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Interbody Fusion Strategies in Thoracic and Sacral Overlap Diseases
Introduction
The thoracolumbar junction is subject to unique biomechanical forces owing to its existence as a transition zone. The thoracic spine is relatively rigid, largely in part to the costovertebral joints as well as coronally oriented facet joints. In contrast, the lumbar spine is more flexible because there are no rib articulations and more sagittally oriented facet joints. Importantly, this relationship may change with aging, as mild to moderate disk degeneration typically increases segmental lumbar motion. However, severe disk degeneration is often accompanied by subchondral sclerosis, ankylosis, and osteophyte formation, which limit lumbar mobility, thus reducing the motion difference of the thoracolumbar spine. In addition, segmental lordosis decreases as the spine progresses toward the apex of kyphosis, underscoring its importance with regard to sagittal balance.
The lumbosacral junction is also subject to “overlap” disease. Historically, lumbosacral fusion has been associated with a high rate of pseudarthrosis. The advent of interbody fusion and the use of pelvic fixation, with either iliac screw fixation or sacral alar iliac screws (S2AI), has improved the rate of arthrodesis across the lumbosacral (LS) junction.
Indications: Thoracolumbar Overlap Disease
Fusion
Bony fusion of this region is often required to maximize patient outcomes. The thoracolumbar junction is subject to higher biomechanical stresses that may lead to increased nonunion at region of the spine. As such, many surgeons support interbody devices to augment fusion rates.
Spondylolisthesis, although more common in the lumbar spine, is a common indication for instrumented fusion. Although the authors’ are unaware of any reports of interbody fusion rates at thoracolumbar junction owing to this pathology, lumbar interbody fusion rates have been shown to be equivalent or better than posterolateral fusion in the lumbar spine. By extrapolation, one may consider this technique to achieve similarly high bony fusion.
Tumor/Infection
Both tumor and infection may involve variable destruction of vertebral bodies and endplates. Malignancies, of which metastases are the most common, typically involve the vertebral bodies and spare the disk. Therefore, vertebral body cage constructs are more commonly needed than interbody cages. However, there may be isolated instances in which interbody cages are helpful if there is disk destruction with relative sparing of the body and endplates.
Spondylodiscitis is often amenable to interbody devices owing to anatomic involvement of this disease. Most commonly, the vertebral body becomes seeded via arterial spread and the disk subsequently is infected through local extension. Because of this, there is often significant disk space collapse, sclerotic endplate changes, and infected body, but with variable degrees of body destruction. If debridement of the collapsed disk space is all that is surgically needed, then an interbody device may be ideal both to restore vertebral body height and to assist in fusion.
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