INTRODUCTION
The fibrous origin of the psoas major (PM) can be divided into 2 groups: anterior, i.e. originating from all anteromedial aspects of lumbar discs and bodies excluding the disc between fifth lumbar/first sacral vertebrae; and posterior, i.e. from all transverse processes of the lumbar vertebrae [12]. The anterior and posterior sites of origin are usually considered as corresponding to the 2 heads of the PM, the superficial head from the anterior group of origin and the deep head from the posterior. The presented heads constitute individual fascicles that fuse and form a common tendon that inserts together with iliacus (IM) tendon onto the lesser trochanter of the femur [12].
Common insertion and shared function — the strongest hip joint flexion, of both PM and IM, lead to associating those muscles together into the iliopsoas muscle complex, which is sometimes considered as being comprised of 3 components, if the psoas minor is present.
Numerous morphological variations have been reported among muscles from different compartments of human body [10, 16, 17, 30], including the iliopsoas muscle complex [1, 3, 7, 24, 28]. There are reports of complete separation or fusion of its components [23], presence of accessory muscular slips [1, 21] or accessory muscles, such as the iliocapsularis [1, 23], accessory iliacus [3], iliacus minimus [25], psoas quartus [18, 24, 28], or psoas tertius [9] and even the presence of an accessory iliopsoas muscle complex [7, 26]. However, on its own, the PM muscle seems a rather constant structure that does not present much morphological variability.
The present report describes the previously unreported occurrence of a variable PM presenting as a 3-headed structure.
CASE REPORT
An unusual muscular structure was revealed during the routine dissection of the right posterior abdominal wall of an 82-year-old male cadaver (Fig. 1). The dissection was performed according to standard techniques for research and teaching purposes at the Department of Anatomical Dissection and Donation, Medical University of Lodz, Poland.
Briefly, the cadaver was positioned in a supine position on the dissection table. To obtain a clear view of the posterior abdominal wall structures, the intestines were separated from the greater omentum, mesentery, and fatty tissue. Subsequently, the components of the lumbar plexus and iliopsoas muscle complex were identified and cleared. The psoas minor remained absent. The PM muscle was noted to have an anomalous morphology.
The identified PM muscle was composed of 3 heads characterised by their muscular origin: superficial, intermediate, and deep. The superficial head originated by distinguishable muscular bands (medial and lateral) from the shafts of Th12 and L1, whose fusion produced a tendon strand. An intermediate head emerged within the tendinous strand of the superficial head, and the deep head originated from the transverse processes of the L1–L5 vertebrae. All 3 heads descended downwards; they were completely distinguishable from one another until the point where the femoral component of the genitofemoral nerve exited from the PM muscle substance. At this point, the intermediate and deep heads joined together and produced an intermediate tendon. The superior head merged with the component developed by the remaining heads via muscular bands, and the fusion was complete at the level of the inguinal ligament. The PM muscle descended further towards the insertion site, the lesser trochanter of the femur, where its tendon was positioned medially without any connection to the laterally inserted IM tendon.
The 3-headed PM muscle was subjected to detailed morphometric measurements and photographic documentation (Fig. 2). All the measurements were taken twice with an accuracy up to 0.1 mm using an electronic caliper (Mitutoyo Corporation, Kawasaki-shi, Kanagawa, Japan), and they are presented in Table 1.
|
Medial band |
|
|
Length |
58.39 mm |
|
Proximal attachment |
|
|
Width |
23.05 mm |
|
Thickness |
0.53 mm |
|
Lateral band |
|
|
Length |
113.15 mm |
|
Proximal attachment |
|
|
Width |
36.19 mm |
|
Thickness |
0.54 mm |
|
Superficial head |
|
|
Length |
207.10 mm |
|
Proximal attachment |
|
|
Width |
7.81 mm |
|
Thickness |
0.33 mm |
|
Intermediate head |
|
|
Length |
164.80 mm |
|
Proximal attachment |
|
|
Width |
36.77 mm |
|
Thickness |
0.41 mm |
|
Deep head |
|
|
Length |
246.32 mm |
|
Proximal attachment |
|
|
Width |
77.39 mm |
|
Thickness |
4.46 mm |
|
Distal attachment |
|
|
Width |
33.31 mm |
|
Thickness |
0.39 mm |
DISCUSSION
As previously stated, numerous anatomical variations of the iliopsoas muscle (IM) complex have been described. Some accessory structures associated with the IM are frequently observed, such as the accessory iliacus [3], iliacus minimus [25], or additional muscular slips from both PM and IM, which interfere with the course of the femoral nerve (FN) [1, 19, 21]. The presence of an accessory iliopsoas muscle complex has also been noted [7, 26]. The PM itself is considered to be relatively constant, and reports about its anatomical variations are rare. Khalid et al. [9] present a case of a psoas tertius that arose from the 12th rib and the transverse process of the L1 vertebrae, split the FN, and fused with the iliopsoas tendon. The psoas quartus was introduced as a structure that arose from substance of the quadratus lumborum and fused with the PM tendon [18, 24, 28].
Jelev et al. [7] reported a similar case to the present study, viz. a PM divided into 3 parts. The proximal attachment of the muscle was split longitudinally, in the frontal plane into parts, which extended downwards to form a muscular belly; however, their attachments and course varied greatly from those in present case. Jelev et al. [7] distinguished the following: a superior part from the L1 vertebral body and L1/L2 intervertebral disc, a middle part from the L2/L3 intervertebral disc, and an inferior part from the lower border of L3 to the S3 vertebrae. The PM muscle described in the present study was composed of a superficial head that originated as 2 bands from the Th12 and L1 shafts, an intermediate head from the tendon strand of the superficial heads, and a deep head from the transverse processes of the L1–L5 vertebrae.
The action of the PM, as a component of the iliopsoas muscle complex, as a primary flexor of the hip joint is well established. However, it has numerous other functions, especially with respect to lumbar spine stability and movement. Nachemson [14, 15] showed that the PM was active during upright standing, forward bending, and lifting, which supported the idea that the vertebral portion of PM takes part in maintaining an upright position as a lumbar spine stabilizer. It was also suggested that the PM has a role in lumbar and pelvic flexion in positions when the femur is fixed, such as in a standing position. Gibbons et al. [4] suggested that the PM might play a dual role in lumbar spine movement — the anterior attachment acting as a flexor and the posterior as an extensor. The PM muscle was also considered to play a part in controlling lumbar lordosis when supporting difficult lumbar loads [2, 20].
In addition, the PM crosses the sacroiliac joint (SIJ) and is suspected to influence it. According to Myers [13], the PM and piriformis act as integral stabilisers in balancing the SIJ. It was previously assumed that the PM assists in the anterior rotation of the hip bone; however, according to Gibbons et al. [4], the force produced by the PM results in posterior rotation. It was also stated that during its contraction, the PM provides a pulling force toward its proximal, i.e. spinal, and distal, i.e. femoral, attachments [13]. Throughout muscle contraction while both attachments are stabilised, the PM is able to exert a force on the hip that might participate in positioning the pelvis and SIJ [13].
It is important to note that the variable, 3-headed PM muscle introduced in this study was observed only in the right posterior abdominal wall. Because such an anomalous muscle was presented unilaterally, it is unclear whether its occurrence could be a cause of significant asymmetry between the function of the right and left PM muscles. It is possible that the occurrence of 3 heads and intermediate tendon strands would disturb the force distribution within the right PM, which would lead to differences in force arrangement patterns between the PM muscles of both sides. Such dissimilarity potentially interferes in functions like stabilisation of lumbar spine, support of lumbar lordosis, or positioning of the pelvis, or it could lead to lower back pain (LBP). The impact of such asymmetry between PM muscles on their function and the presence of LBP needs additional examination, especially since another kind of asymmetry in PM muscle morphology, viz. differences in muscle mass volume, have already been noted among athletes [5, 6, 22] and have been clinically correlated with chronic LBP [20, 29]. Nevertheless, further studies on the impact of the PM on lumbar spine stabilisation and on PM variability and asymmetry are needed in order to investigate the presented suspicions.
Interestingly, an anomalous morphology could not only interfere with PM function associated with the lumbar spine, but also with the SIJ. It can be suspected that activation of the muscle with the structure presented herein could impair load transfer across the SIJ. Studies on LBP based on an active straight leg raise test indicate that such impaired load transfer across the joint might be connected to SIJ pain [27]. It is also unclear whether the unilateral occurrence of such a muscular anomaly additionally contributes to the presentation of SIJ pain. However, the hypothetical link between anomalous PM muscle morphology, impaired load transfer, and SIJ pain needs to be further examined to draw reasonable conclusions.
The cleft of PM is a potential cleft beneath the PM on the side of the L5 vertebra [8]. Numerous anatomical structures, such as great vessels, ascending lumbar vein, iliolumbar vein, obturator nerve, and FN, are distributed within the described cleft [8]. Jianfei et al. [8] highlighted the cleft of PM as clinically relevant in lateral lumbar interbody fusion (LLIF) — a technique that allows the surgeon to access the intervertebral space from a direct lateral approach, either anterior to or through the PM. During the LLIF procedure neurovascular damage of the PM cleft often occurs. The mentioned neural and vascular injuries sustain during the penetration and retraction of the PM; therefore, understanding the anatomical complexity surrounding the PM is crucial in LLIF [8]. It is important to note that variable morphology of the PM, as described in this study, might also impair surgical procedures like LLIF and lead to further complications.
It is also important to accentuate that, together with the obturator internus muscle, the PM is a part of the lateral anatomical limit of the pelvic sidewall (PSW) [11]. Surgical procedures, such as treatment of gynaecological tumours, excluding tumours diagnosed at the earliest stages, and patients’ desire for fertility preservation, metastatic iliac lymph nodes, recurrences located near the PSW, deep infiltrating endometriosis (e.g. within the sacral plexus), or procedures such as a laterally extended endoplevic resection or a laterally extended parametrectomy, often require a dissection of the PSW [11]. Any kind of a variability within the PM muscle potentially disturbs the surgical treatment and leads to further complications; therefore, knowledge of its variation seems important to consider different approaches during surgery.
To prevent the mentioned complications, a variable muscle might be noticed during pre-surgery visualisation, e.g. during ultrasound imaging; however, in current scientific literature there is a noticeable scarcity of reports on morphological variations of PM on ultrasound or other imaging studies. Therefore, further studies on this subject are needed.
CONCLUSIONS
We report a case of a 3-headed psoas major composed of superficial, intermediate, and deep heads with an atypical site of origin and an anomalous structure. In the authors’ opinion, this morphological variation is significant not only because the psoas major is considered a structure with rather constant anatomy, but also because its rare occurrence may interfere in the functions of the muscle and potentially interfere with various surgical procedures. The circumstances surrounding of the anatomical variability of the psoas major and its influence in muscle function needs further exploration.
ARTICLE INFORMATION AND DECLARATIONS
Ethics statement
The cadavers were the property of the Department of Anatomical Dissection and Donation, Medical University of Lodz. Informed consent was obtained from all participants before they died.
Author contributions
Marta Pośnik — project development, data collection and management, data analysis, and manuscript writing.
Nicol Zielinska — data collection and analysis and manuscript editing.
Łukasz Olewnik — data collection and analysis and manuscript editing.
Mariola Głowacka — data collection and analysis and manuscript editing.
Piotr Łabętowicz (MD, PhD) — numerous consultations, observations, suggestions related to the paper, data analysis, and manuscript editing.
Acknowledgements
The authors sincerely thank those who donated their bodies to science so that anatomical research could be performed.
Funding
The authors have no financial or personal relationship with any third party whose interests could be influenced positively or negatively by the article’s content. This research received no specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Conflict of interest
The authors declare that they have no competing interests.
