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The prospective cohort study.The objective was to evaluate the relationships between local pressure changes of the intervertebral foramen during lumbar spine extension and lumbar foraminal morphology.The physiological states of lumbosacral nerve roots in the vertebral foramen remain controversial.We evaluated 56 lumbosacral vertebral foramens in 21 patients with L4-degenerative spondylolisthesis. All patients underwent L4-5 posterolateral fusion (PLF). The local pressure of the intervertebral foramen was measured intraoperatively, and measurement was performed before and after L4-5 PLF. We defined the changes in the ratio of local pressure between lumbar flexion to extension as percent pressure. The sagittal angular motion, distance between the inferior cortex of the cranial pedicle and superior cortex of the caudal pedicle, posterosuperior margin of the superior vertebral body and superior articular facet, posteroinferior edge of the superior vertebral body and inferior articular facet, and the intervertebral disc height were measured using preoperative functional plain radiographs and CT images.The average local pressure of the intervertebral foramen significantly increased during lumbar extension. However, the L4-5 vertebral foraminal pressure after PLF were nearly identical. There was no significant correlation between percent pressure and lumbar range of motion. Furthermore, there were no significant correlations between percent pressure and each morphologic parameter of the lumbar foramen.There were no significant relationships between the lumbar foraminal morphology and intervertebral foraminal pressure changes during lumbar extension, and L4-5 vertebral foraminal pressure was not affected by the lumbar posture after L4-5 posterior fusion. On the basis of the results, the external dynamic stresses on the nerve roots in the vertebral foramen might be improved by lumbar posterior fusion using instrumentation without direct decompression of the vertebral foramen.