Effect of the screw type (S2-alar-iliac and iliac), screw length, and screw head angle on the risk of screw and adjacent bone failures after a spinopelvic fixation technique: A finite element analysis

Abstract

PurposeSpinopelvic fixations involving the S2-alar-iliac (S2AI) and iliac screws are commonly used in various spinal fusion surgeries. This study aimed to compare the biomechanical characteristics, specifically the risk of screw and adjacent bone failures of S2AI screw fixation with those of iliac screw fixation using a finite element analysis (FEA).MethodsA three-dimensional finite element (FE) model of a healthy spinopelvis was generated. The pedicle screws were placed on the L3-S1 with three different lengths of the S2AI and iliac screws (60 mm, 75 mm, and 90 mm). In particular, two types of the S2AI screw, 15°- and 30°-angled polyaxial screw, were adopted. Physiological loads, such as a combination of compression, torsion, and flexion/extension loads, were applied to the spinopelvic FE model, and the stress distribution as well as the maximum von Mises equivalent stress values were calculated.ResultsFor the iliac screw, the highest stress on the screw was observed with the 75-mm screw, rather than the 60-mm screw. The bones around the iliac screw indicated that the maximum equivalent stress decreased as the screw length increased. For the S2AI screw, the lowest stress was observed in the 90-mm screw length with a 30° head angle. The bones around the S2AI screw indicated that the lowest stress was observed in the 90-mm screw length and a 15° head angle.ConclusionsIt was found that the S2AI screw, rather than the iliac screw, reduced the risk of implant failure for the spinopelvic fixation technique, and the 90-mm screw length with a 15° head angle for the S2AI screw could be biomechanically advantageous.

Topping-off surgery vs posterior lumbar interbody fusion for degenerative lumbar disease: a finite element analysis

“…A six-degree-of-freedom rigidly fixed constraint was applied on the lower surface of the L5 lumbar vertebra according to the previous literature [11, 12] to verify that the lower surface of L5 vertebral would not produce displacement and rotation when moment was added.…”

“…The BOND connection was used between the vertebral body and the intervertebral disc. The facet joint surface was subjected to frictional simulation and the friction coefficient was set as 0.2 [4–9, 11–13]. “No Separation” was used to connect the surfaces of Coflex with those of the spinous processes.…”

Topping-off surgery vs posterior lumbar interbody fusion for degenerative lumbar disease: a finite element analysis

“…A 500N pre-compressive load equal to two thirds of the human body mass (50kg, 500N) [5] was applied on the upper surface of L3 lumbar vertebra. An additional moment of 10N*m (10000N*mm) was exerted in each direction as described in the literature [5][6][7][8][9][10][11][12] to simulate lumbar anteflexion, rear protraction and rotation ( Figure 1.2).…”

“…A six-degree-of-freedom rigidly fixed constraint was applied to the lower face of L5 lumbar according to the previous literature [11,12] to verify that the lower face of L5 vertebral would not produce displacement and rotation with added moment.…”

“…The BOND connection was used between the vertebral body and the intervertebral disc. The facet joint surface was subjected to frictional simulation and the friction coefficient was set as 0.2 [4][5][6][7][8][9][11][12][13]. "No Separation" was used to connect the surfaces of Coflex and those of the spinous processes.…”