At L4/5, the surgical corridor approaches the disc space through the natural space between psoas and the left common iliac vessels. If the disc space is oblique, the skin incision should be one-third below and two-thirds above the anterior projection of the disc onto the abdominal skin. For L4/5, this is usually 20 mm in front of the anterior superior iliac spine toward the umbilicus (Fig. 15.2). A single- or double-level operation can comfortably be done within a 60 mm skin incision utilizing the so-called sliding window in the relaxed abdominal wall [14]. For horizontal lying discs, center the incision on the disc. Three or four levels may require a longer skin incision although in scoliosis cases, access on the concave side can often be done with a surprisingly small incision. For more than two levels, we use the same skin incision but usually split the deeper two muscles twice having extended the external oblique split.

All muscles are incised following the line of their fibers and bluntly dissected.

Iliohypogastric or ilioinguinal nerves may be encountered and mobilized typically beneath the internal oblique muscle. The transversalis fascia is opened as laterally as possible to avoid the peritoneum, be particularly careful in the thin. If one removes the self-retainer retractors at this point, one can usually just feel psoas through the fat with the tip of a finger. This provides an immediate indication of correct direction. The retroperitoneal fat is “paddled” backward using a pair of swabs on sticks. Initial dissection is posterolateral and then vertical to push the retroperitoneal fat (with peritoneum and ureter) anteromedially, until psoas muscle comes into view (Fig. 15.3a, b).

The muscles of the lateral abdominal and pelvic walls should not be stripped clean because of the risk of injuries to the cutaneous nerves traveling in the retroperitoneal space on the abdominal wall [4]. Similarly the psoas fascia should be kept intact as this protects and retains the genitofemoral nerve (GFN) on its anterior surface. The GFN is usually seen and retracted with psoas.

At this stage, psoas (and GFN) is gently retracted posteriorly with a handheld retractor and psoas followed carefully around its anterior surface to reach the spine. Be aware that the psoas may overhang the spine in the lateral position, so follow it closely back to the spine.

In order to expose the spine, gentle dissection is required on the medial aspect of psoas through retroperitoneal fat and fascia, retracting peritoneum with ureter on its surface and passing laterally to the great vessels.

A single L-shaped blade Curvy™ retractor (Relax Retractors, Sydney, Australia) is applied medially with its back retracting the retroperitoneal fat while protecting the vessels and its orthogonal blade pushing psoas posteriorly. A separate straight handheld retractor is used to retract psoas (Fig. 15.4a, b).

A long smooth-ended dissector and a Yankauer suction tip are useful to dissect the loose fatty connective tissue between psoas and the fat over the vessels, to reveal the spine, the disc space, the sympathetic chain, and the segmental vessels. Occasionally the fascia needs to be torn. At this stage, other anatomy on display may include lymphatics, lymph nodes, small bridging vessels, and occasionally tiny nerve branches of unclear origin. Below the L4/5 disc, the iliolumbar vein may be seen. On the spine, one can see the vertical fibers of the anterior longitudinal ligament (ALL). It saves time to identify the disc space early by feel (it’s the ridge) or x-ray. One then concentrates solely on this area as diverging from this path may lead to unpleasant encounters with segmental vessels (in the valleys). The Curvy™ retractor is positioned medially over the disc space with the “leg” over the disc and the “foot” lying in a medial-lateral direction protecting the iliolumbar vein inferiorly. The Curvy™ fixation screw can be inserted into the target disc for temporary fixation, freeing up a hand (Fig. 15.5).

The most medial psoas attachments to the disc margin are separated with a dissector or Cobb elevator to allow more psoas retraction. The disc is then incised with a knife to about 20–25 mm behind the ALL. A limited discectomy is performed, and the reverse tooth of the selected G clamp blade is placed beneath the lateral uncut annulus. This blade is attached to rest of the G clamp which is then compressed to retract psoas. The extent of the cut in the annulus limits the posterior extent of psoas retraction, which provides an end point for retraction to avoid compression of the lumbar plexus.

The sympathetic chain can be mobilized usually medially by dividing its tiny branches, the rami communicantes (Fig. 15.6).

At this stage, with psoas retracted and the end plates identified, the initial retraction with Curvy™ blade on the disc is improved with repositioning of the blade lateral to the sympathetic trunk with its screw inserted into the L5 body to secure the blade. The medial blade usually lies close to the posterior edge of the ALL (Fig. 15.7).

A lateral x-ray is then taken to verify the limit of the posterior retraction, as retraction to about mid body is sufficient and adequate for disc preparation; this can be adjusted if required. Only a limited amount of psoas retraction is needed for thorough disc preparation and contralateral release. Both retractors, the Curvy and G clamp are now stable without requiring table mounting.

The iliac vessels, if seen are protected under the medial retractor blade.

The iliolumbar vein at L5 usually sits in the middle of the body; rarely it may overlie the disc space in which case it can be ligated between vessel clips. The vessel is always divided prior to retracting the common iliac vein to expose the L5/1 disc or a transitional L4/5 level. It is noteworthy that the lateral position and oblique approach reveal a greater length of the iliolumbar vein making ligation much easier than during supine L4/5 ALIF surgery.

This phase of the operation is the same as per transpsoas except that surgeons need to be constantly aware that the tools are oblique and over insertion through the annulus can enter the foramen or even the spinal canal. Osteophytes can be removed by means of heavy rongeurs.

Initial discectomy with large pituitary rongeurs and disc curettes is performed.

“Dingo” instruments are shaped so that the surgeon is still working orthogonally to the spine as the handle and the terminal end of the instrument are in the same line. The offset (dog-leg) serves to avoid the iliac crest.

A Cobb elevator is placed in the disc space and under AP x-ray control, impacted through the contralateral annulus and any bridging osteophytes (Fig. 15.8).

After completion of the discectomy and with disc distraction, the oblique trajectory also allows for potential direct visualization of the thecal sac and contralateral foramen to allow decompression under direct vision. Pituitaries at these depths are best used slowly.

Disc removal and Endplate preparation are done with great care to ensure complete disc removal (avoids pushing disc out on the contralateral side) and to preserve the end plates.

Dingo design implant inserters allow for a standard lateral cage or hyperlordotic implant to be inserted across the disc space gaining bilateral cortical endplate coverage (Figs. 15.9, 15.10, and 15.11).