Purpose: The purpose of this biomechanical study was to assess: (1) the effect of thoracic vertebral compression fracture (VCF) on kyphosis and physiologic compressive load path, and (2) the effect of balloon kyphoplasty and spinal extension on restoration of normal geometric and loading alignment.

Methods: Six fresh human thoracic specimens, each consisting of three adjacent vertebrae were used. In order to create a VCF, IBTs were placed transpedicularly into the middle VB and cancellous bone was disrupted by inflation of IBTs. After cancellous bone disruption the specimens were compressed using bilateral loading cables until a fracture was observed. Fracture reduction by spinal extension, and then by balloon kyphoplasty was performed under a physiologic compressive preload of 250 N. The vertebral body heights, kyphotic deformity, and location of compressive load path were measured on video-fluoroscopy images.

Results: The VCF caused anterior VB height loss of 3715%, middle-height loss of 3416%, segmental kyphosis increase of 147.0 degrees, and vertebral kyphosis increase of 135.5 degrees (p< 0.05). The compressive load path shifted anteriorly by 20% of A-P endplate width in the fractured and adjacent VBs (p=0.01). IBT inflation alone restored anterior VB height to 918.9%, middle-height to 9114%, and segmental kyphosis to within 5.65.9 degrees of pre-fracture values. The compressive load path returned posteriorly in all three VBs (p=0.00): the load path remained anterior to the pre-fracture location by 9–11% of the A-P endplate width. The extension moment fully restored the compressive load path to its pre-fracture location. Under this moment, the anterior and middle VB heights were restored to 858.6% and 749.4% of pre-fracture values, respectively. The segmental kyphosis was fully restored to its pre-fracture value; however, the middle height and kyphotic deformity of the fractured VB remained smaller than the pre-fracture values (p< 0.05).

Conclusions: An anterior shift of the compressive load path in VBs adjacent to VCF can induce additional flexion moments. The eccentric loading may contribute to the increased risk of new VB fractures adjacent to an uncorrected VCF deformity. Extension moment could fully correct the segmental kyphosis but could not restore the middle height of the fractured vertebral body. Balloon kyphoplasty reduced the VCF deformity and partially restored the compressive load path to normal alignment.