Morphologic changes in the lumbar intervertebral foramen due to flexion-extension, lateral bending, and axial rotation - An in vitro anatomic and biomechanical study

Study Design. A biomechanical and anatomic study with human cadaveric lumba r spine. Objectives. The purpose of this study is to examine the morphologic changes in the intervertebral foramen during flexion, extension, lateral bending, and axial rotation of the lumbar spine and to correlate these changes with the flexibility of the spinal motion segments. Summary of Background Data. Previous studies showed morphologic changes in the intervertebral foramen during flexion and extension; however, those cha nges during lateral bending and axial rotation were not well known. Methods. There were 81 motion segments obtained from 39 human cadaveric lum bar spines (mean age 69 years). The motion segments were imaged with CT sca nner with 1-mm thick consecutive sections. For biomechanical testing each m otion segment was applied with incremental pure moments of flexion, extensi on, lateral bending, and axial rotation. Rotational movements of the motion segment were measured using VICON cameras. After application of the last l oad, the specimens were frozen under load, and then CT was performed with t he same technique described above. Six parameters of the intervertebral for amen were measured, including foraminal width (maximum and minimum), forami nal height, disc bulging, thickness of ligamentum flavum, and cross-section al area of the foramen. Results. Flexion increased the foraminal width (maximum and minimum), heigh t, and area significantly while significantly decreasing the disc bulging a nd thickness of ligamentum flavum (P < 0.05). However, extension decreased the foraminal width (maximum and minimum), height, and area significantly. Lateral bending significantly decreased the foraminal width (maximum and mi nimum), height, and area at the bending side, whereas lateral bending signi ficantly increased the foraminal width (minimum), height, and area at the o pposite side of bending. Likewise, axial rotation decreased the foraminal w idth (minimum) and area at the rotation side significantly while significan tly increasing the foraminal height and foraminal area at the opposite side . The percent change in the foraminal area was found significantly correlat ed with the amount of segmental spinal motion except for the extension moti on. Conclusions. This study showed that the intervertebral foramen of the lumba r spine changed significantly not only on flexion-extension but also on lat eral bending and axial rotation. The percent change in cross-sectional fora minal area was correlated with the amount of segmental motion except for ex tension motions. Further studies are needed to assess the morphologic chang es in the intervertebral foramen in vivo and to correlate clinically.