[Biomechanical analysis of lumbar spine after implantation of a disk prosthesis and supplementary spinal fusion for management of complications]

Abstract

Study design: The aim of our investigations was to study the question as to what extent the mono- and multisegmental mobility of the lumbar spine (LS) differs after implantation of a disc prosthesis (DP) and if, in case of postoperative complications (dislocation or migration of the disc prosthesis), a sufficient stability of the motion segment can be achieved with a spinal fusion. To answer these questions we examined in corpse the mobility of the LS before and after DP implantation as well as after spinal fusion by using a load simulator for mobility and stability investigations of the lumbar spine.

Method: After radiological exclusion of higher degenerative changes (X-ray in two planes) and exclusion of high-grade osteoporosis by using a pQ-CT scan with measurement of the bone mineral density (T score > -3.0), the first step was the multisegmental measurement of the thirteen unfixated LS specimens without DP by automatic electromechanical load-bearing. The measurements of mobility of the LS were done in flexion, extension, side-bending and side-rotation with a reproducible load of 10 Nm. The analysis of the movements during flexion and extension as well as side-bending and side-rotation were done by an ultrasonic detection system (zebris). In the second step the excursions were measured after implantation of the DP in L3/4 and in the third step after additional dorsal spinal fusion.

Results: After implantation of the artificial disc in L3/4 the mobility in side-bending and side-rotation increases in comparison to the LS without surgery. For side-bending we found an increasing amount up to +74 % for the single motion segment L3/4 and +3 % for the total LS, respectively, for the side-rotation an increase of mobility up to +72 % for the single motion segment L3/4 and +30 % for the total LS. On the other hand a decreased mobility of -11 % for flexion and extension was found with an artificial disc in comparison to the lumbar spine. After additional dorsal spinal fusion of L3/4 the mobility obviously decreases in all directions (flexion/extension -74 %, side-bending -75 %, side-rotation -51 %).

Conclusion: The DP preserves (flexion/extension) and, respectively, improves (side-rotation/side-bending) the mobility of the LS. In cases of complications like dislocation or migration of the DP and secondary degenerative changes, the dorsal spinal fusion with restricted movements of the LS afterwards is a possible treatment solution.