High-resolution ultrasound of spigelian and groin hernias: a closer look at fascial architecture and aponeurotic passageways

Abdominal wall hernias represent a common issue in clinical practice, with a prevalence amongst the general population to be estimated at around 5%^(1,2). The vast majority (85%) of hernias occur in the groin, whereas only 15% of them are found in other areas of the anterior abdominal wall⁽²⁾. Diagnostic imaging is increasingly used for investigating patients with suspected abdominal wall hernias, especially when atypical bumps or symptoms may lead the clinician to suspect other etiologies⁽³⁾. In this clinical context, ultrasound (US) has been regarded as a low-cost, rapid and very effective technique which is able to identify and categorize different types of herniations in exquisite anatomical detail^(4–6). The main advantage of US over computed tomography or MR imaging relies on the ability of this technique to provide a dynamic evaluation which may help to detect intermittent extrusions during the application of provocative maneuvers (i.e. Valsalva) and/or different patient positioning (i.e. supine vs. standing). The aim of this review is to place a specific focus on the complex fascial and aponeurotic anatomy of the anterior abdominal wall that is pertinent to spigelian, inguinal and femoral hernias. The anatomical background and the relevant US findings of the aponeurotic architecture and portals of herniation will be described using schematic drawings, static images and video clips.

A deep knowledge of the anatomy of the anterior abdominal wall is mandatory to perform a targeted US examination in patients with suspected ventral hernias. On each side of the linea alba, five paired muscles can be recognized in the abdominal wall: two of them – the rectus abdominis and the smaller pyramidalis – lie closer to the midline, with their major axis oriented in the craniocaudal direction. The other three muscles – obliquus externus, obliquus internus and transversus abdominis – are located more laterally: they are large, flattened, and more obliquely oriented. From the anatomical point of view, the right and left rectus abdominis muscles are separated in the midline by a vertical band of connective tissue which is referred to as the linea alba. They take their origin from the cartilages of the 5^th to 7^th ribs and the xyphoid process, and attach distally into the symphysis pubis, the pubic crest, and the pubic tubercle. The rectus abdominis is characterized by a series of transversely oriented connective tissue bands (tendinous intersections) that divide the muscle belly into parallel strips. In addition, it is enveloped and stabilized by the aponeuroses of the obliquus externus, obliquus internus, and transversus abdominis muscles. The pyramidalis is a small rudimentary muscle which lies distally, covering the anteroinferior aspect of the rectus. It has a triangular shape, arises from the linea alba, and attaches into the symphysis pubis and the pubic tubercle. The obliquus externus takes its origin from the lateral aspect of the last seven/eight ribs. It travels from cranial to caudal, and from lateral to medial, converging into a large and flattened aponeurosis that approaches the midline, crossing over the rectus abdominis and blending with the contralateral muscle at the level of the linea alba. The obliquus externus also has a distal insertion at the level of the groin. The obliquus internus muscle originates from the anterior and middle thirds of the iliac crest, the lateral half of the inguinal ligament and, through the thoracolumbar fascia, from the spinous processes of the last two lumbar and first sacral vertebrae. Its fibers diverge in a large fan and form a broad aponeurotic tendon that can be subdivided into proximal, intermediate and distal parts. The proximal part inserts into the costal cartilages of the last three/four ribs. In its superior two thirds, the intermediate part runs horizontally, and splits into posterior and anterior layers that envelop the rectus abdominis, whereas its inferior third simply crosses over this muscle, merging with the obliquus externus aponeurosis. The distal part of the obliquus internus aponeurosis inserts into the pubic crest and the pectineal line through a conjoint tendon with the transversus abdominis (Fig. 1). The transversus abdominis muscle is the innermost of the series of flat muscles of the anterior abdominal wall. It has an extensive origin from the internal aspect of the last six costal cartilages, the transverse processes of the lumbar vertebrae through the thoracolumbar fascia, the deep aspect of the anterior iliac crest, and the lateral half of the inguinal ligament. The transversus abdominis muscle travels from posterior to anterior to converge in a large aponeurosis that inserts into the linea alba. Its myotendinous junction has a medial concave shape that is commonly referred to as the spigelian or semilunaris line (Fig. 2). The area between the spigelian line and the lateral border of the rectus abdominis is called the spigelian fascia (Fig. 2). While the superior part of the aponeurosis runs on the undersurface of the rectus abdominis to reach the midline and insert into the linea alba, the inferior lies on the anterior aspect of the rectus. The arcuate line (Douglas line) indicates the transition between the intermediate and distal parts of the obliquus internus aponeurosis and the proximal and distal parts of the transversus abdominis aponeurosis. It demarcates the point at which the inferior epigastric vessels pierce the undersurface of the rectus (Fig. 2). The distal part of the transversus abdominis aponeurosis also shows a common insertion with the obliquus internus into the pubic crest and the pectineal line.

Fig. 1.

Fig. 2.

The spigelian hernia derives from peritoneal infolding through a defect in the spigelian fascia^(7–9). This typically occurs along the Douglas line, as the change in orientation of the aponeuroses of the obliquus internus and transversus abdominis is supposed to cause focal weakening of the abdominal wall⁽¹⁰⁾. The sac of spigelian hernias may contain the omentum, small bowel or colon⁽¹⁰⁾. Surgical repair is generally indicated due to a high risk of incarceration (17–24% of cases)⁽¹⁰⁾. Regarding presentation symptoms, patients often complain of pain at the level of the abdominal wall, and report an intermittently palpable mass. The clinical diagnosis is often challenging, as the protrusion may be hidden by the overlying normal aponeurosis of the obliquus externus. A low diagnostic accuracy of CT and US has also been reported due to the transitory presentation of this type of hernia that may or may not be present at the time of imaging^(11–15). Placing the US transducer at the level of the inferior third of the rectus abdominis, the inferior epigastric vessels should be identified first, as they run vertically alongside the muscle undersurface. These vessels should then be followed on their short-axis from distal to proximal until they pierce the deep fascia of the rectus abdominis at the Douglas’ arcade (Fig. 3). From this level, the probe should be moved externally on transverse planes, looking at the three-layered complex of aponeuroses of the obliquus externus, obliquus internus, and transversus abdominis. The myotendinous junction of the transversus abdominis indicates the level of the spigelian (semilunaris) line, whereas the area intervening between this line and the lateral margin of the rectus abdominis represents the spigelian fascia (Fig. 4). The spigelian hernia can be found at the level of the Douglas’ arcade through a defect in the spigelian fascia (Fig. 5 and Video 1). If the suspected hernia is not visualized at this level, the spigelian fascia should be extensively examined during the Valsalva maneuver to avoid missing small or atypically located protrusions. Compression maneuvers are important to assess reducibility.

Fig. 3.

Fig. 4.

Fig. 5.

Several muscles, tendons and ligaments contribute to building up the complex three-dimensional architecture of the groin. A thorough anatomical knowledge is, therefore, a critical prerequisite to perform a correct evaluation and increase the examiner’s confidence in diagnosing regional hernias. After inserting into the anterior third of the iliac crest and the anterior superior iliac spine, the obliquus externus aponeurosis gets thicker and directs inferomedially to reach the pubic tubercle, forming a strong tendinous arch between the two pelvic bones, which is referred to as the inguinal ligament (Fig. 1). Approaching the pubis, some fibers of the inguinal ligament reflect posteriorly to insert into the pectineal line. They form the lacunar ligament (Gimbernat’s ligament) and delimit the medial wall of the femoral ring (Fig. 2). Close to its insertion into the pubic tubercle, the inguinal ligament also exhibits a thin extension (pectineal ligament or Cooper’s ligament) which diverges toward lateral, and attaches onto the superior pubic ramus. This latter tendinous extension forms the floor of the femoral ring. In close proximity to the lateral side of the rectus abdominis, an opening in the distal obliquus externus aponeurosis contours the superficial inguinal ring a few millimeters cranial to the insertion of the inguinal ligament into the pubic tubercle. This opening is bounded by lateral, medial and posterior walls: the lateral wall consists of vertically-oriented fibers of the obliquus externus blending with the underlying inguinal ligament, and the medial wall derives from more transversely-oriented fibers approaching the midline and inserting into the symphysis pubis (Fig. 1). Some of these latter fibers cross the midline and reach the pectineal line, forming the posterior wall (Colles’ ligament) of the contralateral superficial inguinal ring. The distal aponeurosis of the obliquus internus and transversus abdominis share a common insertion into the pubis which is commonly referred to as the conjoint tendon or inguinal falx (Fig. 6). It lies superficial to the lateral boundary of the rectus abdominis, thus reinforcing the abdominal wall in the groin area. Finally, the deepest structure of the abdominal wall is the fascia transversalis, which lies on the posterior aspect of the transversus abdominis muscle. Looking at this fascia from the abdominal cavity, two thickened bands can be noted: the medial (Henle’s ligament) is adjacent and parallel to the lateral margin of the rectus abdominis and corresponds, in a more superficial plane, to the inguinal falx; the lateral (Hesselbach’s ligament) runs close and parallel to the inferior epigastric vessels (Fig. 2). These ligaments show an undefined proximal origin, and insert distally into the inguinal ligament, playing an important role as groin stabilizers. The internal inguinal ring can be considered an opening of the fascia transversalis: it is located on a plane crossing the midpoint of the inguinal ligament, approximately 11 mm above it, just lateral to the Hesselbach’s ligament that contributes to its medial boundary. The inguinal canal travels through different layers of the abdominal wall, running from the deep (superolateral) to the superficial (inferomedial) inguinal rings. It houses the spermatic cord in men, as it travels from the abdomen to the scrotum, and the round ligament in women, directed to the labia majora. The canal has been described as having superior, inferior, anterior and posterior walls: the inferior wall is made of the inguinal ligament; the superior is composed of the inferior boundaries of the obliquus internus and transversus abdominis; the anterior is represented by the inferior part of the obliquus externus aponeurosis. The posterior wall has a more complex structure: i) its lateral third consists of the fascia transversalis reinforced by the Hesselbach’s ligament and the inferior epigastric vessels; ii) its middle third is made up of the fascia transversalis, and iii) its medial third is composed of the fascia, the conjoint tendon, and Henle’s ligament, which reinforce its superficial and deep aspects, respectively. Medial to the Hesselbach’s ligament and the inferior epigastric vessels, the middle third of the posterior wall represents a weak area of the abdominal wall (Hesselbach’s triangle), as the fascia transversalis lacks any reinforcement there (Fig. 2 and Fig. 6). This is the area through which direct hernias come out. Indirect inguinal hernia descends instead into the inguinal canal, passing through the deep inguinal ring. When dealing with a suspected groin hernia, it should be kept in mind that small protrusions may be unnoticed at rest, with the patient lying supine on the bench. To avoid false negatives, the groin area should also be systematically investigated during the Valsalva maneuver or when standing. US imaging demonstrates the inguinal ligament as an undefined thin hyperechoic band running from the anterior superior iliac spine to the pubic tubercle (Fig. 7). The external iliac vessels can be easily seen, as they cross underneath the middle third of the ligament. Just proximal to it, US can demonstrate the origin of the inferior epigastric vessels which turn cranially to run alongside the undersurface of the rectus abdominis. The inferior epigastric vessels represent the main landmark to identify the Hesselbach’s triangle, which is located in the area intervening between them and the lateral margin of the rectus abdominis. Lateral and slight superior to the origin of the inferior epigastric vessels, the spermatic cord can be identified as an ill-defined hyperechoic structure housing vessels (i.e. spermatic, deferential and cremasteric arteries, venous plexus) and thin hypoechoic non-vascular tubular structures related to the deferens and small nerves (i.e. genital branch of the genitofemoral, ilioinguinal running on its external surface). The inguinal canal begins at the deep inguinal ring and descends parallel to the inguinal ligament, pointing toward the pubic tubercle to end at the superficial inguinal ring, where the spermatic cord passes into the subcutaneous tissue (Fig. 8 and Fig. 9).

Fig. 6.

Fig. 7.

Fig. 8.

Fig. 9.

Indirect inguinal hernias enter the inguinal canal through the deep inguinal ring, lateral to the position of the inferior epigastric vessels. In males, they traverse the canal and may extend beyond the external inguinal ring down to the scrotum (Fig. 10A and Fig. 10B). Indirect inguinal hernias are five times more common than direct hernias, and are reported to carry a moderate risk of strangulation⁽¹⁶⁾. The pathogenesis of indirect hernias differs between the children age group and the adult population. In children, herniation may be secondary to incomplete closure of the processus vaginalis, which is the part of the fascia transversalis that travels with the testes during their descent into the scrotum^(17,18). Although a higher patency rate of the processus vaginalis is also found in adults^(17,19), a “shutter and sphincter” mechanism has been postulated in this latter age group to prevent the protrusion of the abdominal content when the intraabdominal pressure increases. This mechanism is mediated by the Hesselbach’s ligament that is closely associated with the deep inguinal ring. When the transversus abdominis contracts, the ligament pulls the ring superolaterally, causing its narrowing and leading to an obstructed migration of the peritoneal content⁽²⁰⁾. The relaxation of abdominal muscles with aging in conjunction with factors that increase the intraabdominal pressure and/or contribute to collagen degradation, such as coughing, straining, weightlifting and smoking, are implicated in decreasing the efficiency of this mechanism^(21,22). US can categorize inguinal hernias as of indirect type when the sac is shown entering the inguinal canal through the deep inguinal ring (Video 2). The level of this ring can be recognized starting at the point where the inferior epigastric vessels arise from the medial aspect of the external iliac vessels. Sweeping the probe a few centimeters proximal to this level, the deep inguinal ring is easily found lateral to the external iliac artery and vein (Fig. 8). The probe should then be oriented obliquely along the inguinal canal to be in axis with the herniation path. Alternatively, one could scan the inguinal canal in its short-axis to identify the oval echogenic profile of the spermatic cord as a landmark. Moving proximally, the spermatic cord is seen crossing over the external iliac vessels and then pointing towards depth at the level of the deep inguinal ring. Indirect inguinal hernias include a wide range of content, such as retroperitoneal fat, omentum, bowel loops, appendix (Amyand’s hernia), bladder, Meckel’s diverticulum (Littré’s hernia), gonads or the antimesenteric wall of a bowel loop (Richter’s hernia)⁽¹⁶⁾.

Fig. 10.

Direct inguinal hernias protrude through the Hesselbach’s triangle, a weak area delimited laterally by the inferior epigastric vessels and the Hesselbach’s ligament, medially by the lateral side of the rectus abdominis reinforced by the conjoint tendons (anteriorly) and the Henle’s ligament (posteriorly), whereas its base consists of the inguinal ligament (Fig. 10A and Fig. 10C). The Hesselbach’s triangle can confidently be identified with US looking at the level of origin of the inferior epigastric vessels. On this scanning plane, the triangle is demonstrated as a fat-filled area extending between them and the rectus abdominis (Fig. 9A). The prevalence of direct hernias markedly increases with age as a result of degeneration and structural derangement of the fascia transversalis⁽²³⁾. The pathogenesis of this condition seems to be related to the chronic continuing compression exerted by the abdominal content against the fascia transversalis, which shows chronic inflammatory infiltrates, microvascular damage, venous congestion, and impaired tissue perfusion^(24–26). These changes may lead to fascial weakening and breaking, with subsequent prominence of the abdominal content to impact against the deep aspect of the inguinal canal. Direct inguinal hernias most commonly occur in men, and have a low risk of strangulation, possibly because of their relatively large neck and the fact that they do not penetrate the inguinal canal⁽²⁷⁾. US imaging visualizes the hernia as it expands lateral to the echogenic spermatic cord and medial to the landmark of the inferior epigastric vessels. Direct inguinal hernias typically appear as a bulge of fatty tissue or bowel causing “ballooning” of the posterior wall of the inguinal canal at the level of the Hesselbach’s triangle (Video 3). The content of the inguinal canal may appear compressed and stretched laterally by the bulk of the hernia.

Distal to the inguinal ligament, the fascia transversalis continues down into the thigh, where it is referred to as the “femoral sheath”. It crosses over the femoral vessels for a few centimeters before fusing with their fascial envelopes. The “femoral canal” is delimited by the femoral sheath and located just medial to the femoral vein: it is bordered by the inguinal ligament, the pubic ramus covered by the Cooper’s ligament proximally and the pectineus muscle distally, and the Gimbernat’s (lacunar) ligament (Fig. 11). The femoral canal communicates with the abdominal cavity through the so called “femoral ring” that represents the passageway through which femoral hernias extrude (Fig. 11). Inferior to the inguinal ligament, several groups of lymph nodes can be visualized in the crural area. In normal states, these lymph nodes may appear oval or elongated in shape, and can be easily recognized as consisting of a peripheral hypoechoic rim (cortical layer) and a fatty hyperechoic center (hilum).

Fig. 11.

Femoral hernias are found below the inguinal ligament, and they protrude into the femoral canal through the femoral ring. Compared with inguinal hernias, femoral hernias are relatively uncommon, with a reported prevalence of approximately 3% of all groin hernias which need surgical repair⁽²³⁾. Most occur in women as a probable result of their hormonal status and pregnancy-related changes which may lead to derangement and weakening of the supporting connective tissue around the femoral ring⁽²⁸⁾. Although distinguishing inguinal from femoral hernias is not straightforward on clinical grounds regardless of the experience of the examining physician, a prompt diagnosis of the latter is crucial, as femoral hernias are associated with higher rates of acute complications⁽²⁹⁾. US imaging has been reported to be highly accurate in differentiating between femoral and inguinal hernias if the examination is performed with an appropriate scanning technique and in experienced hands^(30,31). Since the risk for strangulation of femoral hernias accounts for 22% and 45% over three and twenty-one months respectively, whereas it occurs in only 3% and 4.5% of cases of inguinal hernias over the same time frame, it seems advisable to follow the policy of screening patients with suspected groin hernias systematically with US^(32,33). The examination should be performed by orienting the probe in the long-axis of the inguinal ligament, close to the pubic tubercle. Sweeping the probe slightly lateral and caudal to it, the common femoral artery and vein are visualized as they run underneath the ligament and superficial to the pubic ramus. The femoral ring can be recognized as the area bounded by the femoral vein laterally, the inguinal ligament anterosuperiorly and the Cooper’s ligament posteriorly (Fig. 11A). When the bulk of a femoral hernia extrudes into the canal, the femoral vein collapses as a result of direct compression to the vessel walls (Fig. 12). During the Valsalva maneuver, the intermittent and reversible extrusion of the hernia with transient collapse of the vein can be nicely observed in dynamic scans (Video 4).

Fig. 12.

A clear differentiation amongst anterior abdominal wall and groin hernias is a critical issue, as these conditions often require different approaches and treatments. US imaging may be regarded as a useful complement to the physical examination for hernia characterization. Familiarity with the gross anatomy of the abdominal wall and the groin is a prerequisite to enhance the diagnostic confidence of the examiner when evaluating patients with suspected hernias.