Does an Annular Puncture Influence the Herniation Path?: An In Vitro Mechanical and Structural Investigation.

STUDY DESIGN: A study of mechanically induced herniation in punctured ovine discs followed by structural analysis. OBJECTIVE: To investigate whether an annular puncture influences the path that herniation takes by providing direct passage for nucleus through the annulus and therefore whether it increases the risk of acute herniation from overload at the site of damage independent of any longer-term degeneration. SUMMARY OF BACKGROUND DATA: Ten years after treatment with discography both degenerative changes and frequency of herniation have been shown to increase compared to untreated discs. Although the effect of an annular puncture over time has been widely investigated the question of whether it increases the risk of acute herniation has not been resolved. METHODS: The posterolateral annuli of healthy ovine lumbar discs were punctured with either a 25-gauge (n = 8) or a larger 18-gauge (n = 8) needle and then compressed in a flexed posture of 10° until initial indications of failure. The entire volume of the disc was visually assessed for structural damage by obtaining progressive, full transverse cross-sections of its entire height thus exposing all regions of the disc. RESULTS: There was no association between the 25-gauge puncture and disc disruption and herniation. In contrast, nuclear material was observed to migrate through the 18-gauge needle puncture. Disruption of the lateral inner annulus was observed in 12 out of the 16 discs tested. CONCLUSION: The risk of acute herniation through the puncture site is dependent on the needle diameter used. Under the conditions employed the lateral inner annulus remains the site most vulnerable to disruption independent of the presence of a posterolateral puncture. LEVEL OF EVIDENCE: N /A.

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.