BIOMECHANICS
In comparison to the thoracolumbar junction the low lumbar spine is protected by the pelvis and the strong ligamentous and muscular attachment. Injuries in the low lumbar spine involve the transfer of high amounts of energy. Falls, motor vehicle accidents or major crush injuries occur. As noted flexion distraction injuries (AO Type B) are rare.
The anterior weightbearing structures are frequently compromised in such injuries. Type A fractures will result in varying degrees of vertebral body injury. Fracture dislocation with displacement results in significant disc discruption and loss of load bearing capacity. These anterior column defects make management options more difficult. Anterior column deficiency in the acute stage has implications for sagittal plane deformity, failure of posterior instrumentation systems, and altered posterior elements loading with accelerated spinal stenosis. Any coronal plane deformity will also result in asymmetrical facet loading with likely accelerated degenerative change. The sloping superior dome of the sacrum results in translational deformities at the lumbosacral junction.
When instrumentation placement is planned the surgeon should be aware that distal attachment sites of the sacrum are mechanically weak in comparison to pedicle fixation within the proximal lumbar spine. The distal fixation sites may be further exposed to failure within increasing anterior column deficit. The site of low lumbar fracture adjacent to the sacropelvic complex has implications for bracing. Biomechanical evidence demonstrates increased forces transferred through the lumbosacral junction when TLSO braces are used. Bracing to immobilise the lumbosacral junction requires pelvic immobilisation with the inclusion of a single thigh in the cast or brace