Pre-psoas (Oblique) Lateral Interbody Fusion at L5-S1

Introduction

The pre-psoas oblique lateral approach to the lumbar spine was first described by Mayer et al. in 1997. At that time, the approach was described only for the L2-5 disk spaces and the authors recommended an anterior transabdominal approach to the L5-S1 disk space owing to the anatomic considerations surrounding its access. The aortic bifurcation and iliocaval junction typically occur at or just below the L4 vertebral body and, as the iliac vessels course inferolaterally from their origin, they commonly overlie the anterolateral aspect of the L5-S1 disk space ( Fig. 13.1 ).

After the initial description of oblique lateral interbody fusion (OLIF) by Mayer, there were only scattered reports of OLIF in the literature while alternative interbody fusion techniques (transforaminal lumbar interbody fusion [TLIF], anterior lumbar interbody fusion [ALIF], posterior lumbar interbody fusion [PLIF]) were predominantly used to access the L5-S1 interspace. This was true until 2012, when a retrospective study of 179 patients who underwent OLIF was published. This study included six patients who had two-level interbody fusions from L4-S1. The authors also introduced a “sliding window” mini-open technique to access multiple disk levels through one small incision. To our knowledge, this is the first reported L5-S1 OLIF procedure in the literature, although the specific technique has since been modified by other authors to facilitate easier access to the L5-S1 level.

In 2014, another group performed a cadaveric study attempting to access L2-S1 disks from a lateral decubitus position. They found that in all of their 20 specimens they were able to access the L5-S1 disk space medial to the iliac vessels, elucidating the surgical corridor used for modern L5-S1 OLIF. The authors noted that an advantage of this technique is in being able to access all levels from L2-S1 while keeping the patient in a lateral decubitus position without a break in the table. It has only been since the publication of this study and the discovery of this corridor that reports of isolated L5-S1 OLIFs have begun to appear in the literature. In addition, at least one company (Medtronic Inc., Memphis, TN) has begun producing and marketing a retractor and instrumentation system designed specifically for this procedure.

One case report described two OLIFs performed at L5-S1 accompanied by posterior fixation and reported good results without complications. In their discussion, the authors supported the concept of being able to access disks at multiple levels and perform fusion from L1-S1 with the patient in the lateral decubitus position. Another case report presented OLIFs performed at L2-3, 3-4, 4-5, and 5-1 in a single procedure with good results. The authors also commented on the benefit of being able to perform the L5-S1 interbody work through the same incision as the other levels.

From an anatomic standpoint, a recent retrospective magnetic resonance imaging study explored the oblique access to L5-S1, which the authors defined transversely from the midsagittal line of the inferior endplate of L5 to the medial border of the left common iliac vessel and vertically to the first vascular structure that crossed midline. They found sufficient anatomic access to the L5-S1 interspace in 69% of patients analyzed and observed that the lower the iliocaval junction was, the less probable it was that sufficient access was present. This study failed to take into account the additional space gained by intraoperative mobilization and retraction of the iliac vessels, but interestingly, they found that in 13% of patients with no anterior access to the L5-S1 disk, an oblique corridor could be delineated between the psoas and the iliac vessels similar to that previously described by Silvestre et al.

The benefits of L5-S1 OLIF may be largely in contrast to other procedures. Some authors discuss the benefits of OLIF’s oblique pre-psoas approach in contrast to a lateral transpsoas approach (lateral lumbar interbody fusion [LLIF] eXtreme LIF [XLIF]/DLIF), as OLIF does not require dissection or splitting of the psoas muscle. This may theoretically decrease postoperative pain and avoid injury to the psoas and lumbar plexus, which may obviate the need for intraoperative neuromonitoring. Specifically, in regards to the L5-S1 level, a nonoblique lateral approach can be extremely difficult or impossible owing to obstruction by the iliac crests.

In contrast to a traditional PLIF, OLIF avoids dissection of the paraspinal muscles, reduces the risk of incidental durotomies, and eliminates the need for nerve root retraction. It also may have some benefit over a direct ALIF, which can lead to injury to abdominal viscera, retrograde ejaculation, and prolonged ileus, while still potentially offering the similar benefits in sagittal balance and restoration of disk height associated with these other interbody approaches.

Surgical Indications

The L5-S1 OLIF has similar indications as other interbody fusion techniques. These include a number of symptomatic pathologies including, but not limited to, degenerative disk disease with disk collapse, spondylolisthesis, discitis, and scoliosis. OLIF at other levels has been reported in the literature for revision of a pseudoarthrosis because it affords good disk space visualization. It may be chosen over other approaches to the L5-S1 disk space for reasons previously discussed, including the ability to access multiple levels through one incision without having to reposition the patient.

Limitations

A number of potential limitations of L5-S1 OLIF exist. Vascular anatomy may in some cases make L5-S1 access a challenge, if not impossible. As previously discussed, a low-lying iliocaval junction may prohibit access to the L5-S1 interspace and can be evaluated with preoperative imaging at the surgeon’s discretion. In trauma patients with substantial pelvic injuries, a lateral decubitus position may be prohibited. A posterior approach which avoids peritoneal manipulation may be preferred in patients with an ostomy or significant abdominal or retroperitoneal pathology. Similarly, an alternative approach that avoids iliac vessel retraction may be preferred in vasculopathic patients with lower extremity arterial insufficiency.

The surgeon may have difficulty using this approach in a morbidly obese patient if the retractor system is not long enough to accommodate the extra depth from the skin to the spine. However, it may also be argued that access and exposure for this approach is often easier in the morbidly obese patient than a traditional posterior approach, or a direct anterior transabdominal approach, owing to the tendency of the abdominal pannus to “fall away” anteriorly when the patient is placed in the lateral position. In the authors’ experience, this has been the case in the moderately and morbidly obese, but begins to lose its advantage in the super-morbidly obese (BMI >50). We recommend measuring along the planned approach trajectory on the preoperative imaging, and comparing the expected depth with the retractor system’s available lengths to minimize the chance of access problems. Accommodation should be made, in a “best guess” manner, for expected shifting of the tissues between intraoperative positioning and the typical supine position in which preoperative imaging is obtained.

Although one might reasonably assume that all aspects of the surgery would be easier in very thin patients, the more extreme end of this spectrum can pose some challenge in the sense that the normal retroperitoneal fat planes used to help proceed with the exposure may be more difficult to identify and stay safely within, potentially increasing the risk of inadvertent entry into the peritoneum or injury to other retroperitoneal structures.

Although spondylolisthesis is an approved indication for the OLIF procedure, surgeon discretion should be used in selecting appropriate cases for OLIF, especially in the early part of the surgeon’s learning curve. The authors advise against attempting to treat Meyerding grade III or higher spondylolisthesis with this technique. Additionally, when approaching L5-S1, we suggest that a dysplastic/congenital spondylolisthesis, with associated anatomic variations (such as domed or rounded S1 superior endplate) should be avoided unless the surgeon has both significant experience treating these types of spondylolisthesis with other techniques and significant experience with OLIF in more conventional cases.

As discussed previously, OLIF may offer the opportunity to complete interbody fusions at multiple levels through a single incision. However, if supplemental fixation beyond what can be achieved through an anterior approach is necessary, then additional incisions, possibly requiring repositioning of the patient, may still be necessary.

Surgeons more familiar with a direct anterior and/or direct lateral approach may find that working in the disk space from an oblique angle can be disorienting. The use of intraoperative image guidance and/or extensive fluoroscopy may be necessary, particularly early in the learning curve, to avoid inadvertent entry into the spinal canal. However, both image guidance and extensive use of fluoroscopy carry well documented risks and costs associated with them.

Another prohibitive factor may be the cost of the procedure or availability of necessary equipment. A specialized retractor system is only available at this time through one device manufacturer and may have limited availability or be expensive to purchase or lease for individual cases. Similarly, if anterior plating systems or specialized cages are desired they present the same challenges.