What stops slipping of a vertebra in spondylolisthesis?

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The shear is normally resisted by the annulus fibrosus, compression of the facets at the apophyseal joints, and by tension in the anterolateral layers of cortical bone in the pars interarticularis.
1. The failure strength and the anterior shear-strength of the disc, spinal ligaments are very high.
2. The ariculation of facet joints (Capacity of the pars for resisting tensile forces anteriorly at L4 and L5 has been reported at up to 2500 N)
3. In case of retrolysthesis; even the posterior displacement of the inferior facets reported to be 6.5 mm at failure.

Anterior displacement in spondylolisthesis is a slow process. The forces contributing to anterior shear arise from following things:
1. from vertical load on the spine above the lesion
2. from activity in the muscles of the spine and trunk
3. from the effects of movements.

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