**2. Spinal instrumentation**

Spinal instrumentation is designed to stabilize the spine and prevent excessive motion at the affected segment as the bones fuse together. It provides immediate stability of the affected segment of the spine and obviates the need for external fixation devices such as rigid collars, braces, and halo traction. This allows for a greater quality of life for patients immediately following surgery. These internal fixation devices such as screws, plates, and rods are affixed to the vertebral bodies and combine to form an instrumentation construct. This construct takes the load-bearing responsibility of the affected spinal segment until fusion has occurred. The instrumentation construct is therefore a temporary load-bearing adjunct to fusion (Benzel 2004). When arthrodesis is achieved, the fusion mass will become the principal bearer of load on the FSU, and the instrumentation becomes obsolete. Therefore, a principle of spine surgery is that spinal instrumentation is to maintain spinal stability until fusion occurs.

Modern spinal fixation devices have been made of biocompatible metallic alloys such as stainless steel and titanium. While these have become standard due to their strength and fatigue resistance, they possess radiographic challenges and biomechanical limitations. First, and foremost, metallic materials are radiologically problematic. Artifacts created by metals in computerized tomography (CT) and magnetic resonance (MR) imaging, as well as obfuscation of bone by metals in planar x-rays, inhibit visualization of new bone growth thus, making it difficult for physicians to assess progression of arthrodesis. Second, implants that are stiffer than natural bone are subject to so-called stress shielding. When two bones are fused together, the state of compressive forces between them determines the extent of modeling or remodeling. According to Wolff's law, bone apposition within a fusion graft is governed in part by the state of compressive stresses within it. Because of this it has been proposed that stiff metallic implants may shield the graft from the stress required for fusion, delaying or preventing the process. This can induce iatrogenic effects such as device-related osteopenia, intervertebral device protrusion into a neighboring vertebral body, and fracture or instability (Lippman et al. 2004). Implants developed out of less stiff materials may prevent this stress shielding and foster better clinical outcomes compared to traditional devices.
