Lumbar intradiscal pressure measured in the anterior and posterolateral annular regions during asymmetrical loading

OBJECTIVE: To analyze the effect of asymmetrical loading on intradiscal pressure. DESIGN: Human cadaveric lumbar spines were instrumented with multiple pressure sensors and subjected to external mechanical loads. BACKGROUND: Tears and radial fissures in the posterolateral annulus with no evidence of annular protrusion or nuclear extrusion are frequently observed in non-degenerated intervertebral discs. Cadaveric studies have shown that asymmetrical loads lead to posterolateral herniation. Regional overload may be responsible for a progressive structural weakness of the posterolateral annular fibres. METHODS: Three needles each equipped with three independent pressure sensors were inserted in the midplane of the L(3)/L(4) intervertebral disc (n = 16) in the anterior, right posterolateral and left posterolateral regions. Axial rotation was applied in the upright, flexed and extended positions and the pressures recorded. RESULTS: The largest intradiscal pressure increase was observed in the posterolateral inner annular regions, more so in flexion than extension, when combined with axial rotation. Significant centripetal pressure gradients were found only in the posterolateral needles during the upright and flexed positions. CONCLUSIONS: When applying compression and axial rotation, the posterolateral inner annular zones of the intervertebral disc show high stress peaks and centripetal pressure gradients. Asymmetrical loads (rotation) combined with postural changes in the sagittal plane increase these effects, and may be responsible for a chronic mechanical overload of these regions. RELEVANCE: Our findings suggest a predilection for the posterolateral inner annular regions to mechanical failure, especially under asymmetrical loading.