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Posterior Sacrococcygeal Plexus: Application to Spine Surgery and Better Understanding Low-Back Pain
Journal Pre-proof The posterior sacrococcygeal plexus: application to spine surgery and better understanding low-back pain Shogo Kikuta, DDS, PhD, Joe Iwanaga, DDS, PhD, Koichi Watanabe, MD, PhD, Robert Haładaj, MD, Grzegorz Wysiadecki, MD, Aaron S. Dumont, MD, R. Shane Tubbs, PhD, PA-C PII:
S1878-8750(19)33090-6
DOI:
https://doi.org/10.1016/j.wneu.2019.12.061
Reference:
WNEU 13908
To appear in:
World Neurosurgery
Received Date: 31 October 2019 Revised Date:
10 December 2019
Accepted Date: 11 December 2019
Please cite this article as: Kikuta S, Iwanaga J, Watanabe K, Haładaj R, Wysiadecki G, Dumont AS, Tubbs RS, The posterior sacrococcygeal plexus: application to spine surgery and better understanding low-back pain, World Neurosurgery (2020), doi: https://doi.org/10.1016/j.wneu.2019.12.061. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Original Article The posterior sacrococcygeal plexus: application to spine surgery and better understanding lowback pain
Shogo Kikuta DDS, PhD2, Joe Iwanaga DDS, PhD1-3, Koichi Watanabe MD, PhD3, Robert Haładaj MD,4 Grzegorz Wysiadecki MD, 4Aaron S. Dumont, MD1, R. Shane Tubbs PhD, PAC1,5
The affiliations and addresses of the authors 1. Department of Neurosurgery, Tulane University School of Medicine, New Orleans, LA, USA 2. Dental and Oral Medical Center, Kurume University School of Medicine, Kurume, Fukuoka, Japan 3. Division of Gross and Clinical Anatomy, Department of Anatomy, Kurume University School of Medicine, Kurume, Fukuoka, Japan 4. Department of Normal and Clinical Anatomy, Medical University of Lodz, Łódź, Poland 5. Department of Anatomical Sciences, St. George’s University, St. George’s, Grenada, West Indies.
Running title Posterior sacrococcygeal plexus Corresponding author: Joe Iwanaga, DDS, PhD Department of Neurosurgery, Tulane University School of Medicine, 131 S. Robertson St.
1
Suite 1300, New Orleans, LA 70112, USA [email protected] Tel: 5049885565 Fax: 5049885793
Key Words Posterior sacrococcygeal plexus; sacral dorsal rami; low-back pain; entrapment; middle clunial nerve; anatomy, surgery
2
Abstract The sacral dorsal rami form the posterior sacrococcygeal plexus (PSCP), which has been scantly studied. This study’s goal was to clarify the PSCP’s detailed anatomy and discuss its clinical relevance. Ten sides of five, fresh-frozen cadavers were dissected for this research. After the muscles covering the sacrum were removed, the PSCP was identified and traced under the operating microscope until the entire plexus was exposed. The contributions to this plexus and its relations to surrounding anatomical structures were recorded. The PSCP was found on all sides and was composed of a medial trunk (MT), communicating branches, and a lateral trunk (LT). Each sacral dorsal ramus’ MT formed a series of loops created by adjacent sacral dorsal rami placed between the transverse tubercles and the posterior sacral foramina. The MT, communicating branches, and the LT demonstrated potential entrapment sites. To our knowledge, this is the first anatomical study that provides detailed images that indicate three potential sites where surrounding structures could entrap the PSCP. Knowledge of its detailed anatomy might help in better understanding low-back pain, targeting pain sources and guide spine surgeons for avoiding injury to these nerves.
Keywords: Posterior sacrococcygeal plexus; sacral dorsal rami; low-back pain; entrapment; middle clunial nerve; anatomy, surgery
1
Introduction The sacral spinal nerves’ dorsal rami emerge from the dorsal sacral foramina and divide into medial and lateral branches [1]. The medial branches are tiny and supply the multifidus lumborum muscles, while the lateral branches form loops by joining each other on the posterior surface of the sacrum. The lateral branches travel posterior to the sacrotuberous ligament (STL) and form secondary loops beneath the gluteus maximus (GM). The neural loops that the lateral branches form are referred to as the posterior sacrococcygeal plexus (PSCP) [2] or posterior sacral nerve plexus [3]. Following this description, scant anatomical studies have demonstrated sites where surrounding ligaments, such as the long posterior sacroiliac ligament (LPSIL) located between the posterior superior iliac spine (PSIS), and the largest of the transverse tubercles in the lateral sacrum may entrap the PSCP [2-8]. Some have suggested that the plexus’ entrapment by an overlying ligament could induce low-back pain [7,9,10]. In addition, spine fixation techniques, such as S2 alar iliac screw placement, could damage the PSCP iatrogenically. Therefore, a better understanding of the PSCP’s anatomy could help elucidate low-back pain syndrome and avoid iatrogenic injury by sacro-iliac fixation during spine surgery. However, to our knowledge, only a few descriptions of the PSCP have been made [2-4]. Therefore, this study’s goal was to revisit the PSCP’s anatomy and topography and discuss its clinical relevance. Materials and Methods The PSCPs of ten sides from five, fresh-frozen Caucasian cadavers (two males and three females) were dissected. The mean age at death was 84 years (range from 76-101 years). None of the specimens had a previous history or signs of surgery relevant to the areas dissected. Institutional Review Board/ethics committee’s approval was unnecessary as this was a cadaveric study.
2
In the prone position, the first skin incision was made along the midline from the level of the spinous process of the L5 vertebra to the coccyx. The thoracolumbar fascia, erector spinae muscle, and multifidus muscle were retracted laterally, and the posterior sacral foramen at each level was identified. Once the sacral dorsal rami had been found at all sacral levels, these nerves were traced distally. After the PSCP was exposed completely, the contribution to the plexus and relations to surrounding anatomical structures were recorded, particularly the interosseous sacroiliac ligament (ISIL), LPSIL, and STL. Finally, the diameter of each branch of the plexus was measured using a microcaliper (Mitutoyo, Kanagawa, Japan). Contributions to the muscles that cover the posterior aspect of the sacrum, i.e., multifidus and erector spinae, were excluded in this study, as they were unrelated to the plexus’ formation. The nerves were traced by separating carefully from the muscle fibers when needed. A surgical microscope (OPMI CS NC31, Carl Zeiss, Oberkochen, Germany) was used for all procedures in this dissection. All quantitative results are presented as basic descriptive statistics. A one-way analysis of variance with Scheffé’s post hoc test and Fisher’s exact test were used to compare the data, and statistical significance was set at p<0.05. The study followed the Declaration of Helsinki (64th WMA General Assembly, Fortaleza, Brazil, October 2013), and applied the AQUA Checklists [11,12]. Results The sacral dorsal rami formed the PSCP on all sides, in which the rami of S1 to S5 formed the plexus on five sides and that of S1 to the coccygeal nerve (Co) on five sides. It comprised three neural structures: the medial trunk (MT), communicating branches, and the lateral trunk (LT: Fig. 1). The three structures were defined as follows:
3
•
The MT is located between the transverse tubercles and posterior sacral foramina and forms a series of loops created by connections between adjacent sacral dorsal rami.
•
The communicating branches connect the MT and LT.
•
The LT is formed by the communicating branches and runs outside the transverse tubercles. The MT was formed by contributions derived from S1 to Co on five sides (50%), and
from S1 to S5 on three sides (30%). On two sides (20%), there was no MT at S3-S4 and S4-S5. Part of the ISIL often covered the MT (Fig. 2) and its average diameter was 0.5 ± 0.2 mm. The communicating branches arose from the MT or from the posterior sacral foramen directly adjacent to the LT. They passed deep to the LPSIL consistently and traveled between the transverse tubercles (Fig. 3). Various contributions to the LT were observed, and sixteen LTs in total were identified. The sacral dorsal rami of S1-S2 formed the LT in 37.5% (6/16) of cases, S1-S3 in 12.5% (2/16), S1-S4 in 31.3% (5/16), and S2-S4 in 18.8% (3/16). Most of the S1-S2 LTs formed ran obliquely toward the midline and pierced the gluteus maximus. In other patterns with more than three sacral dorsal rami, the LT ran downward while joining each communicating branch, coursed deep to the aponeurosis of the gluteus maximus or part of the STL, and then pierced the gluteus maximus (Fig. 4). The LT’s diameter was 1.1 ± 0.2 mm, significantly greater than that of the MTs (p<0.05). On both sides of one specimen, a small branch from the S4 sacral ventral ramus passed inferior to the STL and joined the LT to form the PSCP (Fig. 5). Interestingly, in one case on the right side, a small branch from the LT anastomosed with a branch of the perforating cutaneous nerve from the ventral sacral rami superficial to the STL (Fig. 6). The space in the depth of the
4
ligament was filled with fat tissue. No obvious site where ligaments grossly compressed the plexus was identified on any specimen. Finally, some branches of the LTs penetrated the GM as the middle clunial nerves and supplied the overlying gluteal skin.
Discussion The sacral dorsal rami form the PSCP in a complex manner, and the relations with the overlying muscles and surrounding ligaments of the posterior sacral surface is highly variable [2]. Only few studies have mentioned the anatomy of the PSCP. Horwitz investigated the posterior sacral and coccygeal plexuses’ detailed anatomy in 30 cadavers [3]. This author showed that the upper four sacral dorsal rami divided into small medial branches that end at the posterior sacral muscles and large lateral branches. The latter branches first formed loops by anastomosing with each other and this description is equivalent to the MT that we defined. Consecutively, the lateral branches joined and formed secondary loops lateral to the transverse tubercles, which are equivalent to the LT described in our study. Willard et al. [2] reported similar findings, although the lateral branches were presented as lateral divisions. Horwitz found that the S2 and S3 sacral dorsal rami formed the secondary loops, i.e., the LT, in 43.7% (25/60) of cases, followed by S1 and S3 in 33.3% (20/60) [3]. Willard et al. reported that the S1 to S3 sacral dorsal rami formed the LT consistently [2]. However, in this study, the S1 and S2 sacral dorsal rami formed the LT most commonly, followed by S1 to S4. Interestingly, in the current study the LT pathway differed from earlier reported patterns; the LT traveled obliquely toward the midline inferolaterally when it was formed by S1 and S2, but consistently traveled downward when it is formed by more than three sacral dorsal rami. The difference among these descriptions
5
made by various authors could be attributable to the difficulty of dissection because of the PSCP’s complex nature and deep position. The relations among the PSCP and the posterior sacroiliac joint’s ligaments are an important finding with respect to clinical relevance. Several anatomical studies have indicated that the lateral branches’ pathway penetrates the LPSIL or descends within the STL [3,5,6,8]. However, there have been few detailed descriptions of the course of the PSCP. Willard et al. found that the sacral dorsal rami’s lateral divisions pass through two different anatomical tunnels, the first located between the LPSIL and SI joint, the second between the thoracolumbar fascia and the STL [2]. In our study, the PSCP had three potential sites at which different structures could entrap/compress it. The first site was at the MT where part of the ISIL often covered it. The second site was at the communicating branches, which always passed deep to the LPSIL and across between the transverse tubercles. The third potential site was at the LT, which passed under the gluteus maximus’ aponeurosis or part of the STL. Neural entrapment of these types could be an uncommon cause of low-back pain [7,9,10]. Strong and Davila published the first report of clunial nerve syndrome with low-back pain [13]. Talu et al. found that the superior clunial nerves could become entrapped easily while passing through the fascia near the iliac crest [14]. Moreover, several anatomical studies have reported middle clunial nerve entrapment by a ligament, particularly the LPSIL [5,6,8]. Talu et al. [14] interpreted the PSCP’s communicating branches as the middle clunial nerves, but the terminology should be based on the innervation area. Tubbs et al. found that the middle clunial nerves would be less likely to become entrapped, as they had no osteofibrous tunnels or compression sites [7].
6
Clunial nerve entrapment is a potential cause of low-back pain but is difficult to diagnose [7,9,10]. Aota et al. suggested that trigger points confirmed by LPSIL palpation or pain alleviation demonstrated after local anesthetic injection could be used as diagnostic methods [15]. Recently, Matsumoto et al. demonstrated that middle clunial nerve neurolysis, which severs the LPSIL covering the middle clunial nerves, was effective in eleven patients who had shown no improvement after middle clunial nerve blockade [16]. Invasive surgery, such as nerve decompression, may be considered when less invasive procedures, such as steroid injection and nerve blockade, have no effect [7,9,17,18]. Lastly, surgical procedures of the dorsal sacrum, such as S2 iliac alar screw placement, might injure the PSCP’s branches, particularly when screws are placed in close proximity to the dorsal sacral foramina (Fig. 7). Screw placement to allow sacro-iliac fixation should be performed with the anatomy of the PSCP in mind to avoid iatrogenic injury. Conclusion This paper details the PSCP’s anatomy and the sites where surrounding structures potentially may entrap it. We believe that these findings might also help elucidate low-back pain with an unclear etiology and serve as a guide to surgeons during surgery over the posterior sacrum so that iatrogenic injury to the PSCP and its branches can occur. Acknowledgments The authors thank those who donated their bodies for anatomical research.
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References 1.
Standring S. Gray’s Anatomy e-Book: The Anatomical Basis of Clinical Practice, 41st ed. London: Elsevier Health Sciences; 2015.
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Willard F, Carreiro JE, Manko W. The long posterior interosseous ligament and the sacrococcygeal plexus. In: Third Interdisciplinary World Congress on Low Back and Pelvic Pain. Rotterdam; 1998;
3.
Horwitz MT. The anatomy of (A) the lumbosacral nerve plexus—its relation to variations of vertebral segmentation, and (B), the posterior sacral nerve plexus. Anat Rec 1939;74:91-107.
4.
Grob KR, Neuhuber WL, Kissling RO. Innervation of the sacroiliac joint of the human. Z Rheumatol 1995;54:117-22.
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McGrath C, Zhang M. Lateral branches of dorsal sacral nerve plexus and the long posterior sacroiliac ligament. Surg Radiol Anat 2005;27:327-30.
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McGrath C, Nicholson H, Hurst P. The long posterior sacroiliac ligament: A histological study of morphological relations in the posterior sacroiliac region. Joint Bone Spine 2009;76:57-62.
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Tubbs RS, Levin MR, Loukas M, et al. Anatomy and landmarks for the superior and middle cluneal nerves: Application to posterior iliac crest harvest and entrapment syndromes. J Neurosurg Spine 2010;13:356-9.
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Konno T, Aota Y, Saito T, et al. Anatomical study of middle cluneal nerve entrapment. J Pain Res 2017;10:1431-5.
9.
Berthelot JM, Delecrin J, Maugars Y, et al. A potentially underrecognized and treatable cause of chronic back pain: Entrapment neuropathy of the cluneal nerves. J Rheumatol
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1996;23:2179-2181. 10.
Iwanaga J, Simonds E, Patel M, et al. Anatomic study of superior cluneal nerves: Application to low back pain and surgical approaches to lumbar vertebrae. World Neurosurg 2018;116:e766-8.
11.
Henry BM, Marcinów A, Pekala P, et al. Polish translation of the Anatomical Quality Assurance (AQUA) Checklist: New guidelines for reporting in original anatomical studies. Folia Med Cracov 2017;57:105-16.
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Tomaszewski KA, Henry BM, Kumar Ramakrishnan P, et al. Development of the Anatomical Quality Assurance (AQUA) checklist: Guidelines for reporting original anatomical studies. Clin Anat 2017;30:14-20.
13.
Strong EK, Davila JC. The cluneal nerve syndrome: A distinct type of low back pain. Ind Med Surg 1957;26:417-29.
14.
Talu GK, Özyalçin S, Talu U. Superior cluneal nerve entrapment. Reg Anesth Pain Med 2000;25:648-50.
15.
Aota Y. Entrapment of middle cluneal nerves as an unknown cause of low back pain. World J Orthop 2016;7:167-70.
16.
Matsumoto J, Isu T, Kim K, et al. Surgical treatment of middle cluneal nerve entrapment neuropathy: Technical note. J Neurosurg Spine 2018;29:208-13.
17.
Maigne JY, Doursounian L. Entrapment neuropathy of the medial superior cluneal nerve. Nineteen cases surgically treated, with a minimum of 2 years’ follow-up. Spine (Phila Pa 1976) 1997;22:1156-9.
18.
Mahli A, Coskun D, Altun NS, et al. Alcohol neurolysis for persistent pain caused by superior cluneal nerves injury after iliac crest bone graft harvesting in orthopedic surgery:
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Report of four cases and review of the literature. Spine (Phila Pa 1976) 2002;27:E478-81.
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Figure Legends Figure 1 Posterior view of the posterior sacrococcygeal plexus. Black arrows, communicating branch; green arrows, lateral trunk; white arrows, medial trunk.
Figure 2 Posterior view of the PSCP’s medial trunk on the left side. A: Before the ISIL was removed (white dashed lines), this ligament covered the medial trunk. B: After the ISIL was removed (white dashed lines), the medial trunk was identified. ISIL, interosseous sacroiliac ligament; PSCP, posterior sacrococcygeal plexus.
Figure 3 Posterior view of the PSCP’s communicating branches on the left side. A: Before the LPSIL was removed (white dashed lines), it covered the communicating branches. B: After the LPSIL was removed (white dashed lines), the communicating branches (black arrows) were identified. LPSIL, long posterior sacroiliac ligament; PSCP, posterior sacrococcygeal plexus; PSIS, posterior superior iliac spine.
Figure 4 Posterior view of the PSCP’s communicating branches on the left side. A: The lateral trunk (green arrow) passes deep to the distinct fibrous tissue (white dashed lines). B: After the fibrous tissue was removed, the lateral trunk was identified. PSCP, posterior sacrococcygeal plexus; PSIS, posterior superior iliac spine.
11
Figure 5 Inferolateral view of the PSCP on both sides of the same cadaver. A small branch from the S4 sacral ventral ramus joins the lateral trunk. Green arrows, lateral trunk; orange arrow, a small branch from S4 sacral ventral ramus; yellow arrow, S4 sacral ventral ramus.
Figure 6 Lateral view of the communication between the lateral trunk and the perforating cutaneous nerve. Green arrows, lateral trunk; red arrow, communicating branch.
Figure 7 Schematic drawing of the PSCP with inset of S2 iliac alar screw placement. Note the close relation between the screw trajectory and overlying nerve plexus.
12
Author contribution Shogo Kikuta Protocol/project development, Data collection, Data analysis, Manuscript writing J Iwanaga Protocol/project development, Data collection, Data analysis, Manuscript writing Koichi Watanabe Data collection, Manuscript editing Robert Haładaj Data analysis, Manuscript editing Grzegorz Wysiadecki Data analysis, Manuscript editing Aaron S. Dumont Protocol/project development, Manuscript editing RS Tubbs Protocol/project development, Data collection, Manuscript editing
Abbreviation
GM; gluteus maximus ISIL; interosseous sacroiliac ligament LPSIL; long posterior sacroiliac ligament LT; lateral trunk MT; medial trunk PSCP; posterior sacrococcygeal plexus PSIS; posterior superior iliac spine STL; sacrotuberous ligament
S1878-8750(19)33090-6
DOI:
https://doi.org/10.1016/j.wneu.2019.12.061
Reference:
WNEU 13908
To appear in:
World Neurosurgery
Received Date: 31 October 2019 Revised Date:
10 December 2019
Accepted Date: 11 December 2019
Please cite this article as: Kikuta S, Iwanaga J, Watanabe K, Haładaj R, Wysiadecki G, Dumont AS, Tubbs RS, The posterior sacrococcygeal plexus: application to spine surgery and better understanding low-back pain, World Neurosurgery (2020), doi: https://doi.org/10.1016/j.wneu.2019.12.061. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Original Article The posterior sacrococcygeal plexus: application to spine surgery and better understanding lowback pain
Shogo Kikuta DDS, PhD2, Joe Iwanaga DDS, PhD1-3, Koichi Watanabe MD, PhD3, Robert Haładaj MD,4 Grzegorz Wysiadecki MD, 4Aaron S. Dumont, MD1, R. Shane Tubbs PhD, PAC1,5
The affiliations and addresses of the authors 1. Department of Neurosurgery, Tulane University School of Medicine, New Orleans, LA, USA 2. Dental and Oral Medical Center, Kurume University School of Medicine, Kurume, Fukuoka, Japan 3. Division of Gross and Clinical Anatomy, Department of Anatomy, Kurume University School of Medicine, Kurume, Fukuoka, Japan 4. Department of Normal and Clinical Anatomy, Medical University of Lodz, Łódź, Poland 5. Department of Anatomical Sciences, St. George’s University, St. George’s, Grenada, West Indies.
Running title Posterior sacrococcygeal plexus Corresponding author: Joe Iwanaga, DDS, PhD Department of Neurosurgery, Tulane University School of Medicine, 131 S. Robertson St.
1
Suite 1300, New Orleans, LA 70112, USA [email protected] Tel: 5049885565 Fax: 5049885793
Key Words Posterior sacrococcygeal plexus; sacral dorsal rami; low-back pain; entrapment; middle clunial nerve; anatomy, surgery
2
Abstract The sacral dorsal rami form the posterior sacrococcygeal plexus (PSCP), which has been scantly studied. This study’s goal was to clarify the PSCP’s detailed anatomy and discuss its clinical relevance. Ten sides of five, fresh-frozen cadavers were dissected for this research. After the muscles covering the sacrum were removed, the PSCP was identified and traced under the operating microscope until the entire plexus was exposed. The contributions to this plexus and its relations to surrounding anatomical structures were recorded. The PSCP was found on all sides and was composed of a medial trunk (MT), communicating branches, and a lateral trunk (LT). Each sacral dorsal ramus’ MT formed a series of loops created by adjacent sacral dorsal rami placed between the transverse tubercles and the posterior sacral foramina. The MT, communicating branches, and the LT demonstrated potential entrapment sites. To our knowledge, this is the first anatomical study that provides detailed images that indicate three potential sites where surrounding structures could entrap the PSCP. Knowledge of its detailed anatomy might help in better understanding low-back pain, targeting pain sources and guide spine surgeons for avoiding injury to these nerves.
Keywords: Posterior sacrococcygeal plexus; sacral dorsal rami; low-back pain; entrapment; middle clunial nerve; anatomy, surgery
1
Introduction The sacral spinal nerves’ dorsal rami emerge from the dorsal sacral foramina and divide into medial and lateral branches [1]. The medial branches are tiny and supply the multifidus lumborum muscles, while the lateral branches form loops by joining each other on the posterior surface of the sacrum. The lateral branches travel posterior to the sacrotuberous ligament (STL) and form secondary loops beneath the gluteus maximus (GM). The neural loops that the lateral branches form are referred to as the posterior sacrococcygeal plexus (PSCP) [2] or posterior sacral nerve plexus [3]. Following this description, scant anatomical studies have demonstrated sites where surrounding ligaments, such as the long posterior sacroiliac ligament (LPSIL) located between the posterior superior iliac spine (PSIS), and the largest of the transverse tubercles in the lateral sacrum may entrap the PSCP [2-8]. Some have suggested that the plexus’ entrapment by an overlying ligament could induce low-back pain [7,9,10]. In addition, spine fixation techniques, such as S2 alar iliac screw placement, could damage the PSCP iatrogenically. Therefore, a better understanding of the PSCP’s anatomy could help elucidate low-back pain syndrome and avoid iatrogenic injury by sacro-iliac fixation during spine surgery. However, to our knowledge, only a few descriptions of the PSCP have been made [2-4]. Therefore, this study’s goal was to revisit the PSCP’s anatomy and topography and discuss its clinical relevance. Materials and Methods The PSCPs of ten sides from five, fresh-frozen Caucasian cadavers (two males and three females) were dissected. The mean age at death was 84 years (range from 76-101 years). None of the specimens had a previous history or signs of surgery relevant to the areas dissected. Institutional Review Board/ethics committee’s approval was unnecessary as this was a cadaveric study.
2
In the prone position, the first skin incision was made along the midline from the level of the spinous process of the L5 vertebra to the coccyx. The thoracolumbar fascia, erector spinae muscle, and multifidus muscle were retracted laterally, and the posterior sacral foramen at each level was identified. Once the sacral dorsal rami had been found at all sacral levels, these nerves were traced distally. After the PSCP was exposed completely, the contribution to the plexus and relations to surrounding anatomical structures were recorded, particularly the interosseous sacroiliac ligament (ISIL), LPSIL, and STL. Finally, the diameter of each branch of the plexus was measured using a microcaliper (Mitutoyo, Kanagawa, Japan). Contributions to the muscles that cover the posterior aspect of the sacrum, i.e., multifidus and erector spinae, were excluded in this study, as they were unrelated to the plexus’ formation. The nerves were traced by separating carefully from the muscle fibers when needed. A surgical microscope (OPMI CS NC31, Carl Zeiss, Oberkochen, Germany) was used for all procedures in this dissection. All quantitative results are presented as basic descriptive statistics. A one-way analysis of variance with Scheffé’s post hoc test and Fisher’s exact test were used to compare the data, and statistical significance was set at p<0.05. The study followed the Declaration of Helsinki (64th WMA General Assembly, Fortaleza, Brazil, October 2013), and applied the AQUA Checklists [11,12]. Results The sacral dorsal rami formed the PSCP on all sides, in which the rami of S1 to S5 formed the plexus on five sides and that of S1 to the coccygeal nerve (Co) on five sides. It comprised three neural structures: the medial trunk (MT), communicating branches, and the lateral trunk (LT: Fig. 1). The three structures were defined as follows:
3
•
The MT is located between the transverse tubercles and posterior sacral foramina and forms a series of loops created by connections between adjacent sacral dorsal rami.
•
The communicating branches connect the MT and LT.
•
The LT is formed by the communicating branches and runs outside the transverse tubercles. The MT was formed by contributions derived from S1 to Co on five sides (50%), and
from S1 to S5 on three sides (30%). On two sides (20%), there was no MT at S3-S4 and S4-S5. Part of the ISIL often covered the MT (Fig. 2) and its average diameter was 0.5 ± 0.2 mm. The communicating branches arose from the MT or from the posterior sacral foramen directly adjacent to the LT. They passed deep to the LPSIL consistently and traveled between the transverse tubercles (Fig. 3). Various contributions to the LT were observed, and sixteen LTs in total were identified. The sacral dorsal rami of S1-S2 formed the LT in 37.5% (6/16) of cases, S1-S3 in 12.5% (2/16), S1-S4 in 31.3% (5/16), and S2-S4 in 18.8% (3/16). Most of the S1-S2 LTs formed ran obliquely toward the midline and pierced the gluteus maximus. In other patterns with more than three sacral dorsal rami, the LT ran downward while joining each communicating branch, coursed deep to the aponeurosis of the gluteus maximus or part of the STL, and then pierced the gluteus maximus (Fig. 4). The LT’s diameter was 1.1 ± 0.2 mm, significantly greater than that of the MTs (p<0.05). On both sides of one specimen, a small branch from the S4 sacral ventral ramus passed inferior to the STL and joined the LT to form the PSCP (Fig. 5). Interestingly, in one case on the right side, a small branch from the LT anastomosed with a branch of the perforating cutaneous nerve from the ventral sacral rami superficial to the STL (Fig. 6). The space in the depth of the
4
ligament was filled with fat tissue. No obvious site where ligaments grossly compressed the plexus was identified on any specimen. Finally, some branches of the LTs penetrated the GM as the middle clunial nerves and supplied the overlying gluteal skin.
Discussion The sacral dorsal rami form the PSCP in a complex manner, and the relations with the overlying muscles and surrounding ligaments of the posterior sacral surface is highly variable [2]. Only few studies have mentioned the anatomy of the PSCP. Horwitz investigated the posterior sacral and coccygeal plexuses’ detailed anatomy in 30 cadavers [3]. This author showed that the upper four sacral dorsal rami divided into small medial branches that end at the posterior sacral muscles and large lateral branches. The latter branches first formed loops by anastomosing with each other and this description is equivalent to the MT that we defined. Consecutively, the lateral branches joined and formed secondary loops lateral to the transverse tubercles, which are equivalent to the LT described in our study. Willard et al. [2] reported similar findings, although the lateral branches were presented as lateral divisions. Horwitz found that the S2 and S3 sacral dorsal rami formed the secondary loops, i.e., the LT, in 43.7% (25/60) of cases, followed by S1 and S3 in 33.3% (20/60) [3]. Willard et al. reported that the S1 to S3 sacral dorsal rami formed the LT consistently [2]. However, in this study, the S1 and S2 sacral dorsal rami formed the LT most commonly, followed by S1 to S4. Interestingly, in the current study the LT pathway differed from earlier reported patterns; the LT traveled obliquely toward the midline inferolaterally when it was formed by S1 and S2, but consistently traveled downward when it is formed by more than three sacral dorsal rami. The difference among these descriptions
5
made by various authors could be attributable to the difficulty of dissection because of the PSCP’s complex nature and deep position. The relations among the PSCP and the posterior sacroiliac joint’s ligaments are an important finding with respect to clinical relevance. Several anatomical studies have indicated that the lateral branches’ pathway penetrates the LPSIL or descends within the STL [3,5,6,8]. However, there have been few detailed descriptions of the course of the PSCP. Willard et al. found that the sacral dorsal rami’s lateral divisions pass through two different anatomical tunnels, the first located between the LPSIL and SI joint, the second between the thoracolumbar fascia and the STL [2]. In our study, the PSCP had three potential sites at which different structures could entrap/compress it. The first site was at the MT where part of the ISIL often covered it. The second site was at the communicating branches, which always passed deep to the LPSIL and across between the transverse tubercles. The third potential site was at the LT, which passed under the gluteus maximus’ aponeurosis or part of the STL. Neural entrapment of these types could be an uncommon cause of low-back pain [7,9,10]. Strong and Davila published the first report of clunial nerve syndrome with low-back pain [13]. Talu et al. found that the superior clunial nerves could become entrapped easily while passing through the fascia near the iliac crest [14]. Moreover, several anatomical studies have reported middle clunial nerve entrapment by a ligament, particularly the LPSIL [5,6,8]. Talu et al. [14] interpreted the PSCP’s communicating branches as the middle clunial nerves, but the terminology should be based on the innervation area. Tubbs et al. found that the middle clunial nerves would be less likely to become entrapped, as they had no osteofibrous tunnels or compression sites [7].
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Clunial nerve entrapment is a potential cause of low-back pain but is difficult to diagnose [7,9,10]. Aota et al. suggested that trigger points confirmed by LPSIL palpation or pain alleviation demonstrated after local anesthetic injection could be used as diagnostic methods [15]. Recently, Matsumoto et al. demonstrated that middle clunial nerve neurolysis, which severs the LPSIL covering the middle clunial nerves, was effective in eleven patients who had shown no improvement after middle clunial nerve blockade [16]. Invasive surgery, such as nerve decompression, may be considered when less invasive procedures, such as steroid injection and nerve blockade, have no effect [7,9,17,18]. Lastly, surgical procedures of the dorsal sacrum, such as S2 iliac alar screw placement, might injure the PSCP’s branches, particularly when screws are placed in close proximity to the dorsal sacral foramina (Fig. 7). Screw placement to allow sacro-iliac fixation should be performed with the anatomy of the PSCP in mind to avoid iatrogenic injury. Conclusion This paper details the PSCP’s anatomy and the sites where surrounding structures potentially may entrap it. We believe that these findings might also help elucidate low-back pain with an unclear etiology and serve as a guide to surgeons during surgery over the posterior sacrum so that iatrogenic injury to the PSCP and its branches can occur. Acknowledgments The authors thank those who donated their bodies for anatomical research.
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Figure Legends Figure 1 Posterior view of the posterior sacrococcygeal plexus. Black arrows, communicating branch; green arrows, lateral trunk; white arrows, medial trunk.
Figure 2 Posterior view of the PSCP’s medial trunk on the left side. A: Before the ISIL was removed (white dashed lines), this ligament covered the medial trunk. B: After the ISIL was removed (white dashed lines), the medial trunk was identified. ISIL, interosseous sacroiliac ligament; PSCP, posterior sacrococcygeal plexus.
Figure 3 Posterior view of the PSCP’s communicating branches on the left side. A: Before the LPSIL was removed (white dashed lines), it covered the communicating branches. B: After the LPSIL was removed (white dashed lines), the communicating branches (black arrows) were identified. LPSIL, long posterior sacroiliac ligament; PSCP, posterior sacrococcygeal plexus; PSIS, posterior superior iliac spine.
Figure 4 Posterior view of the PSCP’s communicating branches on the left side. A: The lateral trunk (green arrow) passes deep to the distinct fibrous tissue (white dashed lines). B: After the fibrous tissue was removed, the lateral trunk was identified. PSCP, posterior sacrococcygeal plexus; PSIS, posterior superior iliac spine.
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Figure 5 Inferolateral view of the PSCP on both sides of the same cadaver. A small branch from the S4 sacral ventral ramus joins the lateral trunk. Green arrows, lateral trunk; orange arrow, a small branch from S4 sacral ventral ramus; yellow arrow, S4 sacral ventral ramus.
Figure 6 Lateral view of the communication between the lateral trunk and the perforating cutaneous nerve. Green arrows, lateral trunk; red arrow, communicating branch.
Figure 7 Schematic drawing of the PSCP with inset of S2 iliac alar screw placement. Note the close relation between the screw trajectory and overlying nerve plexus.
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Author contribution Shogo Kikuta Protocol/project development, Data collection, Data analysis, Manuscript writing J Iwanaga Protocol/project development, Data collection, Data analysis, Manuscript writing Koichi Watanabe Data collection, Manuscript editing Robert Haładaj Data analysis, Manuscript editing Grzegorz Wysiadecki Data analysis, Manuscript editing Aaron S. Dumont Protocol/project development, Manuscript editing RS Tubbs Protocol/project development, Data collection, Manuscript editing
Abbreviation
GM; gluteus maximus ISIL; interosseous sacroiliac ligament LPSIL; long posterior sacroiliac ligament LT; lateral trunk MT; medial trunk PSCP; posterior sacrococcygeal plexus PSIS; posterior superior iliac spine STL; sacrotuberous ligament