INTRODUCTION
Numerous muscles in several compartments of the human body have been found to demonstrate anatomical variations [22, 35, 43, 44, 46, 58, 80], and these include the muscular part of the pelvic girdle, which can display additional muscles, anomalous heads, accessory tendinous slips or both unilateral and bilateral absence [6, 37, 61, 67]. As such variation is commonly associated with clinical conditions, there is a need to better understand their occurrence.
A diagnostic method finding increasing use among clinicians around the world is ultrasound imaging. This method is progressively used in musculoskeletal imaging because it shows muscles, tendons, and joints during movement, shows tendon clearly, can be used during invasive procedures, and is painless, non-invasive, cheaper, and easier to perform than MRI [36].
The aim of this review was to present some of many variations of the pelvic girdle muscles that might surprise clinicians during imaging by their diverse morphology. Descriptions of morphological variability of presented structures were assigned according to the most frequent nomenclature as parts of respective compartments — iliopsoas muscle complex, superficial gluteal muscles, and deep gluteal muscles.
ILIOPSOAS MUSCLE COMPLEX
Iliacus muscle and psoas major
The iliopsoas muscle complex, comprising the iliacus muscle and psoas major, is known as the strongest hip flexor. In some cases, the complex also includes the psoas minor (Fig. 1).
The iliacus muscle is a triangular muscle with a vast origin from the superior two-thirds of the iliac fossa, as well as the inner lip of the iliac crest, lateral aspect of the sacrum, anterior sacroiliac ligament, and iliolumbar ligament, which inserts onto the lesser trochanter of the femur [40].
The psoas major fibrous origin can be divided into 2 groups: the anterior group including all anteromedial aspects of the lumbar discs and bodies except the disc between fifth lumbar/first sacral vertebrae, and the posterior group including all transverse processes of lumbar vertebrae [59]. Those sites of origin constitute individual fascicles that join and form a common tendon that inserts together with the iliacus tendon onto the lesser trochanter of the femur [59].
Numerous variations of both the iliacus muscle and psoas major have been reported. Aleksandrova et al. [2] introduced a 10-type classification of the iliacus muscle based on dissectional findings and extensive literature search, that considers accessory slips of both iliacus muscle, the psoas major, and their relation to the femoral nerve (FN). The mentioned types are presented in Table 1.
|
Type |
Description |
Mentioned by |
|
A |
Partial agenesis — missing slips from the anterior and middle parts of the iliacus muscle, replaced by fibrous slip. |
Aleksandrova et al. [2] |
|
B |
Complete separation of psoas major and iliacus muscle. |
Le Double [14] Kopsh et al. [34] Macalister [39] |
|
C |
Complete fusion of iliacus muscle and psoas major with normal course of femoral nerve. |
Fabrizio [18] |
|
D |
Additional, aberrant slips of higher origin, which can also include extremely rare psoas quartus and rare psoas tertius. |
Clarkson and Rainy [11] Tubbs et al. [70] |
|
E |
Presence of iliacus minor - detached portion of iliacus that arises from anterior inferior spine of the ilium with site of insertion onto anterior-trochanteric line, or ilio-capsularis, with a similar origin but different insertion, i.e. into the ilio-femoral ligament. |
Babst et al. [7] Ward et al. [76] |
|
F |
An iliacus muscle that consists of deep and superficial layers. |
Macalister [39] Jelev et al. [26] |
|
G |
Presence of large, aberrant, superficial slips of iliacus muscle. |
Rao et al. [56] |
|
H |
Presence of one, large aberrant slip with femoral nerve that runs between the slip and iliacus. |
D’Costa et al. [12] |
|
I |
Presence of two muscular slips with a split femoral nerve. |
Jelev et al. [26] |
|
J |
Presence of a small muscular slip piercing through the femoral nerve. |
Aleksandrova et al. [2] |
The psoas tertius and psoas quartus are both quite rare. The latter, first described by Clarkson and Rainy [11], arises as a slip from the quadratus lumborum muscle and fifth lumbar vertebra transverse process, which fused with other psoas tendons at the inguinal ligament level. In the same cadaver, Clarkson and Rainy [11] reported the presence of a psoas tertius that arose from the 12th rib and first lumbar vertebra transverse process; this also fused with other psoas tendons at the level of the inguinal ligament. Tubbs et al. [70] also described a psoas quartus that arose from the third lumbar vertebra transverse process and the quadratus lumborum, and then fused with the psoas major and iliacus muscle via muscular fibres at the level of the inguinal ligament (Fig. 2). Wong et al. [79] found a multiple FN to be associated with the presence of the psoas quartus. In this case, the anomalous muscle originated only from the anteromedial surface of the quadratus lumborum muscle and fused with the psoas major tendon at the inguinal ligament level. However, a split FN was found, but due to the presence of the psoas tertius [28], in this case, the muscle variant arose from 12th rib and first lumbar vertebra transverse process, as noted by Clarkson and Rainy [11], but pierced the FN and joined the iliopsoas tendon. The psoas tertius and quartus match type D in the Aleksandrova et al. [2] classification.
Jelev et al. [26] present a psoas major divided longitudinally into 3 parts: a superior part originating from the first lumbar vertebral body and the intervertebral disc between the first and second lumbar vertebrae, a middle part arising from the second and third intervertebral disc, and an inferior part originating from the third lumbar to third sacral lower borders of the vertebrae. In the same cadaver, Jelev et al. also reported the presence of an accessory iliopsoas muscle; this originated from the left transverse process of the third lumbar vertebra and from the intertransverse ligament between the third and fourth lumbar vertebrae, and inserted via a common tendon with an accessory iliacus muscle between the iliacus muscle and psoas major (Fig. 3). An accessory iliacus muscle arose from the middle third of the iliac crest and inserted as previously described [26]. This case was classified as type I in the classification by Aleksandrova et al. [2].
Some studies have examined the muscular bundles of accessory iliopsoas muscle that cover the FN. Unat et al. [73] classified those into 2 main types based on dissections of 50 cadavers and distinguished a sheet muscle pattern (89.5%) and a slip muscle pattern (10.5%). A detailed description of the mentioned classification is provided in Table 2.
|
Pattern |
Description |
Occurrence |
|
|
1 — sheet muscle pattern |
Iliac type |
Accessory muscle formed mostly from the front fibres of the iliacus originating from the middle third of inner lip of iliac crest, which covered the FN like a sheet. |
42% |
|
Psoas type |
Accessory psoas formed mostly from posterior bundles of the psoas and formed dense muscle bundles, which covered the FN like a sheet. |
25% |
|
|
Iliacus and psoas major origin type |
Sheet-like, wide accessory muscle comprising both the anterior iliacus muscle bundles and posterior bundles of the psoas. |
22.5% |
|
|
2 — slip muscle pattern |
Found only in 2 cases: first — the muscular slip originated from the iliacus muscle, crossed the FN and then rejoined the iliacus muscle; in the second instance, the muscular slips were extensive and divided into branches in the iliac fossa, with the accessory muscle connected to the anterior site of quadratus lumborum via an aponeurosis band |
10.5% |
|
The presence of an accessory iliacus muscle or accessory psoas major, like additional muscular slips or sheets, psoas tertius, or quartus, may cause tension of the FN, resulting in neuropathy [26, 73]. The existence of such variants that could additionally compress the FN should be especially considered and ruled out in patients with FN paralysis/neuropathy caused by iliac haematoma, trauma, or vessel catheterization [51, 64]. The ilio-capsularis may have clinical significance during hip surgery. Babst et al. [7] suggest that it may be important in dysplastic hips, where it could help support the femoral head in a deficient acetabulum. They note that it could be atrophied in stable or well-constrained hips and hypertrophic in dysplastic ones.
The iliopsoas tendon also presents certain variability. It was found to be present as a double band in 64% of cases, a single band in 28.3%, and a triple band in 7.5% [49]. The morphological variability of iliopsoas might cause several clinical problems in patients complaining of pain referred from the hip joint to the knee joint and lumbar dermatomes [26].
Variations in the number of the bands forming the iliopsoas tendon can also be considered as clinically significant, especially in patients with internal snapping hip, which occurs during flexion or abduction or external rotation, when the medial part of the iliacus is restricted between the pubic bone and the iliopsoas tendon [13]. Interestingly, snapping hip syndrome is treated by iliopsoas tendon release and is associated with frequent recurrence of symptoms. According to Philippon et al. [49], this might be caused by overlooking the accessory tendinous slip during procedure.
The morphological variability of both iliacus muscle and psoas major might be considered as potential traps in ultrasonography. Firstly, additional muscular or tendinous bands could be overlooked during imaging, especially since such the structures tend to vary greatly in size. Fundamentally, as described by Olewnik et al. [45], some types of such slips are not even visible in US examination or occur only as secret anisotropy; however, there are no studies about the visibility of such bands of iliopsoas in ultrasound image to confirm these suspicions. Secondly, if additional heads or accessory muscles contribute to FN compression, it is possible that they could be mistaken as tumours or cysts because some can cause not only FN neuropathy but also contribute to snapping hip syndrome [74]. Unfortunately, there are no reports about the presence of additional muscular/tendinous bands of the iliopsoas muscle complex, neither the accessory iliacus nor psoas major, in ultrasound images; however, because ultrasound is needed to guide iliopsoas tendon release when treating internal snapping hip syndrome [35], further information regarding such imaging is needed.
Psoas minor
The psoas minor is located on the posterior abdominal wall. When present, it is included with the psoas major and iliacus in the iliopsoas muscle complex, i.e. the most powerful flexor of the thigh.
The psoas minor muscle is a small, flat, fusiform muscle that occurs in 30 to 60% of the population [30, 69]. It usually originates from the 12th thoracic vertebra, first lumbar vertebra, and the intervertebral disc between them; it forms a short and slender muscular belly that descends inferiorly and becomes a long tendon that inserts into the pectineal pubic line, iliopectineal eminence, and the iliac fascia [54].
Numerous morphological variations of the psoas minor have been noted. The muscle might be totally replaced by a tendon, or the tendinous portion might be exceptionally long, i.e. with the tendinous part comprising more than 57% of the total muscle length [69]. There are also mentions of alternative insertion sites including the inguinal ligament, neck of the femur, or lesser trochanter [21]. Additional attachment can be present between the fifth lumbar vertebra and the sacrum as a result of bifurcated tendinous insertion [21]. Psoas accessories have also been identified. Joshi et al. [27] present an instance of such a muscle that arose from the deep surface of psoas minor tendon and spread anteriorly to the psoas major.
A rarely observed variation is the occurrence of a double-headed muscle. Protas et al. [54] presented a case study in which such a variation was noticed on the left side of the posterior abdominal wall. The muscle was composed of a lateral head that originated from the first lumbar vertebral body and from the medial head that arose from the fourth and fifth lumbar vertebral bodies, and from the intervertebral disc between them [54]. The heads merged and formed a long tendon that attached onto the iliopectineal eminence [54] (Fig. 4).
It is possible that anatomical variations of the psoas minor muscle may be responsible for psoas minor syndrome, manifesting as a pain in the iliac fossa usually caused by increased tension of the muscle [21]. These symptoms, which can be misdiagnosed as diverticulitis or appendicitis, occur due to compression of the retroperitoneal neurovascular structures [33, 54]. It can also be speculated that morphological variation might contribute to psoas compartment syndrome due to the anatomical locations; this seems quite likely because the psoas minor lies so close to the neurovascular structures of the posterior abdominal wall that the tendon of the psoas minor was previously mistaken for the genitofemoral nerve [3, 39]. However, to confirm these suspicions, further studies must be undertaken. These should also investigate the relationship between these variations and the spread of malignancies and infections to the retroperitoneal region [21].
In clinical situations such as those mentioned above, imaging studies are frequently employed in the diagnosis process, with ultrasound imaging being increasingly common due to its easy access and low cost. Although several morphological variabilities of the muscles and tendons were successfully noted and diagnosed with the use of US imaging, it might not be the best option when it comes to psoas minor variation: while both psoas muscles appear as typical instances of hyperechoic striations on a hypoechoic background, the psoas minor is not identified as a separate structure [30]. Therefore, it may be difficult to conform whether neurovascular compression is caused by morphological variability, e.g. psoas accessories or additional psoas minor heads. Nevertheless, at this point, anatomical variations can only be suspected to cause psoas minor syndrome or psoas compartment syndrome, and speculation about the use of imaging in such instances needs to be supported by evidence from scientific research and further investigations.
Although morphological variability of the described muscles might be hard to visualise during an ultrasound examination procedure, there are techniques used for imaging the structures of the described compartment. Muscles of iliopsoas muscle complex, especially the psoas major, can be observed during US procedure when the patient is positioned in a supine position and with the use of a high-frequency linear probe placed slightly cephalad to the iliac crest, or when the patient is positioned in lateral position, with the use of a low-frequency convex probe vertically attached above the iliac crest [72]. According to Balius et al. [9], the FABER manoeuvre can be used to achieve the long-axis view of the iliopsoas tendon. This technique involves placing the lateral aspect of the studied limb on top of the contralateral knee at the level of the suprapatellar recess, with involvement of hip flexion and adduction followed by external rotation of the hip. The transducer should be placed over the femoral head in a slightly oblique way, following the trajectory of the iliopsoas tendon distally to its insertion [9] (Tab. 3).
|
Muscle |
Morphological variations presented in literature |
Clinical meaning |
|
|
Psoas major and iliacus muscle |
Psoas tertius |
Origin from the 12th rib and first lumbar vertebra transverse process; fusion with other psoas tendons at the level of inguinal ligament [11]. |
|
|
Psoas quartus |
Origin — quadratus lumborum and fifth lumbar vertebra transverse process; fusion with other psoas tendons at the inguinal ligament level [11]. |
||
|
Origin — the third lumbar vertebra transverse process and the quadratus lumborum; fusion with the psoas major and iliacus muscle via muscular fibres at the level of inguinal ligament [70]. |
|||
|
Origin only from the anteromedial surface of the quadratus lumborum muscle; fusion with the psoas major tendon at the inguinal ligament level [79]. |
|||
|
Accessory iliacus muscle |
Origin from the middle third of the iliac crest; insertion onto the lesser trochanter of the femur, between the psoas major and the iliacus muscle [26]. |
||
|
Iliacus minor |
Detached portion of iliacus that originates from anterior inferior spine of the ilium and inserts onto anterior-trochanteric line [2]. |
||
|
Ilio-capsularis |
Origin from anterior inferior spine of the ilium; insertion onto the ilio-femoral ligament [2]. |
||
|
Other types of additional muscular slips of iliacus muscle and psoas major presented in Table 1 and Table 2. |
|||
|
Divided tendon |
Presence of tendon that consists of 2 or 3 bands [49]. |
||
|
Psoas minor |
Psoas accessories |
Origin from the deep surface of psoas minor tendon, muscle spread anteriorly to psoas major [27]. |
|
|
Double-headed psoas minor |
Lateral head — origin from the first lumbar vertebral body and from the medial head – origin from the fourth and fifth lumbar vertebral bodies and from the intervertebral disc between them; tendon attached onto the iliopectineal eminence [54]. |
||
SUPERFICIAL GLUTEAL MUSCLES
The superficial gluteal muscle group is composed of the gluteus maximus muscle, gluteus medius muscle, gluteus minimus muscle, and the tensor fasciae latae (Fig. 5). However, the tensor fasciae latae does not appear to present any clinically significant anatomical variability.
The most superficial muscle of this region is the gluteus maximus muscle. It originates from the posterolateral aspect of the sacrum and coccyx, sacrotuberous ligament, gluteal surface of ilium, thoracolumbar fascia, and gluteal aponeurosis and inserts by broad aponeurosis onto gluteal tuberosity and the iliac tract [40]. There are many reports of additional origins, including the superficial lamina of the posterior layer of the thoracolumbar fascia, posterior sacroiliac ligament, aponeuroses of erector spinae, or latissimus dorsi [42]. A coccygeal attachment can be absent or wildly variable in size, origin from the gluteal aponeurosis [42]. The distal attachment can be either muscular or tendinous. An ascending tendon that attaches along the lateral lip of the linea aspera and extends down the femur length can be observed just as a direct insertion into the bone [55, 65]. The gluteus maximus muscle can also be divided into sacroiliac and coccygeofemoral parts [68] or cranial (fibrous) and caudal (muscular) parts, as reported by Kirici and Ozan [31].
Sen et al. [60] presented 2 cases of an accessory muscle originating from the gluteus maximus muscle: the gluteoperinealis. In the first case, the bilateral muscle originated from the posteromedial aspect of the gluteus maximus, extended alongside the ischial tuberosity, and inserted to the perineal body. In the second case, it extended from the fascia of the gluteus maximus and attached to the corpus cavernosum of the penis on the left side and perineal body on the right [60].
According to anatomy textbooks, the gluteus medius muscle arises as a broad insertion from the gluteal surface of the ilium between the anterior and posterior gluteal line and inserts onto the lateral side of the greater trochanter of the femur mainly by 2 tendinous components [40]. Most variabilities of this muscle concern its origin because it can arise from the entire length of the iliac crest as well as from the anterior three-quarters [19]. Distally it can attach to the superior, posterior, or posterosuperior trochanteric facet, or even the apex of the greater trochanter when 2 parts of the distal attachment are not distinguished [16, 19, 42]. The gluteus medius muscle might be composed of up to 4 anatomical compartments [19]. This muscle can also be fused with the gluteus minimus muscle distally with its tendon or with the creation of a separate muscular bundle; however, complete fusion of both muscles is rare [42]. There are also reports of fusion with the piriformis muscle, or continuity of the gluteus medius tendon alongside the vastus lateralis or gluteus minimus muscle [23].
The gluteus medius accessories has been described as arising from the iliac crest under the tensor fascia latae and gluteus medius muscle, with a distal attachment onto the greater trochanter [42]. This muscle was recently found by Orthaber et al. [47] alongside another variation of the gluteus minimus muscle: the gluteus quartus.
The gluteus minimus muscle originates from the gluteal surface of the ilium, between the anterior and inferior gluteal lines, and inserts into the anterolateral aspect of the greater trochanter [40]. The gluteus minimus muscle is mostly variable distally; some reports indicate the anterosuperior angle, and lateral and superior aspects of the greater trochanter as sites of distal attachment. This muscle can also attach to the anterior and/or superior aspect of the hip capsule via muscular or tendinous attachment.
An accessory muscle associated with the gluteus minimus muscle is the gluteus quartus, first described in humans by Macalister in 1866 [38]. Since its first description, many variants of this muscle have been described with variable origins, including the anterior inferior and superior iliac spine, deep inner lamina of gluteus minimus, or lateral edge of iliac crest, and various insertions, such as the hip joint capsule, gluteus minimus tendon, or great trochanter [42].
The morphological variability of the gluteal muscles is regarded as clinically significant, especially in surgery. The presence of additional muscles, such as the gluteus quartus or gluteus medius accessories, might contribute to hip joint instability because they cause an imbalance in muscle mass that leads to a mismatched pattern in force distribution during activities such as walking [47]. As such, imaging is crucial before total hip replacement, because such anomalous muscles can surprise clinicians during surgery, especially since the gluteus medius accessories might cross the lateral approach and the gluteus quartus in the antero-lateral approach [47]. The gluteoperinealis might also be an unwelcome surprise during radical perineal prostatectomy, gynaecological operations, urethroplasty, or perineal body reconstruction [60]. Importantly, in addition to the presence of additional muscles of the gluteal region, variations in their proximal and distal attachments might contribute to deep gluteal syndrome (DGS), a form of non-discogenic sciatic nerve entrapment in the subgluteal space known for its multifactorial aetiology [24].
During diagnostic imaging, a lack of awareness of the extreme anatomical variation of the gluteal muscles might easily lead to misinterpretation of such variability for other cause of DGS, such as tumour infiltration. It is also possible that when presenting as painless swelling, the presence of accessory muscles in the gluteal area might be misdiagnosed as soft-tissue Ewing sarcoma during imaging, because such a neoplasm might present itself as a gluteal mass [17]. However, further imaging studies, especially those associated with ultrasound, are needed to confirm these possibilities. Gluteus maximus muscle morphology can be visualised with use of a linear probe [29], and for imaging of the gluteus medius muscle and gluteus minimus muscle, a curvilinear transducer is advised [77]. To achieve a better view, the patient should be positioned lying on their side, test leg up, with the test-leg hip in neutral flexion/extension, neutral rotation, and 20° of adduction, and the knee in full extension [77] (Tab. 4).
|
Muscle |
Morphological variabilities presented in the literature |
Clinical meaning |
|
|
Gluteus maximus muscle |
Gluteoperinealis |
Origin from posteromedial aspect of the gluteus maximus, extension alongside ischial tuberosity; insertion to the perineal body [60]. |
|
|
Origin from the fascia of the gluteus maximus and attachment to the corpus cavernosum of the penis [60]. |
|||
|
Origin from the fascia of the gluteus maximus and attachment to the perineal body [60]. |
|||
|
Gluteus medius muscle |
Gluteus medius accessories |
Arising from the iliac crest under the tensor fascia latae and gluteus medius muscle, with a distal attachment onto the greater trochanter [47]. |
|
|
Gluteus minimus muscle |
Gluteus quartus |
Variable origins: anterior inferior and superior iliac spine, deep inner lamina of gluteus minimus or lateral edge of iliac crest, and various insertions: hip joint capsule, gluteus minimus tendon or great trochanter [42]. |
|
DEEP GLUTEAL MUSCLES
According to current nomenclature, the group of deep gluteal muscles is composed of the piriformis muscle (PM), obturator internus (OI), superior gemellus muscle (GS), inferior gemellus muscle (GI), and quadratus femoris muscle (QF) (Fig. 6).
Piriformis muscle
The PM originates from the anterior surface of the sacrum and the sacrotuberous ligament; it forms a pear-shaped belly and attaches to the superior border of the greater trochanter of femur [40]. Because the PM leaves the pelvis through the greater sciatic foramen and almost fills it, this muscle is considered as a landmark of the gluteal region: the superior gluteal nerve and vessels emerge superior, and the inferior nerve and vessels emerge inferior to the PM [40].
The PM presents extensive variability, and it can occur as undivided or divided or split into separate heads [42]. Natsis et al. [41] reported that the PM was doubled in 12 out of 294 (4.1%) studied limbs with different arrangements of the 2 heads: in 7 limbs, the second muscle belly was located inferior to the first, and in 5 cases, the second belly was located deeper than the first. Interestingly, 3 piriformis bellies were noted in 4 limbs (1.4%) [41]. Where separate bellies are present, each can produce separate tendons that blend to a common insertion [48].
The PM can be absent [15] or fused via fibers with other pelvic girdle muscles, including the gluteus minimus muscle [20], gluteus medius muscle [20], GS [78], or OI [50, 63]. Arora et al. [5] reported a rare conglomeration between the PM and gluteus maximus muscle called the gluteopiriformis. The proximal end of this anomalous muscle was attached to the inferomedial part of the gluteus maximus muscle and distally fused with PM above the apex of the greater trochanter of the femur [5]. It is also possible to find an additional muscle positioned above the PM (Fig. 7). This can be an additional slip, as noted by Ravindranath et al. [57], which arose from the sacrotuberous ligament in 2 cases and from the fascia overlaying the gluteus medius muscle in another case; all cases ran to a more significant muscle that arises from border/posterosuperior aspect of the greater sciatic foramen or inferior to posterior iliac spine [41, 53, 71]. Such a muscle was reported to attach onto the greater trochanter independently or together with the PM [53, 71].
The variability of PM is usually perceived relative to that of the sciatic nerve (SN) [41, 75]. In 1937 Beaton and Anson [10] conducted a classification that depicted anatomical variations between the SN and the PM, which is still applied in recent studies [41, 75]. The types introduced by Beaton and Anson [10] are presented in Table 5.
|
Type |
Description |
|
I |
Undivided SN below the undivided PM. |
|
II |
Undivided PM with SN nervous divisions below and between fibres. |
|
III |
Undivided PM with SN nervous divisions below and above the muscle. |
|
IV |
Undivided SN positioned between PMs heads. |
|
V |
SNs nervous divisions above and between PMs heads. |
|
VI |
Undivided SN positioned above the PM. |
The relationship between the PM and sciatic nerve plays an important role in piriformis syndrome (PS), a subgroup of DGS [24]. The frequency of PS varies from 5% to 36%; however, it is speculated that this syndrome is rather underdiagnosed [24]. This condition manifests as chronic pain in the hip area, which is worsened by hip movement, and the inability to sit for a prolonged time; the patient also reports pain while getting up, which may radiate into the back of the thigh but may also occur in the lower leg at dermatomes L5 or S1 [25]. PS can have primary causes due to anatomical variations or variable attachments, and secondary causes associated with precipitant causes [24]. Clear anomalous variability of the PM, such as junctions between muscles, additional muscles, anomaly course, and numerous additional heads might result in compression of the sciatic nerve that results in the described symptoms [41, 63, 78].
Currently there is no gold standard method of visualisation when it comes to the PS diagnosis, although ultrasound imaging is widely applied for the evaluation of neuropathies caused by entrapments such as PS [62]. Since US can provide not only real-time but also dynamic assessment, it seems to be a good choice for imaging the surprising variability of the piriformis as a cause of entrapment. PM can be visualised as a deep hypoechoic structure characterised by marble appearance, using a curvilinear transducer firstly placed in the lateral margin of the sacrum and then moved inferolaterally toward the greater trochanter until the medial end of the transducer remains at the lateral end of the sacrum [66].
Quadratus femoris muscle, obturator internus, and gemelli muscles
The QF muscle is not considered as variable. It originates from the lateral border of the ischial tuberosity and inserts onto the intertrochanteric crest of the femur [40]. However, there are reports of unilateral and lateral absence [37] as well as duplication, where both muscles arise from the ischial tuberosity and are both attached to the intertrochanteric crest [67].
The OI originates from the inner surface of the obturator membrane and the bones that form the boundaries of obturator foramen, viz. the inferior ramen of the pubis, ischial ramus, pelvic surface of the hip bone, and upper part of the greater sciatic foramen, and insert onto the medial surface of the greater trochanter of the femur [40]. Accessory slips arising from the sacrotuberous ligament/inner surface of the ischium or from typical origin of the muscle have been observed [42]. Kirici et al. present a case of OI with variable course, i.e. the muscle passed posteriorly to the midpoint of the sacrotuberous ligament and not via the lesser sciatic foramen, as usual [32].
The GS originates from the ischial spine and attaches onto the medial surface of the greater trochanter of the femur [40]. This muscle has been found to be absent both bilaterally and unilaterally [8]. There are also reports about a doubled GS. Arifoglu et al. [1] presented an interesting case in which a doubled GS occurred together with a doubled PM. Two separate GSs originated at the posterior aspect of the ischial spine closely to one another and inserted more or less at the same point [1] (Fig. 8). Additionally, the prevalence of fusion of GS and PM was estimated as 29% [78].
GI originates from the ischial tuberosity and attaches to the medial surface of the greater trochanter of the femur [40]. Although reports about the absence of the GI are extremely rare, recently Abdulhameed et al. [1] presented an interesting case of the bilateral absence of both gemelli muscles, with no evidence of fusion with PM or QF. In the case of a doubled GI, the upper part might originate from the ischial spine and the inferior from the ischial tuberosity [42].
Because the site of insertion for both gemelli muscles and the obturator internus is the greater trochanter of the femur, they can fuse and attach as one tendon, as reported in 36% of cases by Shinohara [61]. In another study, 64% of identified gemelli tendons covered the obturator internus tendon [61]. Aung et al. [6] also found that gemelli tendons may fuse posteriorly to the obturator internus. Additionally, the obturator internus tendon might be fused with either the PM [63] or gluteus medius muscle [78].
Anatomical variability of deep gluteal muscles of the pelvic girdle might be a cause of gemelli-obturator internus syndrome, which is a possible diagnosis of previously described DGS [24]. The SN passes between the PM and GS or OI; as such, it is possible to observe a scissor-like type entrapment of the nerve [24]. Anomalous attachments of the described muscles might also cause penetration of the SN [24]. Supernumerary muscles such as the additional GS might place pressure on the nerve, which would probably be aggravated by external rotation of the hip [4]. Since diagnosis and management of such anomalies is identical to those that cause PS, it is highly probable that anomalies of the gemelli or OI might surprise clinicians during imaging in response to suspected PS (Tab. 6).
|
Muscle |
Morphological variations described in literature |
Clinical meaning |
|
|
PM |
Gluteopiriformis |
Proximal attachment — inferomedial part of the gluteus maximus muscle; distal fusion with PM above the apex of the greater trochanter of femur [5] |
|
|
Unnamed additional muscle |
Positioned above the PM; additional slip, arising from the sacrotuberous ligament/fascia overlaying the gluteus medius muscle, attached to another muscle, that arises from border/posterosuperior aspect of the greater sciatic foramen or inferior to posterior iliac spine and inserts onto the greater trochanter [41, 53, 57]. |
||
|
QF |
Duplication of QF |
Two muscles arising from the ischial tuberosity and attached to the intertrochanteric crest [67]. |
|
|
OI |
Accessory slips |
Arising from the sacrotuberous ligament/ inner surface of the ischium/ from typical origin of the muscle [42]. |
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Variable course |
Instead of typical passing through the lesser sciatic foramen, OI passes posteriorly to the midpoint of the sacrotuberous ligament [32]. |
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GS |
Duplication of GS |
Two separate GS with origin onto posterior aspect of the ischial spine closely to one another; inserted at the medial surface of the greater trochanter of the femur [4]. |
|
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GI |
Duplication of GI |
Upper part originates from the ischial spine and inferior from the ischial tuberosity; insertion: medial surface of the greater trochanter of the femur [1]. |
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CONCLUSIONS
Pelvic girdle muscles present certain anatomical variability that has been implicated in numerous clinical syndromes, especially those connected with nerve compression. There are procedures in which US imaging of muscles from described compartments is a typical method of evaluation, and there are standard techniques of such visualisation. However, anomalies in this region, such as additional bands, accessory muscles, and supernumerary heads are poorly documented in ultrasound and other imaging studies. Further imaging studies of the described structures are needed to properly distinguish such variabilities to prevent any confusion that may result in misdiagnoses and surprises on the operating table.
ARTICLE INFORMATION AND DECLARATIONS
Funding
None.
Conflict of interest
None declared.
