Annular bulge contours from an axial photogrammetric method

Objective. To determine the characteristic contour of annular bulge in the lumbar spine under compression using a non-contacting technique. Design. Transverse bulge of the annulus in lumbar motion segments was measured from photographs taken through an acrylic load plate. Background. Annular bulge is considered a potential factor in nerve root impingement. However, consensus is lacking regarding which annular region bulges most. Previous studies have relied on markers in discrete locations or on contacting techniques, both of which limit the accuracy of bulge measurements. Reporting of continuous contours around the periphery of the disc has been limited to individual cases; characteristic behavior derived from a statistical treatment of the data is unknown. Methods. Sixteen lumbar discal motion segments were loaded to 2500 N in compression. Axial photographs were taken in the pre-loaded and loaded states and digitized with an effective accuracy of 0.05 mm at 5 ° intervals. The bulge was averaged over the sample of specimens at each interval to provide a mean-value plot in the transverse plane. Regional variations were evaluated statistically. Results. It was found that the bulge was greatest posterolaterally (0.93 mm, SD 0.52 mm), and smallest anteriorly (0.37mm, SD 0.26 mm; corrected p < 0.05), with an average of all data points of 0.67 mm, SD 0.42 mm. The restraining influence of the posterior longitudinal ligament, while not statistically significant, was evident in magnified view of the bulge contour. These regional differences were greater in the lower than in the upper lumbar segments. Conclusions. Herniation posterolaterally may be associated with relatively large annular bulge in that region, although normal bulge is only of the order of 1/100th the diameter of the disc.