Effect of the In Situ Screw Implantation Region and Angle on the Stability of Lateral Lumbar Interbody Fusion: A Finite Element Study

Abstract

Objective: To investigate the effect of the in situ screw implantation region and angle on the stability of lateral lumbar interbody fusion (LLIF) from a biomechanical perspective.Methods: A validated L2-4 finite element (FE) model was modified for simulation. The L3-4 fused segment undergoing LLIF surgery was modeled. The area between the superior and inferior edges and the anterior and posterior edges of the vertebral body (VB) is divided into four zones by three parallel lines in coronal and horizontal planes. In situ screw implantation methods with different angles based on the three parallel lines in coronal plane were applied in Models A, B, and C (A: parallel to inferior line; B: from inferior line to midline; C: from inferior line to superior line). In addition, four implantation methods with different regions based on the three parallel lines in horizontal plane were simulated as types 1-2, 1-3, 2-2, and 2-3 (1-2: from anterior line to midline; 1-3: from anterior line to posterior line; 2-2: parallel to midline; 2-3: from midline to posterior line). L3-4 ROM, interbody cage stress, screw-bone interface stress, and L4 superior endplate stress were tracked and calculated for comparisons among these models. Results:The L3-4 ROM of Models A, B, and C decreased with the extent ranging from 47.9% (flexion-extension) to 62.4% (lateral bending) with no significant differences under any loading condition. Types 2-2 and 2-3 had 45% restriction, while types 1-2 and 1-3 had 51% restriction in ROM under flexion-extension conditions. Under lateral bending, types 2-2 and 2-3 had 70.6% restriction, while types 1-2 and 1-3 had 61.2% restriction in ROM. Under axial rotation, types 2-2 and 2-3 had 65.2% restriction, while types 1-2 and 1-3 had 59.3% restriction in ROM. The stress of the cage in types 2-2 and 2-3 was approximately 20% lower than that in types 1-2 and 1-3 under all loading conditions in all models. The peak stresses at the screw-bone interface in types 2-2 and 2-3 were much lower (approximately 35%) than those in types 1-2 and 1-3 under lateral bending, while no significant differences were observed under flexion-extension and axial rotation. The peak stress on the L4 superior endplate was approximately 30 MPa and was not significantly different in all models under any loading condition.

The Influence of Screw Positioning on Cage Subsidence in Patients with Oblique Lumbar Interbody Fusion Combined with Anterolateral Fixation

“…In fact, when the screw was inserted from the middle of one side of the vertebral body and penetrated the cortex from the middle of the contralateral vertebral body, rigid tricortical fixation was formed due to the trajectory close to the endplate, which could both provide better initial stiffness under all loading conditions and provide superior protective effects on the interbody cage and screw‐bone interface under lateral‐bending conditions. The technique of tri‐cortical pedicle screw (TCPS) has been used to improve the anchoring strength in the vertebrae 37 . In practice, the technique of TCPS can be applied to enhance the grip strength of the screws and reduce the risk of CS.…”

“… 35 , 36 In addition, Zhu et al suggested that the screw penetrated the cortex of the opposite vertebral body, which was consistent with our usual implantation method. 37 In fact, when the screw was inserted from the middle of one side of the vertebral body and penetrated the cortex from the middle of the contralateral vertebral body, rigid tricortical fixation was formed due to the trajectory close to the endplate, which could both provide better initial stiffness under all loading conditions and provide superior protective effects on the interbody cage and screw‐bone interface under lateral‐bending conditions. The technique of tri‐cortical pedicle screw (TCPS) has been used to improve the anchoring strength in the vertebrae.…”

“…The technique of tri‐cortical pedicle screw (TCPS) has been used to improve the anchoring strength in the vertebrae. 37 In practice, the technique of TCPS can be applied to enhance the grip strength of the screws and reduce the risk of CS.…”