Posterolateral Disc Prolapse in Flexion Initiated by Lateral Inner Annular Failure: An Investigation of the Herniation Pathway.

STUDY DESIGN: Structural investigation of mechanically induced herniations in ovine lumbar motion segments.

OBJECTIVE: This new study addresses the question of whether there are regions other than the posterior and posterolateral aspects that are implicated in the initiation of disc disruption and herniation.

SUMMARY OF BACKGROUND DATA: Flexion in combination with compressive loading will induce disc herniations in healthy motion segments in vitro. Although it is widely accepted that the posterior and posterolateral regions of the disc are the primary sites of herniation much less is known as to whether other regions of the disc might be involved in the herniation process.

METHODS: Healthy ovine lumbar motion segments (n = 14) were flexed 10° and compressed at a rate of 40 mm/min up to point of failure. The discs were macroscopically analyzed using progressive transverse sectioning to obtain a more global picture of internal disc disruption and herniation.

RESULTS: A high prevalence of disruption in the lateral annulus was found associated with circumferential tracking of nucleus between the annular layers toward the posterolateral and posterior regions. In all tests this lateral disruption did not cause any discernible external change in the lateral disc periphery after the removal of load. After imposing the predetermined flexion the applied compression also induced a forward anterior shear of the superior vertebra of approximately equal magnitude to the axial compressive displacement.

CONCLUSION: The vulnerability of the lateral annulus to disruption is thought to arise from the overloading of its differentially recruited oblique/counteroblique fiber sets, this in turn generated by anterior shear developed in the flexed, compressed motion segment. This lateral annular disruption, followed by circumferential tracking of nuclear material and resulting in either contained or uncontained extrusions in the posterior or posterolateral annulus, highlights the complexity of the herniation process.

LEVEL OF EVIDENCE: N/A.