Abstract
Background
Fracture of an osteoporotic vertebral body reduces vertebral stiffness and decompresses the nucleus in the adjacent intervertebral disc. This leads to high compressive stresses acting on the annulus and neural arch. Altered load-sharing at the fractured level may influence loading of neighbouring vertebrae, increasing the risk of a fracture ‘cascade’. Vertebroplasty has been shown to normalise load-bearing by fractured vertebrae but it may increase the risk of adjacent level fracture. The aim of this study was to determine the effects of fracture and subsequent vertebroplasty on the loading of neighbouring (non-augmented) vertebrae.
Methods
Fourteen pairs of three-vertebra cadaver spine specimens (67-92 yr) were loaded to induce fracture. One of each pair underwent vertebroplasty with PMMA, the other with a resin (Cortoss). Specimens were then creep loaded at 1.0kN for 1hr. In 17 specimens where the upper or lower vertebra fractured, compressive stress distributions were measured in the disc between adjacent non-fractured vertebrae by pulling a pressure transducer through the disc whilst under 1.0kN load. These ‘stress profiles’ were obtained at each stage of the experiment (in flexion and extension) in order to quantify intradiscal pressure (IDP), the size of stress concentrations in the posterior annulus (SP) and compressive load-bearing by anterior (FA) and posterior (FP) halves of the vertebral body and by the neural arch (FN).
Results
No differences were found between Cortoss and PMMA so all data were pooled. Following fracture, IDP fell by 26% in extension (P=0.004) and SP increased by more than 200% in flexion (P=0.01). FA decreased from 55% to 36% of the applied load in flexion (P=0.002) and from 36% to 27% in extension (P=0.002). FN increased from 17% to 31% in flexion (P=0.006) and from 22% to 37% in extension (P=0.008). Vertebroplasty reduced stress concentrations in the disc and restored load-bearing towards pre-fracture values.
Conclusion
Vertebral fracture transfers compressive load from the anterior vertebral body to the posterior vertebral body and neural arch of adjacent (non-fractured) vertebrae. Vertebroplasty largely restores normal load-sharing at both the augmented and adjacent levels and in doing so may help reduce the risk of a spinal fracture cascade.
