Optimal Intervertebral Sealant Properties for the Lumbar Spinal Disc: A Finite-Element Study

BACKGROUND In the lumbar spinal column, an annular disruption may be sealed after annulotomy to prevent further prolapse and instability. We investigated the biomechanical effects of various material properties of an injectable sealant. METHODS We used a 3-dimensional, nonlinear, osteoligamentous, experimentally validated finite-element model of the L3-L5 spine segment to study annulotomies of varying sizes and locations in the L3-L4 annulus followed by replacement with isotropic sealants (plugs) with a Young's modulus of 0.4, 2.0, 4.0, 6.0, and 40.0 MPa. Annulotomies in the region of the posterior longitudinal ligament were studied with and without the ligament in place. Intact, destabilized, and repaired models were subjected to 400 N compression and 12.7 Nm moment in all loading modes to compute plug forces, plug stresses, motion characteristics, and annulus bulge. RESULTS Changes in sealant stiffness minimally affected the overall motion characteristics of the segment. Increases in shear stress and von Mises stress were proportional to the stiffness of the sealant. The von Mises stress was inversely proportional to plug size. Removal of portions of the posterior longitudinal ligament did not significantly alter motion between spinal segments or stress in the annulus fibrosus. Removal of portions of the ligament increased the disc bulge when plugs were less stiff. Intradiscal pressure decreased when an annulotomy was created. The sealant generally restored nucleus pressure to a degree proportionate to sealant stiffness. CONCLUSIONS Minimizing sealant stresses as well as expulsion and separation forces should lead to a minimal Young's modulus. Sealant materials with a Young's modulus close to 6 MPa are most appropriate. The allowable variation in material properties is reduced with increased annulotomy size. Removal of posterior longitudinal ligament only allows increased sealant bulge when the sealant's modulus of elasticity is very low. This removal does not affect spinal unit biomechanics or annulus stress in annulotomy or annulotomy with sealant repair.