• Bronfort G
• Ilharreborde B
• Zhao KD
• Berglund LJ
• An N

• Association of Chiropractic Colleges / congresses

METHODS: Fifteen motion segments from 9 fresh, frozen (-20°C) cadaveric lumbar spines were prepared in the standard manner. Specimens with severe osteoporosis, post-traumatic deformity or pathological disease were excluded. Upper and lower vertebral bodies of each motion segment were mounted in circular acrylic fixtures using polymethylmethacrylate. Fixtures were attached to a custom testing device to apply pure bending moments and simultaneous axial compression or distraction. A high-pressure transducer (Model OrthoAR, Medical Measurements, Inc., Hackensack, NJ) measured nuclear pressure and estimated compressive stress perpendicular to the transducer membrane. It was passed through each disc in the mid sagittal plane and extracted at 2 mm/second to produce a "stress profile" for 5 loading conditions: 300N compression (simulated non-weight bearing); 500N compression (simulated relaxed standing); 90N distraction (simulated axial distraction); 90N distraction and 5Nm extension (simulated extension-distraction); and 90N distraction and 5Nm flexion (simulated flexion-distraction). Disc degeneration was graded by 2 observers. Grades were collapsed into "low degeneration" (grades 1-normal and 2-mild) and "high degeneration" (grades 3-moderate and 4-severe) groups for analysis.

RESULTS: Mean age of cadavers was 66.4 years (range 40 to 82). The 9 spines yielded 15 L1-2 to L4-5 motion segments (8 low degeneration and 7 high degeneration discs). Disc levels were reasonably distributed. Neutral distraction yielded the lowest mean nuclear pressure (1.9 +/- 13.4 kPa). Nuclear pressure became negative in 4 of 8 low degeneration discs but only 1 of 7 high degeneration discs. Greater decreases in nuclear pressure (difference between 300N compression and the distracted conditions) occurred in the low degeneration group compared to the high degeneration group and the magnitude of the difference was similar in low and high degeneration groups for all 3 distracted conditions. The stress (pressure) distribution patterns for flexion-distraction and extension-distraction were similar in low degeneration discs. Discs with high degeneration had markedly less stress (pressure) in the posterior annulus than in the nucleus and anterior annulus during flexion-distraction. Extension-distraction resulted in a more even distribution of stress but more in the posterior annulus and nucleus than in the anterior annulus.

DISCUSSION: The only simulated condition associated with negative intradiscal pressures in this model was axial distraction. The distribution of stress in discs with lower grades of degeneration was similar during flexion-distraction and extension-distraction. In contrast, discs with higher grades of degeneration distributed forces differently, possibly due to an inability to generate or sustain high nuclear pressure. In those discs, flexion-distraction produced a stress gradient that was lowest in the posterior annulus while extension-distraction produced more stress in the posterior annulus than in the nucleus and anterior annulus.

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