Optimal stiffness of a pedicle-screw-based motion preservation implant for the lumbar spine.
OBJECTIVE
Pedicle-screw-based dynamic implants are intended to preserve intervertebral mobility while releasing certain spinal structures. The aim of the study was to determine the as yet unknown optimal stiffness value of the longitudinal rods that fulfils best these opposing tasks.


METHODS
A finite element model of the lumbar spine was used which includes the posterior implant at level L4/5. More than 250 variations of this model were generated by varying the diameter of the longitudinal rods between 6 and 12 mm and their elastic modulus between 10 MPa and 200 MPa. The loading cases flexion, extension, lateral bending and axial rotation were simulated. Evaluated optimization criteria were the ranges of motion, forces in the facet joints, posterior bulgings of the intervertebral disc and the intradiscal pressures. Various objective functions were evaluated.


RESULTS
The results show that the objective values depend more on the axial stiffness of the rods than on bending and torsional stiffness, rod diameter and elastic modulus. The optimal stiffness value for most of the investigated objective functions is approximately 50 N/mm and is achieved, e.g. using a rod diameter of 6 mm and an elastic modulus of 50 MPa. The design with the least axial stiffness was the best one with regard to the mobility. The forces in the facet joints and the intradiscal pressures were reduced mostly by an implant with the highest axial stiffness. When minimal posterior disc bulging was the criterion, the optimal axial stiffness was also approximately 50 N/mm.


CONCLUSIONS
The optimal axial stiffness of a pedicle-screw-based motion preservation implant for the lumbar spine is approximately 50 N/mm.
Julius Wolff Institute, Charité-Universitätsmedizin Berlin, Augustenburger Platz 1, 13353, Berlin, Germany. antonius.rohlmann@charite.de
Antonius
A
Rohlmann
Thomas
T
Zander
Georg
G
Bergmann
Hadi N
HN
Boustani
D001696:Biomechanical Phenomena; D001858:Bone Nails; D001863:Bone Screws; D003198:Computer Simulation; D055119:Elastic Modulus / Q000502:physiology*; D020342:Finite Element Analysis; D006801:Humans; D008159:Lumbar Vertebrae / Q000502:physiology* / Q000601:surgery*; D008954:Models, Biological; D019736:Prostheses and Implants; D016059:Range of Motion, Articular / Q000502:physiology*; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods