With a growing elderly population demanding a longer active lifestyle, the impetus has been placed on spine surgeons to use innovations in technology to provide less invasive solutions to increasingly complex spinal deformities. Asymmetric degeneration of disc spaces in the thoracolumbar/lumbar spine is believed to be one of the causes that result in adult degenerative scoliosis/deformity. Symptoms of this class of spinal deformity may range from relatively asymptomatic to axial or radicular pain in 90% of patients [7
]. In many cases, patients with ASD are opting for surgical intervention when conservative measures fail. Traditional goals of adult deformity surgery are correction of coronal and sagittal balance and obtaining a solid fusion. However, treatment of adult spinal deformity is constantly evolving, and radiographic goals such as pelvic tilt <25 degrees and LL = PI ± 9 degrees have been established [4
]. For the purposes of this study, we focus here on improving sagittal balance to an SVA <5
Although the importance of sagittal plane deformity has been well studied, especially in the context of flat back syndrome, we now have a guideline to keep sagittal balance, or SVA less than 5
cm to optimize clinical outcomes [25
]. Sagittal plane correction is traditionally accomplished through posterior shortening techniques, such as a Smith-Peterson (SPO) or pedicle subtraction osteotomy (PSO), which, although effective, may be associated with significant morbidity [29
]. Another option is the release of the anterior longitudinal ligament. Although not a new concept, it is relatively infrequently practiced because of significant approach-related morbidity [32
]. Recently, however, the lateral retroperitoneal/retropleural approach to the thoracic and lumbar spine has provided spine surgeons with another, less invasive option in scoliosis surgery [37
]. It has been shown previously that minimally invasive spine surgery results in less blood loss, reduced muscle dissection/trauma, shorter hospital stays, and faster mobilization and recovery after surgery [38
5.1. Technical Aspects of MIS ALL Release
In this study we describe our experience with MIS ALR, and through our results show that it is not only a safe option but also one that provides significant improvements in sagittal balance with low morbidity. As with all new MIS techniques, there is a steep learning curve, and the most important factor is understanding the procedure and the surrounding anatomy. In addition to the usual risks associated with the lateral approach, unique perils associated with ALR include great vessel injury and damage to the sympathetic plexus [22
]. However, the anatomical dissection plane is ventral to the ALL and dorsal to the sympathetic plexus and great vessels, making injury less likely. In addition, we avoid electrocautery and use a modified 15 blade to cut the ALL to minimize damaging surrounding tissues. Placing the patient in the left lateral decubitus position also allows better control of the inferior vena cava. In order to perform the final release of the ALL, use of the intradiscal distractor may avoid blind sectioning of the ligament on the contralateral side. After completion of the ALR, placing the hyperlordotic cage in the middle of the disc space may increase the disc height and provide indirect foraminal decompression. The cage is then secured into place with a screw to avoid anterior migration into the peritoneum.
5.2. Clinical Implications on MIS ALL Release
Although the average follow-up time in our series is only 9 months, we believe that this is adequate for the purposes of identifying the safety and efficacy of this procedure since the main results and complications occur during the immediate postoperative period. Ongoing analysis of these patients is being performed with the purpose of a future study to evaluate if the sagittal balance is maintained.
Short-term complications include vascular and neural injuries, while long-term complications include subsidence, pseudarthrosis, and adjacent segment failure, which are not unique to this surgery.
Length of hospital stay in our cohort was 8.3 days, similar to the 7.9 days reported by Schwab et al. in the group of patients without complications [39
]. In our practice, surgery for adult deformity is generally done in two stages. The lateral approach for placement of interbody cages as well as sectioning the ALL if necessary is done during the first stage, and posterior fixation with either percutaneous or open pedicle screw placement is performed in the second stage. We recommend breaking large deformity cases into two stages for various reasons, including ease on patient in terms of operating time and related complications and ease on surgeon preventing long and difficult procedures. Another important factor is the ability to reassess the patients' spinopelvic parameters between stages to customize planning for the posterior procedure and assess need for hybrid constructs including an MIS SPO or open laminectomy/instrumentation versus percutaneous screw fixation.
Mean blood loss in our cohort during stage 1 was 125
cc, and during stage 2 was 530
cc, with a total of 655
cc. The International Spine Study Group recently published a study demonstrating that greater intraoperative blood loss (>2.4
L) is a major risk factor for perioperative complications [39
]. This is another benefit of correcting sagittal balance through a minimally invasive approach. The global lumbar lordosis in our group improved by 24 degrees, from 24 to 48 degrees, while segmental lordosis at the levels where an ALR was performed improved an average of 17 degrees. Given that the SVA improved by 4.9
cm, from 9 to 4.1, and within the range established by Lafage et al., we expect postoperative outcome scores to be improved [40
]. This is in fact the case, as VAS and ODI scores improved by 26 and 18%, respectively. Another likely contribution to the overall patient improvement is that the pelvic tilt improved on average by 7 degrees, from 32 to 25 degrees, also within the recommended range of less than 25 degrees.
All techniques for restoration of sagittal balance have significant risk of complications and are technically challenging [14
]. We attribute our low rate of complication to multiple factors, the most important of which is understanding the regional anatomy. Before attempting an ALR which has a steep learning curve, we spent time in cadaveric dissection in the lab, isolating the sympathetic plexus as it runs along the anterolateral border of the lumbar vertebral bodies in order to determine if there was a plane between the ALL and the sympathetic plexus/great vessels. Only when it was determined to be feasible in cadaveric specimens were we willing to try it in our patients [15
]. After performing MIS LIF for five years on over a thousand levels and multiple dissections and publications, we felt confident in our ability to perform an ALR. As of yet there are no cases of subsidence, which is a potential complication with hyperlordotic cage placement.
In our cohort of 7 patients, we had no complications except for a superficial wound infection treated successfully with a wound washout and short course of intravenous antibiotics.
During the dissection there is no adequate proximal and distal control of the great vessels. In case of injury to the aorta or inferior vena cava, the surgeon would likely need to extend the skin incision anteriorly and perform direct compression of the vascular structures followed by proximal and distal control, followed by direct repair of the defect.
The low number of patients (n = 7) in our cohort was a potential limitation in this study; however, we believe that it was adequate to demonstrate our results as well as the feasibility of this technique. At this point we must again stress the importance of understanding the surrounding anatomy if attempting an ALR, as its true safety lies in the hands of the surgeon performing the procedure. The mean follow-up time of 9 months is also slightly below the 2-year standard, but in the context of a new surgical technique is sufficient for our goals. We believe that this technique requires further study to be able to draw general conclusions, and we will continue to follow these patients to assess long-term outcomes.