Abstract
Summary Statement
Burst fractures were simulated in vitro on human cadaveric spine segments. Displacement of the facet joints and pedicles were measured throughout the fracture process showing how these bony structures behave when an impact load is delivered.
Introduction
Burst fractures account for almost 30% of all spinal injuries, which may result in severe neurological deficit, spinal instability and hence life impairment1. The onset of the fracture is usually traumatic, caused by a high-energy impact loading. Comminution of the endplates and vertebral body, retropulsion of fragments within the canal and increase of the intrapedicular distance are typical indicators of the injury. Experimental and numerical studies have reported strain concentration at the base of the pedicles, suggesting that the posterior processes play a fundamental role in the fracture initiation2,3. However, little is known about the dynamic behaviour of the vertebra undergoing an impact load. The aim of this study was to provide an in vitro cadaveric investigation on burst fracture, focusing on the widening of the facet joints and pedicles during the fracture development.
Methods
Eight three-adjacent-vertebrae segments (T9-T10-T11, T12-L1-L2, L3-L4-L5) were harvested from three human spines preserving the ligaments and intervertebral discs. A testing frame was designed to hold the sample whilst undergoing an axial impact load (delivered through a drop-weight rig). Lateral displacement was recorded by two transducers (LVDT) sampled at 5000 Hz and data were used to calculate the percent maximum dynamic widening (MW) and percent residual widening after the impact (RW). LVDTs were positioned in contact with the most lateral region of the cranial facet joints where the central vertebra was lumbar; or posteriorly to the base of the pedicles for thoracic. Samples were divided into two groups to achieve two different grade of severity of the fracture by delivering two different amount of energy: High (HE) and Low (LE). Samples underwent HR-pQCT scanning prior and after fracturing to assess percent canal narrowing (CN), intrapedicular distance and grade the fracture. Differences between results were assessed using Mann-Whitney U test.
Results
Burst fractures were induced in all the samples (fragment retropulsion was present in all HE samples). The median energy delivered to each group was 206J (HE) and 148J (LE) which led to a significant difference in the median CN (HE: 32.4%; LE: 11.8%; p=0.029). No significant difference was found between HE and LE in terms of MW (p=0.11), or RW (p=0.85). Furthermore, MW and CN were poorly correlated (R2=0.13). In all the cases, the first peak in the widening data coincided with MW (median 12.8%, range 4.3–21.8%). RW measurements (median 2.8%, range −1.3–11.5%) were validated against HR-pQCT scans showing excellent agreement (R2=0.93).
Discussion/Conclusion
Results from this study provided further insight on the burst fracture process supporting the wedging effect of the adjacent facet joints when the impact load is transmitted. Indeed, the pedicles were forced to widen up to a critical value (MW), after which they fractured. Further experiments will help clarifying the influence of the amount of energy delivered.
