The role of stiffness in lumbar fusion

Many factors direct instrumented lumbar fusion. As to nonbiological factors, the resultant stiffness of the instrumented lumbar segment has been used as the standard criterion assessing fixation technique and its follow-on fusion possibility. This stiffness, however, is more pertinent to arthrodesis than to fusion, which is unpredictable if the stresses at the fusion bone interface (FBI) have not been well controlled. For bone fusion, new bone formation at FBI must happen. Nonbiologically, the existence of substantial stimulating stresses (SSS) at FBI is the conclusive factor according to Wolff's law; furthermore, a successful fusion depends on the condition that rupturing stresses are always weaker than bone strength; therefore, bounded stresses at FBI is another conclusive factor according to failure theories in mechanics of materials. Arthrodesis attenuates stresses originated by external lumbar loading, acting as a "double-edged sword" which can help prevent bone rupture yet may also retard the bone fusion. Arthrodesis helps resist external disturbances to FBI; it helps maintain the condition at FBI, no matter whether the condition is beneficial or deleterious to bone fusion. Consequently, stiffness - the quality index of arthrodesis is not a good criterion for assessing fixation technique for lumbar fusion. New criterion is warranted.