Biomechanical changes after the augmentation of experimental osteoporotic vertebral compression fractures in the cadaveric thoracic spine.

BACKGROUND CONTEXT: Osteoporotic compression fractures are an important public health concern, leading to significant morbidity, mortality and economic burden. Cement augmentation procedures used to treat these fractures alter the biomechanics of the fractured segment, which could promote adjacent failure. However, if alignment is improved or restored, there will be less risk of adjacent failure. PURPOSE: To determine the effects of load (compression/flexion), adjacent vertebral location (superior/inferior) and augmentation on vertebral segment stiffness and adjacent vertebral strain in the upper and lower thoracic spine. STUDY DESIGN: Human cadaveric thoracic spine segments were tested under load control before and after the creation of experimentally augmented vertebral compression fractures. METHODS: Six T1-T5 and six T8-T12 segments were obtained from eight thoracic spines with known bone mineral density (BMD). Rosette strain gauges were applied to T2, T4, T9 and T11 to measure strain adjacent to the experimental fracture sites T3 and T10. Two compression fractures were created in succession, the first in flexion preceded by a weakening defect in T3 and T10 and the second created in an adjacent vertebra in compression without prior weakening. The first fracture was reduced with the inflatable bone tamp (IBT) and augmented with cement. Compression and flexion tests were performed before and after the first fracture while measuring vertebral cortical shear strain on T2, T4, T9 and T11 and stiffness of the entire segment. Strain and stiffness were compared by using a repeated measures analysis using adjacent vertebral location (superior/inferior), augmentation and load (compression/flexion) as factors. RESULTS: The mean BMD was 0.61+/-0.11 g/cm(2) (T1-T5) and 0.78+/-0.07 g/cm(2) (T8-T12). Stiffness in compression and flexion increased with load (p<.05, and p>.27, respectively). Augmentation reduced compressive and bending stiffness (p=.23, and p=.19, respectively), whereas the adjacent vertebral strain increased (p>.11). The adjacent strain in flexion was much greater than in compression (p<.03). Cement augmentation caused greater amounts of inferior than superior adjacent strain (p>.19). The applied moment at first fracture was 2.98+/-1.28 Nm (T1-T5) and 8.44+/-1.02 Nm (T8-T12). The compressive load at second fracture was 1122+/-993 N (T1-T5) and 2906+/-1008 N (T8-T12). Adjacent vertebral strain during the second compression and flexion tests exceeded that during the first compression and flexion tests (p=.11). Adjacent vertebral strain at second fracture exceeded that at first fracture (p=.007) and was greater on the superior adjacent vertebra than the inferior (p=.47). CONCLUSION: With axial compressive loads, the addition of flexion increases fracture risk. Cement augmentation of a fractured vertebral segment reduces stiffness while increasing both the superior and inferior adjacent cortical strain. This increment in strain that is greatest on the inferior adjacent vertebra effectively redistributes loads from the superior adjacent vertebra to the inferior adjacent vertebra, sparing the superior adjacent vertebra from failure.