space was more safely approached from a more anterior trajectory, namely, at the midpoint of the vertebral body. Benglis et al., Banagan et al., and Guerin et al. likewise noted a ventral progression of the lumbar plexus with lower disk levels [ 25 , 26 , 29 ].

AP fl uoroscopic imaging is obtained to ensure a true lateral position. On AP the spinous pro­cesses at the levels of interest should be perfectly centered between their respective pedicles. If there is a large discrepancy, the patient is reposi­tioned accordingly. If there is a minor discrep­ancy, the bed can be rotated to obtain a true AP

Though injury to the plexus is most frequently encountered while dissecting through the psoas muscle, the nerves traveling outside of the psoas in the retroperitoneal space are also at risk of injury during the abdominal dissection. Dakwar et al. performed cadaveric studies looking at the trajec­tories of the major motor and sensory branches of the lumbar plexus outside of the psoas muscle and within the retroperitoneum and abdominal wall [ 28 ]. They found that four plexus branches were at risk of injury during dissection through the abdom­inal wall and retroperitoneal space: subcostal, ilio­hypogastric, ilioinguinal, and lateral femoral cutaneous nerves. Injury to these nerves can occur during the fl ank or lateral incision and can lead to abdominal wall paresis and sensory defi cits in the corresponding dermatome.

In order to avoid injury to these nerves, once the external oblique muscle fascia is sharply opened, blunt muscle dissection should be per­formed until the retroperitoneal cavity is identi­fi ed. Bovie cauterization and bipolar must be limited, and any electrocautery should be limited to bipolar electrocoagulation. If a nerve is dis­covered during dissection, it can be carefully dissected and mobilized. Once the retroperito­neal space is encountered, blunt dissection of the peritoneal contents from posterior to anterior is performed until the transverse process and psoas

muscle are encountered [ 28 ]. If using fi nger dis- section, one must be careful not to avulse any presumed fi brous bands as they could be nerves.

Some dilators have directional stimulation that allows the operator to test where the lowest threshold (and thus the nerve) is located relative to the dilators. If a threshold less than 10 mA is encountered, the dilators can be repositioned away from the stimulated nerves. Tohmeh et al. prospectively studied the EMG threshold values of 102 consecutive patients undergoing LLIF. They found that 55.7 % of the cases had alert-level EMG feedback (thresholds less than 10 mA) with initial dilator placement, which lead to repositioning. Transient motor defi cits occurred in three patients (2.3 %) with all resolv­ing by 6 months. Of note, two of the three patients had spontaneous EMG (S-EMG) activity in the affected myotome during the surgery while trial spacers were being inserted. Despite these prom­ising fi ndings, t-EMG is limited to motor nerves only leaving the sensory nerves vulnerable. Thus, Tohmeh et al. found that 17.6 % of their patients experienced thigh sensory defi cits immediately postoperatively. However, t-EMG does not warn of stretch injury, and false negatives may occur from shunting of the stimulation through the blades [

When further visualization of the disc space after inserting the working retractor into the psoas muscle, it is important to only open the anterior blade. Opening the posterior blade can lead to stretch injury of the plexus and/or can crush the plexus between the blades of the retractor and the transverse processes. Also the length of surgery has been found to correlate with increased extremity symptoms, presum­ably from longer periods of psoas and plexus retraction [ 8 ]. Thus, once the retractor is opened, the disc space must be prepared in a timely fashion.

position as determined by fl uoroscopy, the psoas muscle is bluntly dissected in a longitudinal fash­ion exposing the disc space (Fig. 12.4 ). Any nerves will likely be clearly visualized or at least amenable to “mapping” of the surface of the psoas muscle to detect the proximity of any lum­bosacral plexus nerves.

main function is limiting psoas muscle and plexus injury. During this approach, the ante­rior aspect of the psoas muscle is exposed, dis­section is performed between the anterior border of the psoas muscle and the sympathetic trunk, and the entire psoas muscle is retracted posteriorly while the sympathetic trunk is retracted anteriorly [ 39 , 40 ]. This has the advantages of being further away from the plexus than the standard lateral approach. Also, no dissection through the psoas muscle is needed limiting any injury to it. However, this approach has its own set of challenges includ­ing: manipulations of the sympathetic trunk, rocking of the inserter posteriorly to obtain correct positioning of the cage, and closer proximity to the great vessels.