10), or at an angle of 20 degrees, representing vertebrae adjacent to a wedge fracture (20 degrees groups, n = 10). Vertebral failure load and stiffness were the most important outcome measures. Results. The failure load was significantly higher (P = 0.028) when tested at 0 degrees (2854 N, SD = 622 N), compared with vertebrae tested at 20 degrees (2162 N, SD = 670 N). Vertebrae were also significantly stiffer (P < 0.001) when tested at 0 degrees (4017 N/mm, SD = 970 N/mm) than those tested at 20 degrees (2478 N/mm, SD = 453 N/mm). Conclusion. The failure load of osteopenic/osteoporotic vertebrae was 24% lower under off-axis loads (20 degrees) than under axial loads (0 degrees). 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.