ORIGINAL ARTICLE
`
`The Relationship of Intrapsoas Nerves During a Transpsoas
`Approach to the Lumbar Spine
`Anatomic Study
`Daniel K. Park, MD,* Michael J. Lee, MD,w Eric L. Lin, MD,z Kern Singh, MD,*
`Howard S. An, MD,* and Frank M. Phillips, MD*
`
`Study Design: A cadaveric study.
`
`Objective: To define the relationship of the lumbar exiting nerve
`root and trunks within the psoas muscle with reference to the
`radiographic center of the intervertebral disc, the recommended
`disc access point for the minimally invasive lateral transpsoas
`approach.
`
`Summary of Background Data: The transpsoas approach to the
`lumbar
`intervertebral body disc is a minimally invasive
`approach used for interbody fusion. This approach carries the
`potential risk of injury to the intrapsoas nerves. There are no
`published studies investigating the locations of the intrapsoas
`neural elements with reference to the transpsoas access corridor
`developed during minimally invasive lateral approaches to the
`disc.
`
`Methods: Ten human cadaveric specimens were analyzed. A
`guide wire was placed in each disc space center under lateral
`fluoroscopic guidance as has been recommended for disc access
`in the transpsoas fusion technique. Using calipers, the distances
`from the exiting nerve and trunk to the wire were measured.
`
`Results: In general, the nerve trunk was a mean of 14 mm
`posterior to the center of the disc and was a mean of 5 mm closer
`to the center of the disc than the exited nerve. The trunks were
`closer to the center of the disc caudally in the lumbar spine, with
`the distance ranging from a mean of 16.4 mm at L2-3 to 10.6 mm
`at the L4-5 level. The intrapsoas location of the exited nerve was
`less variable and was greater than 15 mm from the projected
`center of the disc. At L4-5, the trunk approximated the center of
`the disc in 15% of specimens.
`
`Conclusion: This study suggests that the intrapsoas nerves are a
`safe distance from the radiographic center of the intervertebral
`disc in a majority of cases; however, anatomic variations in the
`
`Received for publication January 2, 2009; accepted April 7, 2009.
`From the *Rush University Medical Center, Chicago, IL; wUniversity of
`Washington Medical Center, Seattle, WA; and zSt Jude’s Medical
`Center, Fullerton, CA.
`Source of Support: NuVasive Corporation.
`Permission to reproduce copyrighted material.
`IRB equivalent approved.
`Reprints: Daniel Park, MD, Rush University Medical Center, Midwest
`Orthopedics, 1725 West Harrison Street, Suite 1063, Chicago, IL
`60612 (e-mail: danparkmd@gmail.com).
`Copyright r 2010 by Lippincott Williams & Wilkins
`
`J Spinal Disord Tech  Volume 23, Number 4, June 2010
`
`location of these nerves place them at injury risk in a small
`number of cases. These results suggest that neural monitoring
`whereas traversing the psoas may be important to enhance the
`safety of
`the
`transpsoas approach. Care
`is particularly
`warranted at the L4-5 level.
`
`Key Words: lumbar nerve trunk, lumbar plexus, psoas muscle,
`lumbar interbody fusion, trans-psoas approach, nerve injury
`
`(J Spinal Disord Tech 2010;23:223–228)
`
`Lumbar spine fusion has been successfully used in the
`
`treatment of instability, deformity, and degenerative
`disc disease.1–4 In recent years,
`interbody fusion has
`become a popular technique with touted benefits that
`include eliminating the disc as a potential pain generator,
`high rates of successful fusion, and restoring the inter-
`lordosis.5–8 Traditional
`vertebral height and lumbar
`techniques to achieve interbody fusion include anterior
`lumbar interbody fusion and posterior or transforaminal
`lumbar interbody fusion. In addition, a lateral retro-
`peritoneal approach to the lumbar spine with posterior
`retraction of the psoas muscle off the vertebral bodies has
`been used.9,10
`Recently, a novel minimally invasive retroperitoneal
`direct lateral transpsoas approach to interbody arthrod-
`esis has been described.11–13 This technique, first termed
`as extreme lateral intebody fusion (XLIF), provides the
`ability to achieve an interbody arthrodesis whereas
`avoiding the risks implicit with an anterior or posterior
`approach to the interbody space. Furthermore,
`this
`technique uses a minimally invasive approach through
`the retroperitoneal space. The theoretical advantages of
`minimally invasive surgery include less tissue trauma, less
`postoperative pain, shorter hospital stay, and faster
`return to activities of daily living.14 In addition, the
`lateral minimally invasive approach does not require the
`need for an access surgeon. During the lateral approach
`the radiographic center of the disc is identified. Then by
`using various dilators, the intervertebral disc space is
`accessed through the psoas muscle. The expansion of a
`blade retractor system then provides a working channel
`and direct visualization of the disc. Standard intradiscal
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`Park et al
`
`J Spinal Disord Tech  Volume 23, Number 4, June 2010
`
`FIGURE 1. Radiograph with guide wire placed in the middle of the disc (A). Gross picture of the guide wire placed through the
`psoas muscle (B).
`
`instruments are then used for disc space preparation,
`arthrodesis, and interbody implant insertion.
`When approaching the intervertebral disc via a
`transpsoas approach, the exiting nerve from the inter-
`vertebral foramen and trunks of the lumbar plexus that
`reside in the psoas are at risk. In general, the neural
`structures reside in the posterior third of the psoas
`muscle15–18; therefore, transpsoas access has been recom-
`mended through the anterior portion of the muscle to
`avoid neurologic injury. Although anatomic studies have
`defined the exiting nerve and plexus, the precise location
`of the nerves at various levels in the lumbar spine with
`reference to the radiographic center of the intervertebral
`disc have not been described.
`The purpose of
`this study was to define the
`relationship between the lumbar exiting nerve roots and
`trunks within the psoas muscle and the working corridor
`used for the lateral approach to the disc. Measurements
`were on the basis of projected center of the disc from a
`lateral fluoroscopic image as would be used during the
`typical
`lateral transpsoas interbody fusion procedure.
`This study also examines the effects of hip flexion and
`extension on the relationship between the disc center and
`the intrapsoas neural tissues.
`
`METHODS
`Ten fresh-frozen cadaveric specimens (9 females and
`1 male) were obtained for this study. Five of these
`specimens had intact proximal femurs in addition to the
`torso and pelvis whereas the other 5 specimens (1 male)
`only contained the torso and pelvis. Before dissection, the
`specimens were slowly thawed at room temperature. The
`specimens were then placed in the lateral decubitus
`position. The lateral retroperitoneal approach was then
`used to access the psoas muscle and lumbar spine. With
`the typical transpsoas interbody fusion procedure, the
`center of the disc is targeted with sequential dilators
`guided by neural monitoring. To mimic the targeted
`docking location, a guide wire was placed colinear with
`the disc space and in the anterior-posterior and cranio-
`caudal center of the L1-2, L2-3, L3-4, and L4-5 disc
`spaces under lateral fluoroscopic guidance (Fig. 1). To
`maintain the anatomic relationship of the lumbar nervous
`tissue to the psoas muscle, minimal dissection was then
`carried out to identify the exiting nerve nerve and the
`associated lumbar trunk at each disc level. The shortest
`distance from the guide wire to the respective exiting
`nerve/trunk was measured using digital calipers (Mitu-
`toyo, Kanagawa, Japan) (Fig. 2). At each level except
`
`FIGURE 2. Guide wire placed at a safe distance away from the nervous tissue (arrow) (A). The shortest distance between the wire
`and the nervous tissue was measured (double arrow) (A and B). In some cases, the wire was placed through nervous tissue (*) (B).
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`J Spinal Disord Tech  Volume 23, Number 4, June 2010
`
`Anatomic Study of the Intrapsoas Nerves
`
`FIGURE 4. Illustration demonstrating the theoretical distance
`from the guide wire at the center of the MaXcess dilators to
`the posterior margin of the posterior blade. This distance is the
`radius of the last dilator (6 mm) plus the width of the posterior
`blade (1.27 mm) = 7.27 mm. As a measure of neural safety,
`this number was rounded up to 8 mm as the minimum
`required distance from the center that the neural structures
`must be found without having to be retracted during the
`surgery.
`
`the psoas muscles are separated using
`the fibers of
`sequential dilators, which split the psoas and spread the
`surrounding tissue. An access retractor is inserted over
`the final dilator. The XLIF (NuVasive, Inc, San Diego,
`CA) retractor system used by the senior author (F.M.P.)
`has an outer diameter of 12 mm. The posterior blade of
`the access retractor has a thickness of 1.27 mm. On the
`basis of the size of the final retractor (Fig. 4), neural
`structures more than 8 mm from the center of the disc
`(and therefore the center of the access retractor) would
`not be at risk of direct traction by the retractor blade. If a
`neural structure was within 8 mm of the disc a higher
`degree of neural retraction would be required for the
`retractor to be safely placed. Therefore if the distance
`from the nerve tissue to the center of the disc was less
`than 8 mm, it was classified as ‘‘at risk’’. The data were
`then compared using unpaired 2 sample t
`testing.
`Statistical significance was defined as a P value less
`than 0.05.
`
`FIGURE 3. Anatomic illustration of the exiting lumbar nerve
`and trunks and identification of how distances of each were
`measured from the disc center. Black arrow as trunk and gray
`arrow as exiting nerve.
`
`L1-2, where the lumbar plexus is not yet formed, 2 neural
`structures were identified. The corresponding exiting
`nerve root was first identified, and the associated nerve
`trunk, which comprised a plexus of the superior exiting
`nerve was then identified (Fig. 3). Both the left and right
`sides were dissected in a similar manner. Lastly,
`if
`proximal femurs were intact, measurements were made
`with the hip in extension and flexion. Because the guide
`wire might impede the movement of the psoas muscle, the
`wires were individually removed when hip flexion and
`extension was performed and replaced in the middle of
`the disc under fluoroscopic guidance.
`The data were then analyzed for occurrences of
`actual nerve penetration and risk of nerve injury by the
`guide wire. When performing the transpsoas procedure,
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`Park et al
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`J Spinal Disord Tech  Volume 23, Number 4, June 2010
`
`TABLE 1. Average Distance of Nervous Tissue to Disc and
`Percentage of Nervous Tissue ‘‘at Higher Risk’’ or Pierced by
`the Initial K Wire
`
`Disc
`Space
`
`L1-2
`L2-3
`L2-3
`L3-4
`L3-4
`L4-5
`L4-5
`
`Nerve
`
`L1 exiting
`Trunk
`L2 exiting
`Trunk
`L3 exiting
`Trunk
`L4 exiting
`
`Average
`(mm)
`
`Standard
`Deviation
`(mm)
`
`Percent
`Closer
`Than 8 mm
`
`Percent
`Pierced
`by K Wire
`
`0
`5
`0
`5
`0
`25
`5
`
`0
`0
`0
`5
`0
`15
`0
`
`18.7
`16.4
`20.9
`14.9
`20.0
`10.6
`16.2
`
`4.2
`4.1
`4.5
`4.9
`4.4
`7.0
`5.2
`
`RESULTS
`
`Location of the Exiting Nerve and Roots
`In general,
`the nerve trunk was a mean of
`14.0 ± 5.9 mm posterior from the center of the disc and
`was 5 mm closer to the projected center of the disc based
`off a lateral radiograph than the exited nerve root (mean;
`19.0 ± 4.8 mm) (Table 1, Fig. 5). The nerve trunks were
`closer to the disc center caudally in the lumbar spine with
`this distance ranging from a mean of 16.4 ± 4.1 mm at
`L2-3 to 10.6 ± 7.0 mm at L4-5 level. The intrapsoas
`location of the exited nerve root was less variable and in
`general was greater than 15 mm from the projected center
`of the disc at all anatomic levels. In 1 case, the L2-3 nerve
`trunk was penetrated during fluoroscopically guided wire
`placement at the L3-4 disc, whereas 3 (15%) L3-4 nerve
`trunks were pierced although by accessing the L4-5 disc
`level.
`
`Neural tissue ‘‘at risk’’, that is close enough to the
`center of the disc that direct neural retraction is required
`
`TABLE 2. Distance of Nervous Tissues Comparing From Right
`to Left
`
`Disc
`Space
`
`L1-2
`
`L2-3
`
`L2-3
`
`L3-4
`
`L3-4
`
`L4-5
`
`L4-5
`
`Nerve
`
`L1 exiting
`
`Trunk
`
`L2 exiting
`
`Trunk
`
`L3 exiting
`
`Trunk
`
`L4 exiting
`
`Right/
`Left
`
`Average
`(mm)
`
`Standard
`Deviation
`(mm)
`
`R
`L
`R
`L
`R
`L
`R
`L
`R
`L
`R
`L
`R
`L
`
`19.2
`18.3
`16.1
`16.7
`21.3
`20.6
`13.7
`16.1
`19.7
`20.2
`10.8
`10.4
`15.9
`16.4
`
`4.6
`3.4
`4.0
`4.0
`4.4
`4.3
`5.6
`3.3
`4.6
`3.9
`8.0
`5.4
`5.8
`4.2
`
`P
`
`0.19
`
`0.29
`
`0.31
`
`0.054
`
`0.40
`
`0.40
`
`0.37
`
`for the placement of the final retractor, was observed in
`5% of approaches at the L2-3 and L3-4 levels. At L4-5,
`the nerves were felt to be at risk of having to be directly
`retracted in 25% of approaches (Table 1).
`
`Right and Left Differences
`There were no significant differences in right and
`left measurements of the location of the neural tissue
`relative to the disc spaces (Table 2). Furthermore, the
`same number of specimens and same incidence of nerve
`tissue penetration by the guide wire occurred on the right
`and left.
`
`Flexion and Extension Differences
`In general, the intrapsoas nerves migrated anteriorly
`with hip flexion, although we did not detect a statistically
`significant difference between flexion and extension
`(P>0.2) (Table 3).
`
`TABLE 3. Distance of Nervous Tissue Comparing Flexion and
`Extension
`
`Nerve
`
`Flexion/
`Extension
`
`Average
`(mm)
`
`Standard
`Deviation
`(mm)
`
`Disc
`Space
`
`L1-2
`
`L2-3
`
`L2-3
`
`L3-4
`
`L3-4
`
`L4-5
`
`L4-5
`
`FIGURE 5. Average distances of nerve trunks (A) and exiting
`nerve (B).
`
`Trunk
`
`Trunk
`
`L1 exiting Flexion
`Extension
`Flexion
`Extension
`L2 exiting Flexion
`Extension
`Flexion
`Extension
`L3 exiting Flexion
`Extension
`Flexion
`Extension
`L4 exiting Flexion
`Extension
`
`Trunk
`
`18.0
`18.9
`16.2
`16.4
`19.9
`21.1
`14.0
`14.8
`19.4
`19.8
`7.1
`8.5
`13.5
`14.0
`
`3.7
`3.8
`4.0
`4.6
`4.0
`4.9
`5.3
`3.7
`3.9
`3.0
`8.0
`7.0
`5.1
`6.5
`
`P
`
`0.25
`
`0.43
`
`0.20
`
`0.23
`
`0.40
`
`0.31
`
`0.36
`
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`

`J Spinal Disord Tech  Volume 23, Number 4, June 2010
`
`Anatomic Study of the Intrapsoas Nerves
`
`DISCUSSION
`This anatomic study defines the location of the
`neural elements in the psoas muscle with reference to
`minimally invasive lateral interbody fusion. The measure-
`ments performed were referenced from the center of the
`disc on the basis of a lateral fluoroscopic image, as would
`be clinically used during the transpsoas procedure. The
`nerve trunks are closer to the center of their respective
`discs than the corresponding exited nerve root. At L4-5,
`the nerve trunk was in the path of the initial guide wire
`approach directed towards the center of the disc in 3 of 20
`approaches and the neural elements would require direct
`retraction for placement of the final retractor in 25% of
`cases. The neural proximity was substantially less at the
`levels cephalad to L4-5.
`Successful
`results with the minimally invasive
`transpsoas approach have been reported recently. Ozgur
`et al14 reported on 13 patients treated with single level or
`multilevel XLIF procedures. The patients experienced
`significant relief of pain and improvement in functional
`scores. Pimenta et al have reported results in patients who
`underwent XLIF from L2 to L5 for degenerative
`scoliosis.13,19 The patients with 2-year follow-up demon-
`strated significant improvement in Visual Analog Scale
`pain scores and Oswestry scores whereas coronal and
`sagittal alignments sustained improvement at follow-up.
`As is recommended with the XLIF procedure, continuous
`free run neuromonitoring as well as triggered electro-
`myographics off all dilators and the final retractor was
`used, and the procedures were reportedly performed
`without intraoperative complications. There was a 4%
`incidence of pseudarthrosis and 1 patient (4%) had a late
`occurring subsidence at 6 months follow-up, but was
`asymptomatic and untreated.19
`Infrequent complications have however been re-
`ported with the transpsoas procedure. Wright reported on
`the first 145 patients undergoing XLIF, with 2 transient
`genitofemoral injuries and 5 patients experiencing tran-
`sient hip flexor weakness.20 The author felt that real-time
`neural monitoring was essential as
`the monitoring
`detected a nearby intrapsoas nerve in 46% of the cases.
`Ozgur et al14 reported no neurologic complications in
`the author’s first 13 XLIF patients. Recently, Knight and
`colleagues reported postoperative meralgia paresthetica
`in 10% of cases and L4 nerve root injuries in 3.4% of 58
`patients undergoing a lateral
`transpsoas
`interbody
`arthrodesis. Despite the risk of nerve injury, none of
`these complications affected patient satisfaction.21 Im-
`portantly, in this study varied designs of retractors were
`used and not all dilators and retractors used provided real
`time electromyographic feed back. The complication rate
`for minimally invasive lateral approaches is similar to that
`reported with an endoscopic lateral approach for lumbar
`interbody fusion reported.21 In contrast, the reported
`incidence of neurologic injury which includes retrograde
`ejaculation, femoral nerve palsy, impotence, and peroneal
`palsy from 1963 to 1990 was 5.1% when the traditional
`open retroperitoneal approach to interbody fusion was
`used.22
`
`In the literature, there are few studies that describe
`the location of the nerve roots within the psoas muscle.
`Bae et al18 performed a morphometric cadaveric study,
`which examined the lumbar nerve root anatomy for the
`purposes of determining a safe zone when addressing
`extraforaminal disc herniations. In this study, the authors
`described the distances between the nerve to the superior
`aspect of the transverse process, to the inferior aspect
`of the pedicle, and the superior articular process. In
`addition, they measured the distance between the dorsal
`lateral most aspects of the intervertebral disc and the
`nerve. These measurements are not particularly relevant
`to the transpsoas procedure. In a histologic study
`identifying the relationship of the neural elements to the
`disc at various levels in the lumbar spine, Moro et al17
`reported that all intrapsoas nerve roots were located over
`the posterior half of the vertebral body at the L4-5 disc
`space and above. Below that disc level, nerve roots or
`plexus were found over the anterior half of the L5 body.
`Both of these studies lack normative reference values for
`where the nerve root lies at the disc space in reference to
`the fluoroscopic center of the disc.
`Proponents of
`the transpsoas procedure have
`recommended targeting the fluoroscopic center of the
`lateral aspect of the disc as the initial disc access point.
`Certainly moving the initial access point and subsequent
`working portals further anterior would reduce the risks of
`neural injury. However, as one moves anterior away from
`the center of
`the disc,
`the side-to-side (left-to-right)
`dimensions of the disc get smaller, so that a smaller and
`therefore less stable interbody implant is required. In
`addition, the blood vessels and peritoneal cavity are at
`risk anteriorly.
`In this study, the location of the intrapsoas nerves
`was similar with a left or right-sided approach. This
`finding is not surprising as nervous tissue is likely
`symmetrical in the body. We observed that the intrapsoas
`nerves migrated anteriorly with hip flexion, although we
`did not detect a statistically significant difference between
`flexion and extension. The lack of significance may reflect
`the study being underpowered to detect a difference. Also
`because these were cadaveric specimens, the mobility of
`the psoas may have been restricted relative to the in vivo
`situation. Purported benefits of hip flexion during this
`procedure include relaxing the psoas muscle making for
`less psoas trauma during the transpsoas approach and
`also permitting the nervous tissue to be more mobile and
`easier to retract.
`In summary, this study suggests that although in a
`majority of lateral transpsoas interbody surgeries, the
`intrapsoas nerves are a safe distance from the disc access
`pathway, the anatomic variations in location of these
`nervous tissue place them at risk of injury in a small
`number of cases. In particular, surgeons should be
`cognizant of the higher risk of nerve injury at lower
`levels, especially at L4-5. To reduce this risk, real time
`neuromonitoring may be an important element of this
`procedure. Although clinical studies to date suggest a risk
`profile that is comparable or lower than that associated
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`Park et al
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`J Spinal Disord Tech  Volume 23, Number 4, June 2010
`
`further
`with other interbody arthrodesis techniques,
`studies will be required to precisely define the clinical
`benefits of different techniques of neural monitoring with
`the minimally invasive transpsoas technique.
`
`ACKNOWLEDGEMENT
`We appreciate NuVasive for providing the cadavers
`for this study.
`
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