Introduction: Lumbosacral pseudoarthrosis and instrumentation failure is common with long segment constructs. Optimizing lumbosacral construct biomechanics may help to reduce failure rates. The influence of iliac screws and interbody type on lumbosacral stability and instrumentation bending strain is not well established.
Methods: Fourteen human cadaveric spine (L1-ilium) specimens were prepared and potted at L1 and ilium. Specimens were equally divided into either an L5-S1 ALIF or TLIF group. All specimens underwent testing in the following conditions: 1) Intact 2) L2-S1 Pedicle Screw Fixation (PSF) 3) L2-ilium (PSF-I) 4) PSF+ALIF (ALIF-S) or TLIF (TLIF-S) 5) PSF-I + ALIF (ALIF-I) or TLIF (TLIF-I). Pure moment bending (7.5Nm) in flexion(F), extension(E), lateral bending (LB), axial rotation (AR) and axial compressive (C) loads(400N) were applied to all conditions and range of motion (ROM), SS, and RS were measured. Statistical comparisons were performed using one-way ANOVA (p<.05).
Results: ALIF-S and TLIF-S provided similar decreases in ROM as TLIF-I (p>.05). Compared to PSF, iliac screws significantly decreased SS during bending in all directions (p<.01) except lateral bending (p>.16) but increased RS in flexion and extension (p<.03). ALIF-S provided similar decreases in SS as TLIF-I in all directions (p>.40). TLIF-S had more SS than TLIF-I in F,E,AR (p<.02). ALIF-S had significantly less rod strain than TLIF-I in F,E,C (p<.02) while TLIF-S had less rod strain only in E (p=.04). Compared to PSF-I, ALIF-I decreased the RS (p<.03) but TLIF-I did not have a significant effect (p>.64).
Conclusions: Iliac screws diminish SS during pure moment bending in all directions except LB but increase RS. ALIF-S provides comparable decreases in SS as TLIF-I and has significantly less RS, perhaps obviating the need for iliac fixation in long segment fixation. Iliac screw induced RS is only significantly reduced with ALIF but not TLIF.
Patient Care: Optimization of lumbosacral junction biomechanics with
appropriate combination of ALIF,TLIF, or iliac screws may help reduce pseudoarthrosis in long segment constructs.
Learning Objectives: By the conclusion of this session, participants should be able to:
1)Understand the impact of ALIF, TLIF, and iliac screws on range of motion, sacral screw strain, and rod strain at the lumbosacral junction
2) Understand that iliac screws decrease sacral screw
strain but increase rod strain
3) Understand that ALIF but not TLIF is protective of the increased rod strain induced by iliac screws
4)Understand that ALIF with sacral screws only has a comparable biomechanical profile to the posterior only construct TLIF with supplemental iliac screw fixation
References: 1. Boachie-Adjei O, Dendrinos GK, Ogilvie JW, Bradford DS. Management of adult spinal deformity with combined anterior-posterior arthrodesis and Luque-Galveston instrumentation. J Spinal Disord. 1991; 4:131-141.
2. Cunningham BW, Sefter JC, Hu N, Kim SW, Bridwell KH, McAfee PC. Biomechanical comparison of iliac screws versus interbody femoral ring allograft on lumbosacral kinematics and sacral screw strain. Spine. 2010;35(6):E198-205.
3. Emami A, Deviren V, Berven S, Smith JA, Hu SS, Bradford DS. Outcome and complications of long fusions to the sacrum in adult spine deformity: Luque-Galveston, combined iliac and sacral screws, and sacral fixation. Spine. 2002;27: 776-786.
4. Fleischer GD, Kim YJ, Ferrara LA, Freeman AL, Boachie-Adjei O. Biomechanical analysis of sacral screw strain and range of motion in long posterior spinal fixation constructs: effects of lumbosacral fixation strategies in reducing sacral screw strains. Spine. 2012; 37(3):E163-9.
5. Guler UO, Cetin E, Yaman O, et al. European Spine Study Group. Sacropelvic fixation in adult spinal deformity (ASD); a very high rate of mechanical failure. Eur Spine J. 2015;24(5):1085-91.
6. Harimaya K, Mishiro T, Lenke LG, Bridwell KH, Koester LA, Sides BA. Etiology and revision surgical strategies in failed lumbosacral fixation of adult spinal deformity constructs. Spine (Phila Pa 1976). 2011;36(20):1701-10.
7. Kuklo TR, Bridwell KH, Lewis SJ, et al. Minimum 2-year analysis of sacropelvic fixation and L5-S1 fusion using S1 and iliac screws. Spine. 2001;26:1976-1983.
8. Lehman RA Jr, Kuklo TR, Belmont PJ Jr, Andersen RC, Polly DW Jr. Advantage of pedicle screw fixation directed into the apex of the sacral promontory over bicortical fixation: A biomechanical analysis. Spine. 2002;27:806–811.
9. McCord DH, Cunningham BW, Shono Y, Myers JJ, McAfee PC. Biomechanical analysis of lumbosacral fixation. Spine. 1992;17:S235–S243.
10. Sutterlin CE 3rd, Field A, Ferrara LA, Freeman AL, Phan K. Range of motion, sacral screw and rod strain in long posterior spinal constructs: a biomechanical comparison between S2 alar iliac screws with traditional fixation strategies. J Spine Surg. 2016;2(4):266-276.