Sagittal Alignment With Downward Slope of the Lower Lumbar Motion Segment Influences Its Modes of Failure in Direct Compression: A Mechanical and Microstructural Investigation.

STUDY DESIGN: Microstructural investigation of compression-induced herniation of ovine lumbar discs with and without added component of anterior-inferior slope. OBJECTIVE: Does increased shear arising from a simulated component of motion segment slope imitating sacral slope weaken the lateral annulus and increase risk of overt herniation at this same region. SUMMARY OF BACKGROUND DATA: An increase in sacral slope secondary to lordosis and pelvic incidence increases shear stresses at the lumbosacral junction and has been associated with an increase in spondylolisthetic disorders and back injury. The small component of forward shear induced when a segment is compressed in flexion is suggested to cause differential recruitment of the lateral annular fibers leading to its early disruption followed by intra-annular nuclear tracking to the posterolateral/posterior regions. However, the influence of even greater forward shear arising from the added component of slope seen where pelvic incidence and lumbar lordosis are increased in the lower lumbar spine is less understood. METHODS: Ovine motion segments were compressed at 40 mm/min up to failure; 9 with a horizontal disc alignment and 26 with a segment slope of 15° and then analyzed structurally. RESULTS: All the horizontal discs failed (11.8 ±â€Š2.4 kN) via vertebral fracture without any evidence of soft tissue failure even in the lateral aspects of the discs. The increased forward shear resulting from the slope decreased the failure load (6.4 ±â€Š1.6 kN). The sloping discs mostly suffered mid-span, noncontinuous disruption of the lateral annulus with some extruding nuclear material directly from these same lateral regions. CONCLUSION: The increased level of forward shear generated in moderately sloping lumbar segments when compressed was abnormally damaging to the lateral regions of the disc annulus. This is consistent with the view that shear differentially loads the oblique-counter oblique fiber sets in the lateral annulus, increasing its vulnerability to early disruption and overt herniation. LEVEL OF EVIDENCE: N/A.

New evidence for structural integration across the cartilage-vertebral endplate junction and its relation to herniation.

BACKGROUND CONTEXT: The cartilaginous and bony material that can be present in herniated tissue suggests that failure can involve both cartilaginous and vertebral-endplates. How structural integration is achieved across the junction between these two distinct tissue regions via its fibril and mineral components is clearly relevant to the modes of endplate failure that occur. PURPOSE: To understand how structural integration is achieved across the cartilaginous-vertebral endplate junction. STUDY DESIGN: A micro- and fibril-level structural analysis of the cartilage-vertebral endplate region was carried out using healthy, mature ovine motion segments. METHODS: Oblique vertebra-annulus-vertebra samples were prepared such that alternate layers of lamellar fibers extended from vertebra to vertebra. The endplate region of each sample was then decalcified in a targeted manner before being loaded in tension along the fiber direction to achieve incomplete rupture within the region of the endplate. The failure regions were then analyzed with differential interference contrast microscopy and scanning electron microscopy. RESULTS: Microstructural analysis revealed that failure within the endplate region was not confined to the cement line. Instead, rupture continued into the underlying vertebral endplate with bony material still attached to the now unanchored annular bundles. Ultrastructural analysis of the partially ruptured regions of the cement line revealed clear evidence of blending/interweaving relationships between the fibrils of the annular bundles, the calcified cartilage and the bone with no one pattern of association appearing dominant. These findings suggest that fibril-based structural cohesion exists across the cement line at the site of annular insertion, with strengthening via a mechanism somewhat analogous to steel-reinforced concrete. The fibrils are brought into a close intermingling association with interfibril forces mediated via the mineral component. CONCLUSIONS: This study provides clear evidence of structural connectivity across the cartilaginous-vertebral endplate junction by the intermingling of their fibrillar components and mediated by the mineral phase. This is consistent with the clinical observation that in some disc herniations bony material can be still attached to the extruded soft tissue.