METHODS Assessment of qualitative and quantitative changes in the microcirculation of nerve roots, such as the diameter of blood vessels and the low rate of erythrocytes, was observed during lumbar interbody fusion. OBJECTIVE To ascertain changes in the microcirculation of nerve roots before and after retraction during lumbar interbody fusion surgery. BACKGROUND The changes in microhemodynamics caused by nerve root retraction have not yet been elucidated. METHODS Subjects were patients with lumbar spinal canal stenosis who underwent lumbar interbody fusion. Changes in the microcirculation of nerve roots were examined in the L5 nerve root in 26 patients and the S1 nerve root in 3 patients. Through the use of video images captured by contact endoscope and stored in a computer, erythrocytes were automatically followed to measure flow rate and the diameter of blood vessels. RESULTS Plasma skimming, where blood cells and plasma flow separately, was seen in 3 of the 29 patients (10.3%) before retraction of the nerve root and in 8 of the 29 patients (27.6%) following retraction. Intravascular erythrocyte agglutination, when erythrocytes flow in clumps due to changes in the charge state of erythrocytes, was seen in blood vessels larger than 100 microm in 3 patients (10.3%) after retraction. Following nerve root retraction, the flow rate of erythrocytes through blood vessels decreased an average of 23.9% (P< 0.005). CONCLUSIONS A contact endoscope was used to observe the microhemodynamics of nerve roots before and after retraction of the nerve root during lumbar interbody fusion surgery, and a decrease in the flow rate of erythrocytes was observed.