Creep associated changes in intervertebral disc bulging obtained with a laser scanning device.

BACKGROUND Lumbar disc bulging has been determined with different methods in the past. Reported methods of bulging assessment were limited to a direct physical contact, were two-dimensional and were time-consuming. Assessing the three-dimensional contour of a biological object under load would imply that the tissue would creep and therefore changes its contour. For that purpose, we were interested how fast the contour has to be assessed and how creeping would counteract on the intradiscal pressure and disc height. METHODS For that purpose, a laser based three-dimensional contour scanner was developed. This scanner was especially designed to be mounted in a spine tester. For 15 min a static compression of 500 N was applied to seven human lumbar segments having all ligaments, facets and arches removed. Disc height, intradiscal pressure and disc contour were time dependently measured. FINDINGS Load application reduced the disc height by 1.14 mm. The further decrease showed a typical creep behavior whereas the intradiscal pressure slightly but significantly decreased from 0.49 to 0.48 MPa. Cross-sectional disc contours showed that bulging was largest anterolateral followed by the anterior region. The creeping also increased the disc circumference. This effect varied region dependently having a maximum of 0.1 mm posterolateral. INTERPRETATION Results suggest that geometries of biological tissues should be obtained within one minute avoiding superimposing creep effects. This new method might be used to evaluate disc injuries, degeneration and disc treatments. Measuring disc contours under different loads and conditions yields the outer annular strain distribution. This is a prerequisite for the development of cell seeded and tissue engineered implants.

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