Prediction of peripheral tears in the anulus of the intervertebral disc

The mechanical behavior of age-related degeneration in the anulus of lumbar spine segments was investigated under extension with compressive preload by the finite-element method.

To investigate why peripheral tears are initiated in the anterior outer anulus in the early stage of life.

Age-related changes in the geometry, loading conditions, and material-mechanical properties of lumbar spine would change mechanical behavior of the anulus.

Two finite element models of the human lumbar segments (L3-L4), a young spine model for young adults and an old spine model for elder adults, were constructed. The anulus was modeled as laminate composite elements with 16 layers and six materials. The microfailure modes such as fiber breakage, fiber folding, layer (ply) failure, layer folding, and interlaminar delamination were predicted by principal strain of the anulus layer.

Excessively high tensile principal strain in the transverse direction of the anulus layer, indicating layer failure, was predicted in the anterior outer anulus. The strain level was much higher in the young model than in the old model. Compressive principal strain in the transverse direction of the anulus layer, predicting layer folding, was found in the posterior and posterolateral anulus.

Layer failure in the anterior outer anulus could occur in the early stage of life during extension with preload. This could then progress to formation of peripheral tears oriented at right angles to the fiber's direction.