Neuro-navigation assisted pre-psoas minimally invasive oblique lumbar interbody fusion (MI-OLIF): New roads and impediments
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.263262
Source of Support: None, Conflict of Interest: None
Introduction: Minimally invasive spine-oblique lumbar interbody fusion (MIS-OLIF) has emerged as a novel anterolateral, retroperitoneal, “pre-psoas” approach for lumbar interbody fusion for degenerative spinal instability, as well as for correction of deformity in patients without severe canal stenosis. In the last decade, the technique has gained popularity owing to several advantages like the minimal blood loss, minimal tissue dissection, preservation of posterior tension bands, better biomechanical strength, provision of mechanical stability to the lumbar spine, and a larger footprint of the implant, associated with it. It, thus, maximises load bearing on the cortical bone, and provides a better lordotic correction of the lumbar spine. The armamentarium is further boosted by the use of neuro-navigation and neuro-monitoring tools, thereby improving the surgical outcome.
Keywords: Oblique lumbar interbody fusion; retroperitoneal; lateral lumbar fixation; surgical technique; minimally invasive spine surgery
Minimally invasive spine surgery (MISS), that is based on the principles of posterior tension band preservation and least muscle damage, is exemplified in the “oblique approach” to the lumbar spine. The retroperitoneal “oblique” or “pre-psoas” approach, that completely avoids the posterior tension band of the spine, is one of the latest approaches described for achieving anterior spinal fusion. It was first published by Mayer in 1997. The technical modifications and the advancements in instrumentation have largely overcome the difficulties and problems that were initially encountered in the trans-psoas approach.,,,, Neuro-navigation and neuro-monitoring tools further help to improve the surgical outcome. The MIS-OLIF technique defines a working trajectory through a narrow oblique corridor to the lumbar disc space via the retroperitoneal space, between the major vessels and the psoas muscle. [Figure 1] The principle is indirect decompression of the spinal canal, thus, obviating the need to directly work within the spinal canal.
On comparison with other anterolateral approaches (anterior lumbar interbody fusion [ALIF] or lumbar lateral interbody fusion [LLIF]) and posterolateral approaches (posterior lumbar interbody fusion [PLIF] and transforaminal lumbar interbody fusion [TLIF]), OLIF has shown several advantages. It allows the carrying out of a thorough discectomy; and, it provides an ample space for the endplate preparation and the placement of a larger cage in the anterior column, which is the most compressive surface of the spine. Hence, the technique not only helps in achieving a better interbody fusion but also results in maintenance of a good segmental lordosis. The cage is aligned in the transverse plane resting on the endplates, that are considered as the strongest part of the vertebral body. This ensures a proper anchorage of the cage and least chances of subsidence of the contiguous vertebral bodies occurring over a period of time. Another advantage is in the preservation of the posterior ligaments and muscle bands, thereby decreasing the incidence of postoperative back pain. This translates into a very rapid recovery for the patient. The complications related to the cerebrospinal fluid (CSF) leak and injury to the nerve root are minimal. In contrast to the lateral transpsoas approach, the pre-psoas technique avoids injury to the lumbar plexus, and therefore, decreases the chances of anterior thigh pain as well as sensory loss in that region. Moreover, nerve monitoring is not mandatory while performing the OLIF technique. However, its disadvantages include traversing through an unfamiliar anatomy in an oblique corridor (that indirectly increases the radiation exposure for the theatre personnel as repeated imaging may be required for the target vertebral localisation). Although the technique is also suitable for the L1-S1 level, it is preferable for the L2-L5 levels as the ribs hamper the trajectory of approach above the L2 vertebra, and the bifurcation of major vessels hampers the trajectory to L5-S1. The drawbacks of this approach also include a theoretically higher risk of bowel and vessel injury due to the passage though a more anterior corridor, the anatomy of which may be unfamiliar to the surgeon. Navigation assisted OLIF really obviates this risk as it continuously guides the surgeon regarding the angle and depth of the instrument in real time without much radiation exposure .
There is no definitive (level I/II evidence) literature that highlights one approach as being superior to the other in terms of fusion or clinical outcomes. Radiological evidence show that after performing the OLIF procedure, the disc height increases by approximately 60%-80% and the cross sectional area of the thecal sac increases by 20% - 30%. The fusion rate after this approach is comparable to the procedure of ALIF with minimal morbidity, due to the choosing of a safe trajectory. The fusion rates have been found to be comparable without the associated risks of nerve injury, infection or hardware failure.,,
The MIS-OLIF approach to the lumbar spine is a minimally invasive retroperitoneal approach through the anterolateral abdominal wall muscles. The three layers of the muscle encountered are the external oblique, internal oblique, and transversus abdominis. These muscles are arranged in a grid pattern from superficial to deep. Their motor innervation is by the 12th subcostal, Iliohypogastric, and Ilioinguinal nerves. These nerves traverse the muscle layers, following an anterior-inferior oblique course, and they should be identified and protected intra-operatively. Deep to the transverse fascia, the anatomical boundary of retroperitoneal space include the psoas muscle and the spine medially, the peritoneum and abdominal viscera anteriorly, the quadratus lumborum muscle and the iliac muscles posteriorly, the diaphragm superiorly, and the pelvis forms the inferior limit. The retroperitoneal space contains multiple structures that are vulnerable to injury including the kidney proximally; the ureter distally; and, the aorta, inferior vena cava and peritoneum anteriorly. The segmental arteries branch from the aorta and run obliquely upwards from the mid-vertebral body along the lateral surface of the vertebra. At the anterior part of the L4-5 level, the iliolumbar vein occasionally crosses the trajectormy of approach and needs proper delineation to prevent its injury. The psoas muscle, which originates from the transverse processes of L1–L5 shows significant anatomical variation, both in muscle bulk and its ventro-dorsal location. These variations should be noted, as this indirectly suggests the anatomical pattern of the lumbar plexus. The lumbar plexus originates from the ventral rami of the first four lumbar nerve roots and the 12th subcostal nerve. It gives rise to the iliohypogastric nerve, ilioinguinal nerve, lateral femoral cutaneous nerve, genitofemoral nerve, femoral nerve, and obturator nerve. The genitofemoral nerve lies over the psoas muscle, which forms an important landmark during the exposure and should be identified. The rest of the lumbar plexus lies within the psoas muscle. Any retraction of the psoas or traversing through the psoas muscle endangers the lumbar plexus. Anterior to the psoas, along the anterolateral lumbar spine, is the sympathetic chain, that should be carefully protected.
A retrospective analysis of all the patients operated by the minimally invasive OLIF procedure, wherein, neuro-navigation was used intra-operatively, at our institute from June 2017 to March 2019 was conducted. The clinical information, radiological findings, operative details and follow up were recorded from the Hospital Information System. All the patients of spondylolisthesis were graded radiologically according to the Meyerding's grading system.
The patients' demographic profile, clinical history and neurological examination data, the pre- operative Oswestry disability index (ODI) and Visual Analogue Scale (VAS) score, the intraoperative surgical details and the postoperative and follow up outcome, including the clinical as well as radiological data, were recorded and compared with the preoperative details. Complications, readmissions, length of stay, estimated blood loss, and surgical time were also recorded. Static radiographs were used for fusion assessment. The degree of lumbar intervertebral fusion was assessed using the postoperative lumbar spine X-ray or CT scan. The 'Bridwell' grading  was used for assessment: Grade I: Fusion with remodeling and trabeculae; Grade II: Graft intact, not fully remodeled, no radiolucency; Grade III: Graft intact, but with a definite radiolucency; and, Grade IV: Graft definitely not fused, collapsed.
OLIF: When to perform? and, when not to perform?
Primarily, any patient who requires an interbody fusion between the L2-L4 lumbar levels for spinal instability is a candidates for OLIF. Clinically, the patient should have axial back pain, or back pain with claudication, which should be relieved by rest. The patients who are having persistent pain at rest are considered as poor candidates for the indirect decompression procedure utilizing the OLIF technique. The radiologic indications are the presence of lumbar spinal instability due to degenerative lysthesis or lytic lysthesis up to grade 2; degenerative adult scoliosis; degenerative lumbar kyphosis; degenerative disk disease; tuberculous or pyogenic spondylodiscitis; and, adjacent segment disease.,, This approach is contraindicated in patients with the presence of facet fusion; extruded disc; severe osteoporosis; congenital and/or severe spinal stenosis; bony lateral recess stenosis; calcified disk or posterior longitudinal ligament; prior history of retroperitoneal surgery; and, a high grade spondylolisthesis (>grade 3).
The OLIF technique is more suitable for the L2-L5 level, as ribs obstruct the surgical corridor above the L2 level; and, for the L5-S1 disc space, the iliac crest and the course of the iliac vessels become a major obstacle. Sometimes, due to a high iliac crest, an OLIF procedure for even the L4/5 level is contraindicated as the surgical trajectory is obstructed by the sacral ala and a high iliac crest. At the L5/S1 level, OLIF is not an absolute contraindicated, as the level can be accessed by an experienced surgeon after a thorough evaluation of the vascular anatomy. In certain patients, OLIF may be performed with relative ease at the L5/S1 level if the patient has a low bifurcation of the anterior vasculature and a low lying iliac crest. Even with an adequate corridor at the L5/S1 level, the rate of vascular injury can be up to 10%.,
The curvature of the lumbar spine also needs to be noted, although a correction of scoliosis by lumbar interbody fusion may be achieved equally from either the convex or concave side of the curve. OLIF inter-body cages of different sizes and curvature can be obtained before surgery (those having a lordosis angle of 0, 6, 12 and 18 degree, a height ranging from 8mm-18mm, and a width ranging from 20mm-27mm) for an optimal deformity correction.
Positioning of the patient
After induction of general anesthesia, the patient is positioned in a lateral decubitus position [Figure 3]. Although the “left up” or the “right up” position depends upon the surgeon's choice, the right decubitus position is preferable as the space between the iliac vessels and psoas muscle is roomier on the left side. Moreover, it helps to avoid any interference by the liver on right side and keeps the surgeon towards the aorta rather than the fragile inferior vena cava. In the presence of scoliosis, approaching from the side of concavity is sometimes favoured to minimize the number of incisions needed. Axillary padding and applying gel pads at pressure points (between the knees and below the shoulder) are important prerequisites, along with an appropriate strapping of the patient to the operating table. Arms are positioned in a neutral position and legs are flexed at the knee to prevent slippage of the patient. The hip is positioned just below the point of “table break” and is gently flexed to relax the psoas muscle and the femoral nerve. Breaking of the table is done just enough to create a space between the rib and the pelvis. The surgeon stands and operates from the abdominal side. At the same time, the first true lateral position is fluoroscopically confirmed (both in anteroposterior [AP] and lateral views). The pedicles and the transverse processes of both sidex should absolutely overlap on lateral imaging to ensure a true lateral position. We used intraoperative monitoring (IONM- both continuous and triggered electromyogram [EMG]) for posterior percutaneous pedicle screw fixation (PPF) in our series. Conducting the IONM is optional and not mandatory for OLIF. Paralytic medications are avoided as they interfere with the EMG recording.
Step 4: Neuronavigation registration
A 1-2 cm incision is given 5cm supero-lateral to the anterior superior iliac spine (ASIS) and the reference array for navigation is inserted securely. A scout fluoroscopic image is obtained and registered to the navigation software (Medtronic, USA). Thereafter, the entry point and trajectory are planned with navigational assistance. We realized that the problems arising due the unfamiliarity of the surgeon to the oblique corridor of the surgical approach is mitigated by neuronavigation.
Step 5: Surgical landmarks for incision marking
The surface marking of the disc space of interest, lower ribs and iliac crest are placed. The incision is marked 4-6 cm anterior to the midsection of the targetted disc for a single level or two-level approach [Figure 3]. The incision can be horizontal (parallel to the orientation of the disc space) or preferably oblique (parallel to the direction of the external oblique muscle fibers) and up to 2-3cm in length. The navigation guided localization theoretically minimizes the radiation exposure but does carry a minimal error rate.
Step 6: Oblique retroperitoneal access to the intervertebral disc
All the three layers of abdominal muscles are dissected separately. The external oblique fascia is encountered initially and needs a sharp incision, followed by the division of fibers of the internal oblique and transversus abdominis muscles. These muscles are identified and dissected parallel to the direction of their fibers using a blunt muscle splitting technique [Figure 4]. The iliohypogastric or ilioinguinal nerves may be encountered beneath the internal oblique muscle and need a meticulous mobilization. The transversalis fascia, a thin aponeurotic membrane which lies between the inner surface of the transversus abdominis muscle and the parietal peritoneum, is opened as laterally as possible to avoid inadvertent injury to the peritoneum. Thereafter, the yellow retroperitoneal fat is seen. The retroperitoneal fat and peritoneum are swiped away from the posterior-lateral abdominal wall by a blunt finger dissection, until the bulky psoas muscle is seen and palpated. The ureter is generally retracted along with this retroperitoneal fat. On the surface of the psoas, the genitofemoral nerve is appreciated passing downwards. The dissection is further advanced anteriorly until the anterior margin of the psoas muscle is reached. Using navigation, the desired disc space and trajectory is re-confirmed.
Step 7: Placement of the initial probe
The entry point into the disc space must be in the anterior one-third to anterior-half of the disc space. Once the disc space is reached, the first probe dilator is docked within the disc space under neuronavigational assistance. [Figure 4]
Step 8: Placement of dilators and the tubular retractor
A guide-wire is inserted through the probe cannula into the desired disc space. Once the guide-wire is firmly anchored, the first dilator is firmly impacted inside the disc space being guided by the guide-wire. Sequential dilation with progressively larger dilators is used to spread the fibers of the abdominal musculature till a diameter of 22mm of surgical corridor is achieved [Figure 5]a. The tubular retractor is then placed with its vertex centered over the anterior half of the intervertebral disc space [Figure 5]b. The grooves on the dilators should align with the corresponding stability pin channels on the retractor blades. While aligning the retractors, one must remember that the opening between the retractor blades should be parallel to the disc space [Figure 5]c. While fixing the pins, a special focus should be on preserving the genitofemoral nerve, the sympathetic chain, and the segmental blood vessels.
Step 9: Discectomy and the orthogonal manoeuvre
Once the retractors are fixed, the microscope is brought in [Figure 5]d. As the trajectory of surgical approach is oblique, it is imperative to rotate the instruments into a true lateral position vertically across the disc space (the orthogonal manoeuvre) to avoid any injury to the contralateral nerve root [Figure 6]a. 1.5-2 cm annulotomy is done for a thorough endplate preparation for the graft and to facilitate the orthogonal manoeuvre. Different sizes of shavers, serrated curettes, rasps, ring curettes, and combo tools are used to ensure a thorough discectomy and a proper end-plate preparation. After completion of the discectomy, the Cobb's forceps is passed along the end plates to release the contralateral annulus from the end plate by a blunt push. The superior and inferior aspects of the contralateral annulus are then gently removed. This step is critical for ensuring an appropriate distraction of the disc space and the coronal alignment of the contiguous vertebrae.
Step 10: Trialing
After the discectomy has been completed, trials using inter-body templates are tried to select the cage of an appropriate size. The disc space is sequentially distracted with progressive larger inter-body templates to achieve the desired disc space height and foraminal diameter (radiologically coronal and sagittal alignment is also seen) [Figure 6]b. The interbody templates are passed through the retractors obliquely, and then, are turned forwards and downwards across the disc space (executing the orthogonal manoeuvre). A proper template should be centred to the spinous process and should span the entire ring apophysis in order to completely traverse the vertebral body end plate.
Step 11: OLIF implant placement
The adequate sized implant (based upon the trialing technique)
is inserted using the navigated dorsolumbar (DL) inserter system. The DL inserter utilizes sleeves for the graft containment. The sleeves must be retracted to attach the implant. Before inserting the cage, autologous bone is packed in the implant's cavity. To achieve an adequate interbody fusion, the cage is packed with demineralized bone matrix (DBM) or harvested iliac bone graft. Using navigational assistance, placement of the cage is monitored in both coronal and sagittal views to confirm an adequate lordotic and/or scoliosis correction [Figure 6]c. An additional advantage of OLIF is that one can harvest the iliac crest bone graft through the same incision.
Step 12: Fixation
In PPF (utilizing the posterior approach), a separate incision is required in prone position. We placed the percutaneous pedicle screw under navigation guidance with the use of free EMG monitoring to assess the medial pedicle breach [Figure 7]. Navigational guidance not only reduces the time undertaken to perform the surgery, but also enhances the accuracy of the procedure and also reduces the radiation exposure. In LLF, both the lateral lumbar plate [Figure 8] and screws (utilizing the oblique approach) are inserted and fixed on the anterolateral surface of the vertebral body. The long-term results of the fusion rates of LLF are not known. A proper randomised controlled trial to compare the efficacy of LLF with the conventional posterior transpedicular lumbar screw fixation should be undertaken in the future.
To obviate the limitation of LLF/PPF techniques in terms of two column negotiation, we used a novel concept of RPSF in two patients. Through the same retroperitoneal approach used for discectomy, from the anterolateral surface of the vertebral body, the lateral inter-body plate is fixed. Then, two reverse pedicle screw are inserted, which ensure the engagement of the three vertebral columns. The trajectory of the superior screws are directed at 20 to 30 degrees upward direction (as visualised in the antero-posterior view of the lumbar spine) towards the contralateral pedicle. The trajectory of the inferior screws were directed parallel to the upper endplate towards the contralateral pedicle. This step can be done either utilizing navigational assistance or fluoroscopic guidance. Our technique of RPSF, not only provides a three column fixation, but also obviates the need for changing the patient's position to a prone one. Thus, it also helps in reducing the operative time.
A few case examples with post-operative images and the degree of fusion achieved on follow up imaging after the patient had undergone the OLIF procedure are demonstrated in [Figure 10].
[Table 1] shows the demographic and clinical details of patients (n = 15) operated through the navigated OLIF procedure. The mean age was 52.5 ± 9.6 9 years (range: 36-65; 8 male: 7 female patients). According to the Meyerding's grading system, we operated upon 11 cases of grade 1 spondylolisthesis and 4 cases of grade 2 spondylolisthesis. A patient had both grade I and II spondylolisthesis, at different levels. 13 patients had improvement in their mechanical back pain and neurogenic claudication. Two patients had improvement in back pain, but on one side, lumbar radiculopathy persisted, which was treated conservatively. 8 patients underwent PPF, 5 patients underwent LLF, and in 2 patients, RPSF was done. In one patient (after LLF), the graft remained in situ but the screw and plate got dislodged and had to be removed. Radiologically, all the patient had reduction in their spondylolisthesis, as seen on their follow up visit. Clinically, 3 patients had post-operative transient gastroparesis, which improved on medical management. 1 patient (after the PPF procedure) required a screw revision. One patient complained of pain on the medial aspect of thigh. The pain was intermittent and sharp shooting but decreased gradually on analgesic intake. One patient had intra-operative transient hypotension (ITH) (perhaps related to sympathetic chain injury).
In the 12 patients, according to the Bridwell's fusion grading, 2 patients showed Grade I fusion, 7 showed grade II fusion and 3 patient had grade 3 fusion on follow up visit [ranges of follow up: 1 to 10 months (mean 5.7 ± 3.3 months)]. There was no difference in the fusion achieved on radiological assessment at follow up visits between the PPF and the LLF groups. The mean hospital stay was 8 ± 3.7 days (median stay: 7 days) and the mean operative time was 191.4 ± 25.6 minutes (median: 195 minutes).
The mean pre-operative ODI score was 35 ± 6.1, which improved to 14.6 ± 4.1 at follow-up Similarly, the mean pre-operative VAS score was 7 ± 0.7, which improved to 3.3 ± 0.4 at the follow-up visit.
The overall incidence of all complications from the previous studies utilizing this procedure ranges from 0% to 52.9%.,,,, Due to the prolonged retraction of the psoas muscle, postoperative pain in the groin or thigh area (muscle spasm), transient paraesthesia/numbness and weakness are the most common complications reported in the literature, which spontaneously resolve after 2 weeks to 3 months., Very rarely, segmental arterial, iliac vein or iliolumbar vein injury and peritoneal laceration may occur inadvertently during exposure of the disc space, especially when excessive retraction of the vascular structures or the peritoneum has been applied. Silvestre et al., reported complications in 179 patients upon employing the OLIF technique. In his study, the most common complications were incisional pain in 2.2% of patients, lower extremity symptoms due to sympathetic chain injury, and vascular injuries in 1.7%.
Pitfalls and pearls
An indirect decompression and correction of the sagittal and coronal alignment are best achieved with OLIF. Compare to the lateral lumbar interbody fusion (LLF), it has the advantage of a very minimal risk of psoas muscle damage and lumbar plexus injury. Thus, the approach may be easily used even without the presence of intraoperative nerve monitoring. Navigational assisted OLIF continuously guides the surgeon regarding the trajectory of approach and the depth of the instrument in real time. It also minimises the radiation exposure for the surgeon. The duration of surgery also decreases, once the surgeon gains sufficient experience and becomes well-versed with the anatomy of the structures encountered. The MIS-OLIF technique, for vertebral levels L2–L5, has shown encouraging early surgical outcomes (with regard to the fusion rate and a rapid recovery). However, there is still need for evidence-based studies with a larger sample size to justify its efficacy and feasibility.
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Conflicts of interest
There are no conflicts of interest.
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