Biomechanical Comparison of Unilateral and Bilateral Pedicle Screws

1 Objective. Little is known about the biomechanical effectiveness of transforaminal lumbar 2 interbody fusion (TLIF) cages in different positioning and various posterior implants used 3 after decompressive surgery. The use of the various implants will induce the stability change 4 in stresses and range of motion (ROM) at the surgical and adjacent segments. Unilateral 5 pedicle screw with or without supplementary facet screw fixation in the minimally invasive 6 TLIF procedure has not been ascertained to provide adequate stability without the need to 7 expose on the contralateral side. This study used finite element (FE) models to investigate 8 biomechanical differences in stability and stress on the neighboring structures after TLIF 9 cages insertion in conjunction with posterior fixation. 10 Mehtods. A validated FE model of L1-S1 was established to implant three types of cages 11 (TLIF with a single moon-shaped cage in the anterior or middle portion of vertebral bodies, 12 and TLIF with a left diagonally placed ogival-shaped cage) from the left L4-5 level after 13 unilateral decompressive surgery. Further, the effects of unilateral versus bilateral pedicle 14 screw fixation (UPSF vs. BPSF) in each TLIF cage model was compared to analyze 15 parameters, including stresses and ROM on the neighboring annulus, cage-vertebral interface 16 and pedicle screws. 17 Results. All the TLIF cages positioned with BPSF showed similar ROM (<5%) at surgical 18 and adjacent levels, except TLIF with an anterior cage in flexion (61% lower) and TLIF with 19 a left diagonal cage in right lateral bending (16% higher) at surgical level. On the other hand, 20 the TLIF cage models with left UPSF showed varying changes of ROM and annulus stress in 21 extension, right lateral bending and right axial rotation at surgical level. In particular, the 22 TLIF model with a diagonal cage and UPSF increased segmental motion mostly in 23 contralateral axial rotation (32% higher) and lateral bending (59% higher). Prominent stress 24 shielded to the contralateral annulus, cage-vertebral interface, and pedicle screw at surgical 25 level. A supplementary facet screw fixation shared stresses around the neighboring tissues 26 and revealed similar stability to those models with BPSF. 1 Conclusions. TLIF surgery is not favored for asymmetrical positioning of a diagonal cage 2 and UPSF used in contralateral axial rotation or lateral bending. Supplementation of a 3 contralateral facet screw is recommended for the stability of TLIF construct 4