Development of the prone lateral lumbar interbody fusion: a comparison of lateral spine surgery techniques

Introduction

Lumbar fusion surgery has been widely utilized for the treatment of refractory degenerative and deforming disease of the lower back in the ageing population. Over many decades, various surgical approaches have been developed to access the lumbar spine and disc space, each with its own unique advantages and disadvantages (1). With recent advancements in navigation technology, robotics, and minimally invasive techniques, more attention has turned towards improving surgical technique and access to the intervertebral disc (IVD) while optimizing postoperative outcomes and minimizing collateral soft tissue injury and surgical complications.

The prone lateral lumbar interbody fusion (LLIF) has emerged as a relatively new approach to lateral spine surgery that offers many potential advantages over traditional lateral techniques (2,3). The purpose of this article is to review the advantages and disadvantages of prone LLIF and compare them to previous lateral techniques.

Traditional LLIF

The traditional LLIF, often described as direct LLIF (DLIF) or extreme LLIF (XLIF), was developed to address the shortcomings of the anterior lumbar interbody fusion (ALIF) (4). It is a minimally invasive technique that utilizes a trans-psoas approach to access the IVD space with the patient in the lateral decubitus position. Because of the trans-muscular nature of the approach, the technique reduces the morbidity associated with traditional retroperitoneal and transperitoneal approaches by avoiding gastrointestinal and vascular structures that are seen in the ALIF approach and manipulation of the dura and encased cauda equina nerve roots seen in a posterior approach for transforaminal and posterior lumbar interbody fusion (TLIF/PLIF).

The indications for the trans-psoas LLIF also need to be understood in the context of a patient’s psoas morphology and the modified Moro’s classification. The modified Moro’s classification is based on the anatomic shape (i.e., low/normal versus high-rising/teardrop-shaped psoas morphology) and anterior-to-posterior position of the psoas major muscle relative to the IVD space (5,6). Patients with more tear-dropped and anterior psoas morphology are a relative contraindication for trans-psoas LLIF and may instead benefit from a pre-psoas LLIF approach such as an oblique lumbar interbody fusion (OLIF) to avoid the risk of lumbar plexus injury (6).

Another advantage of the lateral approach is that it offers ease of access from T12/L1 to the L4/5 disc space while also preserving the anterior and posterior longitudinal ligaments, thus maintaining ligamentous stability at the treated levels. Moreover, lateral approach to the lumbar spine allows for the use of wider cages with a large bone graft window and increased endplate contact that can help restore disc height, promote bony integration, provide indirect decompression, and improve malalignment in deformity cases (7).

Lastly, while the patient is in the lateral decubitus position, the primary surgeon can perform either DLIF/XLIF or OLIF depending on the psoas morphology (5,6). In this position, OLIF can also be performed at the L5/S1 disc space although this may be technically challenging given the anatomic variations of the vascular anatomy and unfamiliar patient positioning for both the access and spine surgeon (8). As such, previous studies have described staying lateral to or between the bifurcated common iliac vessels to access the disc space (9). When determining the feasibility of utilizing the L5/S1 OLIF, the primary surgeon must consider the size of the vascular corridor, the location of the left common iliac vein, and the presence of a fat plane between the left common iliac vein and IVD (8,9).

Despite these advantages, there are several limitations to this approach. First, because the trans-psoas LLIF is performed in the lateral decubitus position, the primary surgeon must reposition the patient prone for pedicle screw fixation and decompression if necessary. This, in turn, increases operative time and reduces surgical efficiency. Patients may also experience postoperative hip flexion weakness, pain and paresthesia due to blunt dissection of the psoas muscle and potential stretch neuropraxia to the lumbar plexus (10). As well, the lateral decubitus position remains inherently unstable compared to the prone or supine position, making precise instrumentation difficult (7). Some methods to eliminate this risk include the use of bean bags, heavy adhesive, tape, and/or LLIF-specific positioners. Once properly secured, the lateral position often provides great stability due to gravitational forces keeping the retractor in place.

Further, accessing the L5/S1 level through a trans-psoas LLIF approach is contraindicated due to the risk of iliac vessel mobilization and presence of the iliac crest restricting access to and visualization of the IVD space (7). In cases with a narrow vascular corridor, midline left common iliac vein, and adhesions between the vasculature and IVD, the L5/S1 OLIF is not recommended due to high risk of vascular injury (8). Additionally, L5/S1 OLIF may be technically difficult in patients with a high sacral slope and restrictive pubic symphysis (8). Therefore, when considering a trans-psoas LLIF approach with plans for extension to the L5/S1 level in which OLIF is not feasible, the patient typically will need to be repositioned supine for L5/S1 ALIF with vascular surgeon assistance (7,8). The additional wound closure, repositioning, re-prepping and draping and additional surgical personnel can add significant operative time.

Single-position surgery lateral lumbar interbody fusion (SPS LLIF)

Considering the limitations associated with trans-psoas LLIF, SPS LLIF emerged as a technical modification that has been popularized in both the primary and revision settings (11). SPS LLIF is performed with the patient in the lateral decubitus position with additional posterior pedicle screw instrumentation performed simultaneously, thereby obviating the need for further repositioning. Similar to the trans-psoas LLIF approach, the primary surgeon has the flexibility to perform a DLIF/XLIF or OLIF while the patient is in the lateral decubitus position (5,6). Access to the lumbosacral junction while in the lateral decubitus position also remains viable in the form of an L5/S1 OLIF as previously discussed (11). Newer technology such as computer-assisted navigation and robotics have also been used to help facilitate accurate pedicle screw placement through separate percutaneous incisions posteriorly with minimally invasive techniques (11,12). An all-lateral approach has been associated with decreased operative time, estimated blood loss, and length of hospitalization compared to traditional techniques (13).

While SPS LLIF offers several advantages over trans-psoas LLIF, there remain several disadvantages. First, not unique to the trans-psoas LLIF approach, the SPS LLIF still carries the inherent risk of femoral neuropraxia due to blunt dissection of the psoas muscle as previously discussed (10). Second, due to the continued lateral positioning in this technique, the pedicle screw trajectory is in the horizontal plane as opposed to the more familiar vertical trajectory most spine surgeons are trained on. Placement of the “downside” screws is often difficult given the oblique and upward trajectory needed and often having the surgical table blocking instruments (12). Choi et al. published their experience performing SPS LLIF with pedicle screw fixation in 35 patients (169 pedicle screws) and found a 4.1% (7/169) breach rate (14). Most of their breach screws were found on the “downside” pedicle screw (6/7), all of which had a breach <2 mm and occurred while using robotic-assisted technology for pedicle screw placement. Similarly, Blizzard et al. retrospectively studied 72 patients (300 pedicle screws) who underwent minimally invasive SPS LLIF or OLIF with available 6-month postoperative computed tomography scans (11). They identified a breach rate of 5.1% (13/254) with 10/13 breaches measuring <2 mm with 2 patients (2.8%) requiring reoperation for radicular pain secondary to the mal-positioned pedicle screws. Therefore, pedicle screw placement on the “downside” can be challenging, awkward, and unfamiliar for most surgeons ergonomically. Given the inherent instability of the body in the lateral decubitus position, any slight unrecognized change in patient position intraoperatively can often cause failure of navigation and lead to malposition of pedicle screws, breach, and potential devastating neurologic compromise (7). However, these risks may be mitigated by the use of bean bags, heavy adhesive tape, and/or LLIF-specific positioners, which use gravitational forces to keep the retractors held in place.

Another major disadvantage is the lack of direct access to the posterior column. Posterolateral fusion with bone graft is generally not possible. Rather, only anterior column fusion can be achieved (3). For the same reasons, while the surgeon can apply indirect decompression through restoration of disc height from the lateral approach and cage insertion, direct decompression and laminectomy of the neural elements would be extremely challenging without repositioning the patient.

Spinal deformity correction may also be suboptimal without the assistance of gravity and the patient’s body weight to help provide natural lumbar lordosis (15). This is further compounded by the lack of posterior access and inability to perform bony osteotomies to provide further realignment. Overall sagittal realignment is more difficult to achieve with the patient in the lateral position (15).

Prone lateral lumbar interbody fusion

Within recent years, the prone LLIF was developed and introduced to minimize the limitations seen with the previous approaches while maintaining the same advantages offered by the standard trans-psoas LLIF (16). With this technical modification, the patient is placed in the prone position on an open Jackson frame, which is a familiar position for most spine surgeons and offers a variety of surgical techniques that can be performed (16). From the prone position, both the flank and back are prepped into the surgical field to allow for simultaneous access to both the anterior column via the trans-psoas LLIF and the posterior column via traditional dorsal approach, respectively (Figure 1).

Figure 1 The image demonstrates the advantage of prone single-position lateral lumbar interbody fusion having simultaneous access to both the lateral and posterior incisions. This image is published with the patient's consent.

The trans-psoas LLIF approach can be performed while the surgeon is seated and table tilted away, which may improve comfort, ergonomics, and visualization of the disc space. The option to perform the prone LLIF with two primary surgeons (i.e., one performing the lateral approach and another simultaneously performing the posterior approach) greatly improves efficiency and decreases operating time by allowing for simultaneous access to the posterior and lateral spine while minimizing the need to reposition and re-drape the patient intraoperatively if a single surgeon were performing the case (3,16). Familiarity with the prone position is also conducive to minimally invasive techniques and navigation/robotics in a more stable patient position, which may improve the accuracy of pedicle screw placement (16). In addition, spine surgeons can perform direct decompression, posterior column osteotomies, and achieve posterolateral fusion as indicated (17). Surgeons may also access the L5/S1 level via a PLIF/TLIF approach through the same posterior incision or in a minimally invasive fashion (18). In the case of revision surgery, that same posterior incision may also be used to extend the previous instrumentation.

The prone position offers additional anatomic advantages from a deformity standpoint. With the help of gravity and the weight of the patient’s abdomen hanging freely from the table, the prone position allows for improved correction of lumbar lordosis (15). Not only does the patient’s lordosis naturally improve, the iliopsoas muscle and lumbar plexus shift posteriorly with the hips fully extended which may decrease the risk of approach-related plexus injury (19). This, however, does not preclude the risk of femoral neuropraxia in prone LLIF as one study found a 34% incidence (10/29) of thigh pain or weakness in their cohort of prone LLIF patients at 6 weeks after surgery that resolved by 3 months postoperatively (20).

An important consideration of any trans-psoas approach regardless of positioning is the use of intraoperative neuromonitoring (IONM) to minimize the risk of femoral neuropraxia (21). Femoral neuropraxia, which presents clinically as thigh pain and/or hip flexion weakness, is a well-documented complication following lateral spine surgery that may be seen with excessive and/or prolonged retraction during surgical exposure. This risk is especially important at the L4/5 level where the lumbar plexus transitions from a dorsal to a more ventral position within the psoas muscle and can directly run across the disc space. The utility of IONM in any trans-psoas LLIF procedure would be to detect acute sensory and/or motor nerve conduction changes so that the primary surgeon may employ intraoperative countermeasures, such as promptly adjusting or removing the retractor to reduce the strain on the femoral nerve before the damage becomes irreversible (21). Therefore, IONM use is advisable for any laterally based lumbar spine surgery involving a trans-psoas approach.

Another anatomic consideration during the prone LLIF is that the peritoneum translates ventrally with gravity, which theoretically decreases the risk of gastrointestinal and ureteral complications. However, Dodo et al. demonstrated that this displacement may not be to a large enough extent to eliminate injury risk to visceral organs (22). Therefore, careful preoperative planning is still warranted to prevent inadvertent extraspinal injury during the procedure.

Nonetheless, several studies have shown that the prone LLIF technique is associated with improved deformity correction and lumbar lordosis given the assistance of gravity on the free abdomen, better subjective patient reported outcomes, and decreased operative time and length of hospital stay without increased complication rates compared to traditional lateral surgery (2,3).

While there are many technical advantages to prone LLIF, there are a few important disadvantages to consider. First, those more familiar with the trans-psoas lateral approach via lateral decubitus position may find that exposure and retraction is more difficult in the prone position. With the patient in the lateral decubitus position, gravity helps to move excess adipose tissue from the operative field and reduce the depth of the approach needed to access the IVD space. In the lateral position, the shorter tubular retractors remain in line with gravity, providing a degree of stability that is taken away once the patient is placed in the prone position as the retractor is now perpendicular to gravity and tends to drift ventrally. In contrast to lateral positioning, prone positioning may cause ventral migration of the retractor due to gravity which can reduce the surgical field, dislodge the retractor, or even risk injury to peritoneal contents (23). Prone lateral retractor placement is especially challenging in patients who are obese and have large abdominal girth. In such cases, the use of longer retractor blades is necessary for adequate disc visualization, but this often can lead to a smaller working space, increased tubular drift from gravity, and difficult ergonomics (18).

Another disadvantage to prone LLIF is the steep learning curve for surgeons unfamiliar with the procedure. In previous studies demonstrating a single surgeon’s experience naïve to prone LLIF, most complications were found within the first 30 cases (24). However, this gradually improved with a higher case volume leading to decreased retractor time as the surgeon became more comfortable with the procedure. Another single surgeon study noted a minimal learning curve which the authors attributed to the surgeon’s prior extensive experience with SPS LLIF (25).

Conclusions

While many surgical approaches to the IVD have been employed, the prone LLIF has been shown to be the latest modification in lateral spine surgery that has proven to be safe, effective, and mitigates many of the critical disadvantages to previous techniques (Table 1). As more surgeons begin to utilize the prone LLIF in clinical practice, further study is warranted to better understand the nuances between the different surgical approaches, delineate its clinical indications, and improve on its current limitations.

Acknowledgments

Funding: None.

Footnote

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