Normal Mechanics of Thoracic and Lumbar Spine
Lumbar spine is much more flexible than thoracic spine and the thoracolumbar junction represents a biomechanical transition zone between the rigid thoracic spine and flexible lumbar spine. The thoracic spine is rigid by virtue of presence of the rib cage, thoracic musculature, and facet joint configuration.
Normally the thoracic spine has a normal kyphosis whereas lumbar spine has lordosis normally. The thoracolumbar junction represents the area of change in sagittal alignment between the kyphotic thoracic segment and a lordotic lumbar region.
For example, between T1 and T6, there is approximately 4 degrees of flexion-extension permitted at each intervertebral segment, whereas approximately 12 degrees is permitted at T12 and L1. In contrast, lateral bending between vertebrae decreases from proximal to distal. There is 8 degree of lateral bending allowed between thoracic levels, but only 2 degrees in the upper lumbar segments.
This is due to more saggittal orientation of the facet joints in the lumbar spine.
Mechanics of Injury
Various forces act on the spine in trauma including axial loading, flexion, extension, shear, and axial rotation. Most often, damage occurs from a combination of these forces.The extent of the fracture depends largely on the energy and speed with which the force is delivered.
Flexion forces variety of injury patterns. What would be the type of injury would be dictated by the axis of rotation.
If the center of rotation is near the posterior longitudinal ligament, there will be a compressive load applied to the anterior vertebral body and a corresponding distraction force within the posterior elements. Thus the anterior column would fail in compression and posterior column would fail in distraction.
However, when the rotation centers about a point anterior to the spine, as in seat-belt injury, both anterior and posterior elements fail under distraction. This leads to substantial midsubstance ligament failure and/or avulsion fractures.
Extension-type injuries are not as common as flexion injuries. These injuries produce tensile forces in the anterior spine and compressive or tensile forces applied to the posterior elements.
Pure axial loads lead to burst fractures however new evidence suggests that adding some extension load is necessary to produce the characteristic burst fracture.
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