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Integrated Screw/Plate Interbody Fusion Devices
Introduction
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 .
TABLE 18.1
Clinical Studies Featuring Integrated ALIF Devices
| 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 |
| ROI-A | ||||||||
| 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 85.2% satisfied one patient with postoperative sciatica owing to screw impingement |
BMP-2, bone morphogenetic protein-2; ODI, oswestry disability index; PEEK, polyetheretherketone; VAS, visual analogue score.
SynFix-LR
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.
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