COMPARATIVE STUDY OF LUMBAR PLEXUS PATH ON THE LEFT AND RIGHT SIDES THROUGH THE PSOAS MUSCLE

ABSTRACT Objective: Spine surgery with a minimally invasive lateral approach and validate possible anatomical differences between the right and left sides. Methods: Four measurements (cm) were taken on 38 cadavers: the distance between the lumbar plexus and the transverse process (L4-L5) and the distance between the lumbar plexus and the midline of the lumbar spine, both on the right and left sides. Results: The mean distance between the lumbar plexus and the transverse process of L4-L5 was 1.03 cm and the distance to the midline was 3.99 cm for the right side. The averages of the left side were 1.13 cm and 3.38 cm, respectively. There is statistical difference between the sides (p<0.05) using the non-parametric Wilcoxon test. Conclusions: The authors suggest that the transverse process might be used as an anatomical landmark to define the surgical approach through the psoas muscle. Level of Evidence IV; Cadaveric study.


INTRODUCTION
The anterior approach to the lumbar intervertebral disc has several biomechanical advantages over the posterior approach, but also presents technical difficulties in the management of vascular structures not always familiar to the spine surgeon.Lateral access emerged as an alternative to these two approaches, reducing the vascular risk and also offering ample access to the intervertebral disc.][3][4][5][6][7] A part of the technique involves inserting the surgical instrumentation through the psoas major muscle, which may incur some type of injury, the most serious of which would be damage to any of the nerve structures that pass inside of it, the lumbar plexus. 8he lumbar plexus can be found crossing the fibers of the psoas major muscle.It is part of the lumbosacral plexus and is formed by ventral branches originating from the four primary lumbar nerves with the contribution of the last thoracic branch.They originate as motor and sensory nerves and most of them have both components present..10It is important to note that the lumbar plexus emerges from the lumbar nerve roots dorsally and close to L4 where it begins to become ventral, the only exception being the genitofemoral nerve, anteriorized in relation to the others.This fact will be important in the surgical approach since safety zones are created for the transpsoas approach and the level most frequently accessed is L4-L5, precisely where the nerve roots move anteriorly. 1,5,11here is a specific anatomical relationship between the lumbar plexus and its position within the psoas major muscle, and it remains relatively constant. 9However, it is difficult to visualize the location of the nerves of the lumbar plexus with respect to the space of the intervertebral disc from a lateral view, that is, a safe and easily visible reference to assist the surgeon during surgery is lacking. 5he objective of this study was to propose a safe and practical anatomical parameter that can be used as a reference in MILA surgeries and to validate the possible anatomical differences between the right and left sides of the lumbar plexus.

METHODS
The study was conducted by means of measurements of 18 adult cadavers from the anatomy laboratory of the Pontifícia Universidade Católica do Paraná (PUC-PR) and 20 cadavers from the Universidad Pública de El Alto (La Paz, Bolivia) (n=38), taken by 2 evaluators.
Four measurements were taken in centimeters: the distance between the first nerve of the lumbar plexus and the transverse process of the adjacent L4-L5 and the distance between the first nerve of the lumbar plexus and the midline of the spine in L4-L5, both measured on the right and left sides.
The measurements were taken after dissection of the lumbar spine by lateral approach, exposure of the psoas muscle and its elevation, followed by identification of the lumbar plexus and its path over the L4-L5 disc, and measurement using the pachymeter (Figure 1).

RESULTS
The results of the measurements were described by means, medians, minimum values, maximum values, and standard deviations.The non-parametric Wilcoxon test was used to compare the two sides in relation to the two measurements analyzed.The condition of normality of the variables was evaluated by the Kolmogorov-Smirnov test.Values of p<0.05 indicated statistical significance.The data were analyzed with the IBM SPSS Statistics v.20 software.
The results of the measurements and their statistical evaluation can be seen in Table 1.
In 17 of the 38 cadavers evaluated (44.7%), the measurements of both sides were equal.In 14 (36.8%), the measurements of the left side were greater than the measurements of the right side and in 7, (18.4%) the measurements of the right side were greater than the measurements of the left side, as observed in Figure 2.

DISCUSSION
There is agreement that the safest approach is in the anterior region of the vertebra.However, few studies determine the exact distance and none define a useful reference during the surgical procedure.
The first study to refer to the anatomical positioning of the lumbar plexus was published in 1996 by Hasegawa et al., who determined for the first time that the lumbar plexus is positioned more anteriorly as it branches out. 12Similarly, Moro et al., in 2003 and Benglis et al. in 2009 understood that, because of this anteriorization, the surgical safety zone in L4-L5, when compared to the other spinal safety zones, is much smaller. 2,13he first distance was proposed by Regev et al., in 2009, who found a distance of 19.5 mm between the lumbar plexus and the posterior margin of the terminal plate in L4-L5. 11In that study, a safety zone of 20 mm was defined that was and still is used as a parameter for MILA surgical instrumentation.
Here, it is fitting to question studies, like that of Kepler in 2011, which defined the risk of lesion as 44% on the right side and 21% on the left side. 14With this, it is understood that the value of 20 mm is clearly overestimated and can lead to more plexus injuries than anticipated.
The distance found in this study is much less, which is consistent not only with clinical studies that evaluate complications, but also with other publications, like that of 2011 by Lu et al. that defined a distance between the lumbar plexus and the transverse process of 0.49 and 0.84 cm in L4. 9 The description of each of the studies and their respective results are shown in Table 2.
Notice that most of them, with the exception of 3, do not provide an exact value of where to position the surgical instrumentation.][16][17][18] The method of each study is directly related to its respective results.Studies conducted with magnetic resonance can have significant differences from those with direct measurements in cadavers, which may explain the differences between the values.This study has several limitations, including the small sample size and the method itself that can determine small changes in the measurement due to the position of the pelvis and lower limbs, anatomical dissection, and tissue retraction.
It is understood that the most critical zone for intervention is L4-L5, since the safety corridor is very small (when compared to the others), especially on the right, where there is a risk of nerve lesion posteriorly and lesion of the vena cava anteriorly.In this study, we found values of 1.03 cm for the right side and 1.13 cm for the left side as the margins of safety From the transverse process of the closest vertebra.It should be emphasized that on the left, where most approaches are performed, the lumbar plexus is slightly anteriorized and the point of entry for the instrumentation should be anterior to the lumbar plexus.If we consider that in some cases this distance was greater than 2.5 cm, this distance would be the safest region for initiating the dissection of the L4-L5 intervertebral disc and identifying the plexus.In this way, a work area posterior to the lumbar plexus, where the risk of postoperative deficit is high, would be avoided.This safe area can be defined by the arithmetic means of all the measurements (Figure 3).
The statistical analysis showed a statistically significant difference between the right and left sides (p=0.020).Therefore, greater attention and a slight anteriorization of the point of entry in L4-L5 on the left are required.
We suggest the transverse process as a useful and practical surgical reference during the execution of the MILA technique.
The lumbar plexus of L1 to L5 begins more dorsally and at L4 begins its ventralization.
Moro T et al. 13 2003 Cadaver -30 adult cadavers The distance between the anterior and posterior margins of the vertebral body was divided into 4 equal zones (I, II, III, and IV).
The lumbar plexus is located in zone II and below in L4L5.The genitofemoral nerve was found in zone I.
The safe working zone is above L4L5, excluding the genitofemoral nerve (which is anteriorized before the others).

Benglis DM et al. 2 2009 Cadaver -3 adult cadavers
The relationship between the location of the plexus in the posterior terminal plate and the total length of the disc in radiographical images.

28% (L4L5)
They suggest that the lumbar plexus begins more dorsally (L1L2) and has a more ventral direction during its course, especially in L4L5.Therefore, there is a higher risk of lesion of the genitofemoral nerve and the lumbar plexus.

MRI -100 adults
Anterior to posterior margin of the terminal plate of each vertebral disc.

19.5mm
The safety zone between L4 and L5 is quite reduced in comparison to the other vertebrae (13.1%).Thus, an error in posterior interventions could damage nerves or in anterior interventions, the blood vessels (particularly on the right).
Uribe JS et al. 16 2010 Cadaver -5 cadavers The space between the anterior and posterior margins of the vertebral body was divided into 4 equal zones.
The lumbar plexus is ideally located in zones III and IV (L4L5) and the genitofemoral nerve was found in zone I.
They suggest that the safe work zone would be in zone III (posterior middle fourth) and in L4L5 between zones II and III.
Guérin P et al.The anterior edge of the psoas major muscle should be used as a reference to estimate the position of the lumbar plexus -when the anterior edge of the psoas muscle is 10 mm anterior to the AIP, the position of the lumbar plexus will be approximately 20 mm posterior to the AIP.Patients with neurological structures at risk were defined as those who had lumbar plexus/femoral nerve less than 20 mm from the anterior intervertebral plane.
Lu S et al.Anterior posterior diameter of each vertebral body (measured between the anterior edge of the foramen and the anterior edge of the vertebral body).
No nerve roots were found in the anterior 33% of the L4L5 intervertebral space.
The safe working zone would be the anterior half of the intervertebral disc.

Figure 1 .
Figure 1.Marking the measurement site.Cadaver in dorsal decubitus with the cephalic region on the right.A indicates the lumbar plexus at its exit, B marks the transverse process of L4, and C, the psoas major muscle folded back.The measurement is represented by the line.In the image, the measurement of the left side is being taken.Source: Produced by the author.

Figure 2 .
Figure 2. Representation of the distribution of the values of the distance between the lumbar plexus and the transverse process of L4-L5.
Distance from the lumbar plexus to the L4-L5 transverse process (cm)

Figure 3 .
Figure 3. Arithmetic means of the distances found in the study, comparing the right and left sides of each specimen.

Table 1 .
Statistical results in centimeters.

Distance from the lumbar plexus to the L4-L5 transverse process p value* n Mean Median Minimum Maximum Standard Deviation
*Non-parametric Wilcoxon test, p<0.05.

Table 2 .
Results of the main similar studies already conducted.