Stiffness of prosthetic nucleus determines stiffness of reconstructed lumbar calf disc.
BACKGROUND
Currently, artificial spinal discs require transection or partial removal of the annulus fibrosis in order to excise the nucleus and implant a prosthetic nucleus or implant a total disc device, respectively. Preservation of the annulus for prosthetic disc replacement maintains the function of the annulus and may improve annulus load sharing with the prosthesis.


OBJECTIVE
To quantify the biomechanical characteristics of an annular sparing intervertebral prosthetic disc (IPD) in a lumbar calf spine model. The aim of the study was to determine whether altering the stiffness of the elastic component of this unique prosthesis would correspond to changes of the overall reconstructed disc.


METHODS
A biomechanical study was conducted in vitro using cadaveric calf spines such that each specimen served as its own control. Investigations were performed at the Minneapolis Medical Research Foundation, Orthopaedic Biomechanics Laboratory.


METHODS
Six L45 or L56 motion segments (from which the posterior elements had been removed) were studied in axial compression, sagittal and lateral bending and torsion. These load states were applied to the intact, denucleated and prosthetically reconstructed disc using four IPDs of differing stiffness.


RESULTS
Load-displacement testing demonstrated that stiffer IPDs resulted in a decreased range of motion and neutral zone, and greater stiffness of the reconstructed disc. Disc reconstruction with the stiffest IPD approximated the behavior of the intact disc.


CONCLUSIONS
The overall biomechanical characteristics of a reconstructed disc are related to the stiffness of a nucleus prosthesis. The similarities in the mechanical behavior of reconstructed and intact discs suggest that additional feasibility studies for the annulus-sparing IPD are warranted.
Buttermann
Glenn R
GR
Midwest Spine Institute, 1950 Curve Crest Boulevard, Stillwater, MN 55082, USA. butte011@tc.umn.edu
Beaubien
Brian P
BP
D000818:Animals; D019643:Arthroplasty, Replacement / Q000295:instrumentation* / Q000379:methods; D002417:Cattle; D004548:Elasticity; D066298:In Vitro Techniques; D007403:Intervertebral Disc / Q000503:physiopathology / Q000601:surgery*; D007595:Joint Prosthesis; D008159:Lumbar Vertebrae / Q000601:surgery*; D016059:Range of Motion, Articular; D016474:Weight-Bearing