Rationale for Dynamic Stabilization II—SoftFlex System

Rationale for Dynamic
Stabilization II—SoftFlex System

Dilip K. Sengupta

The Requirements of a Dynamic Stabilization System

The concept of nonfusion and motion preservation has generated considerable clinical interest in the surgical treatment of chronic low back pain in recent years. Fusion has been the mainstay of surgical treatment in the last 3 decades, and the common experience is that, although fusion works for many patients with back pain, a successful fusion does not guarantee a successful clinical outcome with pain relief. Even if it works, most surgeons believe that successful fusion may increase the incidence of, or at least accentuate, adjacent segment disease.^1,2 Prosthetic disk or nucleus replacements are attractive alternatives, but these are not applicable in the presence of significant facet arthrosis and instability of the motion segment.^3,4 All these factors have raised the demand for a dynamic stabilization system to restore stability in an unstable, painful motion segment, to protect an adjacent segment from previous or concomitant fusion, or to salvage a failed disk or nucleus prosthesis without losing motion.

Instability

In degenerative lumbar spine disorder, “instability” as a cause of chronic low back pain is not well understood. Degeneration in the lumbar spine often results in decreased range of motion (ROM).5 Most authors describe spinal instability as an abnormal motion.^6–9 However, it is not an increase in the quantity of abnormal motion but in the abnormal quality of motion that characterizes spinal instability.^6,10–12 Mulholland and Sengupta¹³ hypothesized that the mechanism of pain production related to the instability is in fact abnormal load transmission; the abnormal motion leads to an abnormal distribution of load across the vertebral end plate, which is pain sensitive. The aim for motion preservation by dynamic stabilization is therefore to permit normal motion as much as possible but to limit any abnormal motion, and more importantly to unload or to uniformly load the disk and prevent any large spikes of spot loading at any stage throughout the ROM.

The Requirements of a Dynamic Stabilization System

Even such a well-defined goal of dynamic stabilization may not be adequate to understand exactly what is needed from dynamic stabilization unless the abnormal quality of motion is defined or can be determined. The characteristics of quality of motion that can be determined in the laboratory are ROM in each individual direction (like flexion, extension, etc.), neutral zone (NZ) motion, stiffness [load-deformation (L-D) curve], and instantaneous axis of rotation (IAR). The ROM, NZ motion, and L-D curve are easy to determine in the laboratory, whereas IAR is difficult to determine and not easily reproducible. A more practical alternative is to determine a continuous pressure tracing in the center of the disk during motion, measurement of which is more precise and reproducible. Although the pressure profile does not indicate the location of the IAR, it is a sensitive parameter, which may sharply reflect any abnormal load transmission through the disk as a result of an abnormal IAR or quality of motion during the ROM.

The biomechanical requirements of an ideal dynamic stabilization system may be summarized as follows:

1. Preserve motion and prevent any abnormal motion

2. Unload the disk and prevent any abnormal load distribution throughout the ROM

The clinical requirements of an ideal dynamic stabilization device include:

1. Minimally invasive surgical insertion

2. Survive fatigue failure

3. Maintain normal resting posture of the spine—no excessive kyphosis or lordosis

4. Easy salvage (e.g., conversion to fusion in case of failure)

A classification of the various dynamic stabilization systems has been described by the author.14

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