Background: There is an increased incidence of fractures in untreated adjacent vertebrae after vertebroplasty.
Objective: To introduce unconstrained 6 degrees of freedom biomechanical testing to investigate whether vertebroplasty lowered the fracture strength of adjacent untreated vertebrae under physiological loading conditions and to describe the observed fracture pattern.
Methods: Three-level spinal segments (T10-12 and L1-3) from 6 spines were tested under unconstrained axial compression in which shear forces and torque were minimized using a 6-degrees of freedom robotic arm. Fracture initiation loads and ultimate failure loads of lumbar segments were predicted from the corresponding thoracic segments by assuming constant fracture stress along the spinal column. The predicted values were compared with postvertebroplasty experimental values of the lumbar spine segments. Plain radiographs were taken at 600-N increments to record the developing fracture pattern.
Results: All 6 vertebroplasty group specimens experienced reductions in fracture strengths ranging from 27.4% to 47.6% with an average decrease of 32.6% (P < .002) and reductions in ultimate failure load ranging from 1.6% to 47.3%, with an average decrease of 34.7% (P < .003) compared with predicted values from the nonvertebroplasty group. In all vertebroplasty group specimens, the superior and inferior endplates of the untreated middle vertebral body (L2) were deflected, whereas 5 of the 6 nonvertebroplasty group specimens did not show any evidence of endplate deflection.
Conclusion: Vertebroplasty altered the load transfer along the anterior spinal column, thereby statistically significantly increasing fracture risk and ultimate failure load of the untreated adjacent vertebrae. The radiographic findings support the endplate deflection fracture mechanism as the cause of adjacent fractures after vertebroplasty.