IN-VITRO FLEXIBILITY IN THE LUMBAR SPINE BEFORE AND AFTER TOTAL DISC ARTHROPLASTY

Abstract

Introduction: Degenerative disc desease is one of the most frequently encountered spinal disorders. The intervertebral disc is a complex anatomic and functional structure, which makes the development of an efficient artificial disc a challenge [1].

Based on the complexity of the anatomical structures and the nearly unknown loading conditions at the moment only contradictory knowledge exists about the kinematics after TDA and in particular the location of the center of rotation in the human lumbar spine [2].

The objective of our study was to evaluate the kinematics of the human lumbar spine and the ability of TDA to restore the native conditions in regard to range of motion (ROM), neutral zone (NZ) and center of rotation (COR).

Material and Methods: In-vitro flexibility testing on functional spinal units (FSU) out of 12 fresh frozen lumbar spines has been performed. The FSU (L2/L3 and L4/L5) were tested first in the native condition, followed by nucleotomy and partial annulus resection and also after TDA with activ L (lumbar artificial disc, Aesculap Germany).

Therefore a spinal simulator has been customized, applying pure moments for flexion/extension, lateral bending and axial rotation (+/−7.5Nm) and axial preload (FP=400N) with a defined velocity (1°/s). The instantaneous COR has been calculated based on the velocity pole method using a 3D ultrasonic motion analysis system, measuring the twelve components of motion.

Results: The TDA with activ L leads to a good restoration of ROM and NZ in all loading directions under in-vitro flexibility testing. The instantaneous COR is exemplary described for the native condition under flexion/ extension in the sagittal plane. For the native condition the COR is located in the center of the inferior vertebral endplate. After nucleotomy the COR shifts dorsally into the region of the spinal cord and a significant grade of instability has been measured.

After insertion of the lumbar artificial disc the instability can be reduced to the native grade of motion and the COR is located again in the main axis of the spinal column in the upper third of the inferior vertebra.

Conclusion: The instantaneous COR has been estimated in-vitro for the different loading situations in the human lumbar spine before and after TDA. Based on the newly introduced method further optimizations of TDA devices can be undergone in regard to the particular aspect of physiological kinematics.