Lumbar Interbody Fusion L3–L4, L4–L5

(1)

Department of Orthopaedics, University of North Carolina, Chapel Hill, NC, USA

(2)

Wake Orthopaedics, WakeMed Health and Hospitals, Raleigh, NC, USA

Keywords

Lateral lumbar interbody fusionExtreme lateral interbody fusionLumbosacral plexussurgical technique

History of the Direct Lateral Approach

The first laparoscopic lumbar discectomy was described in 1991 [1, 2], and minimally invasive lumbar surgery has continued to evolve. Stemming from the initial laparoscopic lumbar discectomy were the laparoscopic anterior lumbar approach and mini-open anterior lumbar interbody fusion which were complicated by sexual dysfunction, visceral damage, and large vessel bleeding [3, 4]. First described in 2001, the lateral lumbar interbody fusion (LLIF) also known as extreme lateral interbody fusion (XLIF) has become increasingly popular as it avoids the aforementioned complications of anterior intra-abdominal procedures [3, 5]. Since the introduction of the LLIF technique, reported outcomes include decreased blood loss, decreased operative times, short hospital stays, and less postoperative pain [3, 6, 7] with comparable fusion rates to the anterior lumbar interbody fusion (ALIF) [8, 9]. Furthermore, advantages include indirect decompression, coronal and sagittal plane correction, and stabilization through a less invasive approach [2]. Compared to the posterior approaches, the LLIF does not require retraction of nerve roots or the cauda equina, and leaves bony and ligamentous structures intact [10].

Anatomy and Anatomic Considerations

The LLIF approach involves using a lateral retroperitoneal transpoas corridor [11], and an understanding of anatomy to the iliopsoas muscle and nerves of the lumbar plexus is vital in order to avoid complications. The direct lateral approach is primarily concerned with the psoas major and psoas minor portions of the iliopsoas [10]. The psoas muscle originates on the transverse processes and lateral borders of the vertebral bodies of T12–L5 [10]. As shown in cadaveric studies, the lumbar plexus is generally found within the psoas muscle between the transverse processes and vertebral body and dorsal to the posterior fourth of the vertebral bodies [12, 13]. The nerve roots exit along the medial edge of the psoas and course anteriorly as they move distally [11, 13]. The iliac vessels course more laterally at more caudal levels [11]. Safe anatomical zones at the disc spaces have been defined using cadavers and show that the anterior 3/4 of the disc space at L3–L4 and the anterior 2/3 of the disc space at L4–L5 are generally free of motor nerves [2, 10, 14]. The genitofemoral nerve is at particular risk at the anterior quarter of the vertebral body at L3–L4 and L4–L5, and the plexus overall is at greatest risk at the L4–L5 level [12]. Furthermore, following the skin incision, care needs to be taken to avoid the subcostal nerves supplying the abdominal wall muscles [10].

Operating Room Setup and Operative Technique

The C-arm is placed across the surgeon with the monitor at the side. The patient is placed in the lateral decubitus position on a radiolucent table with the knees slightly flexed to relax the psoas muscle. The greater trochanter is placed at the table break. The patient is then secured to the table. The table is then flexed to increase the distances between the iliac crest and rib cage which is particularly important when approaching L4–L5. Neuromonitoring is mandatory using the direct lateral approach [6] and a twitch test should be performed prior to initial incision to ensure that no neuromuscular blocking agent has been administered. An anterioposterior (AP) orientation on fluoroscopy should be obtained with the spinous processes in the midline and with the pedicles symmetric. Next, the C-arm should be rotated 90 degrees to obtain a true lateral, confirmed when the pedicles are superimposed on one another and the endplates and posterior cortices are linear. The table rather than the C-arm should be adjusted to obtain the true AP and true lateral images.

Next, the surgical site is prepped, and the appropriate level is identified on the lateral view. Two K-wires are crossed slightly posterior to the midpoint of either L3–L4 or L4–L5, and this area is marked on the patient’s lateral side. If a second incision is used for instrumentation, it is made posterior to the lateral incision between the erector spinae muscles and the abdominal oblique muscle. An approximately 2 cm posterolateral (PL) incision is made, and then blunt dissection is carried out using blunt scissor and finger dissection. Fingers are used to advance through the abdominal wall musculature and the retroperitoneal space is accessed. Next, the peritoneum is gently swept off the abdominal wall to allow the contents of the abdominal wall to fall forward. The transverse processes are then palpated along with the origin of the psoas muscle. Next, fingers are used to pass through the PL incision to the lateral incision, bluntly sweeping peritoneum off the underportion of the lateral incision entry point and allowing safe passage while making the lateral incision. The lateral incision is then made in a similar fashion as the posterolateral incision.

After the initial surgical approach is complete, dilators are then introduced. The first dilator is inserted through the PL incision and guided to the lateral border of the psoas muscle. The level of the first dilator is then confirmed under fluoroscopy. Neuromonitoring should then be established through the dilator using the EMG-stimulating surface found on the dilator and stimulation should be maintained through psoas dilation. Use blunt dissection through the psoas fibers and advance the dilator toward the lateral disc paying close attention to the neuromonitoring feedback. The position of the dilator is confirmed on fluoroscopy and secured with a K-wire placed midway into the disc. Sequential dilation is performed and a retractor is placed over the last dilator. The position of the retractor is then confirmed on fluoroscopy. The retractor is then stabilized using an articulating arm. Using an EMG-stimulating probe, the absence of nerves within the surgical field is verified.

Next, the disc spaced is prepared. An ipsilateral annulotomy is performed and the disc space is evacuated, avoiding damage to the endplate and decreasing the risk of implant subsidence. The contralateral annulus is then released ensuring parallel distraction. An implant of the correct size is then carefully chosen and gently impacted while observing nerve activity. If desired, supplemental fixation is then performed using anterolateral plating, unilateral pedicle screws and rod fixation, facet screw fixation, or interspinous fixation. The retractor is then slowly removed, and the disc space and psoas muscle examined for bleeding. The muscles of the abdominal wall are sutured and the skin is closed in a standard fashion. Postoperatively, the patient should be encouraged to mobilize. Side effects include hip flexion weakness from violation of the psoas muscle which typically resolves and sensory disturbances from irritation of sensory nerves [3, 10].

Tips and Tricks

Appropriate preoperative planning and careful review of preoperative imaging is mandatory, especially when performing a direct lateral approach at the L4–L5 level.
Pelvic morphology often dictates whether or not access to the L4–L5 level can be accomplished. Radiographs.
Review of the cross-sectional anatomy in Fig. 19.1 demonstrates an L4–L5 level that can be accessed on the left side only. Figure 19.2 demonstrates an L4–L5 level that will be difficult to access from either side due to the height of the pelvic brim and the spinal deformity. Pre-operative MRI is imperative. This includes review of the psoas morphology as well as evaluation of the great vessels. At the level of L4–L5, there is more anatomic variance. Figure 19.3 demonstrates a psoas morphology that is amenable to direct lateral approach; however, the IVC is lateralized and at increased risk for injury. If a direct lateral approach is performed at this level, it is the author’s preference to approach the at-risk vessel with the retractor so that direct vision can be used to avoid injury as the vessel is at greatest risk with a contralateral release with the Cobb. Figure 19.4 demonstrates an anteriorly positioned psoas, “mickey mouse ears.” This patient would not be candidate for a direct lateral approach as the risk to the lumbar plexus is too great. This psoas anatomy does not allow the retractor to be docked at or behind the “30 yard line.”
Efficiency is critical during the procedure. Prolonged times with the retractor open in the psoas can result in increased thigh pain and increased risk of neurologic injury or postoperative palsy. We attempt to limit time with the retractor open to approximately 15–20 minutes per level.
It is important to remove the retractor slowly and observe for bleeding. Large retroperitoneal hematomas have been reported, and these are likely secondary to injury to the segmental artery without adequate hemostasis.
Careful repair of the abdominal wall musculature is important to prevent postoperative hernia.