Advantages of Unilateral Transpedicular Screw Fixation in Transforaminal Lumbar Interbody Fusion - Biomechanical Study with Fin

INTRODUCTION: Transforaminal lumbar interbody fusion (TLIF) with transpedicle screw (TPS) fixation was accepted as a minimally invasive surgery to treat degenerative disc disease for decompression and maintaining stability. According to previous researches, spinal fusion using bilateral TPS (BTPS) fixation with single or paired cages could sufficiently maintain vertebral stabilities, but the unilateral TPS (UTPS) fixation was considered that it could not afford sufficient stabilization. However, in our clinical observations, good postoperative performances were found in the further follow-ups (Fig.1). It was believed that application of unilateral approach could reduce the incision and the period for recovery, but still the biomechanical evidences for its capability to maintain stability were required. Purpose of this study was to evaluate the biomechanical performances of single cage insertion with BTPS and UTPS fixations using finite element method. Furthermore, analyses of different vertebral levels for UTPS surgery were also compared. Fig. 1 Radiographs of spinal fusion with unilateral transpedicular screw fixation on L45. Spinal structure remained stable in the follow-ups. METHODS: Finite element model of human spine (L2-S1) was reconstructed and validated for simulation with ANSYS Workbench 11.0. Comparisons of fusion with oblique single cage insertion (Fig. 2a) and BTPS/UTPS (Fig. 2b and 2c) fixation on L45 and biomechanical behaviors of unilateral pedicle screw fixation on different fusion level (L45 and L5S1, Fig. 3) were performed and evaluated. With fixation of the bottom of S1 vertebral body, a 150N compressive load was applied as weight of upper trunk, and additional 10N-m moment for simulating flexion, extension, both sides of lateral bending and axial rotation. Maximum compressive stresses (minimal 3 principal stress) on vertebral bodies (inferior side), and von Mises stresses on cage and TPS were evaluated Fig.2 (a)cage position, (b)BTPS and (c)UTPS fixation on L45. Fig. 3 UTPS fusion on (a)L4-L5 and (b)L5-S1 RESULTS SECTION: Both groups with UTPS fixation showed higher von Mises stresses on TPS than on either side in BTPS group (Table 1). Severe increases (over 100%) of stress were found in flexion and right lateral bending. Differences between UTPS on L45 and L5S1 were not toward identical tendency. All values of von Mises stresses on TPS were not exceeded the yielding stress (criteria for implant failure) of titanium alloy. High compressive stresses on inferior vertebral bodies after implantation were found in all groups (Fig. 4a), and generally UTPS group is higher than BTPS. But significant high compressive stress was found in BTPS group under flexion (27.307 MPa). Comparing with stresses on the cage (Fig. 4b), highest von Mises stress was also found in BTPS group under flexion (150.23 MPa). Stresses on cages were almost vanished under extension in all groups. Distributions of stress on vertebral bodies and cages were shown in Fig.5. Table 1 von Mises stresses on TPS (unit: MPa) Fig. 4 (a) Compressive stress on L5 for intact, BTPS, and UTPS groups. (b) von Mises stresses on cages in 3 groups. (unit: MPa) Fig. 5 von Mises stress distributions under flexion for (a) BTPS on L45, (b) UTPS on L45, (c) UTPS on L5S1, and extension for (d) BTPS on L45, (e) UTPS on L45, and (f) UTPS on L5S1. (unit: Pa) DISCUSSION: There’s an interesting observation for asymmetrical fixation with UTPS: significant high stress on cage and inferior vertebral body from BTPS under flexion could be reduced. Symmetrical fixation method (BTPS) tended to restrict the forward flexion of the motion unit to be symmetrical on coronal and transverse planes, but the supporting component (i.e. cage) was inserted obliquely. So BTPS would force the contact to occur at the front tip of cage under flexion, which led to highstress concentration that was not observed in traditional surgical approach with insertion of paired cages. Since the oblique cage insertion was an asymmetrical approach, UTPS either on L45 or L5S1 could just give an adequate assistance for supporting the structure under flexion according to current study. In the opinion of mechanical balance, asymmetrical fixation may be applicable for TLIF, the current trend of minimally invasive spinal fusion surgery. However, vanish of stresses on the cages remained a problem that cage dislodgements would occur during extension due to the lack in normal contact force. Positioning of cage to increase the stress on cage under extension is necessary to be concerned in further studies. BTPS_L45 (L) BTPS_L45 (R) UTPS_L45 UTPS_L5S1 flexion 37.786 43.514 78.784 88.912 extension 88.097 88.216 127.45 100.74 left bending 40.039 32.209 69.749 82.794 right bending 28.798 34.507 71.821 60.934 left rotation 43.943 62.955 76.512 72.35 right rotation 62.239 44.779 80.78 62.819