Fill of the nucleus cavity affects mechanical stability in compression, bending, and torsion of a spine segment, which has undergone nucleus replacement.
METHODS
Axial loading, rotation, and bending were applied to human cadaveric lumbar segments to investigate the changes in disc mechanics with denucleation and incremental delivery of a novel hydrogel nucleus replacement.


OBJECTIVE
The purpose of this study was to investigate the effect of nucleus implant injection pressure/volume relationships on the quasi-static mechanical behavior of the human cadaveric lumbar intervertebral disc to determine if intact biomechanics could be reproduced with nucleus-implanted discs.


BACKGROUND
Previous studies have shown that volumetric filling of the nucleus cavity with a compliant nucleus replacement device will affect compressive stiffness of the implanted intervertebral disc, but data regarding restoration of mechanics through cavity pressurization are lacking.


METHODS
A total of 12 intact lumbar anterior column units were loaded in series in axial loading, axial rotation, lateral bending, and flexion/extension (FE). Each segment was fully denucleated and implanted with a hydrogel nucleus replacement using pressurization between 12 psi and 40 psi. Range of motion (ROM), neutral zone (NZ), energy dissipation (HYS), disc height (DH), and stiffness were compared among the intact, denucleated, and implanted conditions.


RESULTS
Denucleation significantly destabilized the segments compared to intact controls as shown by increased ROM, NZ, and HYS, and decreased DH and stiffness through the NZ. As the nucleus cavity was repressurized with increasing volumes of hydrogel implant, the segments were stabilized and DH was restored to the intact level. No significant differences from intact were observed in any loading direction for ROM, NZ, or DH after the segments were implanted with the nucleus replacement device using inflation pressures between 20 psi and 40 psi.


CONCLUSIONS
Compliant nucleus replacement using inflation pressures of 20 to 40 psi resulted in restoration of intact mechanics. Mechanical function was dependent on the volume of implant injected into the nucleus cavity.
Arthur
Amy
A
Synthes Spine, West Chester, PA, USA.
Cannella
Marco
M
Keane
Michael
M
Singhatat
Wamis
W
Vresilovic
Ed
E
Marcolongo
Michele
M
D019643:Arthroplasty, Replacement; D001696:Biomechanical Phenomena / Q000502:physiology; D019245:Compressive Strength / Q000502:physiology; D006801:Humans; D020136:Hydrogel, Polyethylene Glycol Dimethacrylate; D007403:Intervertebral Disc / Q000503:physiopathology* / Q000601:surgery; D008159:Lumbar Vertebrae / Q000503:physiopathology* / Q000601:surgery; D009068:Movement / Q000502:physiology; D016059:Range of Motion, Articular / Q000502:physiology*; D013314:Stress, Mechanical; D016474:Weight-Bearing / Q000502:physiology
D020136:Hydrogel, Polyethylene Glycol Dimethacrylate