The influence of intervertebral disc shape on the pathway of posterior/posterolateral partial herniation

Basic scientific investigation employing the medical imaging techniques of contrast-enhanced plane film radiographs and computed tomography in addition to gross dissection techniques on a sample of spines.

To document the specific posterior/posterolateral pathway of mechanically-induced intervertebral disc (IVD) herniation as the result of repetitive loading and disc geometry.

In vitro cadaveric and animal investigations have indicated that the posterior/posterolateral aspects of the IVD are most susceptible to damage leading to herniation, and that cyclic bending is the most potent variable influencing herniation. The IVD in horizontal cross-section ranges in shape from ovoid to limacon (kidney-shaped) which influences stress distributions. The purpose of this investigation was to determine the role of the IVDs shape and size on influencing the pathway of herniation.

Compressive loads (1472 N) in conjunction with 7000 repetitive cycles of flexion-extension were applied to 22 porcine motion segments. Computed tomography images and contrast-enhanced plane file radiographs, in addition to dissection techniques were used to evaluate the progression of herniations. A logistical regression assessed the links between endplate size and shape, and the probability of a specific herniation type (directionally diffuse or directionally concentrated).

A total of 18 out of the 22 specimens exhibited detectable anular damage in the posterior/posterior lateral direction. Of the 18 specimens, 17 were partial herniations while one incurred a full herniation. IVD shape was found to be predictive of the pathway of herniation (P = 0.0329); oval IVD shapes were more likely to herniate in a directionally diffuse manner (6/18), while limacon IVDs were more likely to herniate in a directionally concentrated manner (12/18).

The shape of the IVD appears to be predictive of the pathway of IVD herniation.