Clinical StudiesRestoring geometric and loading alignment of the thoracic spine with a vertebral compression fracture: effects of balloon (bone tamp) inflation and spinal extension
Introduction
Vertebral compression fractures (VCFs) in the elderly are an important public health problem because of the large size of the affected population and because of the devastating influence of VCFs on morbidity, mortality and social costs. The majority of osteoporotic VCFs occur in the thoracolumbar junction (T12–L1) and the mid-thoracic spine (T7–T8) [1], [2], [3]. Thoracic VCFs typically lead to increased kyphosis, which may worsen over time in persons with prevalent VCFs [4]. Spinal kyphosis resulting from osteoporotic VCFs has been shown to adversely affect quality of life, physical function, mental health and survival [5], [6], [7]. Furthermore, the occurrence of prior VCFs with the resulting kyphotic deformity is a strong risk factor for additional vertebral fractures in osteoporotic patients [3]. The increased risk of fracture is thought to be the result of alterations in spinal load distribution as a consequence of a kyphotic deformity [8].
In recent years, as we have gained an understanding of the adverse consequences of spinal kyphosis, the kyphoplasty procedure has been developed in an attempt to treat the fracture pain and spinal deformity resulting from osteoporotic VCFs. With this procedure, an inflatable bone tamp (IBT) is percutaneously introduced into the fractured vertebral body and is then inflated in an attempt to reduce the fracture deformity. The reported percentage of kyphosis reduction in clinical studies has ranged from 39% to 65% [9], [10], [11], [12], [13], while the restoration of vertebral body height ranged from 35% to 68% [10], [12], [14]. Spinal extension inducing “postural reduction” of VCFs has also been reported and in a prospective study of 41 consecutive patients with 65 VCFs who underwent vertebroplasty, McKiernan et al. [15] achieved improvement of kyphotic deformity in 23 fractures using spinal extension. In a recent, prospective clinical study Kim et al. [16] have described the ability of postural reduction (using a pillow under the VCF level) in achieving significant correction of the anterior vertebral body height and vertebral kyphotic deformity in 90% of acute osteoporotic compression fractures the onset of which was less than 8 weeks.
Restoration of the vertebral height and sagittal alignment are believed to mitigate adverse biomechanical consequences of osteoporotic VCFs [15]; however, there are no biomechanical data on how the VCF deformity and its correction affect the loads in the adjacent vertebral bodies. Proponents of kyphoplasty have suggested that by restoring normal spinal alignment the risk of new VCFs in adjacent vertebrae may be reduced. Although previous laboratory studies have investigated the reduction of osteoporotic VCF with balloon kyphoplasty [17], [18], [19], [20], these studies were performed on individual vertebral bodies. As a result, the effects of balloon kyphoplasty on the angular alignment and loading of adjacent segments under physiologic compressive preloads have not been reported.
We hypothesized that 1) kyphotic deformity caused by a thoracic VCF will shift the physiologic compressive load path anteriorly in the fractured vertebra as well as in the adjacent vertebrae and (2) deformity correction will restore the load path to its prefracture alignment. The specific objectives of this study were to measure 1) the effect of thoracic VCFs on segmental kyphosis (geometric alignment) and the shift of the physiologic compressive load path (loading alignment), 2) the effect of fracture reduction by balloon (bone tamp) inflation in restoring normal geometric and loading alignment and 3) the effect of spinal extension alone on fracture reduction and restoration of geometric and loading alignment.
Section snippets
Specimens
Fresh frozen human cadaveric thoracic spines from two women and three men (age, 81±10 years) were used. The specimens were radiographically screened to ensure that there were no previous fractures. They were thawed at room temperature (20 C), 24 hours before testing. The paravertebral muscles were dissected, while keeping the discs, ligaments and posterior bony structures intact.
Six thoracic specimens were obtained from five donor spines. Each test specimen consisted of three adjacent vertebrae
Fracture loads
The mean (±1 SD) value of the applied load for fracture creation was 528±164 N. No fractures in the adjacent vertebral bodies were observed in any of the specimens.
Geometric alignment
In the middle vertebral body of the prefracture specimens, the mean (±SD) values of anterior, middle and posterior vertebral body heights were 21.9±3.2 mm, 20.2±4.1 mm and 25.4±3.1 mm, respectively. The prefracture vertebral kyphosis was 5.8±1.7 degrees, and the segmental kyphosis was 1.0±3.6 degrees.
The thoracic VCF caused
Discussion
The presence of VCFs with the resulting kyphotic deformity is a strong risk factor for additional vertebral fractures. The increased fracture risk is thought to be the result of altered spinal alignment and loading as a consequence of the VCF. The purpose of this study was to investigate the effects of a thoracic vertebral compression fracture as well as fracture reduction using bone tamp inflation on the angular alignment and loading of adjacent segments under physiologic compressive preloads.
Conclusions
The anterior shift of the compressive load path in vertebral bodies adjacent to VCF can induce additional flexion moments on these vertebrae. This eccentric loading may contribute to the increased risk of new fractures in osteoporotic vertebrae adjacent to an uncorrected VCF deformity. Bone tamp inflation under a physiologic preload significantly reduced the VCF deformity (anterior and middle vertebral body heights, segmental and vertebral kyphosis) and returned the compressive load path
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FDA device/drug status: approved for this indication (inflatable bone tamp).
Support in whole or in part was received from The Department of Veteran Affairs, Washington, DC. Nothing of value received from a commercial entity related to this research.