Bilateral pedicle screw fixation provides superior biomechanical stability in transforaminal lumbar interbody fusion: a finite element study.
BACKGROUND CONTEXT
Transforaminal lumbar interbody fusion (TLIF) is increasingly popular for the surgical treatment of degenerative lumbar disease. The optimal construct for segmental stability remains unknown.


PURPOSE
To compare the stability of fusion constructs using standard (C) and crescent-shaped (CC) polyetheretherketone TLIF cages with unilateral (UPS) or bilateral (BPS) posterior instrumentation.


STUDY DESIGN
Five TLIF fusion constructs were compared using finite element (FE) analysis.


METHODS
A previously validated L3-L5 FE model was modified to simulate decompression and fusion at L4-L5. This model was used to analyze the biomechanics of various unilateral and bilateral TLIF constructs. The inferior surface of the L5 vertebra remained immobilized throughout the load simulation, and a bending moment of 10 Nm was applied on the L3 vertebra to recreate flexion, extension, lateral bending, and axial rotation. Various biomechanical parameters were evaluated for intact and implanted models in all loading planes.


RESULTS
All reconstructive conditions displayed decreased motion at L4-L5. Bilateral posterior fixation conferred greater stability when compared with unilateral fixation in left lateral bending. More than 50% of intact motion remained in the left lateral bending with unilateral posterior fixation compared with less than 10% when bilateral pedicle screw fixation was used. Posterior implant stresses for unilateral fixation were six times greater in flexion and up to four times greater in left lateral bending compared with bilateral fixation. No effects on segmental stability or posterior implant stresses were found. An obliquely-placed, single standard cage generated the lowest cage-end plate stress.


CONCLUSIONS
Transforaminal lumbar interbody fusion augmentation with bilateral posterior fixation increases fusion construct stability and decreases posterior instrumentation stress. The shape or number of interbody implants does not appear to impact the segmental stability when bilateral pedicle screws are used. Increased posterior instrumentation stresses were observed in all loading modes with unilateral pedicle screw/rod fixation, which may theoretically accelerate implant loosening or increase the risk of construct failure.
['Ambati|Divya V|DV|', 'Wright|Edward K|EK|', 'Lehman|Ronald A|RA|', 'Kang|Daniel G|DG|', 'Wagner|Scott C|SC|', 'Dmitriev|Anton E|AE|']
D001696:Biomechanical Phenomena D002102:Cadaver D003198:Computer Simulation D020342:Finite Element Analysis D006801:Humans D016268:Internal Fixators D008159:Lumbar Vertebrae D008953:Models, Anatomic D065289:Pedicle Screws D016059:Range of Motion, Articular D013123:Spinal Fusion D014057:Tomography, X-Ray Computed
Pedicle screw fixation Transforaminal lumbar interbody fusion Finite element method Interbody cage Lumbar spine Spine biomechanics