Mechanical Load Study of Lumbar Center of Rotation and Lordosis and Its Potential Relationship to Formation of Rotatory Olisthesis.

Introduction: Rotatory olisthesis is a common finding of adult degenerative scoliosis. An in vitro model of rotatory olisthesis of the lumbar spine and its correlation with the center of rotation (COR) and lumbosacral lordosis (L3-S1) are presented.

Methods: Different centers of rotation and various angles of lumbosacral lordosis were tested for the production of rotatory olisthesis. The radiological finding of rotatory scoliosis was excacerbated with the center of rotation lying in the posterior column and with a normolordotic or hyperlordotic lumbosacral spine. Twenty-one synthetic models of L3-sacrum with simulation of certain anatomic restraints (ligaments and capsules) and disc disruption at L4-L5 were used. The COR was reproduced at the anterior, middle, or posterior column. A combination of a constant axial load at the lateral side of the upper vertebra and a similar size contralateral-side horizontal axial rotation force at the level of L4 vertebral body were applied to produce a rotatory olisthesis. The loaded specimens were immobilized at the end of the dynamic procedure and were imaged in two planes by fluoroscopy. The lateral and anteroposterior olisthesis at L4-L5 levels as well as L3-L5 scoliotic, L3-S1 lordotic Cobb angles and L4-L5 intervertebral rotation were measured.

Results: The radiologic finding of rotatory olisthesis (translation ≥ 1mm with rotation) appeared in the spinal units with the COR in the posterior column, although olisthesis was less than 1mm in specimens with the COR in the anterior or middle column. With COR in the posterior column and 40° to 60° of L3-S1 lordosis, rotatory olisthesis at the L4-L5 level was produced with anteroposterior olisthesis as the main component (p < .05). In specimens with 20° to 40° lordosis, lateral olisthesis was the main component of rotatory olisthesis (p < .05), and in less than 20° lordosis, rotatory olisthesis was minimum (translation less than 1 mm).

Conclusion: During an in vitro study for the appearance of rotatory olisthesis in the lumbar spine, the COR has been identified to lie in the posterior elements. The main component of rotatory olisthesis is anteroposterior olisthesis and lateral olisthesis in normolordotic and hypolordotic lumbosacral spine, respectively. The described model of rotatory olisthesis in the lumbar spine may serve as a guide for the formation of this deformity and can be the base for future research in the treatment of rotatory olisthesis in degenerative lumbar scoliosis.