Segmental instrumentation is a standard and effective technique in modern spine surgery. It is a form of instrumented spinal fusion that stabilizes the spine and corrects deformities using implanted hardware. This method involves attaching mechanical hardware to multiple vertebral levels to provide immediate stability and hold the corrected alignment. The internal fixation creates an environment where the vertebrae can permanently grow together into a single, solid structure.
Defining Segmental Instrumentation
Segmental instrumentation applies fixation to nearly every vertebral segment within the fusion area. This contrasts with older fixation methods that used only a few anchor points. The term “segmental” refers to attaching hardware at multiple, individual levels, providing comprehensive control over the spinal column.
The system is modular, consisting of interconnected hardware elements made from materials like titanium or cobalt-chrome. Primary components include two parallel metal rods, which act as the main structural beams. These rods connect to the vertebrae via multiple fixation points, typically pedicle screws, hooks, or wires.
Pedicle screws are inserted into the vertebral pedicles, which are strong bony columns. These screws function as anchors, and contoured rods are locked into their heads. This tight, multi-point connection transforms the mobile segments into a rigid construct, essential for successful bone fusion.
Evolution and Purpose in Spinal Correction
Segmental fixation became the standard due to its biomechanical advantages over earlier methods. Historically, devices like the Harrington rod offered limited correction and often resulted in an undesirable “flat back” profile. The introduction of segmental fixation in the mid-1970s pushed the field toward greater stability.
A major advance was the incorporation of the pedicle screw, which provided superior anchoring strength. This allowed for the development of systems, such as the Cotrel-Dubousset instrumentation, capable of achieving three-dimensional correction of spinal deformities. The primary goals are to provide immediate mechanical stability, correct the spinal alignment, and maintain that correction until biological fusion occurs.
The improved stability and multi-level control reduce the risk of hardware failure and pseudarthrosis (failed fusion). By applying corrective forces at multiple points, surgeons can address the side-to-side curvature (coronal plane), the front-to-back curves (sagittal plane), and the twisting rotation (axial plane). This comprehensive, three-dimensional correction leads to better clinical outcomes.
How Segmental Fixation Works
The mechanical action begins with the precise placement of anchors, primarily pedicle screws, into the vertebrae being fused. These screws are strategically positioned to maximize their grip within the dense bone. Once the anchors are in place, the surgeon contours the metal rods to match the desired, corrected shape of the spine.
The rods are connected to the multiple fixation points, creating a strong, supportive scaffold. By rigidly connecting the contoured rods to the anchors, the surgeon can manipulate the spine into the corrected position. This is achieved by pushing, pulling, or rotating the vertebrae using the screws as handles.
The tight locking of the rods to the screws creates a highly rigid construct that resists movement and stress. This internal hardware provides immediate support, holding the spine firmly in the corrected alignment and dramatically reducing motion at the surgical site. This immobilization protects the bone graft material, which is necessary for the graft to successfully bridge the gaps and heal into a single, permanent bone mass.
Conditions Treated Using This Technique
Segmental instrumentation is widely used across the spine, from the cervical to the lumbar regions, to treat complex conditions requiring fusion. Primary applications include the correction of severe spinal deformities, such as adolescent or adult scoliosis (side-to-side curvature) and kyphosis (excessive forward rounding).
The technique is frequently employed to stabilize the spine following significant trauma, such as complex spinal fractures. It is also applied in cases of spinal instability caused by conditions like spondylolisthesis, where one vertebra slips forward over the one beneath it.
Segmental fixation is necessary after surgical procedures that require the removal of significant bone or disc material, such as tumor resections or extensive decompressions. In these instances, the hardware provides the necessary structural support to restore spinal strength and prevent collapse.