Intervertebral disc degeneration can lead to "stress-shielding" of the anterior vertebral body: a cause of osteoporotic vertebral fracture?
METHODS
Mechanical testing of cadaveric lumbar motion segments.


OBJECTIVE
To test the hypothesis that degenerative changes in the intervertebral discs can influence loading of the anterior vertebral body in a manner that makes it vulnerable to fracture.


BACKGROUND
Measurements of systemic bone loss do not fully explain the patterns of osteoporotic vertebral fractures.


METHODS
Thirty-three cadaveric lumbar motion segments (aged 19-82 years) were subjected to 2 kN of compressive loading while positioned to simulate habitual erect standing postures and forwards bending. Intradiscal stresses were measured in each posture by pulling a miniature pressure transducer along the midsagittal diameter of the disc. "Stress profiles" were then integrated over area to calculate the force acting on the anterior and posterior halves of the vertebral body. These forces were subtracted from the applied 2 kN to determine the compressive force on the neural arch.


RESULTS
In motion segments with nondegenerated discs, <5% of the compressive force was resisted by the neural arch, and forces on the vertebral body were always distributed evenly, irrespective of posture. However, with severely degenerated discs, neural arch load-bearing increased to 40% in the erect posture, and the compressive force on the vertebral body was concentrated anteriorly in forwards bending, and posteriorly in erect posture.


CONCLUSIONS
Severe disc degeneration causes the anterior vertebral body to be stress-shielded during the usual erect posture, and yet severely loaded whenever the spine is flexed. This could help to explain why this region is frequently the site of osteoporotic fracture, and why forward bending movements often precipitate the injury.
Pollintine
Phill
P
Department of Anatomy, Rheumatology Unit, University of Bristol, UK.
Dolan
Patricia
P
Tobias
Jon H
JH
Adams
Michael A
MA
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena / Q000379:methods / Q000706:statistics & numerical data; D002102:Cadaver; D019245:Compressive Strength; D006801:Humans; D007403:Intervertebral Disc / Q000502:physiology* / Q000503:physiopathology; D008159:Lumbar Vertebrae / Q000502:physiology*; D008875:Middle Aged; D010024:Osteoporosis / Q000150:complications*; D011187:Posture; D016059:Range of Motion, Articular; D012016:Reference Values; D016103:Spinal Fractures / Q000209:etiology* / Q000503:physiopathology; D013314:Stress, Mechanical; D016474:Weight-Bearing / Q000502:physiology