Segmental vertebral three-dimensional motion in patients with L4 isthmic spondylolisthesis under weight-bearing...

To investigate in vivo 6-degree-of-freedom (DOF) vertebral motion in patients with isthmic spondylolisthesis (IS) during various functional weight-bearing activities.

Fifteen asymptomatic volunteers (mean age 54.8 years) and fourteen patients with IS at L4-5 (mean age 53.4 years) were recruited. The positions of the vertebrae (L4-L5) in the supine, standing, flexion-extension, left-right twisting and left-right bending positions were determined using previously described CT-based models and dual fluoroscopic imaging techniques. Local coordinate systems were established at the center of the anterior vertebra of L4 isthmic spondylolisthesis (AIS), the posterior lamina of L4 isthmic spondylolisthesis (PIS) and the center of the L5 vertebra to obtain the 6DOF range of motion (ROM) at L4-L5 and the range of motion (ROM) between the AIS and the PIS.

The translation along the anteroposterior axis at L4-L5 during flexion-extension, left-right bending and left-right twisting was significantly greater than that of the healthy participants. However, the translation along the mediolateral axis at L4-L5 presented paradoxical motion under different positions: the ROM increased in the supine-standing and flexion-extension positions but decreased in the left-right bending and left-right twisting positions. The separation along the anteroposterior axis during flexion was significantly greater than that during standing, on average, reaching more than 1 mm. The separation along the mediolateral axis during standing, flexion and extension was significantly greater than that in the supine position.

This study revealed the occurrence of displacement between the AIS and PIS, primarily in the form of separation during flexion. Symptomatic patients with isthmic spondylolisthesis exhibit intervertebral instability, which might be underestimated by flexion-extension radiographs.