INTRODUCTION
Low back pain continues to be the primary cause of years lived with disability worldwide with projections continuing to increase with the aging population[1]. Surgical management of degenerative disc disease, spondylolisthesis, facet arthropathy, spinal stenosis, and lumbar deformity is often employed after failed conservative treatment modalities. Historically, patients were treated with posterior approaches to the lumbar spine with direct decompression. Advancement of the field has provided spine surgeons new techniques for indirect decompression with interbody fusions and several approaches to the lumbar spine. Anterior lumbar interbody fusion (ALIF), lateral lumbar interbody fusion (LLIF) also known as direct lateral interbody fusion (DLIF) or extreme lateral interbody fusions (XLIF), and oblique lumbar interbody fusion (OLIF) also known as anterior to psoas (ATP) interbody fusion have gained momentum as minimally invasive techniques for dealing with these pathologies. The purpose of this editorial is to review the LLIF and OLIF indications, associated factors affecting outcomes, and complications.
The LLIF and OLIF procedures were advanced techniques to improve the shortcomings of the posterior and anterior approaches such as postoperative morbidity due to disruption of paraspinal musculature, requirement of an access surgeon, iatrogenic injury to nerves, blood vessels, and viscera[2]. The LLIF/XLIF approach was designed to approach the lumbar spine through a retroperitoneal, transpsoas corridor and first described in 2006 by Ozgur et al[3]. This approach allows spine access from T12/L1 to L4/5 disc space for interbody fusion. Limitations of this approach include L5-S1 pathology, patient anatomy with high iliac crests limiting exposure to L4-5, history of retroperitoneal infection or injury potentially leading to scar adhesions and difficult exposure, severe spondylolisthesis and poor bone quality risking cage subsidence[4]. Specific complications related to LLIF include hip flexor weakness, bowel injury, anterior thigh paresthesias and weakness attributed to neuropraxia and lumbosacral plexus injuries from psoas dissection and retraction[5-7].
The OLIF technique has the ability to elude the psoas by utilizing the space between the great vessels (inferior vena cava and aorta) and the psoas muscle or ATP. This approach provides access from L1 vertebral body to S1 vertebral body. OLIF and LLIF techniques allow surgeons to insert larger cases into the intervertebral disc spaces to support distraction with the preservation of surrounding structures and not damage the posterior tension band from a traditional posterior approach. Indications for both OLIF and LLIF include degenerative disc disease with and without radiculopathy, degenerative scoliosis not requiring corrective osteotomies, spondylolisthesis (mild to moderate), adjacent segment disease, and pseudoarthrosis. Contraindications to OLIF include severe central canal stenosis and high-grade spondylolisthesis[2]. The theoretical benefit to OLIF over LLIF is the preservation of the psoas musculature with expectation of decreased hip flexion weakness, earlier postoperative mobilization from muscle preservation, and lower neurologic injury to the lumbar plexus. Fujibayashi et al[8] discuss the benefits and outcomes of OLIF in a prospective clinical study highlighting the ability to directly visualize critical structures such as the ureter and sympathetic trunk, no need for neuromonitoring and consistent access to L4-5 levels compared to the LLIF. Rates of thigh symptoms postoperatively have been noted to be as high as 61%[9]. However, a systematic review of LLIF complications performed by Hijj et al[5] included 63 articles and 6819 patients found transient and persistent neurologic injury in 36.07% and 3.98% of patients, respectively. On the contrary, neurologic complications related to the OLIF approach through several systematic and retrospective reviews have demonstrated lower reported rates of thigh pain, neurologic numbness and psoas weakness from 0%-14%[2,5,7,8-11].
Collectively, both the OLIF and LLIF allow additional opportunities for surgeons to provide indirect decompression of the spine. Indirect decompression involves insertion of an interbody cage into the intervertebral disc space to restore foraminal and disc height, lumbar and segmental lordosis for correction of coronal and sagittal imbalance and unbuckling of the ligamentum flavum to allow for increased central canal space. The advantage of indirect decompression with these approaches over direct decompression includes minimally invasive techniques with smaller incisions, shorter surgical times, reduced postoperative pain, shorter hospital stays while still relieving foraminal and central canal stenosis. Limthongkul et al[12] performed a randomized controlled trial comparing direct and indirect decompressions with LLIF and found significant postoperative improvements in all their radiographic parameters which included foraminal height, foraminal area, fusion rate, segmental and lumbar lordosis, but no significant difference between groups at any follow up period up to 12 months after surgery[12]. This also included no difference in Oswestry disability index or visual analogue scale of back and leg pain, complication rate or fusion rates between groups. However, there was a significantly lower blood loss, shorter operative time in favor of indirect decompression group but a larger increase in cross sectional area of the thecal sac in the direct decompression group[12].
EDITORIAL DISCUSSION
As these minimally invasive techniques continue to gain traction in spine care, it poses continued topics of discussion regarding the outcomes and comparison to traditional methodologies. The authors of this study have the pleasure to further discuss the paper by Bokov et al[13] “Factors that influence the results of indirect decompression employing oblique lumbar interbody fusion”. Their cross-sectional study investigated 80 patients who underwent OLIF with lumbar spinal stenosis combined with instability of the lumbar spinal segment. Patients underwent single level or bisegmental spinal instrumentation of the OLIF technique with percutaneous pedicle screw fixation. The aim of the paper was to assess factors that influence radiographic and clinical results of the indirect decompression in patients with stenosis of the lumbar spine. Patients underwent preoperative computed tomography scan and magnetic resonance imaging (MRI) that were compared to postoperative computed tomography scans and utilization of the MacNab scale for clinical results. In some cases, MRIs were repeated postoperatively if there was concern for incomplete nerve decompression.
Radiographic findings after indirect decompression using the OLIF technique demonstrated a statistically significant increase in the anterior and posterior disc space height, vertebral canal square, right and left lateral recess canal depth compared to preoperative measurements (P < 0.001 for all parameters). Subgroup analysis was performed between patients that had spondylolisthesis requiring reduction of the slip and those without reduction. They found patients with reduction of the upper vertebral body had a median increase in vertebral canal square of 49.5% (quartile borders of 22.35%, 99.75%), while on the contrary indirect decompression alone had a median vertebral canal square increase of 24.5% (quartile borders of 16.3%, 33.3%). Bokov et al[13] utilized analysis of covariance and goodness-of-fit to assess predictability of outcomes based on vertebral canal square and lateral recess depth and found that the preoperative value of vertebral canal square was a predictable indicator for postoperative outcomes; however, the variability in lateral recess depth from preoperatively to postoperatively may result in unpredictable outcomes[13].
Clinical outcomes were classified based on the MacNab scale and 6/80 (7.5%) patients had unsatisfactory results due to unresolved radiculopathy. Logistic regression with receiver operating characteristics where then used on these patients to identify critical values that predict failure of indirect decompression. The findings showed lateral recess depth less than 3 mm and a spinal canal square less than 80 mm2 to predict inadequate relief of foraminal or central stenosis from indirect decompression. The authors of this editorial review will discuss and analyze the radiographic and clinical outcomes of this study into the context of current literature.
As found in the study by Bokov et al[13], current literature supports improvement in radiographic parameters after indirect decompression with LLIF and OLIF procedures. Fujibayashi et al[8] demonstrated a mean cross-sectional area (CSA) of thecal sac increase from 99.6 mm2 to 134.2 mm2 postoperatively (P < 0.001)[13]. The preoperative to postoperative CSA extension ratio was 30.2% and inversely correlated with preoperative CSA[8]. Lee et al[14] investigated the change in spinal canal in degenerative spondylolisthesis after OLIF in a prospective observational study and found that the canal area gradually increased from 78.5 mm2, to 112.5 mm2 at early follow up and 151.8 mm2 at late follow up (P < 0.001)[14]. Limthongkul et al[15] had similar findings in their retrospective review of indirect decompression on central canal and ligamentum flavum during XLIF/LLIF and OLIFs with a mean CSA preop of 93.1 mm2 to 127.3 mm2 postoperatively[15]. Upper limit findings of CSA of the thecal sac postoperatively have been found to be 143%[16].
The mechanism behind alleviating the central canal stenosis seen in many spinal pathologies is believed to be caused by reduction of the disc bulge, stretching of a buckled ligamentum flavum, atrophy of ligamentum flavum due to spinal stability, facet joint release and spondylolisthesis correction[14]. Studies have shown that there can be a progressive decrease in thickness of the ligamentum flavum over time. The previously mentioned study by Limthongkul et al[15] measured both left and right ligamentum flavum thickness preoperatively and postoperatively and found there to be a significant 17% decrease in thickness (P < 0.00625). Lee et al[14] imaged patients preoperatively, 5 days postoperatively and between 10-14 months postoperatively and found a progressive decrease in thickness from 4.2 mm, 3.7 mm and 2.6 mm (P < 0.001), respectively. Similarly, ten year follow up after ALIF found the average CSA of the ligamentum flavum at the level of the fusion to be significantly less than before surgery[17]. It is hypothesized that mechanical stresses from aging, activity level, facet arthropathy and subsequent instability lead to an inflammatory reaction at the cellular level inducing ligamentum flavum hypertrophy. The normal ligamentum flavum in healthy individuals is an elastic tissue structure made up of 80% elastic fibers and 20% collagen fibers[18]. Histological studies have found cellular pathways linking tumor necrosis factor alpha, transforming growth factor beta, connective tissue growth factor, platelet-derived growth factors, and matrix metalloproteinases to altering the gene expressions of collagen types I and III, stimulating a fibrotic process and hypertrophy of the ligamentum flavum[17,19]. These findings would support that indirect decompression with restoration of disc height unbuckles the ligamentum flavum creating more central canal space. Spinal fusion and unbuckling the ligamentum flavum likely play a role in stabilizing the spine and diminishing the cellular induced inflammatory reaction that results in hypertrophic changes. As stated in the paper by Bokov et al[13], reduction of spondylolisthesis and restoration of disc height correlates with spinal canal square. Furthermore, researchers and spinal surgeons can extrapolate clinical improvements in the setting of central canal stenosis and nerve root compression.
On the contrary, and controversially, indirect decompression may provide inadequate decompression in the setting of lateral recess stenosis. Bokov et al[13] discuss their findings comparing preoperative and postoperative increase in lateral recess depth but note a 7.5% failure rate attributed to persistent radiculopathy and inadequate decompression requiring direct decompression[13]. The conclusion of their estimated regression model finds unpredictable outcomes in patients with lateral recess stenosis undergoing indirect decompression. Studies have noted the incidence of indirect decompression failure, defined by inability to resolve central or foraminal stenosis and resulting in additional direct decompressive surgery for persistent symptoms, to be poorly described but reported between 6.4%-29%[20-22] and even as high as 71%[23]. Both patient factors and procedure related factors have been investigated to localize the limitations of the procedures utilizing indirect decompression.
Regarding patient factors, Wang et al[22] prospectively enrolled 45 patients involving 101 spinal levels undergoing indirect decompression via XLIF/LLIF procedures and found that bony lateral recess stenosis was an independent predictor for failure to achieve adequate spinal decompression. Their failure population did have a significantly smaller preoperative central canal diameter, foraminal height and disc height compared to those who did not fail indirect decompression. A review by Lu et al[24] found that preoperative anteroposterior and dural area plays a vital role in OLIF patient outcomes. They found an anteroposterior spinal canal diameter less than 6.545 mm and dural area less than 34.43 mm2 led to poor outcomes after indirect decompression[24]. Additional researchers have attributed locked facet joints from severe degeneration to limit the amount of distraction allowed from the interbody implant, decreasing the change between preoperative and postoperative disc height, foraminal height, and foraminal area[20,21,23]. However, it is also important to consider patient’s bone quality as Yingsakmongkol et al[24] found low bone mineral density (T-score < 2.1) to be a risk factor for cage subsidence and low postoperative disc height (< 10 mm) and subsequent failure to indirect decompression when restoration of radiographic disc height and foraminal height is not sufficient.
Surgical and technical factors have also been evaluated to understand their effects on improving patient’s radiographic parameters and subsequent related outcomes. Previous investigations had previously believed that cage size, specifically width, allowed more dispersed bony contact to restore and maintain the decompressive effects of improved disc and foraminal height while avoiding subsidence[20,21,25]. Park et al[26] found that there was a correlation with cage placement in the middle of the vertebral body producing greater foraminal height improvements and subsequently improvements in visual analogue scale for leg pain. Additionally, they concluded distraction of the middle and posterior vertebral column instead of the anterior vertebral column allowed for increased facet distraction, cross sectional area of the thecal sac and foraminal/disc height providing more successful radiological outcomes after OLIF procedures[26].
Continued research highlights the complexity of decision making of spine surgeons regarding which patients are ideal candidates for indirect decompression through minimally invasive techniques. Criteria and algorithms have been developed to better differentiate these candidates. A series of investigators recommend patients to have dynamic clinical symptoms where 50% of their pain subsides with supine positioning, no weakness less than grade 4, reducible disc height defined as having at least 1 mm change from upright to supine radiographs indicating non-locked, distractable facets, no static stenosis such as bony lateral recess, free disc fragment, facet cysts directly compressing the spine/nerve root, lack of congenital or severe spinal stenosis, and absence of frank osteoporosis[25,27]. In the setting of one of these risk factors, discussion with the patient should be directed at preoperative optimization of bone health and discussions regarding treatment options. These discussions may include direct decompression, or staged surgery when indirect decompression is insufficient and direct posterior decompression is required[25].
