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Background
Interbody fusion (arthrodesis) in the lumbar spine is performed to treat painful symptoms caused by instability of the vertebrae, such as spondylolisthesis, spinal stenosis, or degenerative disc disease. Methods of spinal fusion include bone grafts or metal implants placed either anteriorly, posteriorly, or laterally; however, insertion of these implants is not without surgical risk. Numerous open and minimally invasive techniques have been developed but all of these approaches experience the same shortcomings related to biomechanics or inherent iatrogenic destabilization.
In an attempt to alleviate many of the limitations of previous techniques, a pre-sacral approach to the lumbosacral junction has been investigated. Transaxial anterior lumbar interbody fusion is an emerging minimally invasive spinal fusion procedure used to treat patients with chronic lower back pain (Ollendorf, et al., 2011). This procedure is an alternative to traditional fusion techniques that utilize anterior or posterior approaches to directly expose the lumbosacral spine. In the case of transaxial anterior lumbar interbody fusion the spine is accessed percutaneously via the anterior surface of the sacrum.
The axial lumbar interbody fusion (AxiaLIF) system (TranS1, Inc., Wilmington, NC) combines a minimally invasive technique with a novel corridor approach. Through a small para-coccygeal incision, a pre-sacral corridor is percutaneously created for access to the anterior lumbosacral body and, subsequently, to the L5 - S1 intervertebral space. Proponents of this approach report minimal risk to adjacent vital structures and no annular disruption. The AxiaLIF system was cleared for marketing through the Food and Drug Administration (FDA) 510(k) process. The device includes instruments for creating a small pre-sacral axial track to the L5 - S1 vertebral bodies for the insertion of bone graft material into the disc space. The device also includes an anterior fixation rod that is implanted through the same track. According to the FDA 510(k) letter to the manufacturer, the system is indicated for patients requiring fusion to treat pseudoarthrosis, unsuccessful previous fusion, spinal stenosis, spondylolisthesis (grade 1 or 2), or degenerative disc disease defined as back pain of discogenic origin with degeneration of the disc confirmed by history and radiographic studies. The AxiaLIF is not intended to treat severe scoliosis, severe spondylolisthesis (grades 3 and 4), tumor or trauma. Its usage is limited to anterior supplemental fixation of the lumbar spine at L5 - S1 in conjunction with legally marked facet and pedicle screw systems.
According to TranS1's website, the risks of the AxiaLIF system are similar to any spine stabilization surgery (e.g., bleeding, neurological damage, damage to soft tissue, spinal cord impingement or damage, infection, loss of bowel or bladder function, loss of erectile or ejaculatory function, meningitis, or pain). The website states that the risks associated with the implant include: breakage of the implants, loosening or expulsion of the implants possibly causing delayed nerve root impingement or damage, fracture of osseous structures, and bursitis.
The most common comparison of interbody techniques in the literature has been between anterior and posterior interbody techniques. There are no randomized controlled trials comparing the outcomes of lumbar fusion using the pre-sacral technique with standard approach techniques. Current published literature consists of feasibility reports and small non-randomized case series with limited follow-up.
The American Association of Neurological Surgeons/Congress of Neurological Surgeons guidelines for the performance of fusion procedures for degenerative disease of the lumbar spine (Resnick et al, 2005) reported that posterior lumbar interbody fusion, transformal interbody fusion, or anterior lumbar interbody fusion techniques are the treatment options for patients with low-back pain due to degenerative disc disease at one or two levels. The guideline did not mention a pre-sacral approach for spinal fusion or the AxiaLIF system. The guideline stated, "Future studies focusing on patient outcomes are required to establish whether the increased fusion rates seen with interbody techniques are truly associated with improved functional outcomes. Application of reliable, valid, and responsive outcome measures in a multi-center randomized trial would serve to answer this question. In terms of the techniques used to achieve an interbody arthrodesis, it is likely that certain techniques will be more applicable to different patient populations. Future studies should be focused on evaluating the individual techniqes within specific patient populations. Well-designed cohort studies would provide needed Class II medical evidence. Randomized studies would need to include adequate numbers of patients to ensure sufficient power to be able to assess whether the incremental improvement achieved with interbody techniques is clinically significant."
The AxiaLIF percutaneous lumbar interbody fusion procedure may provide an alternative access route to the L5 - S1 inter-space in those patients who may have unfavorable anatomy or contraindications to the traditional open anterior approach (Aryan et al, 2008). However, there is insufficient evidence of the effectiveness of a pre-sacral approach with the AxiaLIF system.
Duan and colleagues (2009) reviewed the feature, biomechanics, and clinical application of percutaneous 360 degree AxiaLIF technique, which is different from other lumbar interbody fusion techniques due to its capability in maintaining the integrity of the bilateral facet joints, the anterior/posterior longitudinal ligament, and the annulus fibrosus. The 3-dimensional AxiaLIF Rod provided axial support and fixation, thus relieving stenosis of the lumbar intervertebral foramen and restoring the intervertebral disc height and the whole height and physiological curvature of the lumbar spine. The recovery of the intervertebral disc height could restore the folded or crumpled flavum, the posterior longitudinal ligament, and the herniated annulus, resulting in the improvement of stenosis symptoms of nerve root canal or central vertebral canal. The authors concluded that percutaneous 360 degree AxiaLIF technique achieves satisfying therapeutic effects, although it has fairly narrow indication and needs long-term follow-up observation.
Luther, et al. (2009) reported on a case series of transaxial lumbar interbody fusion in patients with chronic back pain and radiographic evidence of lumbar degenerative disc disease at L5-S1. Five patients received a single-level fusion and one patient received a two-level fusion. Bony fusion was seen on postoperative imaging in one patient after 12 months. Limitations of this study include its small size, limited followup, lack of comparison group and retrospective nature.
Patel, et al. (2010) conducted a review of the charts of 50 patients who underwent axial lumbar interbody fusion surgery at a single institution between 2006 and 2008. the 50 patients (32 women, 18 men; mean age, 49.29 years) treated with axial lumbar interbody fusion, 48 had preoperative VAS scores and 16 had preoperative ODI scores available. Complete radiographic data were available at the preoperative, initial postoperative, and final postoperative time points for 46 patients (92%). At last follow-up (average, 12 months), ODI scores were reduced from 46 to 22, and VAS scores were lowered from 8.1 to 3.6. Of the 49 patients with postoperative radiographs, 47 (96%) went on to a solid fusion. There were no significant differences between pre- and postoperative disk space height and lumbar lordosis angle. The most common complications were superficial infection and pseudoarthrosis. Other complications were rectal injury, hematoma, and irritation of a nerve root by a screw. Limitations of this study include its retrospective nature, small size, limited duration of followup and lack of comparison group.
Tobler et al (2011) reported the 2-year clinical and radiographic outcomes associated with a L5-S1 interbody fusion procedure that employs an axial presacral surgical approach. A total of 156 patients from 4 clinical sites were selected for inclusion if they underwent a L5-S1 interbody fusion via the presacral approach with the AxiaLIF system and had both pre-surgical and 2-year radiographic or clinical follow-up. Back pain and functional impairment were evaluated with an 11-point numeric scale and the Oswestry Disability Index (ODI), respectively, pre-operatively and at 2 years. Standard radiographic imaging techniques were used to determine fusion status. Marked clinical improvements were realized in back pain severity and functional impairment through 2 years of follow-up. Mean pain scores improved from 7.7 +/- 1.6 (n = 155) pre-operatively to 2.7 +/- 2.4 (n = 148) at 24 months, reflecting an approximate 63 % overall improvement (p < 0.001). Mean ODI scores improved from 36.6 +/- 14.6 % (n = 86) pre-operatively to 19.0 +/- 19.2 % (n = 78) at 24 months, or approximately 54 % (p < 0.001). Two-year clinical success rates on the basis of change relative to baseline of at least 30 % were 86 % (127 of 147) and 74 % (57 of 77) for pain and function, respectively. The overall radiographic fusion rate at 2 years was 94 % (145 of 155). The authors concluded that findings from this clinical series of patients treated with a pre-sacral interbody fusion procedure, stabilized with the AxiaLIF rod, reflect favorable and durable outcomes through 2 years of follow-up. Moreover, they noted that while the results from the current study are encouraging, especially in contrast to previously published systematic reviews and randomized controlled trials (RCTs), they must be interpreted with caution. In fact, Andersson et al (2006) noted that there was an identifiable trend for RCTs to report a smaller magnitude of improvement in most clinical outcomes compared with other types of trials. For example, there was a 36 % median reduction in pain severity realized after spinal fusion in RCTs compared with a median reduction of 53 % for retrospective case series. It is probable that the selection criteria utilized in the current series effectively excluded the worst cases by requiring 2 years of post-operative radiographical or clinical follow-up. Consequently, the authors acknowledged that this retrospective study offers only level IV evidence of clinical effectiveness. However, it does provide a foundation for future studies to confirm or modify the current findings. Tobler et al (2011) also stated that "[t]he minimally invasive presacral interbody fusion is a procedure that allows preservation of the annulus and all paraspinous soft tissue structures. The distinctness of the procedure requires that surgeons be well familiarized with the presacral anatomy. Additionally, the methodology to prepare the interspace for fusion requires that the surgeon relies on fluoroscopic imaging as opposed to direct visualization of the disc space. These procedural characteristics have resulted in a level of uncertainty among surgeons about widely adopting this technique. The distinct limitations of the current study notwithstanding, the clinical and radiographic findings remain promising but require corroboration in additional patient groups".
Gundanna et al (2011) evaluated complications associated with axial interbody lumbar fusion procedures using the AxiaLIF system in the post-marketing period. Between March 2005 and March 2010, 9,152 patients underwent interbody fusion with the AxiaLIF system through an axial presacral approach. A single-level L5 to S1 fusion was performed in 8,034 patients (88 %), and a 2-level (L4 to S1) fusion was used in 1,118 (12 %). A pre-defined database was designed to record device- or procedure-related complaints via spontaneous reporting. The complications that were recorded included bowel injury, superficial wound and systemic infections, transient intra-operative hypotension, migration, subsidence, pre-sacral hematoma, sacral fracture, vascular injury, nerve injury, and ureter injury. Complications were reported in 120 of 9,152 patients (1.3 %). The most commonly reported complications were bowel injury (n = 59, 0.6 %) and transient intra-operative hypotension (n = 20, 0.2 %). The overall complication rate was similar between single-level (n = 102, 1.3 %) and 2-level (n = 18, 1.6 %) fusion procedures, with no significant differences noted for any single complication. The authors concluded that the 5-year post-marketing surveillance experience with the AxiaLIF system suggested that axial interbody lumbar fusion through the pre-sacral approach is associated with a low incidence of complications. The overall complication rates observed in this evaluation compares favorably with those reported in trials of open and minimally invasive lumbar fusion surgery. The major drawback of this study was that fusion rates were not reported in this series. Another drawback was the spontaneous complication-reporting mechanism, which may under-estimate the true incidence of complications.
An assessment of the Axialif procedure by the Australian Safety and Efficacy Register of New Interventional Procedures – Surgical (ASERNIP-S) (Leopardi, 2010) noted the lack of high quality studies of the Axialif procedure and the need for long-term studies. The assessment concluded: "Overall, the AxiaLIF procedure appears to offer some symptom improvement in patients suffering from back pain, without major compromise to their safety. High-quality comparative studies are needed to completely assess the safety and efficacy of the AxiaLIF procedure."
Guidance from the National Institute for Health and Clinical Excellence (NICE, 2011) concluded that current evidence on the efficacy of transaxial interbody lumbosacral fusion is limited in quantity but shows symptom relief in the short term in some patients. The guidance stated that evidence on safety shows that there is a risk of rectal perforation. Therefore this procedure should only be used with special arrangements for clinical governance, consent and audit or research. The guidance noted the need for further research into transaxial interbody lumbosacral fusion, and stated that research outcomes should include fusion rates, pain and functional scores, quality of life measures and the frequency of both early and late complications. Specialist advisors to NICE listed key efficacy outcomes as improvement in low back pain and radiological evidence of fusion. Adverse events reported in the literature include rectal perforation and presacral abscess, rectal injury, pseudarthrosis, and intractable pain. One specialist advisor to NICE reported anecdotal events of infection and peritoneal injury and another reported pelvic abscess in one patient. The specialist advisors to NICE considered theoretical adverse events to include infection, bowel or bladder perforation, vascular injury, nerve damage, damage to parasympathetic plexus, cerebrospinal fluid leak due to dural tear, implant migration, loosening or breakage, and difficulty in revision. The guidance noted that transaxial interbody lumbosacral fusion was used as the sole procedure in a minority of the patients reported in the literature. Most patients had other adjunctive procedures and this made the evidence difficult to interpret.
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