The goals of an interbody fusion device is to restore disk height, create or maintain segmental lordosis, maintain sagittal balance, and restore weight bearing to the anterior column while bony fusion develops. Numerous cage options exist, including cages that require posterior fixation, stand-alone cages without integrated fixation devices, and stand-alone cages integrated with screws and/or anterior plates for fixation.
Interbody cages that require posterior fixation often result in higher fusion rates than stand-alone interbody cages, except those described with LT cages and bone morphogenetic protein-2 (BMP-2, Medtronic, Memphis, TN). However, the additional fixation may result in increased surgical morbidity and approach-related complications. Therefore, there has recently been an increased interest in stand-alone interbody cages.
Although stand-alone cages decrease the additional morbidity associated with additional posterior fixation, biomechanical studies have demonstrated that stand-alone interbody cages without any type of fixation are weakest in sheer strength and do not adequately increase stiffness in axial rotation or extension prior to the development of bony fusion. The absence of rigid fixation may impair fusion rates. Although additional anterior plating devices are effective, they require increased exposure and have a large anterior profile, which may result in soft tissue and vessel injury.
In order to obtain the benefits of a single-approach and negate the exposure necessary for additional plating devices, interbody devices with integrated bone fixation have been developed. These devices are generally composed of a polyetheretherketone (PEEK) interbody cage with three or four integrated locking screws that thread through the cage and into the adjacent inferior and superior vertebral bodies. This addition has minimized the occurrence of anterior migration and has increased biomechanical stiffness, theoretically increasing capacity for fusion.
In this chapter, we discuss the currently available integrated interbody devices from both a biomechanical and clinical perspective.
Anterior Lumbar Interbody Fusion
Most integrated interbody devices have been developed for the anterior lumbar interbody fusion (ALIF) technique. After surgery, both initial vertebral segment stability and loading transmission of the ALIF device play essential roles in the rate of fusion and development of adjacent level pathology. ALIF reinforces and stabilizes the anterior column, restores disk and foraminal height, and increases segmental lordosis.
Before the development of integrated interbody cages, stand-alone cages were kept in place by compressive forces of the spine. However, implantation of an interbody device during ALIF necessitates cutting the anterior longitudinal ligament, which is a major stabilizer in extension movements. Without this anterior tension band, the superior vertebral body may “lift off” of a nonintegrated interbody device in extension movements. The lack of rotational stiffness supplied by nonintegrated interbody devices allows for similar movements. To address the need for restoration of the anterior tension band, stand-alone cages with intrinsic plates and screws have been developed ( Fig. 18.1 )
The clinical summaries of stuides utilizing integrated ALIF are listed in Table 18.1 .
|Device||Author||Study Design||Patients ( N )||Follow-up||Graft substrate||Fusion rate||Outcome||Pertinent Notes|
|SynFix-LR||Seipe et al.||Prospective||71||Minimum 12 months||PEEK||97.3%||Significant improvement in VAS and ODI||Segmental lordosis improvement 16.1 to 26.7 |
2 cases required revision surgery
|Cho et al.||Retrospective||28||Mean 27.3 months||PEEK||86.7%||Significant improvement in VAS back and Leg and ODI||SynFix vs. Stabilis |
Only SynFix-LR maintained disk and foraminal height over follow-up time
|Strube et al.||Prospective||80||Mean 47 months||PEEK||70.6%||Significant improvement in VAS, ODI||SynFix-LR compared to ALIF and pedicle screws |
Higher satisfaction rate, less blood loss and shorter OR time in SynFix LR
|Schimmel et al.||Retrospective||95||Mean 47.7 months||PEEK||76%||ODI improvement after initial surgery, but less improvement after revision|
|Behrbalk et al.||Retrospective||25||Mean 16.7 months||PEEK plus BMP-2||90.6%||None||15.6% graft subsidence at 16.7 months|
|STALIF||McCarthy et al.||Retrospective||85||Mean 19 months||PEEK||65%||None||STAFLIF vs. ALIF and pedicle screws |
23% single level 60% dual level had nonunion
|Allain et al.||Prospective||65||12 months||PEEK||96.3%||Significant improvement in VAS, ODI and SF-36||Cage subsidence, one requiring post instrumentation, one have persistent L5 paresthesia, one having migration of the superior plate|
|Flouzat-Lachaniette||Retrospective||51||12 months||PEEK |
Iliac crest autograft vs. rhBMP-2
|88.7% in ICAG; 71% in BMP-2||Not reported|
|Hartshill Horshoe||Madan et al.||Retrospective||56||Mean 36 months||Titanium plus ICAG||100%||85.2% satisfied||No subsidence over 3 years |
one patient with postoperative sciatica owing to screw impingement
The SynFix-LR (DePuy Synthes, West Chester, PA) integrated ALIF cage is the most studied integrated interbody cage to date. The device consists of a trapezoid PEEK interbody cage with an integrated anterior titanium alloy stabilization plate and four titanium alloy fix-angled screws that lock into the superior and inferior endplates. The device and integrated screws and plate are entirely within the disk space, minimizing the anterior footprint needed during exposure.
Several studies have investigated the biomechanical properties of the SynFix-LR device. Schleicher et al. found that the SynFix-LR device significantly decreased range of motion (ROM) in extension, flexion, lateral bending, and axial rotation compared to an intact spine. Cain and colleagues noted the same biomechanical advantages and also found a decreased neutral zone and elastic zone in the involved intervertebral segment in comparison to an intact spine. In contrast, the authors found that an anterior interbody cage without integrated screws, but with the addition of posterior transarticular facet screws, did not significantly reduce ROM, neutral zone, or elastic zone in extension or axial rotation. However, the authors did note that there was more stability in spines with the SynFix-LR device and transarticular facet screws compared with the SynFix-LR device only, but this was not statistically significant. Similarly, the SynFix-LR device was statistically equivalent to an anterior interbody cage with posterior pedicle screws. In agreement with the aforementioned study, Choi et al. also found that SynFix-LR had a similar ROM compared with an anterior interbody cage with posterior pedicle screw fixation. The authors noted that the SynFix-LR device also resulted in a similar decrease in ROM of superior and inferior vertebral segments, although the SynFix-LR did cause more same-level facet strain compared with the anterior cage with pedicle screws. Interestingly, the anterior cage with pedicle screws caused more facet strain at adjacent-level facet joints compared with the SynFix-LR device. Furthermore, SynFix-LR had a more anatomic load distribution, whereas the combined anterior-posterior approach resulted in a more posterior load distribution compared with an intact spine.
Multiple studies have compared SynFix-LR with the Stabilis (Stryker, Kalamazoo, MI) interbody device. Stabilis is an earlier, nonintegrated, stand-alone ALIF device consisting of a titanium cage shaped as a threaded cylinder that anchors to the adjacent vertebral bodies owing to the compressive forces of the spine. Chen et al. found that the SynFix-LR device had more stability than Stabilis in flexion/ extension, lateral bending, and axial rotation in both osteoporotic and normal bone. Furthermore, SynFix-LR caused less stress and more even stress distribution on the annulus in all motions compared to Stabilis. Furthermore, SynFix-LR caused less facet joint contact force in extension and axial rotation. Notably, the authors determined that restriction of ROM was decreased by an average of 15.9% when using the SynFix-LR device in osteoporotic compared to normal bone, with most noticeable decreases occurring during extension. The authors stated that the device might be better suited for younger patients and those with a higher bone density.
In addition to the aforementioned biomechanical studies investigating SynFix-LR, multiple studies have examined clinical outcomes with the device. Siepe et al. found that patients treated with stand-alone SynFix-LR had significantly improved visual analogue scale (VAS) and Oswestry Disability Index (ODI) scores at mean follow-up of 35.1 months. Overall, 77.5% (55/71) patients reported satisfaction with their procedure. The authors noted that segmental lordosis significantly increased from 16.1 degrees preoperatively to 26.7 degrees postoperatively. Fusion was achieved in 36/37 (97.3%) of cases with adequate radiographic follow-up. One patient required revision operation for back pain, two patients had a postoperative S1 radiculopathy, and one patient had a transient L5 radiculopathy.
Cho et al. retrospectively studied 28 patients who received L5-S1 ALIF using Stabilis or SynFix-LR for the treatment of lumbar intervertebral foraminal stenosis. Clinical outcomes were similar between devices with both devices demonstrating improved ODI, VAS back, and VAS leg pain scores at average 27.3-month follow-up. However, the authors noted that, although both devices caused a significant increase in disk height and intervertebral foraminal height in the immediate postoperative period, only the SynFix-LR device maintained these improvements over the follow-up period.
Strube et al. prospectively compared outcomes in 40 patients receiving stand-alone SynFix-LR with 40 patients receiving ALIF with transpedicular fixation. The authors found that VAS and ODI scores were significantly improved in both groups but significantly better in the SynFix-LR group at 12-month follow-up. Furthermore, the SynFix-LR group had a significantly higher satisfaction rate, less blood loss, and shorter operative time than the ALIF with transpedicular fixation group. Fusion rates were similar between groups.
In contrast to the excellent outcomes in the aforementioned studies, Schimmel et al. investigated 95 patients treated with SynFix-LR and reported 26 (27%) patients required reoperation at average follow-up of 47.7 months. Most of the reoperations were for symptomatic pseudoarthrosis. The authors noted that the average ODI scores significantly improved after the initial operation, but that reoperation equated to worsened outcomes. The authors proposed that the lower stiffness and hydrophobic characteristics of PEEK lead to insufficient initial stability, thus creating suboptimal conditions for bony bridging with stand-alone SynFix-LR cages. In contrast, Behrbalk and colleages investigated outcomes in patients operated on with SynFix-LR and supplemental recombinant human bone morphogenic protein-2 (rhBMP-2) and found a 90.6% fusion rate in 32 levels receiving stand-alone SynFix-LR. Five patients (15.6%) demonstrated graft subsidence at mean follow-up of 16.7 months. Of these, three resulted in nonunion that required revision operation. However, no patient had rhBMP-2-related complications.
Multiple studies have reported device-related complications with the SynFix-LR. Thaler et al. reported two cases of postoperative bleeding from the right and left inferior epigastric arteries occurring from using SynFix-LR at the L5-S1 level. The authors stated that the divergent screws required more extensive mobilization, and thereby injury, of surrounding structures compared with standard ALIF without plate addition. Furthermore, Lastfogel et al. reported three incidences of sacral fracture after L5-S1 ALIF with SynFix-LR for the treatment of isthmic spondylolisthesis. All three cases occurred within 45 days of the operation. The authors noted that there is an axial compression force perpendicular to the sacral endplate and a shear force parallel to it. Additionally, the sloping face of the sacral endplate results in the SynFix-LR implant being oriented such that the superior screws are in the coronal plane and the inferior screws lie in the axial plane. In this configuration, the inferior endplates take on an axial load perpendicular to the screw insertion, which may result in sacral fracture. Kwon et al. reported a vertebral fracture of the L4 vertebral body after fusion of the L3-4 level for degenerative spondylolisthesis with the SynFix-LR device. The authors believed that instability from the degenerative facet joints led to excessive compression and shear load anteriorly, resulting in failure of the cage. The authors recommended additional pedicle screw fixation for patients requiring operation for degenerative spondylolisthesis.
Two studies have investigated the Stalif (Surgicraft Ltd., Redditch, UK) ALIF interbody device. Stalif is composed of a PEEK interbody cage with spaces for nonintegrated screws. In a biomechanical study, Schleicher and colleagues found that Stalif was statistically similar to SynFix-LR during extension, flexion, and axial rotation, but that SynFix-LR created more rigidity during these movements. Furthermore, the authors found that lateral bending was significantly more rigid with the SynFix-LR device.
McCarthy et al. investigated 37 patients receiving ALIF with the Stabilis device and found that 5/22 (23%) patients receiving single-level and 9/15 (60%) patients receiving two-level ALIFs with the device had pseudoarthrosis, whereas fusion occurred in all 30 patients who received an ALIF cage with posterior pedicle screws.
Multiple studies have investigated the ROI-A (LDR Medical, Troyes, France) integrated ALIF device. The ROI-A device consists of a PEEK cage with an intracorporeal locking system (VerteBRDIGE, LDR Medical). The proposed advantage of this locking system is that it requires less exposure than cages with integrated bone screws, which necessitate high-angle trajectories for insertion.
Freeman et al. investigated biomechanics of the ROI-A integrated cage and found that it significantly reduced motion in flexion-extension and lateral bending compared with an intact spine. However, there was no significant increase in rigidity in axial torsion. Additionally, the authors found that the deployment of the integrated plates did not significantly add to rigidity of the cage. Furthermore, addition of pedicle screws and rods decreased motion significantly in all planes compared with ROI-A alone.
Allain et al. investigated the ROI-A and found that 52/54 (96.3%) of patients achieved fusion at 1 year. Furthermore, ODI scores improved significantly at 12-month follow-up with 88.7% of patients reporting satisfaction with their surgery and 80.4% stating that they would have the surgery again. Cage-related complications included one patient having migration of the superior plate, one demonstrating cage subsidence, one requiring additional posterior instrumentation, and one having persistent L5 parasthesias postoperatively.
Flouzat-Lachaniette et al. investigated rhBMP-2 compared with autologous iliac crest bone graft in patients receiving ALIF with the ROI-A device and found that 88.7% of patients receiving iliac bone graft achieved fusion, whereas only 71.0% of patients receiving rhBMP-2 achieved fusion ( P = .001). Furthermore, the bone quality was significantly better in patients receiving iliac bone graft, as there was a central void in 98.1% of fusions receiving rhBMP-2 compared with only 44.4% of fusions receiving iliac bone.
Other Integrated ALIF Devices
Multiple studies have reported on numerous other integrated ALIF devices. Most of these studies have investigated PEEK cages with integrated screws for fixation.
Voronov et al. researched the PILLAR SA (Orthofix, Lewisville, TX) integrated PEEK ALIF device and found that the integrated cage decreased flexion, extension, lateral bending, and axial rotation compared with intact spinal segments with a physiologic preload of 400 N. However, the addition of bilateral pedicle screws and rods reduced the motion significantly more. Furthermore, the authors found that a 360-degree fusion consisting of an ALIF cage with posterior instrumentation was more rigid than the PILLAR SA device alone in flexion, extension, and lateral bending. However, axial rotation was similar between groups.
Similarly, Kornblum et al. investigated an integrated PEEK ALIF cage, the Brigade Standalone ALIF System (NuVasive, Inc., San Diego, CA). Biomechanical testing determined that the addition of three integrated screws to the Brigade device provided more stability than the device without screws. A fourth screw did not add significant stiffness to the construct. Furthermore, the device demonstrated equivalent stability to an ALIF construct with supplemental posterior fixation in lateral bending and axial rotation. However, the Brigade system with integrated screw fixation allowed more flexion/extension than an ALIF cage with additional anterior plate fixation and an ALIF cage with additional pedicle screw fixation. In addition, the authors determined that adding a spinous process plate to the integrated cage provided the most rigid construct in flexion and extension, although this was statistically similar to a nonintegrated ALIF cage with bilateral pedicle screw fixation. Furthermore, ALIF with pedicle screw fixation decreased lateral bending more than the integrated cage with additional spinous process plating, although the two groups had a statistically similar decrease in axial rotation.
In another biomechanical study of an integrated PEEK ALIF device, Drazin et al. demonstrated that the Independence (Globus Medical, Inc., Audubon, PA) integrated device was superior to an anterior spacer with posterior pedicle screws and an anterior graft with anterior tension band plate and screws. The integrated spacers demonstrated no subsidence, but rather at the bone-screw interface. The authors believed this superior outcome was owing to the angulation of the screws that were inserted at 30 degrees to 45 degrees of the endplate as opposed to parallel. Therefore, these screws must actually tear bone to fail. The authors noted a decreased stiffness with increasing sacral slope and suggested a stand-alone not be used for sacral slopes greater than 40 degrees.
Beaubien et al. researched an investigational PEEK cage with screw integration and found that the device decreased ROM in flexion/extension, lateral bending, and axial torsion compared with an intact spine. The greatest contribution of the integrated screws was in limiting torsional motion and pullout. The authors found that the investigational device was similar to threaded titanium cages, but better resisted anterior displacement.
One author investigated another integrated PEEK ALIF device with a novel fixation system. Yeager et al. researched the Solus PEEK interbody cage with integrated counter-rotating fixation blades (Alphatec Spine, Carlsbad, CA). They found that the integrated device with additional interspinous process clamp significantly reduced lateral bending and axial torsion, whereas a nonintegrated PEEK interbody cage with interspinous process clamp did not.
In previously mentioned study, Chen et al. also investigated the Latero (A-Spine ASIA, Taipei, Taiwan) integrated ALIF device, which consists of a lateral vertebral plate that is bent in the coronal plane to fixate a titanium trapezoid cage. The authors found that Latero had a similar ROM restriction to a nonintegrated ALIF cage with posterior pedicle screw instrumentation. Furthermore, Latero caused superior rigidity in flexion and extension when compared with the SynFix-LR integrated cage. However, Latero was similar to SynFix-LR in bilateral axial rotation and left lateral bending, while being less rigid in right lateral bending. The discrepancy seen in lateral bending is caused by the asymmetric design of the Latero model and the fact that the vertebral plate is inserted on the left side. The authors also noted a decreased spinal stiffness in osteoporotic bone when using Latero, which was especially prevalent in right lateral bending. In addition, the Latero device created minimal annulus stress that was superior to the SynFix-LR device in flexion and extension, and similar to an ALIF cage with posterior instrumentation in all ROM. Furthermore, the Latero device distributed little force to the same-level facet joints in extension, and facet contact force was similar to SynFix-LR in bilateral axial rotation. Notably, the superior endplate of the Latero model was more stressed than all other models in axial rotation, possibly owing to the smaller contact surface of the Latero cage. Additionally, the SynFix-LR device created higher stress on the screw fixation than the Latero device created on the fixation plates in all motions.
Another author investigated a separate integrated titanium ALIF device. Madan et al. studied 29 patients with the Hartshill Horseshoe cage, which is stabilized by inserting screws through holes in the implant. All patients achieved fusion with no implants demonstrating subsidence over an average follow-up of 3.0 years. Furthermore, 23 (85.2%) patients were satisfied with their surgical outcome. One patient did suffer from postoperative sciatica owing to screw impingement.
Femoral ring allografts (FRAs) are commonly used alternatives to synthetic interbody devices. They are vertical rings that incorporate into the fusion mass over time. In an attempt to gain the benefits of an integrated device in the cheaper FRA, Kuzhupilly et al. performed a biomechanical study investigating FRAs (Musculoskeletal Transplant Foundation, Edison, NJ) with and without integrated crossed anterior screws. The authors found that intact spines had stiffer flexion compared with the integrated construct while being statistically similar to the FRAs without screws. However, the integrated FRA was stiffer in extension compared with the screw-less FRA. Lateral bending motion was similar between the integrated FRA, the FRA without screws, and the intact specimens. Conversely, torsion was stiffer in the integrated and nonintegrated FRAs when compared with intact spines.
Lateral Interbody Fusion
The eXtreme lateral interbody fusion (XLIF) technique is among the newest interbody fusion techniques. The XLIF approach allows for abundant disk removal and placement of a large stable interbody implant at the apophyseal ring where the bone is strongest. Furthermore, this approach allows for retention of the anterior longitudinal ligaments and annulus and provides a large fusion surface area. As this is a relatively new technique, integrated interbody devices are limited.