The transpsoas approach, which can also be used in the setting of revision arthroplasty or in the conversion to lumbar arthrodesis, involves accessing the anterior spine through the iliopsoas muscle. This way, dissection around the retroperitoneal vessels, as well as in most abdominal structures (e.g., visceral organs, ureters, hypogastric plexus), can be avoided. Risk to the lumbar nerves and plexus is present with this approach. Therefore, careful neurophysiologic monitoring is critical, and comprehensive anatomic data on the lumbar plexus are important .
Location of the lumbar plexus and its clinical meaning
Lumbar plexus nerves were found to be arranged regularly: the nerve roots exit the intervertebral foramina and go down from L1 to L5, the upper nerve roots are always lateral, and the lower nerve roots are opistho-medial. Such anatomical arrangement indicates that with the anterior lumbar approach, injury to the lateral lumbar nerve at the lower level of the lumbar vertebrae would cause symptoms related to lower lumbar plexus injury. This was kept in mind when the injury location of the lumbar plexus was determined. The lumbar nerve goes downward from an angle of 20° at the upper level to 40° at the lower level. According to such an arrangement, dissection of the major psoas muscle should be done at the lower border of the transverse process to avoid injury to the nerve.
Selection of the landmark for anterior approach to the lumbar vertebrae
The lumbar plexus serves as one of the most important anatomical structures of the anterior approach, and its injury greatly affects patients. Therefore, its location should be considered from the beginning of the approach decision and before the exposure of the lumbar vertebrae. However, the lack of reliable anatomical landmarks at the posterior belly presents problems. This study confirms that the lumbar plexus is closely related to the transverse process. The transverse process, located between the major psoas muscle and the lumbar quadrate muscle, could serve as the landmark for the lumbar plexus during operation. Several important notes were remembered in the course of the study: (1) dissection and exposure of the major psoas muscle should be marked by the lateral border of the transverse process and vertebra pedicle; (2) the self-help retractor should not be placed at the superior articular process and transverse process; (3) the proximal end of the vertebra pedicle should always be approached from the dorsal surface of the lumbar vertebrae; and (4) even though it is easy to approach the lumbar plexus, the lumbar nerve root should be exposed completely to avoid injury to the lumbar plexus at the extreme lateral approach to the intervertebral discs.
Relationship of the major psoas muscle and the lumbar plexus
After exiting the intervertebral foramen, majority of the lumbar nerves traveled through the major psoas muscle. Some of them formed the plexus within the major psoas muscle. Branches then exit from the anterior surface or lateral border of the muscle. Kirchmair et al.  studied the relationship of the lumbar plexus and the psoas muscle and demonstrated that there are two locations of the lumbar plexus that may be encountered in clinical practices: locations within and posterior to the psoas major muscle. However, the latter situation represents a minor variant. At the anterior approach to the lumbar vertebrae, sites that might complicate an injury of the femoral nerve are generally located in the part inferior to the peritoneum, between the peritoneum and the iliac muscle, or between the major psoas muscle and the iliac fascia. Such injuries to the femoral nerve are mainly caused by major psoas muscle hematoma. Sotiris  reported two cases of the anterior approach of lumbar vertebral fusion that complicated a tension injury of the femoral nerve. They determined that this was caused by the extended duration of tension of the major psoas muscle during operation, which resulted in insufficient space for the activity of the femoral nerve. A similar situation was observed in this study. This encouraged us to maintain a relaxed major psoas muscle by bending the hip to avoid excessive tension of the major psoas muscle during anterior approach of the lumbar vertebrae. Thus, there was minimal consequence on the femoral nerve. The lumbar plexuses were wrapped by fascia at the deep surface of the major psoas muscle and tightly connected with it. Owing to this, an injury might occur during manipulation of the major psoas muscle. Excision of the extreme-lateral intervertebral discs requires attention to the relationship of the lumbar plexus to the major psoas muscle. If such disc is inferior to L4, lateral retraction of the major psoas muscle along the transverse process by the interspaces of the major psoas muscle and the lumbar quadrate muscle is required. The lumbar plexus is located just posterior to the major psoas muscle, so as much exposure as possible of the lumbar nerve root was required. The lumbar plexus was found to be closely related to the major psoas muscle in the view of body surface positioning and operational mark. This relationship should always be considered during dissection or traction.
Significance of the anterior approach safety zone
Mono et al.  studied the problem of the safety zone and the lumbar plexus via retroperitoneum by laparoscope. The safety zone was concluded to be L2-L3, as well as L4-L5, if injury to the genifemoral nerve is ignored. At L5-S1, the risk of injury was higher if the routine approach was chosen because of the iliac vessels present at the lateral surface. Another challenge was going through the space between the major psoas muscle and the lumbar quadrate muscle and drawing the major psoas muscle forward to reach the lateral to lumbar vertebra. The nerve roots of L4 and L5, the femoral nerve, and the obturator nerve presented and formed a dangerous area for this approach. Thus, sufficient exposure of the lumbar plexus is required. Entering the lumbar vertebrae required pulling away or incising the major psoas muscle, but it was impossible to localize the exact point of the lumbar plexus during operation. Hence, the position of the lumbar plexus versus the major psoas muscle was considered critical. In this study, entering the retroperitoneum was found to reveal the anterior and posterior borders of the major psoas muscle. According to this finding, the position of the lumbar plexus versus the anterior and posterior borders was thought to serve as a landmark to incise the major psoas muscle and safely enter the lumbar vertebrae or intervertebral discs. Choosing the correct place to incise the major psoas muscle is very important. The approach to such a place should avoid injury to the lumbar plexus during the process of dissecting or passing through the major psoas muscle. According to the sectional anatomy on the lumbar plexus and major psoas muscle done in this study, the location of the lumbar plexus at the major psoas muscle was comparatively constant. This made it possible to determine the 2/3 ventral part of the major psoas muscle and to reach the safe area of the intervertebral space. The lumbar plexus was then drawn backwards. It is suggested that 1/3 dorsal parts of the major psoas muscle must be retained to avoid injury to the nerve roots. In 2004, Bergey  reported on the anterior approach to the major psoas muscle at the lumbar vertebrae by laparoscope. He emphasized incising the major psoas muscle rather than incising at the medial border of the major psoas muscle then drew it. Thus, injury to the lumbar plexus caused by excessive tension of the major psoas muscle can be avoided. He also recommended retaining the 1/3 dorsal part of the major psoas muscle to avoid damage to the nerve roots. Since at the level of L4-L5 intervertebral discs, nerve roots of L3 pass over the lateral border of the intervertebral discs, injury at the 1/2 ventral part of the major psoas muscle was possible. To minimize the risk of Extreme lateral interbody fusion (XLIF) to the lumbar plexus, the dilators should enter the psoas at the junction of the anterior and middle thirds. A radiolucent blade or tubular retractor system is placed over the largest dilator and docked on the lateral aspect of the disc space . These results were similar to our findings.
At the lateral approach, the location of the L4-L5 nerve is either near the inferior border of the vertebral body or near the inferomedial to the major psoas muscle, and not at the upper lumbar vertebrae. Regardless of dissection of the medial border of the major psoas muscle or dissection through the major psoas muscle, exposing the lumbar plexus to reach the lateral and anterior aspect of the vertebrae above L4 was not necessary. This is as long as surgeons are aware that the lumbar plexus is tightly adhered to vertebral body and that appropriate traction should be retained to avoid injury to the lumbar plexus.
Significance of 3D reconstruction of the lumbar plexus
Unfortunately, it is difficult to determine the location and adjacent space relation of the lumbar plexus from sectional images of the anatomical layers. If surgeons want to visualize such space location from a two-dimensional image, they would have to speculate carefully and precisely, but this method is problematic. Currently, this method takes advantage of the 3D reconstruction technique to review the anatomical structures of the lumbar plexus from any angle and thus understand the lumbar plexus and its surrounding tissues. The anatomical data presented herein might prove useful for the anterior approach to the lumbar plexus and help avoid injury during surgery.