Discogenic origins of spinal instability.
METHODS
Cadaveric motion segment experiment.


OBJECTIVE
To show how two physical aspects of disc degeneration (dehydration and endplate disruption) contribute to spinal instability.


BACKGROUND
The origins of spinal instability and its associations with back pain are uncertain. METHODS.: Twenty-one cadaveric thoracolumbar motion segments aged 48 to 90 years were secured in cups of dental plaster and loaded simultaneously in bending and compression to simulate full flexion, extension, and lateral bending movements. Vertebral movements, recorded using a two-dimensional "MacReflex" motion analysis system, were analyzed to calculate neutral zone (NZ), range of motion (ROM), bending stiffness (BS), horizontal translational movements, and the location of the center of rotation (COR). Intradiscal "stresses" were measured by pulling a miniature pressure transducer through the disc along its midsagittal diameter. All experiments were repeated after each of two treatments, which simulated physical aspects of disc degeneration: creep loading to dehydrate the disc and compressive overload to disrupt the endplate. Results were analyzed using ANOVA and linear regression.


RESULTS
Motion segment height was reduced by 1.0 (SD 0.3) mm during creep and by a further 1.7 (0.6) mm after endplate disruption. In flexion and lateral bending, the combined treatments increased NZ and ROM by 89% to 298%, and increased the "instability index" (NZ/ROM) by 43% to 61%. Translational movements increased by 58% to 86%, whereas BS decreased by 42% to 48%. In extension, ROM and NZ were little affected, although the COR moved closer to the apophyseal joints. Measures of instability increased most in lateral bending, and following endplate disruption. Stress concentrations in the posterior anulus fibrosus increased markedly after endplate disruption.


CONCLUSIONS
Two physical aspects of disc degeneration (dehydration and endplate disruption) cause marked segmental instability. Back pain associated with instability may be attributable to stress concentrations in degenerated discs.
Zhao
Fengdong
F
Department of Orthopaedics, Sir Run Run Shaw Hospital, ZheJiang University, HangZhou City, ZheJiang, People's Republic of China.
Pollintine
Phillip
P
Hole
Barny D
BD
Dolan
Patricia
P
Adams
Michael A
MA
D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena / Q000379:methods; D019245:Compressive Strength / Q000502:physiology; D005260:Female; D006801:Humans; D007403:Intervertebral Disc / Q000502:physiology*; D007593:Joint Instability / Q000503:physiopathology*; D016014:Linear Models; D008159:Lumbar Vertebrae / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D013122:Spinal Diseases / Q000503:physiopathology*; D013904:Thoracic Vertebrae / Q000502:physiology*