Vertebral burst fractures: an experimental, morphologic, and radiographic study.

Spinal burst fractures are produced by rapid compressive loading, and may result in spinal cord injury from bone fragments forced from the vertebral body into the spinal canal. This fracture is one of the most difficult injuries of the spine to successfully treat, in part because the biomechanics of reduction and the exact mechanism by which the distraction forces are transmitted to the intracanal fragments of the burst fracture have not been adequately investigated. The authors developed a reproducible technique for creating these fractures in vitro. The fractures produced were identical to those observed in clinical practice, and were used for investigating the mechanics of this fracture and its reduction. This work describes the pathologic anatomy of the burst fracture both on the gross structure and also on microtome sections of the vertebrae, and examines the biomechanics of fracture reduction. The margins of the vertebral bone fragment, which was forced posteriorly into the spinal canal during fracture, were noted to extend far laterally beyond the pedicles. The authors also found extensive damage not only to the disc above the injured level, but also to that below, explaining the clinical observation that disc degeneration frequently occurs at both levels. Examination of anatomic data provided by microtome section supported the hypothesis that the fibers that actually reduce the intracanal fragment originate in the anulus of the superior vertebra in the midportion of the endplate and insert into the lateral margins of the intracanal fragment. Investigations using magnetic resonance imaging confirmed that these obliquely directed fibers account for the indirect reduction of the fragment. The authors' studies demonstrate that the posterior longitudinal ligament provides only a minor contribution in the reduction of the fracture in comparison to the attachments of the posterior portion of the anulus fibrosus. The forces required to reduce this fragment were studied. Distraction was found to be the predominant force required for indirect posterior reduction. This was confirmed by a series of tests using devices that provided segmental fixation. The application of uniform distraction forces was most effective in the posterior reduction of the intracanal fragment.