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00007632-200510010-0002300007632_2005_30_e562_ames_transforaminal_19miscellaneous-article< 77_0_15_6 >Spine© 2005 Lippincott Williams & Wilkins, Inc.Volume 30(19)1 October 2005pp E562-E566Biomechanical Comparison of Posterior Lumbar Interbody Fusion and Transforaminal Lumbar Interbody Fusion Performed at 1 and 2 Levels[Biomechanics]Ames, Christopher P. MD*; Acosta, Frank L. Jr MD*; Chi, John MD, MPH*; Iyengar, Jaicharan BS†; Muiru, William BS‡; Acaroglu, Emre MD§; Puttlitz, Christian M. PhD§From the *Department of Neurological Surgery, †School of Medicine, ‡Department of Biomedical Engineering, and §Department of Orthopaedic Surgery, University of California, San Francisco, San Francisco, CA.Acknowledgment date: January 31, 2005. First revision date: April 7, 2005. Acceptance date: April 8, 2005.The device(s)/drug(s) that is/are the subject of this manuscript is/are not FDA-approved fro this indication and is/are not commercially available in the United States.Corporate/Industry funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.Address correspondence and reprint requests to Christopher P. Ames, MD, Department of Neurological Surgery, University of California, San Francisco, 505 Parnassus Avenue, M-779, San Francisco, CA 94143-0112; E-mail: amesc@neurosurg.ucsf.eduAbstractStudy Design. Biomechanical laboratory study of human cadaveric spines.Objective. To determine the difference in acute stability between posterior lumbar interbody fusion (PLIF) and transforaminal lumbar interbody fusion (TLIF) performed at 1 and 2 levels with and without posterior fixation.Summary of Background Data. Circumferential spinal fusion with both an interbody graft and posterior pedicle screw-rod construct has been advocated to decrease pseudarthrosis rates. Both PLIF and TLIF theoretically allow for 3-column fixation and fusion.Methods. Specimens underwent either PLIF or TLIF at L2–L3 (single-level) and L3–L4 (2-level), both with and without pedicle screw instrumentation. During TLIF, an interbody allograft was placed in the anterior or middle column. Nondestructive, nonconstraining pure moment loading was applied to each specimen.Results. There were no significant differences in the range of motion after either PLIF or TLIF at 1 level. The addition of pedicle screws tended more strongly to increase rigidity after 1-level PLIF compared to TLIF. Position of the TLIF graft did not affect stability. The addition of pedicle screws to a 2-level construct significantly reduced all motions tested.Conclusions. Based on our findings, posterior fixation with a pedicle screw-rod construct is suggested for 1-level PLIF and TLIF, and is necessary to achieve stability after interbody fusion across 2 levels using either technique.Circumferential (360°) spinal fusion with both an interbody graft and posterior pedicle screw-rod construct has been advocated to decrease pseudarthrosis rates and improve clinical outcomes in the treatment of degenerative disease of the lumbosacral spine.1–4 Traditional options for 360° fusion in the lumbar spine have included: (1) sequential anterior lumbar interbody fusion (ALIF) with interbody graft placement combined with supplemental posterior instrumentation, and (2) posterior lumbar interbody fusion (PLIF) at simultaneous posterolateral fusion. Although allowing for unobstructed and direct access to the anterior column for improvement of sagittal balance and neural foraminal height, ALIF often requires the assistance of a vascular or general surgeon to provide exposure, and involves making both an abdominal incision as well as a posterior midline incision.5,6 Moreover, manipulation of the hypogastric plexus can cause retrograde ejaculation in as many as 45% of cases.4,7 Although PLIF with simultaneous posterolateral fusion requires only one surgical approach, it is associated with retraction of the thecal sac and nerve roots, with the attendant potential for injury to the nerve root or conus medullaris (above L3), and cannot be safely used in a repeat procedure secondary to scar tissue formation.8,9 In addition, to achieve adequate graft placement, it is often necessary to violate the structural integrity of both facet joints.To avoid these potential complications of ALIF and PLIF, in 1982, Harms and Rolinger10 suggested a modification of the traditional PLIF that involved the placement of bone graft and titanium mesh, via a transforaminal route, into the disc space that had previously been distracted using pedicle screw instrumentation. Such an approach can be accomplished without exposing more than the ipsilateral foramen, and retraction on the thecal sac is minimal. This approach can be particularly advantageous in the face of scarring after prior surgery. Moreover, the pedicle screw construct can be used intraoperatively via rod contouring to restore lumbar lordosis, while maintaining disc height with a titanium mesh graft that can be positioned in either the anterior or middle columns.11 Transforaminal lumbar interbody fusion (TLIF) also has the additional advantage of sparing the lamina, facet, and pars on the contralateral side, thus providing increased surface area for additional posterolateral fusion.Both PLIF and TLIF theoretically allow for 3-column fixation and fusion. Nevertheless, we hypothesize that lumbar spinal stability will not be significantly enhanced after either single-level PLIF or TLIF without posterior fixation. The goal of this study is to determine the difference in acute stability between PLIF and TLIF performed at 1 and 2 levels via 3 specific aims: (1) to determine the difference in range of motion of the lumbar spine after either 1 or 2-level PLIF and TLIF, (2) to determine the additional stability afforded by pedicle screw instrumentation after 1 and 2-level PLIF and TLIF, and (3) to determine what effect anterior versus middle column graft placement has on 1-level TLIF range of motion.Materials and MethodsSpecimen Preparation.Fourteen fresh frozen human lumbar spines (L1–L5) were used. Lateral roentgenograms were obtained to exclude any severe structural lesions. Dual energy x-ray absorptiometry was performed to determine bone mineral density. Specimens were stored at −20°C. After overnight thawing at 4–6°C, the attached soft tissue was removed, taking care to preserve the joint capsules, ligaments, and bony structures. The specimen then underwent PLIF or TLIF.InstrumentationPLIF. after soft tissue dissection, partial facetectomy and discectomy were performed bilaterally at L2–L3 (single-level PLIF) and L3–L4 (2-level PLIF) for the PLIF group. A ProSpace allograft interbody graft () was then inserted into each disc space bilaterally, according to manufacturer’s instructions (Figure 1 Left). Posterolateral fixation was accomplished with pedicle screw-rod instrumentation of L2–L3 (1-level) and L3–L4 (2 levels). The integrity of the final posterolateral fusion/PLIF construct was confirmed on plain film anteroposterior and lateral images.Figure 1. No caption available.TLIF. After soft tissue dissection, bilateral pedicle screws were inserted in the L2 and L3 (single-level TLIF), or L2, L3, and L4 pedicles (2-level TLIF) to achieve distraction during interbody graft placement. Next, a unilateral facetectomy and discectomy were performed at L2–L3 (1-level) and L3–L4 (2 levels). After discectomy, distraction was applied to the pedicles screws on the contralateral side. The cartilaginous endplate was removed to cancellous bone. Next, a ProSpace kidney bean-shaped interbody allograft (Figure 1 Right) was placed into the disc space(s) in either an anterior or middle column position (single-level). The graft was positioned in the anterior column only in the case of 2-level instrumentation. After graft insertion, distraction was released, and a pedicle screw-rod system was loaded and secured. Final position of the graft and pedicle screws was confirmed with plain radiographs.Biomechanical Testing.After instrumentation, specimens were nondestructively tested (Figure 2). The fixture holding the caudal vertebra was attached to the base of the testing apparatus while loads were applied to the rostral fixture. Nondestructive, nonconstraining pure moment loading was applied to each specimen through a system of cables, pulleys, and weights, as described previously.12–15 The primary advantage of applying pure moments is that the load is independent of the site of application, and the applied load is uniform along the entire length of the spine. Loads were applied about the appropriate anatomic axes to induce 6 different motions: flexion, extension, right axial rotation, left axial rotation, right lateral bending, and left lateral bending. For each loading scenario, 3 preconditioning cycles were applied to the specimen. Loads were applied quasi-statically using 1.0-Nm increments to a maximum of 4.0 Nm. Each load was held for 45 seconds, and data were collected at 60 Hz for 5 seconds.Figure 2.No caption available.Load-dependent, 3-dimensional displacements were calculated using the principles of stereophotogrammetry. Three noncollinear markers were attached to the anterior aspects of the vertebral bodies of L2, L3, and L4. These triads were tracked using a 3-camera system (Motion Analysis System Inc., Santa Rosa, CA). Marker coordinate data were used to calculate rigid body displacements (rotations and translations) using an Eulerian formulation implemented in a custom-designed algorithm (Matlab version 5.3, Mathworks, Natick, MA). The resulting data provided range of motion data across the L2–L3 and L2–L4 segments. Previous experiments in our laboratory have determined that this measurement and computation system is accurate to within ±0.05°.Data Analysis.The range of motion (at 4.0 Nm) of the L2–L3 segment (1-level) and L2–L4 segments (2 levels) of each spine was computed from the marker triad data. Statistical analysis was performed using repeated measure 1-way analysis of variance with the Fischer least significant difference post hoc method for multiple comparisons. Statistical significance was defined as P < 0.05.ResultsAfter testing, no bone fractures were found in any specimen, and no screw, rod, or graft showed signs of fracture, loosening, or breakage. There were no statistically significant differences in flexion-extension, axial rotation, or lateral bending after either PLIF or TLIF at 1 level compared to the intact condition (Table 1). The addition of pedicle screws tended to increase rigidity after 1-level PLIF (P = 0.058) as compared to 1-level TLIF (P = 0.312) (Figure 3). The position of the TLIF graft (anterior vs. middle column) did not affect stability after instrumentation across 1 level. Across 2 levels, there was no difference in stability between PLIF and TLIF (anterior graft placement), with the exception that TLIF significantly reduced flexion-extension motion compared to PLIF (Table 2). Neither PLIF nor TLIF significantly improved rigidity as compared to the intact specimen. The addition of pedicle screws to a 2-level fusion significantly reduced all motions tested after both PLIF and TLIF as compared to the noninstrumented case (Figure 4). Although 2-level TLIF with pedicle screws tended to resist flexion-extension movement better compared to the PLIF construct, this was not found to be statistically significant (P = 0.171). There was otherwise no significant difference in stability between TLIF and PLIF across 2 levels after pedicle screws were added.Table 1. Mean Angular Motion in Each Condition Studied and at Each Level in Degrees ± Standard DeviationFigure 3. Range of motion across L2−L3 for each hardware condition.Table 2. P values for Comparisons of Motion Among Hardware ConditionsFigure 4. Range of motion across L2−L4 for each hardware condition.DiscussionSpinal fusion should provide stability to a spinal segment to allow for interbody graft consolidation and fusion without dislodgment. It is generally held that intervertebral motion should be reduced and, in the limit eliminated, as much as possible for a solid fusion to be achieved. In addition, if the construct does not provide adequate stability, displacement of the graft and eventual instrumentation failure may occur. Thus, interbody fusion procedures that minimize intervertebral motion are considered more likely to achieve a solid arthrodesis.Biomechanical ImplicationsIt was hypothesized that lumbar spinal stability would not be significantly enhanced after either single-level PLIF or TLIF without posterior fixation. This hypothesis proved to be true because there was no significant difference in flexibility across grafted levels for any motion (flexion-extension, lateral bending, or axial rotation) after 1-level PLIF or TLIF compared to the intact specimen. The addition of pedicle screws after single-level interbody graft placement did tend to increase rigidity in all cases, and it strongly tended to decrease flexion-extension after single-level PLIF (P = 0.058). These results imply that although sparing 1 facet joint (TLIF) may decrease flexion-extension across a single segment compared to PLIF, in neither case is the grafted segment without supplemental pedicle screw fixation significantly more rigid than in the noninstrumented case. Thus, from a purely biomechanical perspective, stand-alone PLIF and TLIF constructs (without additional posterolateral fixation) should be avoided because they may ultimately lead to increased rates of pseudarthrosis, graft dislodgment, and/or loosening. In addition, we found that the position of the interbody graft in either an anterior or middle column position during TLIF had no effect on stability. As such, although some have found that sagittal alignment is dependent on anterior graft placement,16 the ultimate position of an interbody graft in either the anterior of middle disc space during TLIF has no significant effect on final stability.After 2-level instrumentation, TLIF with anterior graft placement had a higher stabilizing effect on flexion-extension movement across the operated level than did PLIF, although there was no significant improvement in stability noted after either PLIF or TLIF compared to the intact specimen. Because TLIF has a higher stabilizing effect on flexion-extension compared to PLIF is most likely an outcome of the iatrogenic instability imparted to the functional spinal unit using the PLIF approach (facet and disc removal), an effect that becomes more significant to the regional stability of the lumbar spine when performed at 2 levels. Moreover, unlike PLIF, TLIF allows for the placement of a higher surface area of the interbody cage in the anterior disc space, thus limiting bending in the sagittal plane.11,17 This may not result in a significant difference in flexibility after 1-level instrumentation, however, violation of the bilateral facet joints and anterior disc spaces across 2 levels after the PLIF technique seems to necessitate additional anterior column support to maintain sagittal plane balance. Placement of an interbody graft in the anterior disc space is significantly more difficult in the PLIF compared to the TLIF procedure. Nevertheless, pedicle screw fixation is necessary to achieve adequate stabilization after both interbody fusion techniques.ConclusionsNeither the PLIF nor TLIF approach imparts a significant degree of segmental stability after a 1-level procedure. The addition of a pedicle screw-rod construct after 1-level PLIF or TLIF tends to increase stability, and this effect is more accentuated in a PLIF construct. Moreover, across 1 level, the decision to place an interbody graft in either an anterior or middle position has no effect on spinal stability. Across 2 levels, TLIF with anterior graft placement significantly decreases motion in the sagittal plane compared to PLIF, although neither is significantly more rigid than the noninstrumented spine. Based on our findings, posterior fixation with a pedicle screw-rod construct is suggested for 1-level PLIF and TLIF, and is necessary to achieve stability after interbody fusion across 2 levels using either technique.Key Points * Three-column instrumentation enhances fusion in the lumbar spine. * Placement of lumbar interbody grafts does not enhance spinal stability. * Pedicle screw fixation enhances spinal stability after PLIF and TLIF.References1. 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spines underwent either posterior lumbar interbody fusion or transforaminal lumbar interbody fusion at 1 or 2 levels. Neither approach imparted a significant degree of stability without supplemental pedicle screw fixation. Posterior fixation with a pedicle screw-rod construct is suggested for 1-level posterior lumbar interbody fusion and transforaminal lumbar interbody fusion, and is necessary to achieve stability after fusion across 2 levels.Biomechanical Comparison of Posterior Lumbar Interbody Fusion and Transforaminal Lumbar Interbody Fusion Performed at 1 and 2 LevelsAmes, Christopher P. MD; Acosta, Frank L. Jr MD; Chi, John MD, MPH; Iyengar, Jaicharan BS; Muiru, William BS; Acaroglu, Emre MD; Puttlitz, Christian M. PhDBiomechanics1930