Bulging of the inner and outer annulus during in vivo axial loading of normal and degenerated discs.
METHODS
Comparison of in vivo biomechanical outcomes between experimental and control group animals.


OBJECTIVE
To quantify the in vivo bulging response of the inner and outer annulus in animals with and without disc degeneration.


BACKGROUND
Prior attempts to quantify the load-deformation response of the inner annulus have most often relied on in vitro preparations. Unfortunately, to visualize the inner annulus, these in vitro approaches rely on disc modifications that may result in nonphysiologic behaviors. In response to this problem, in vivo techniques were developed to quantify regional bulging of the inner and outer annulus during applied axial loading.


METHODS
Two groups of pigs were tested: a normal group and a group having disc degeneration that was induced surgically 3 months earlier. Eight adolescent pigs were evaluated and for each animal, a miniature servohydraulic actuator was attached to the third and fourth lumbar vertebrae to deliver a cyclic axial loading protocol (300 N, 1 Hz, 10 cycles) whereas regional deformations of the annulus were visualized ultrasonically via retroperitoneal access.


RESULTS
For the normal animals, image analysis demonstrated a significantly greater bulging of the inner annular region when compared with the outer annular region. In animals with disc degeneration, the inner and outer annular regions were equal in their bulging response, which ranged from 0 bulging to 37% greater than the average response of the normal animals.


CONCLUSIONS
This work supports prior in vitro studies that observed maximal disc bulging in the inner annulus and minimal bulging in the external annulus. Results for this in vivo study suggest that this normal bulging gradient is lost with degenerative disc disease. Compared with in vitro approaches, this new in vivo technique has the potential to demonstrate disc behavior in a variety of loading conditions and/or with a variety of induced disc pathologies.
Faculty of Rehabilitation Medicine, University of Alberta, Edmonton, Canada. kawchuk@ucalgary.ca
Gregory N
GN
Kawchuk
Allison M
AM
Kaigle Holm
Lars
L
Ekström
Tommy
T
Hansson
Sten H
SH
Holm
D000818:Animals; D001696:Biomechanical Phenomena; D004195:Disease Models, Animal; D051445:Fibrocartilage / Q000000981:diagnostic imaging / Q000473:pathology* / Q000503:physiopathology; D007091:Image Processing, Computer-Assisted; D007403:Intervertebral Disc / Q000000981:diagnostic imaging / Q000473:pathology* / Q000503:physiopathology*; D007405:Intervertebral Disc Displacement / Q000000981:diagnostic imaging / Q000473:pathology* / Q000503:physiopathology*; D016059:Range of Motion, Articular / Q000502:physiology; D013131:Spine / Q000000981:diagnostic imaging / Q000473:pathology / Q000503:physiopathology; D034421:Sus scrofa; D014463:Ultrasonography / Q000379:methods; D016474:Weight-Bearing / Q000502:physiology