A Tell-Tale Spine: When a lateral interbody fusion should be performed

by Brie Eteson, Medical Communications, Digital Surgery

Lumbar interbody fusion is a surgery to treat disc problems in the lower back. Fusion surgery locks together two or more bones to stop painful motion and correct alignment. An implant with bone graft is placed in the empty space to restore the height and relieve nerve pinching. During healing, the bones fuse into one solid piece.

There are several approaches to a lumbar interbody fusion. The most common is the standard posterior approach, but this does not provide optimal access to the vertebral bodies and intervertebral discs.1 As an alternative, the lateral approach may be recommended for back or leg pain resulting from injury or degenerative diseases.2 Two lateral approaches are demonstrated in this simulation, the anterior to psoas (ATP) and the lateral lumbar interbody fusion (LLIF).

LLIF is a relatively new approach and aims to have several advantages over the posterior approach. Some of these include:1

  • Reduced risk of visceral and vascular injuries, incidental dural tears, and perioperative infections
  • Preservation of both the anterior and posterior annular and ligamentous structures
  • Favorable long-term outcomes.

However, as with most variations, the lateral approach also has associated complications, such as motor and sensory deficits.1

The lateral approach is also a suitable treatment option for younger and older patients, including fragile patients, due to generally shorter surgical time.

For this approach, working knowledge of retroperitoneal anatomy is crucial. This is due to individual variation, in particular, of the vascular structures and the lumbar plexus. These structures should be studied preoperatively through magnetic resonance imaging (MRI). Intraoperatively, patient positioning and X-ray usage are key to ensuring a safe and effective procedure.

What is the retroperitoneum and what’s in it?

The retroperitoneum is the space behind the abdominal or peritoneal cavity, lying between the posterior parietal peritoneum (anteriorly) and transversalis fascia (posteriorly).3 It is separated into three main anatomical spaces:4 

  • Anterior pararenal space, which is bounded by the parietal peritoneum and the anterior leaf of the renal fascia, and includes:
    • Head, neck, and body of the pancreas
    • Ascending and descending colon
    • Duodenum (except for the proximal first segment)

The retroperitoneal space.

  • Perirenal space, which includes:
    • Adrenal gland
    • Kidney
    • Ureters
    • Renal vessels
  • Posterior pararenal space, which is surrounded by the posterior leaf of the renal fascia and muscles of the posterior abdominal wall, and primarily consists of:
    • Fat
    • Blood vessels lymphatics
  • The great vessel space is a fourth, less well-defined space, which lies anterior to the vertebral bodies and psoas muscles and contains:
    • Aorta
    • Inferior vena cava
    • Surrounding fat

The key anatomy to be aware of in each approach is detailed in full on the relevant module in the app.

Determining the level L3/L4 for a direct lateral approach.

What are the key differences between the anterior to psoas (ATP) approach and the lateral lumbar interbody fusion (LLIF)?

In the ATP approach, the incision line should be marked 5 cm anterior to the midline of the L3/L4 disc. Whereas in the standard lateral approach, the incision line is marked at the level of the L3/L4 disc.

The anterior superior iliac spine (ASIS) is a particularly important landmark in the ATP approach. It is used to determine the horizontal and vertical levels of the incision.

Awareness and protection of the ureter is vital in the ATP approach, as the ureter lies anterior to the psoas muscle.

What are the key surgical objectives outlined in the simulation?

  • Patient positioning
  • X-ray usage and anatomy
  • Direct lateral approach
  • Anterior to psoas approach
  • Dilators and retractors
  • Discectomy
  • Trialing and implant insertion

How is the procedure carried out?

Patient positioning affects the neuromonitoring probe and retractor system compatibility and, more importantly, the access and success of implant placement and fusion. The patient must be positioned in true lateral decubitus, with mild extension. The surgeon should verify correct positioning by taking X-rays showing a true direct lateral view of the vertebra.

After determining the level of the incision, an incision is made, long enough to accommodate the retractor system. This is where an awareness of the surrounding anatomy is crucial to the success of the procedure.

Patient positioning in true lateral decubitus with mild extension on the affected side.

The simulation goes through each anatomical landmark that should be avoided, dissected, or protected. This changes depending on the approach taken and is outlined in each module.

A safe corridor is then mapped out to expose the disc. Insertion of the first dilator is followed by the placement of the probe into the middle third of the disc. 

Once an annulotomy wide enough to accommodate the width of the implant has been created, the endplate elevator or Cobb elevator is passed through to the contralateral annulus. Ensuring the contralateral annulus is released, the disc material should then be cleared out, and the vertebral endplates cleaned.

The implant should be packed with bone graft, which is comprised of cancellous or corticocancellous bone, and inserted into the disc space. The implant should cover the vertebral endplate as much as possible and match its concavity.

Verify the implant position, remove the retractor, and close the subcutaneous tissue and skin.

New bone cells grow around the graft. Several months post-surgery, the bone graft should begin to fuse the two vertebrae, forming one solid piece of bone. This usually takes 3–6 months.2 This simulation depicts a one-level fusion, meaning two bones are joined. However, a multi-level fusion is sometimes required, which joins three bones.2

Determining the correct implant size under AP and lateral image intensification.

Insertion of the implant into the disc space.

Don’t break your back while trying to learn about a lateral lumbar interbody fusion. Use the Touch Surgery™ app for free to get up-to-speed on this procedure.

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How to cite this simulation: Gebhard H, Menezes CM. AO Lateral Lumbar Interbody Fusion (LLIF) L3/L4. Touch Surgery Simulations. https://doi.org/10.18556/touchsurgery/2021.s0178. Published Feb. 26, 2021.


1. Salzmann SN, Shue J, Hughes AP. Lateral lumbar interbody fusion—outcomes and complications. Curr Rev Musculoskelet Med. 2017;10(4):539-46.

2. Mayfield Brain & Spine. Spinal Fusion: Lateral Lumbar Interbody Fusion (LLIF). Mayfield Clinic. https://mayfieldclinic.com/pe-llif.htm. Updated Apr. 2019. Accessed Dec. 11, 2020.

3. Knipe H, Jones J, et al. Retroperitoneum. Radiopaedia. https://radiopaedia.org/articles/retroperitoneum. Accessed Jan. 30, 2021.

4. Lambert G, Samra NS. Anatomy, Abdomen and Pelvis, Retroperitoneum. In: StatPearls (website). Treasure Island, FL: StatPearls Publishing; 2020.

5. Vaccaro A, Kandziora F, Fehlings M, Shanmughanathan R. Thoracic and Lumbar Trauma. AO Foundation. Surgery Reference. https://surgeryreference.aofoundation.org/spine/trauma/thoracolumbar.

6. Vaccaro A, Eck JC. Surgical Atlas of Spinal Operations. New Delhi: Jaypee Brothers Medical Publishers; 2013.