Posterior lumbar interbody fusion (PLIF) is done to obtain interbody vertebral fusion through a posterior approach. The advantage of PLIF over anterior lumbar interbody fusion (ALIF) is the avoidance of vascular and reproductive system complications that can occur with anterior lumbar surgery.1,2 PLIF can be done in lieu of combined anterior and posterior surgeries. The surgical technique for PLIF continues to be modified and refined. Modifications include the addition of pedicle screw instrumentation, the use of engineered interbody devices, and the use of supplemental posterolateral bone grafting to add stability and increase fusion rates.3–5
Transforaminal lumbar interbody fusion (TLIF) is a modification of the PLIF approach that makes use of a unilateral facetectomy to gain access to the disk space. This approach requires significantly less nerve root retraction than does PLIF. Theoretically, TLIF has a potential lower complication rate than does PLIF, and it has been growing in favor.
The indications for PLIF have evolved as biomechanical and physiologic rationales have been investigated and as instrumentation has improved. Currently, PLIF is used to manage a variety of spinal pathologies, including degenerative disk disease, severe instability, spondylolisthesis, deformity, and trauma.
Rationale for Posterior Lumbar Interbody Fusion
PLIF and TLIF allow access to both anterior and posterior spinal columns by way of an all-posterior approach. Fusion is achieved at the site of the intervertebral joint, and the anterior column is reconstructed as the load-bearing structures are reconstructed. The resulting arthrodesis is biomechanically superior to posterolateral fusion (PLF) and creates a biomechanically superior environment for bone healing.6 Interbody fusion also offers increased surface area for bone graft. Because of these factors, PLIF may offer an advantage when performing surgery for pseudarthrosis. In a patient in whom standard PLF has failed, the posterior interbody approach may provide additional support for fusion. Neural foraminal distraction may occur during PLIF, which can result in nerve root decompression and correction of scoliosis or kyphosis.
The risk of injury to anterior vascular structures and sympathetic neural elements is lower with PLIF than with ALIF.7 One study indicated lower fusion rates with ALIF as a stand-alone (noninstrumented) procedure than with instrumented PLIF.8 However, other reports favor instrumented ALIF. In a retrospective study comparing ALIF and PLF, the authors noted significantly less blood loss, need for transfusion, amount of blood transfused, surgical time, and length of hospital stay for patients who underwent anterior fusion (P < 0.01).9 No significant difference was found in rehabilitation needs, complication rates, and clinical outcome.
Indications and Contraindications
The indications for PLIF and TLIF are continually being updated and refined as better outcomes studies become available.10–17 No level I or II studies comparing PLIF with other fusion techniques are available. Neither are there any level I or II studies that compare the results of different interbody devices or the effect of adjunctive instrumentation. Most outcomes are based only on level III case series. Thus, there are no clear evidence-based guidelines to determine when to use PLIF/TLIF over other posterior or anterior fusion methods.
Acceptable indications for PLIF and TLIF include degenerative scoliosis in which neural decompression and correction of deformity or severe spinal instability is desired from an all-posterior approach, high-grade spondylolisthesis, and failure of posterolateral fusion (pseudarthrosis) (Table 1). Relative indications for PLIF include treatment of debilitating low back pain caused by degenerative disk disease,18–22 high degree of spinal instability, low-grade spondylolisthesis, and patients at high risk for nonunion (eg, obese, smoker).12,15,23,24
The neural elements must be mobilized to perform PLIF safely. Severe epidural fibrosis may prevent this, and the resulting risk of nerve injury is a contraindication to PLIF. Severe osteoporosis is considered a contraindication to PLIF and TLIF because of the risk of end-plate and vertebral body fracture with increased stress at the graft site. There also may be increased risk of graft subsidence or migration.25 PLIF generally is not appropriate for surgical management of the upper lumbar spine because of the need to manipulate the conus. However, TLIF may be safe to use because of the decreased need for retraction. Active infection is a contraindication to PLIF/TLIF because of the risk of dural tear and possible meningitis. Other contraindications to PLIF include severe ankylosis, inability to distract the disk space, and the presence of conjoined nerve roots. TLIF may be more amenable in a patient with conjoined nerve roots.
The classic PLIF technique is performed through a wide laminotomy, with resection of the ligamentum flavum and partial or complete removal of the cranial lamina.1 The lower one third of the inferior facet and the medial two thirds of the superior facets are resected to expose the pedicle of the vertebra as far laterally as possible. This wide opening into the spinal canal minimizes the amount of neural retraction required. The traversing nerve root and dural sac are retracted medially. Special attention should be given to identifying and protecting the true axilla of the upper nerve root. Failure to identify it may result in undue tension, which can cause neurologic damage. Retraction of the traversing nerve root and dural sac will aid in visualizing the widest possible field for disk resection. The disk and any osteophytes are then resected as far anteriorly and laterally as possible. This aids in neural decompression as well as disk space visualization.
Disk space distraction is accomplished with either pedicle screw instrumentation or disk space distractors. Because of the risk of screw loosening with levering during pedicle screw manipulation, disk space distractors are preferred. These are designed so that the cage or bone graft can be inserted adjacent to them. The cartilaginous end plates are then removed with specially designed shavers and curets. The bony end plates are preserved to help prevent graft subsidence. The cage or bone graft spacer is then inserted posterolaterally, and supplemental bone graft can be packed around the spacer. PLIF may be supplemented with pedicle instrumentation and PLF to gain biomechanical advantages and increased fusion rates20,26–28 (Figure 1).
TLIF is a modification of the PLIF technique29 involving unilateral total facetectomy. The pars interarticularis is resected, and the inferior facet is removed. The superior facet is then resected until it is flush with the pedicle. The traversing nerve root is mobilized and retracted medially, but to a much lesser degree than with PLIF. Access to the disk can then be obtained transforaminally. Using special curets and shavers, disketomy is performed across to the opposite side. Disk height is reestablished using special distractors or pedicle screws. One or two interbody grafts are placed. When using a single interbody device, emphasis is placed on crossing the midline. The addition of bilateral pedicle screw instrumentation is recommended to restore spinal stability.30
Many reconstruction options are available, including allograft (ie, bone dowel, iliac crest), threaded cages (eg, Bagby and Kuslich [BAK] cage), titanium mesh cages, polymeric rectangular cages (eg, Brantigan cage), autograft (ie, tricortical iliac crest), recombinant human bone morphogenetic protein-2 (rhBMP-2), bioabsorbables, and ceramics1 (Figure 2). The primary goals of using interbody devices are to create immediate and permanent stability, circumvent bone graft resorption, maintain deformity correction, and decrease morbidity of autologous bone graft harvest. No high-quality clinical studies are available to adequately address which reconstruction technique is most efficacious.
In a retrospective study, Barnes et al26 compared the use of allograftthreaded cortical bone dowels with allograft-impacted wedges in PLIF. A statistically significant increase in permanent nerve root injury was noted with the use of threaded cortical bone dowel implants (P = 0.049). No difference was found between the two groups in terms of fusion rates. However, the authors reported nerve root complications resulting from the threaded cortical bone dowel implants. Clinical outcomes were significantly better with the use of impacted wedges (P = 0.016).
In their randomized prospective study, Zhao et al31 reported no overall difference between bilateral cage placement and PLIF using diagonal insertion of a single threaded cage by a posterior approach with unilateral facetectomy. The unilateral cage enabled sufficient decompression and solid interbody arthrodesis while maintaining most of the posterior elements. This procedure was clinically safe and had acceptable outcomes, but the study lacked enough patients for sufficient power.
The question of whether to use one cage or two is still debated. Molinari et al32 compared PLIF using one cage versus two in a study of US servicemen. The patients treated with two cages had a higher rate of dural tears but did not differ from the patients treated with one cage in terms of other complication rates or clinical outcome measures. The use of two cages added an average of $1,728 to the cost of the procedure. Thus, this study indicates that the use of one cage is favorable due to decreased complications and cost.
In a small prospective, randomized, nonblinded trial, Haid et al33 compared PLIF with autogenous iliac crest bone grafting and titanium cages versus PLIF using the same cages with rhBMP-2 in a collagen sponge. No statistically significant differences were found with respect to radiographic fusion and most clinical outcome measures. Two complications were reported related to bone graft harvest. Despite the results, we believe that there is insufficient evidence and long-term follow-up to recommend rhBMP-2 use for PLIF. The potential for ossification into the spinal canal and for seroma or inflammatory reaction is a serious concern that has not been fully studied.
Ceramics have the ability to provide primary stability against compressive loading, and they have been studied in animal models as an autograft substitute. However, studies have shown that biointegration of the osteoconductive cement does not occur fast enough and that shear forces cause early cement fracture, subsequent fragmentation, and gross resorption.34 There is also a possibility for acute inflammation. Thus, ceramics are not recommended for interbody grafting in PLIF.34
There are insufficient controlled studies to demonstrate the efficacy of PLIF and TLIF over other fusion techniques. Rompe et al35 found no difference in clinical outcome in a retrospective study of PLF with pedicle screws done with and without PLIF in patients treated for lumbar spinal degeneration and instability. However, the group treated without PLIF had a higher hardware failure rate. A retrospective study by Lidar et al36 also failed to find a clinical difference in patients who underwent PLIF versus PLF. Disk space height was increased and better maintained in patients who underwent PLIF. No correlation has been found between preservation of disk space height and clinical outcome. In their nonrandomized study comparing instrumented PLIF with nonsurgical treatment for single-level degenerative disk disease in 39 US servicemen, Molinari et al24 reported a high rate of return to functional military duty, pain relief, and satisfaction in the surgical group. Surgically treated patients were less likely to receive a back pain disability discharge or a permanent physical disability profile.21
Lumbar interbody fusion also has been used successfully in the treatment of chronic discogenic back pain. DeBerard et al37 performed a retrospective cohort study comparing posterolateral versus BAK interbody lumbar fusion (including ALIF and PLIF) in workers' compensation cases. The authors found that arthrodesis rates, satisfaction, function, and health were better for the cohort managed with BAK interbody lumbar fusion. This study provides level III evidence suggesting greater efficacy of BAK interbody lumbar fusion versus a posterolateral technique for lumbar fusion in workers' compensation patients.37,38 These findings should be interpreted with caution, however, because the study by De-Berard et al37 is limited by the large loss to follow up (30%), different patient populations, variable fusion assessment methods, and the retrospective nature of the review.
Clinical outcomes studies for TLIF are typically comprised of level III and IV evidence. Most are case series documenting the safety and comparable clinical efficacy of TLIF in treating lumbar disk disease as well as isthmic and degenerative spondyloslisthesis.39–41 Radiographic fusion rates of 89% to 94% have been reported in these studies.39–41
PLIF is frequently used for the treatment of spondylolisthesis, even though there is no solid evidence that clinical outcomes are better following PLIF than other fusion methods. Jacobs et al23 performed a systematic literature review of fusion for low-grade spondylolisthesis. The authors were unable to identify the best surgical technique (PLF, PLIF, or ALIF, with or without instrumentation) for fusion. Instrumentation and interbody grafting may facilitate and maintain reduction and fusion, but there are no studies that establish clinical results (Figure 3). Patient age and motivation to return to premorbid function play a strong role in the success or failure of PLIF. Other retrospective series have reported on the use of TLIF for the management of low-grade spondylolisthesis. TLIF has been shown to be efficacious in obtaining deformity correction, interbody fusion, and patient satisfaction, although no clinical outcomes parameters have indicated superior results with TLIF.41–43
Studies have shown that most patients who experience loss of reduction, nonunion, or implant failure following surgery for high-grade spondylolisthesis report resolution of pain, solid fusion, and no further slip progression after undergoing revision to PLIF.10,44 Reduction, pedicular fixation, and combined posterolateral and interbody fusion may facilitate permanent reduction and stabilization of high-grade spondylolisthesis.10,44
Fritzell et al27 compared ALIF, noninstrumented PLF, and PLIF in a randomized controlled study of patients with chronic disabling low back pain. All surgical techniques were found to reduce pain and decrease disability substantially, and no significant clinical differences were found among the groups. Fusion was better for instrumented than noninstrumented surgery, and interbody fusion led to the highest rates of fusion. However, this did not correlate with clinical outcomes. Complications increased significantly with increasing technical difficulty of the surgical procedure. No significant association between clinical outcome and complications was reported 2 years after surgery.27
Retrospective cohort studies have been done comparing combined anteroposterior fusion with PLIF. PLIF was associated with less surgical time, blood loss, hospitalization time, total costs, and time to return to work; patient satisfaction and fusion rates were comparable.13,45 Higher complication rates and hospital costs were reported in the groups that underwent combined anterior and posterior surgeries.45,46
Madan and Boeree47 prospectively compared two nonrandomized groups of patients who underwent either PLIF or ALIF. Fusion rates, Oswestry scores, and complication rates were comparable in both groups, and the authors concluded that either procedure could be a viable option for the treatment of discogenic back pain. In contrast, Barnes et al8 compared instrumented PLIF with stand-alone ALIF. Threaded cortical bone dowels were used in both techniques. The PLIF group had a much higher fusion rate (95% versus 13%) and better clinical outcomes at 1 year (70% versus 38%). Based on these results, the authors recommend the use of pedicle screws in conjunction with PLIF.
In a recent prospective randomized study, Kim et al48 compared instrumented PLF, PLIF, and PLIF plus posterolateral fusion. At 3-year follow-up, no significant differences were found in either clinical outcome or fusion rates. The patients who underwent PLIF had better sagittal balance on radiographic examination than the patients who underwent PLF. In their retrospective comparison of PLF and PLIF, Yashiro et al28 reported that fusion was improved by 21% in the PLIF group. Additionally, PLIF produced better correction and maintenance of deformity, even in patients with osteoporosis. A higher rate of hardware failure was noted in the PLF group.
Humphreys et al49 compared PLIF with TLIF in a retrospective cohort study of 74 patients. No significant differences were found in surgical time, blood loss, or duration of hospital stay in patients who underwent single-level PLIF and TLIF. However, patients who underwent multilevel TLIF experienced less blood loss than patients treated with PLIF. Of note, no complications were reported in the TLIF group, whereas multiple complications occurred in the PLIF group (eg, radiculitis, hardware loosening/breakage, nonunion). The authors suggest that TLIF is a useful alternative to PLIF because of the lower complication rate and greater versatility.
In PLIF, bone graft is applied directly across the intervertebral joint to create a structurally load-bearing graft that is initially stiff but that also, through compression, promotes a physiologically advantageous environment for bone healing.6,50 PLIF is technically demanding, but it can provide excellent decompression and intervertebral distraction, both anteriorly and posteriorly.1,19,51 Evans50 compared the reconstructed spinal motion segment with a flagpole. The pole (anterior graft) acts to resist compression, whereas the tensioned cables (internally distracted and fixed posterior elements) bear the tensile force (Figure 4).
Because of the destabilizing effects of overresection of the posterior elements, Cloward1 and Lin et al51 recommend preserving as much as possible of the posterior element tissue. Increased dorsal tensioning by preservation of tissues is advantageous but may not always be possible. Thus, internal fixation of the posterior elements also may be necessary.50,52 Evans50 recommends internal fixation of the posterior elements in PLIF because of the surgical disruption of these ligaments. This allows continuity of the posterior force transmission pathway to be maintained and protects the graft from levering, extrusion, and catastrophic failure.50,52 Internal fixation of the posterior elements is particularly important for the surgical management of spondylolisthesis. There is no consistent integrity of the posterior elements; thus, the original holding force is decreased, resulting in increased shear force generated at the intervertebral joint.52
The most important principle regarding bone grafting in PLIF and TLIF is that the graft must be capable of uniformly distributing the compressive interbody force over the end plate to prevent graft-host collapse.50 Physiologic loads borne by the disk and end plates during standing and light activities have been shown to be 1,000 N.53 Supine loads reach approximately 300 N.53 Recent biomechanical studies have been done to test the amount of surface area needed to provide sufficient bone graft and anterior support. Closkey et al54 found that vertebral bodies with graft covering ≤25% of the total end-plate area failed at loads <600 N. In contrast, 88% of the vertebral bodies with at least 30% coverage of the end-plate area were able to carry a load >600 N. The results suggest that to provide a margin of safety, interbody graft area should be significantly greater than 30% of the total end-plate area.
Finite element stress analysis of different interbody devices has shown that interbody fusion cages with larger areas of contact between cage and end plate produce a lower stress distribution pattern and that bony fusion produces a near normalization of physiologic stress distribution.55
PLIF can be modified in numerous ways, including using various graft techniques, incorporating different cage constructs and orientations, and adding posterior instrumentation. Currently, stand-alone PLIF/TLIF is not considered the standard for posterior interbody fusion. Pedicle screw fixation is usually added. Biomechanical studies have demonstrated that combined ALIF/PLIF results in increased stiffness in all bending moments compared with either stand-alone ALIF or PLIF.56 In one study, PLIF plus pedicle screws and rods was significantly stiffer than stand-alone PLIF in all moments except extension (P < 0.01).56 In two cadaveric studies, interbody cages/grafting demonstrated increased stiffness in flexion and lateral bending, and pedicle screw fixation increased the construct stiffness of stand-alone PLIF.57,58 Stand-alone TLIF biomechanics differ slightly from PLIF in that reconstruction with a single cage increases segmental flexibility in axial rotation.30 The addition of unilateral pedicle screws further increases stiffness, but bilateral pedicle screw/rod instrumentation is necessary to restore initial spinal stability. Ames et al58 suggest pedicle screw fixation for all singlelevel PLIF and TLIF and recommend its use for all multilevel interbody fusions because it has proved necessary to restore biomechanical stability of the affected spinal motion segments in multilevel procedures.
Biomechanical construct stiffness is affected by selection of grafting technique, graft or cage type, orientation, and size. Evidence suggests that threaded cages provide a more highly stress-shielded environment within the cage, but this does not appear to create a significant difference in construct stiffness between threaded, nonthreaded, and allograft spacers.57,59
The influence of cage or spacer positioning and the number of devices also has been studied. A single obliquely placed cage has a different stability profile compared with bilateral parallel cage constructs, which may be slightly stiffer.60 The addition of bilateral pedicle screw fixation neutralized these differences.60 Two small cages have been shown to distribute stress equal to one large cage.61 Therefore, the use of two cages in PLIF rather than one (which is the norm for ALIF) appears to have no biomechanical disadvantage. Goh et al62 studied the effect of cage size on bilaterally instrumented facetectomized vertebral motion segments. They found that large cages were necessary to restore normal lateral bending and torsional stiffness in the facetectomized segments but that flexion stiffness could not be restored regardless of cage size. This study indirectly suggests the need for additional fixation (pedicle screws) to restore spinal stability.
There is conflicting evidence regarding the optimum cage position. A correlation has been shown between mean construct stiffness and cage positioning within the sagittal plane that is inversely related to posterior rod strain. Cage strain increases with more anterior positioning and is inversely related to rod strain.61 Polly et al61 reported that placement of the interbody devices as anteriorly as possible showed increased biomechanical benefit. Others who have studied regional strength differences in the lumbar vertebral end plate have found that the posterolateral end plate provides the greatest resistance to subsidence, while the central region provides the least.63–65 Lowe et al63 commented on the need to tailor cage placement to the goals of surgery and the pathology of the patient. When lordosis correction is a major concern, two anterolateral cages can be inserted, and cantilever correction can be achieved through compression (with the possible risk of neuroforaminal compression). When the anulus is pliable, two posterolateral cages can be inserted to provide a fulcrum, thereby achieving maximum distraction.
End-plate preparation in interbody fusion is critical to success but remains controversial. End-plate removal has been assumed to weaken the compressive strength of the vertebral body. However, no clear compromise has been determined between the need to preserve end plates for anterior column support and endplate removal to improve vascularity for enhancement of fusion.63 Hollowell et al66 studied the biomechanical effect of end-plate preparation in thoracolumbar interbody fusion. They concluded that preservation of the vertebral end plate may not offer a significant biomechanical advantage in reconstructing the anterior column and that a titanium cage provided the best resistance to axial load. More recent studies have refuted this claim.25,64,67 Oxland et al25 found that removal of end plates caused mean failure loads to decrease to approximately 33% of intact specimen failure loads. Mean stiffness also decreased significantly (P = 0.01). Greater construct stiffness was obtained when posterior and lateral end plates were preserved. Based on the biomechanical data available supporting bony end-plate preservation, balanced with the desire to promote vascular channel access, we recommend the preservation of as much as possible of the posterolateral end plate. Breeching of the end plate to create vascular access should be done in a small area anteriorly or centrally.
Complications from PLIF can result from a range of factors, including dural tears, neurologic injury, hardware or bone graft migration, dysesthesia, loss of fixation, leg pain, infection, arachnoiditis, and pseudarthrosis.8,21,26,68–70 Molinari and Gerlinger21 reported a 13% incidence of dural tears in a consecutive series of patients treated with PLIF. In a large prospective randomized trial comparing PLF, instrumented PLF, and 360° fusion (ALIF with posterior fusion or instrumented PLIF), the complication rate in the PLF group was 12%, compared with 22% in the instrumented PLF group and 40% in the 360° fusion group.7 In 1999, Okuyama et al70 reported a high rate of complications with PLIF (75 of 148 patients). There was a 9% rate of transient neural palsy, a 5.4% rate of dural tear, and an 8% rate of hardware complications. Barnes et al26 reported a 13.6% rate of permanent neurologic injury with the use of threaded cortical bone dowels. They now recommend the use of impacted wedges. Elias et al68 reported a 16.4% rate of radiculopathy in a series of 61 patients who underwent PLIF. In 2006, Okuda et al69 cautioned that neurologic injuries continue to be the most serious complications with PLIF. In their series of 251 patients, 6.7% had neurologic injury resulting in dysesthesia or motor deficit. Strikingly, 3.5% of these patients sustained permanent severe motor loss (eg, foot drop), and 7% had hardware complications.
Most of the evidence in favor of PLIF and TLIF is retrospective. Biomechanically and biologically, the rationale for their use appears sound. PLIF and TLIF have shown to perform better than PLF with respect to rates of fusion and deformity correction.12,18,37 PLIF and TLIF also have been shown to be more costeffective than anteroposterior 360° fusion.13,45 A significant difference in clinical outcomes between interbody techniques has yet to be identified.38 There is no solid evidence that functional outcome is better following PLIF and TLIF than with other fusion methods.7,20,27 TLIF seems to be better than PLIF because of the decreased complication rate, increased versatility within the lumbar spine, and decreased invasiveness. The patient and the treating physician should carefully discuss the possible advantages and drawbacks of the different surgical options to make an informed decision and develop a treatment plan.
Evidence-based Medicine: Two level I prospective randomized studies (references 7 and 27) and several level II references (references 18, 20, 31, and 38) are cited. Level III/IV cohort and case-control studies predominate. Citation numbers printed in bold type indicate references published within the past 5 years.
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. Fritzell P, Hägg O, Nordwall A, Swedish Lumbar Spine Study Group: Complications in lumbar fusion surgery for chronic low back pain: Comparison of three surgical techniques used in a prospective randomized study. A report from the Swedish Lumbar Spine Study Group. Eur Spine J
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. Dehoux E, Fourati E, Madi K, Reddy B, Segal P: Posterolateral versus interbody fusion in isthmic spondylolisthesis: Functional results in 52 cases with a minimum follow-up of 6 years. Acta Orthop Belg
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. McAfee PC, DeVine JG, Chaput CD, et al: The indications for interbody fusion cages in the treatment of spondylolisthesis: Analysis of 120 cases. Spine
. Sears W: Posterior lumbar interbody fusion for degenerative spondylolisthesis: Restoration of sagittal balance using insert-and-rotate interbody spacers. Spine J
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. Hioki A, Miyamoto K, Kodama H, et al: Two-level posterior lumbar interbody fusion for degenerative disc disease: Improved clinical outcome with restoration of lumbar lordosis. Spine J
. Jacobs WC, Vreeling A, De Kleuver M: Fusion for low-grade adult isthmic spondylolisthesis: A systematic review of the literature. Eur Spine J
. Molinari RW, Sloboda JF, Arrington EC: Low-grade isthmic spondylolisthesis treated with instrumented posterior lumbar interbody fusion in U.S. servicemen. J Spinal Disord Tech
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. Harris BM, Hilibrand AS, Savas PE, et al: Transforaminal lumbar interbody fusion: The effect of various instrumentation techniques on the flexibility of the lumbar spine. Spine
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. Molinari RW, Sloboda J, Johnstone FL: Are 2 cages needed with instrumented PLIF? A comparison of 1 versus 2 interbody cages in a military population. Am J Orthop
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49. Humphreys SC, Hodges SD, Patwardhan AG, Eck JC, Murphy RB, Covington LA: Comparison of posterior and transforaminal approaches to lumbar interbody fusion. Spine
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