Bulging of lumbar intervertebral discs: non-contacting measurements of anatomical specimens

A noncontacting photogrammetric method was used to measure intervertebral disc bulge (deformation perpendicular to the disc surface) at an anterior site and a posterolateral site of 17 human lumbar discs. Specimens were tested in constructs consisting of two lumbar vertebrae and the intervening disc. The posterior elements were removed to give unobstructed visibility of the posterolateral aspects. Stereophotogrammetry was used to record the motion of target points attached to the disc and vertebral body surfaces. In axial loading, the bulging increased linearly with disc compression, which in turn had a nonlinear relationship with the applied load. Axial loads and offset compression loads of physiologic magnitude caused disc bulge that was generally less than 1 mm. Axial torsion of an otherwise unloaded specimen produced minimal change in disc bulge. The disc bulging measured in this study is generally less than that previously reported, especially for angular motions between the vertebrae. This is probably because the photogrammetric method used here is independent of motion of the vertebrae, which can corrupt mechanical, contacting methods of measurement.