DEFINITION

Lateral approach to interbody fusion

Many different names including extreme lateral interbody fusion (XLIF), direct lateral interbody fusion (DLIF), or oblique lumbar interbody fusion (OLIF). It is often called the transpsoas approach because in the lumbar spine, it requires traversing the psoas muscle. The lateral approach can also be used to access thoracic spine as well.

Typically, this technique relies on a combination of neuromonitoring and direct visualization to safely navigate through the lateral lumbosacral neurologic plexus.

ANATOMY

After the superficial dissection, the lateral abdominal wall muscles are split to approach the lumbar spine. This leads directly into the retroperitoneal space.

The psoas muscle flanks the lateral lumbar spine and is covered by a thin, slippery fascia.

Within the psoas muscles traverse the lumbosacral plexus, genitofemoral nerve, and lateral cutaneous nerve (FIG 1).1,7

Moro et al5 identified and counted the location of the lumbar plexus and genitofemoral nerve relative to each disc space in 12 cadavers. Although there are general trends for each disc level in an anterior to posterior

distribution, each individual has significant variability that can be different from the “typical” situation.5

FIG 1 • The lumbar plexus exits the foramen traveling through the psoas muscle while moving more ventral as it progresses caudally. n, nerve; m, muscle.

As one progresses distally in the lumbar spine, the lumbosacral plexus covers more of the ventrolateral aspect of the lumbar spine.

The lateral iliac crests are typically slightly below or at the level of the L4-L5 disc space. However, there is variability between patients, and at times, high iliac crest (or deep seated L4-L5 disc space) may prevent parallel access to the L4-L5 disc space from a direct lateral approach (FIG 2).

When approaching the upper lumbar levels, the ribs may interfere with direct lateral approach to the spine. This may require choosing an incision that is not perfectly lateral to the disc space or excising a rib to improve access (see FIG 2).

Anteroposterior (AP) and lateral lumbar radiographs are useful to assess the rib and iliac crest position relative to the level that needs to be approached.

Aorta, inferior vena cava, and common iliac vessels run on the ventral surface of the anterior longitudinal ligament (ALL). Axial imaging allows preoperative localization of these structures to understand the safe zone for each patient (FIG 3).

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FIG 2 • Lateral lumbar radiographs superimpose the ribs and iliac crest over the disc spaces to determine which levels are accessible with a lateral approach.

FIG 3 • Axial imaging allows preoperative planning of the working window, including evaluating the proximity of the aorta and inferior vena cava.

SURGICAL MANAGEMENT

Indications

Spondylolisthesis low grade Isthmic

Degenerative

Deformity Scoliosis Kyphosis

In combination with pedicle subtraction osteotomy (PSO)

Foraminal stenosis with vacuum disc and instability Adjacent segment disease with instability

Discitis after failure of medical management

Other situations when interbody fusion is necessary

Preoperative Planning

AP and lateral radiographs allow assessment of accessibility of each disc level relative to the iliac crest and ribs.

Determine side of approach. Typically, the disc is approached from the convex side (ie, the more open side of the disc) to facilitate intradiscal work. However, in scoliosis cases, when the approach to the L4-L5 level (the convex side) dictates the side of the approach, this may place the other levels on the concave side. Surgeons often perform the surgery on the same side (convex at L4-L5 and concave at the other lumbar levels) and work around the inconvenience of working in the concavity (FIG 4). In flexible curves, bending the patient can alleviate this problem significantly. Rarely, some surgeons may choose to flip the patient to work on the convex side for the rest of the curve.

Establish a neuromonitoring plan. Typically, electromyography (EMG) monitoring is performed. This can consist of both free-running EMG as well as stimulated proximity sensing. EMGs help to monitor the motor branches of the lumbosacral plexus; however, sensory nerves cannot be monitored. The genitofemoral and lateral cutaneous nerves cannot be monitored.

Ensure the anesthetic plan is compatible with the neuromonitoring plan. The patient must have muscle twitches during the surgery to allow for EMG monitoring.

Positioning

Positioning on the operative table is extremely important in lateral interbody fusion.

Typically, the procedure is performed on a regular operative bed with the capacity to break or flex in the middle. Usually, the table's orientation is reversed such that the base is attached to the feet, allowing the Carm to pass freely underneath the thoracolumbar spine.

The patient is positioned in a true lateral position as close to vertical as possible. This can be fine-tuned under fluoroscopy by tilting the bed.

The iliac crest should be positioned approximately 4 inches cephalad to the center of the break in the table. This allows the flexion of the table to open the disc space while still giving enough room for the C-arm to pass underneath the table to provide a perfect lateral of the L4-L5 disc space (FIG 5A).

Hips are flexed approximately 30 degrees to take tension off the iliopsoas. Knees are flexed approximately 30 degrees to

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compensate for the hip flexion and to keep the feet in a good position on the bed.

FIG 4 • AP lumbar radiographs help determine the best approach side in preoperative planning. The approach side is often determined by the orientation of the L4-L5 disc or by the convexity of the curve. Bending films can be used to determine the flexibility of the curve, which is often reproduced by flexing the operating room table.

Once positioned and prior to flexing the table, the patient is taped to the table. First, tape the patient horizontally at the hip (just inferior to the iliac crests) and at the chest (just below the axilla). Next, a crossing pattern of tape holds the legs in position because the table will be tilted throughout the case (FIG 5B).

FIG 5 • A. The iliac crest is positioned approximately 4 inches cephalad of the break in the table. This allows the disc space to be opened by flexing the table while allowing the C-arm to pass freely underneath the table.

B. Horizontal tape straps are placed at the hip and the chest followed by a crossing pattern of tape on the lower extremities to secure them while the table is tilted during surgery.

After taping, flex the table. Flexing typically requires reverse Trendelenburg at the base while flexing the long end of the table. The goal is to have the involved disc space perpendicular to the floor. Feel the tension of the abdominal oblique muscles on the convex side to determine when sufficient flexion has been achieved, then use reverse Trendelenburg to make the lumbar spine parallel with the floor. Another round of taping of the hip area may be necessary.

Approach

The approach technique in the lumbar spine can be divided into the one-incision or two-incision technique.

With the one-incision technique, a single incision is created directly lateral to the disc space in a position that is as parallel as possible to the two endplates of the disc (FIG 6A).

With the two-incision technique, an initial incision is made posterior to the direct lateral position. Then a second incision is made directly lateral to the spine in a manner similar to the one-incision technique.

The addition of the posterior incision allows finger localization of the retroperitoneal space prior to incising the lateral abdominal musculature and facilitates finger-guided (through the posterior incision) placement of the instruments to the lateral disc space (FIG 6B,C).

The single-incision technique can also be used safely, but extra care must be taken to identify the retroperitoneal space, and a larger incision may be necessary to allow a finger to be placed through the direct lateral incision as opposed to through the posterior incision.

If multiple levels are involved, a single longitudinal incision or multiple small transverse incisions paralleling each of the involved disc spaces can be used. Multiple well-localized transverse incisions at each disc space have the advantage of ensuring one is able to work directly aligned over each disc space without undue soft

tissue tension.6

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FIG 6 • A. Three incisions are planned, each parallel to the respective disc space. This is the one-incision technique as the posterior incision is omitted. B,C. The posterior incision allows finger localization of the disc space and direct finger-guided delivery of instruments to the lateral disc space.

TECHNIQUES

• Localizing the Incision2,6

The C-arm should be turned parallel to the floor (ie, beam is horizontal). Adjustments are made to the table to obtain a perfect AP view of the disc space. The body, pedicles, and spinous processes are used as guides.

Once the true AP is obtained (ie, beam is horizontal in orientation), arc the C-arm 90 degrees into the lateral position. Then adjust the bed angle to get a parallel view of the endplates. Although these steps to align the disc space to the vertical and horizontal planes are not absolutely necessary, it can be very helpful for the surgeon to maintain orientation and help the surgeon stay within the safe zone throughout the case.

TECH FIG 1 • The guidewire is used to localize the incision parallel to the disc space using fluoroscopy at each surgical level.

Using a guidewire or other radiopaque instrument, mark the anterior and posterior border of the involved disc on the skin while the C-arm captures a true lateral of the disc space. Mark the incision such that it exactly parallels the orientation of the disc, which may change at each level based on the changes in lordosis or kyphosis (TECH FIG 1).

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• Approach and Dissection

  Single-Incision Technique

   

  After incising the skin sharply, dissect through the subcutaneous tissue to the fascia overlying the external oblique muscle.

   

  Dissect through the external oblique, internal oblique, and transversus muscles and fascia to enter the retroperitoneum. Sharp, blunt, or electrocautery dissection can be performed through the external layers of the abdominal muscles, but careful blunt dissection is recommended beyond that to minimize chances of damage to the peritoneum. Minimizing disruption to the muscle and innervation of the abdominal wall may reduce the risk of pseudohernia.

   

  Characteristic features of the retroperitoneum include the presence of retroperitoneal fat causing a very slippery feel to the tissues and the ability to feel striations in the psoas musculature. In many situations, it is possible to directly palpate the disc and vertebral body undulations (TECH FIG 2).

  Two-Incision Technique

   

  A longitudinal 4-cm incision is made approximately 5 to 8 cm posterior to the planned transverse incision overlying the disc space.

   

  Dissection is carried through the fascia and abdominal muscles into the retroperitoneal space, which is confirmed by feel.

   

  Approach is made through the skin and subcutaneous tissue down to the level of the fascia through the standard transverse incision.

   

  A finger is passed into the retroperitoneal space via the posterior incision and used to push up underneath the muscle and fascia in line with the transverse incision. Monopolar electrocautery can then be used to safely divide the abdominal muscles and fascia to enter the retroperitoneal space.

   

   

   

  TECH FIG 2 • Gentle finger sweeps allow localization of the retroperitoneal space, psoas, and lateral disc space.

• Placing the Guidewire

   

  After ensuring the approach is retroperitoneal, a small dilator is then placed through the psoas into the disc space. Neuromonitoring during this step will alert the surgeon to proximity of motor nerves and redirection of the dilator may be necessary. Once the dilator is docked on the lateral aspect of the disc and neuromonitoring is safe, then place the guidewire into the disc space.

   

  If the incision was localized correctly, the guidewire will be oriented perfectly vertical and centered in the incision in order to fall at the center of the disc.

   

  Care should be taken to pass the initial dilator to the lateral aspect of the disc by shuttling it with a finger (either through the same incision or the second posterior incision). This will prevent inadvertent bowel injury.

   

  The ideal position for the guidewire varies from level to level. Although it is more convenient to place the guidewire slightly posterior to midpoint, because of the lumbar plexus, it is advisable to start at the midpoint or slightly anterior at L4-L5 or any level at which neuromonitoring warning is present at the more posterior position. The orientation of the guidewire should be in line with the fluoroscopy beam, resulting in a single superimposed circle (TECH FIG 3).

   

   

   

  TECH FIG 3 • Under fluoroscopy, the guidewire is positioned on the lateral disc space just posterior to the midpoint of the disc parallel with the fluoroscopy beam.

   

   

   

• Exposure

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  Once the guidewire is in position, a series of larger soft tissue dilators are used to bluntly spread the soft tissue and psoas muscle. Each dilator connects to the EMG neuromonitoring system to help localize the motor nerves of the lumbosacral plexus.

   

  Starting with the smallest dilator, slide it over the guidewire. Rotate the dilator clockwise and counterclockwise with gentle pressure to advance to the lateral aspect of the disc. Attach the neuromonitoring system during dilation to determine if it is safe to proceed. Ensure the EMG electrode is oriented posteriorly, typically marked by a line on the dilator probe, as this is where the lumbosacral plexus and exiting nerve roots are most likely to be encountered.

   

  Repeat this step until the largest dilator is passed to the lateral disc space and reveals safe monitoring parameters.

   

  Many surgeons prefer the “dock shallow” approach where the dilators are placed on the psoas muscle instead of through the muscle. Once the retractor is placed on the psoas, the dissection can occur under direct visualization. This reduces the risk of neurologic injury, especially to sensory nerves that are not detected by EMG neuromonitoring.

• Docking and Opening the Retractor

   

  Attach the retractor arm to the mount affixed to the anterior side of the bed.

   

  Connecting the neuromonitoring clip to the retractor will allow for neuromonitoring during the placement of the retractor.

   

  Connect the retractor arm to the retractor (TECH FIG 4).

   

  Depending on the system and where you attach the retractor arm, the blades will open anteriorly or posteriorly. Typically in the lumbar spine, it is often more favorable to dock posteriorly and open the

  blades anteriorly.5

   

  Open the blades anteriorly, cephalad, and caudad. Affix the light sources. At this time, using a combination of direct visual inspection and the neuromonitoring probe tip, explore the remaining soft tissue over the lateral disc space. When “shallow docking,” dissection through the psoas may be necessary. Remember, the neuromonitoring system will not detect sensory nerves. It is important to check for nervous structures before proceeding with the discectomy or using bipolar electrocautery.

   

  Once the wound bed is confirmed to be free of nervous structures, the posterior blade can be affixed with the docking blade into the disc space. This should be done under direct vision to avoid entry to adjacent nervous structures. An optional anterior retractor can be placed over the ALL to help retract anterior soft tissues and provide a reference to prevent disruption of the ALL during discectomy.

   

   

   

  TECH FIG 4 • Once the dilator is in place, the blades are oriented so as to open in line with the disc space prior to locking the position with the holding arm.

• Discectomy

   

  With the lateral annulus exposed, perform a rectangular annulotomy and remove the nucleus pulposus with a pituitary rongeur.

   

  Pass a Cobb elevator along the cartilaginous surface of the superior and inferior endplate, with special care not to violate the bony endplate. Use AP fluoroscopy to ensure the Cobb takes down the annulus at both the superior and inferior endplate on the far lateral side (TECH FIG 5). This step allows the disc space to open and allows the cage to pass to the far lateral cortex. Depending on the level, orientation, and shape of the disc endplate, an angled or straight Cobb may be preferred.

   

  Using a combination of curettes and pull shavers, perform a complete discectomy. Rotary shavers can facilitate discectomy; however, judicious use is encouraged as they can result in a high incidence of endplate violation.

   

  Kerrison rongeur is used to débride any overlying annulus or osteophyte at the opening of the discectomy to improve visualization.

   

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  TECH FIG 5 • AP fluoroscopy image is used to monitor the position of the Cobb as the far annulus is released.

• Trialing and Sizing

   

  Once a complete discectomy is performed, size the interbody cage using the sizing trials. In addition to choosing the appropriate height, select the depth and width.

   

  Placing a cage that extends to both lateral rims of the endplates improves coronal plane correction and reduces the potential for cage collapse through the endplate. This can be confirmed on AP fluoroscopy images (TECH FIG 6A).

   

   

   

  TECH FIG 6 • A. AP fluoroscopy is used to confirm that the cage covers both lateral cortices to reduce the chance of collapse. (continued)

   

   

  AP cage size depends on the anatomic situation. Larger cages are biomechanically superior, but the bigger footprint requires more dissection and hence more chance of encountering neurologic structures (TECH FIG 6B).

   

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  TECH FIG 6 • (continued) B. Increased AP width provides increased contact surface area to prevent collapse without compromising fusion surface area. The cage should be centered in the AP direction.

  Anterior Longitudinal Ligament Release

   

  Intentional ALL release can be performed to achieve additional lordosis in cases of sagittal imbalance. This procedure is called the anterior column realignment or ACR by some surgeons.

   

  The ALL release can be done as part of an anterior and posterior case when a moderate degree of correction is required, or it can be done as part of a posterior (osteotomy), anterior, then posterior (compression and fixation) case when even more correction is required.

   

  This requires careful dissection anterior to the ALL and a retractor to protect the aorta and vena cava. Typically, the lateral annulus on both sides is released, the disc space cleaned out, and then trials inserted.

   

  Once reasonable tension is obtained with the initial trials, the ALL is resected under direct vision.

   

  Then taller cages or hyperlordotic cages are trialed and inserted. Restraint should be observed to avoid placing an overly lordotic or tall cage that merely leads to anterior placement of the cage.

   

  Screw fixation of the cage is typically required when the ALL is released to prevent migration.

   

  Hyperlordotic cages of 20 or 30 degrees with vertebral body tabs that allow screw fixation are available that facilitate this procedure.

  Graft Preparation and Delivery

   

  Most available cages are made of polyetheretherketone (PEEK). The cage should be filled with bone graft material. This can be held in place with circumferential sutures during delivery or by using a graft slider.

   

  The graft can be delivered by impacting it into place; however, there is a risk for endplate violation. The graft slider can safely deliver the graft while reducing the chance for endplate violation (TECH FIG 7).

   

  Graft delivery should be checked on AP fluoroscopy to ensure the radiographic markers in the graft extend to or slightly beyond the lateral body cortex.

   

   

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  TECH FIG 7 • The graft slider delivers the graft while preventing endplate violation.

• Closure

Confirm graft location with final AP and lateral fluoroscopic images.

Obtain hemostasis prior to retractor removal. Partially collapse the retractor and gently remove while inspecting for bleeding along the walls.

Layered closure is then performed, including the external abdominal fascia. If the thoracic cavity was entered, a chest tube may be indicated.

Additional Fixation

When posterior instrumentation is planned, additional anterior fixation is not required.

Percutaneous pedicle screws, either unilateral or bilateral, are commonly used to augment the lateral interbody graft. This is usually performed in the prone position following the lateral interbody fusion. Refer to the chapter of pedicle screw instrumentation for additional information.

If the ALL is disrupted intraoperatively, there is a marked increase in the risk of graft dislodgement. In this case, additional anterior fixation may be indicated.

In some instances, vertebral body screw fixation with rod, plate, or through the cage itself can be performed to improve biomechanics. However, this typically requires more dissection of the psoas and increases morbidity.

risk of disruption.

ALL release

• Can be used in cases of sagittal imbalance to achieve additional lordosis

• Use with PSO to achieve additional correction (FIG 7).

• Must protect adjacent vascular structures

• Screw fixation of cage is often required to prevent migration once the ALL is released.

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FIG 7 • Preoperative (A) and postoperative (B) lateral radiographs demonstrate the correction in sagittal balance achieved with an L4 PSO and T12-L1 lateral interbody cage with ALL release.

POSTOPERATIVE CARE

No additional postoperative protocols or restrictions are required after lateral interbody fusion than is standard for posterolateral or anterior fusion.

With both anterior and posterior column support (assuming posterior augmentation is performed), postoperative bracing is typically not necessary.

Generally, because of the minimally invasive retroperitoneal approach, patients typically mobilize quickly and with less pain than open posterolateral fusions.10

OUTCOMES

Less intraoperative blood loss compared with open posterior fusion4

No significant difference in outcomes or complication profile in obese patients9

Early and midterm outcome data for the treatment of adult degenerative scoliosis suggests less morbidity, blood loss, and overall complication rate compared with open posterior fusion historical cohorts.2,8

COMPLICATIONS3,4,9,11

Psoas palsy

Lumbosacral plexus injury Quadriceps palsy

Meralgia paresthetica (lateral femoral cutaneous nerve) Genitofemoral nerve injury

Implant subsidence Broken cage

Cage displacement Endplate violation ALL disruption

Vascular injury Hernia/pseudohernia Bowel injury

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REFERENCES

1. Benglis DM, Vanni S, Levi AD. An anatomical study of the lumbosacral plexus as related to the minimally invasive transpsoas approach to the lumbar spine. J Neurosurg Spine 2009;10(2):139-144.

    

    

2. Dakwar E, Cardona RF, Smith DA, et al. Early outcomes and safety of the minimally invasive, lateral retroperitoneal transpsoas approach for adult degenerative scoliosis. Neurosurg Focus 2010;28(3):E8.

    

    

3. Galan TV, Mohan V, Klineberg EO, et al. Case report: incisional hernia as a complication of extreme lateral

   interbody fusion. Spine J 2012;12(4):e1-e6.

    

    

4. Knight RQ, Schwaegler P, Hanscom D, et al. Direct lateral lumbar interbody fusion for degenerative conditions: early complication profile. J Spinal Disord Tech 2009;22(1):34-37.

    

    

5. Moro T, Kikuchi S, Konno S, et al. An anatomic study of the lumbar plexus with respect to retroperitoneal endoscopic surgery. Spine 2003;28(5):423-428.

    

    

6. Ozgur BM, Aryan HE, Pimenta L, et al. Extreme Lateral Interbody Fusion (XLIF): a novel surgical technique for anterior interbody fusion. Spine J 2006;6(4):435-443.

    

    

7. Park DK, Lee MJ, Lin EL, et al. The relationship of the intrapsoas nerves during a transpsoas approach to the lumbar spine: anatomic study. J Spinal Disorder Tech 2010;23(4):223-228.

    

    

8. Phillips FM, Isaacs RE, Rodgers WB, et al. Adult degenerative scoliosis treated with XLIF: clinical and radiographical results of a prospective multicenter study with 24-month follow-up. Spine 2013;38(21): 1853-1861.

    

    

9. Rodgers WB, Cox CS, Gerber EJ. Early complications of extreme lateral interbody fusion in the obese. J Spinal Disord Tech 2010;23(6): 393-397.

    

    

10. Rodgers WB, Cox CS, Gerber EJ. Experience and early results with a minimally invasive technique for anterior column support through extreme lateral interbody fusion: XLIF. Musculoskelet Rev 2007;1: 28-32.

     

     

11. Tonetti J, Vouallat H, Kwon BK, et al. Femoral nerve palsy following mini-open extraperitoneal lumbar approach: report of three cases and cadaveric mechanical study. J Spinal Disord Tech 2006;19(2): 135-141.