Pyramidalis muscle

The pyramidalis muscle is a small, paired, triangular skeletal muscle located in the lower anterior abdominal wall, anterior to the rectus abdominis and enclosed within the rectus sheath.^[1] It originates from the pubic crest and the anterior surface of the pubic symphysis, with its fibers converging superiorly to insert into the linea alba midway between the pubic symphysis and the umbilicus.^[2] The muscle is innervated by the subcostal nerve (T12 spinal nerve) and vascularized primarily by branches of the inferior epigastric artery.^[2] The primary function of the pyramidalis muscle is to tense the linea alba, which helps reinforce the anterior abdominal wall and maintain intra-abdominal pressure during physiological processes such as forced expiration, defecation, and labor.^[2] Although it generates less than 1% of the force produced by the rectus abdominis, it contributes to overall abdominal wall stability in coordination with other muscles.^[3] Anatomical variations are common, with the muscle absent in approximately 20% of individuals (ranging from 10-70% across populations) and potentially present unilaterally or bilaterally; it is a vestigial structure in humans,^[4] showing no significant differences in morphometry between sexes.^[3] Clinically, the pyramidalis serves as a reliable surgical landmark during procedures like caesarean sections and appendectomies, and it has emerging applications as a source of muscle stem cells for therapies in wound healing, urinary incontinence, and microsurgical reconstructions.^[3]

Anatomy

Attachments

The pyramidalis muscle is a small, triangular, paired muscle located in the anterior abdominal wall, anterior to the rectus abdominis. Its origin is from the anterosuperior aspect of the pubic symphysis and the pubic crest.^[1] The attachment to the pubic symphysis occurs via short ligamentous fibers, while the portion arising from the pubic crest does so through tendinous fibers.^[2] This base forms the wider inferior margin of the muscle, with fibers oriented vertically and converging superiorly without pennation.^[5] From its origin, the muscle fibers course superomedially, forming a narrow apex that inserts into the linea alba, typically at a point midway between the pubic symphysis and the umbilicus.^[4] The insertion blends with the fibrous raphe of the linea alba, contributing to the tensioning of this midline structure.^[2] In anatomical dissections, the pyramidalis measures approximately 5-6 cm in length and 2 cm in width at its base, though these dimensions can vary.^[5]

Relations

The pyramidalis muscle is situated within the rectus sheath of the anterior abdominal wall, lying superficial to the inferior portion of the rectus abdominis muscle and deep to the anterior layer of the rectus sheath.^[2]^[6] It occupies a position anterior to the lower rectus abdominis, with its base attached to the pubic crest and symphysis, and its apex inserting into the linea alba approximately midway between the pubis and umbilicus.^[3]^[1] Laterally, the pyramidalis is bordered by the aponeuroses forming the rectus sheath, derived from the external oblique, internal oblique, and transversus abdominis muscles, which enclose it bilaterally.^[2] Posteriorly, it relates closely to the pubic symphysis and the anterior pubic ligament, with some tendinous connections to the adductor longus tendon.^[6] Superiorly, its insertion blends with the linea alba, positioning it medial to the rectus abdominis margins and without direct contact to the oblique abdominal muscles.^[3]

Innervation

The pyramidalis muscle is primarily innervated by the subcostal nerve, which arises from the anterior ramus of the twelfth thoracic spinal nerve (T12).^[7] This nerve provides motor supply to the muscle after traveling within the rectus sheath, contributing to its role in tensing the linea alba.^[8] In standard anatomical descriptions, the subcostal nerve's ventral branch directly innervates the pyramidalis, often alongside its supply to the lower rectus abdominis and transversus abdominis muscles.^[9] Variations in innervation have been documented, with the nerve to the pyramidalis (NPy) potentially originating from spinal segments T12 to L2.^[10] Common sources include the anterior cutaneous branch of the intercostal nerve (most frequently the subcostal), the ilioinguinal nerve, or the genital branch of the genitofemoral nerve.^[10] In a morphological study of 67 cadavers, the NPy was classified into nine types based on its course and branching, with double innervation observed in approximately 4.5% of cases, involving contributions from transitional and superficial rectus abdominis nerve branches or other lumbar derivations.^[10] Such variability may relate to developmental deviations in the lumbar plexus, influencing surgical approaches in the lower abdomen.^[10] The subcostal nerve is the most common source.

Vascular supply

The pyramidalis muscle primarily receives its arterial blood supply from branches of the inferior epigastric artery, a major vessel arising from the external iliac artery just above the inguinal ligament.^[1]^[2] This supply ensures oxygenation and nutrient delivery to the muscle's anterior abdominal location, supporting its minor role in tensing the linea alba. In a computed tomography angiography study of 50 pyramidalis muscles in Australian females, the most common arterial origin was a dedicated muscular branch of the inferior epigastric artery, observed in 68% of cases.^[11] Less frequent variants included the pubic branch of the inferior epigastric artery (4%), a variant obturator artery (2%), a common trunk from the inferior epigastric artery (2%), and a branch to the rectus abdominis muscle (2%), with supply not discernible in 22% of instances.^[11] Venous drainage parallels the arterial supply, primarily via tributaries of the inferior epigastric vein, which ultimately joins the external iliac vein to facilitate return of deoxygenated blood from the lower anterior abdominal wall.^[12] These vascular patterns highlight the muscle's integration into the broader abdominal wall circulation, with implications for surgical procedures in the region.

Anatomical variations

The pyramidalis muscle exhibits significant anatomical variability in its presence, sidedness, and morphology across populations. A systematic review and meta-analysis of 11 cadaveric studies involving 787 individuals (1,548 sides) reported a pooled prevalence of bilateral presence in 82.3% (95% CI: 76.2–87.6%), unilateral presence in 6.3% (95% CI: 3.3–10.2%), and bilateral absence in 11.3% (95% CI: 7.2–16.2%), indicating an overall presence rate of approximately 90%.^[13] Unilateral occurrences were more frequent on the left side (57.8%, 95% CI: 37.7–77.0%) than the right (42.2%, 95% CI: 23.0–62.3%), based on data from four studies (n=37 cases).^[13] Morphometric variations are common, with the muscle typically presenting as a small triangular structure but showing differences in dimensions. Length ranges from 3.12 cm to 12.50 cm across studies, with high heterogeneity in measurements.^[13] In a cadaveric analysis of 30 adult specimens, mean length was 6.80 ± 2.14 cm on the right and 6.64 ± 2.04 cm on the left, while mean width at the base was 1.87 ± 0.45 cm bilaterally; lengths showed symmetry (R²=0.79, p<0.0001), but widths were asymmetrical (R²=0.10, p=0.1102).^[14] Another study of 60 cadavers confirmed similar dimensions, with mean lengths of 66.2 mm (right) and 64.4 mm (left), widths of 23.4 mm (right) and 22.5 mm (left), and thickness of 4.1 mm bilaterally, noting minimal side-to-side differences.^[15] Gender influences were observed in one cohort, where muscles in males were longer (mean 6.92 ± 2.23 cm) but narrower (mean 1.84 ± 0.52 cm) compared to females (shorter at 6.40 ± 1.81 cm but wider at 1.92 ± 0.31 cm), though no correlations with age, height, or weight were found (p>0.05).^[14] Other variations include occasional asymmetries in size and rare reports of atypical insertions or multiple bellies, though duplications were absent in examined cohorts.^[14]^[15] The muscle consistently maintains a triangular shape with longitudinally oriented fibers, but its rudimentary nature contributes to these inconsistencies, with absence more common in certain ethnic groups as noted in broader reviews.^[13]

Development and evolution

Embryology

The pyramidalis muscle originates from the intraembryonic mesoderm of the anterior abdominal wall during the late embryonic period. It first appears as a distinct structure at Carnegie stage (CS) 20, approximately 51–53 days post-fertilization, in a subset of human embryos examined via high-resolution magnetic resonance imaging (MRI).^[16] By CS 21–23 (53–57 days), the muscle is identifiable in approximately 71.4% of specimens, forming ventrally and inferiorly to the rectus abdominis muscle primordium, in close proximity to the medial aspects of the lower limb muscle groups.^[16] This development aligns with the broader differentiation of the anterior abdominal wall musculature, which begins as a dorsal mesenchymal condensation at CS 15–16 (33–37 days) and progresses to layered formations by CS 17–20, with the pyramidalis emerging as the lateral muscles encircle the rectus sheath.^[17] In the early fetal period, starting around 8–9 weeks gestation (crown-rump length of 39.5–185.0 mm), the pyramidalis muscle achieves a higher prevalence, present bilaterally in 81.4% of fetuses, unilaterally on the right in 5.1%, and absent in 13.6%.^[16] At this stage, it is positioned superficially and ventrally to the rectus abdominis, appearing proportionally larger relative to adult dimensions, with its triangular shape already evident as fibers extend from the pubic crest toward the linea alba.^[17] The muscle's length typically exceeds its width, and key proportional ratios—such as pyramidalis length to rectus abdominis length or to the umbilicus-pubic symphysis distance—remain stable across fetal growth, indicating early establishment of its adult-like morphology.^[16] No significant sexual dimorphisms or lateral asymmetries in presence or positioning are observed during these stages, though absence in a minority of cases underscores its variable ontogeny.^[16] The pyramidalis develops concurrently with the closure of the ventral body wall and the resolution of the physiological umbilical hernia, integrating into the rectus sheath by 11–15 weeks gestation without disrupting surrounding structures.^[17] This timeline reflects its derivation from the same myogenic populations as the rectus abdominis and other abdominal wall muscles, originating from somitic mesoderm migrations.^[17]

Comparative anatomy

The pyramidalis muscle is present in monotremes and marsupials, where it originates from the linea alba and inserts onto the epipubic bones—ossicles extending anteriorly from the pubis that are unique to these basal mammal groups.^[18] In these species, such as the platypus (Ornithorhynchus anatinus) and opossum (Didelphis spp.), the muscle is fan-shaped and contributes to a kinetic linkage system that stiffens the trunk during locomotion by retracting the epipubic bones synchronously with limb movements.^[19] This function integrates with the rectus abdominis and oblique abdominal muscles to support the body between diagonal limbs, enhancing stability in sprawling postures characteristic of these animals.^[19] The epipubic association suggests an evolutionary origin tied to primitive mammalian locomotion, with homologous structures possibly traceable to reptilian hypaxial musculature.^[18] The muscle is absent in most placental (eutherian) mammals, including rodents, carnivores, and artiodactyls, correlating with the loss of epipubic bones in this clade.^[18] However, it reappears irregularly in primates, indicating independent retention or re-evolution. In prosimians (e.g., lemurs) and New World monkeys (platyrrhines, such as howler monkeys), the pyramidalis is well-developed, arising from the pubic crest and inserting into the linea alba, where it tenses the abdominal wall to aid in mammary gland compression for milk expression.^[20] Among great apes, it is observed in chimpanzees (Pan troglodytes) and gorillas (Gorilla gorilla) but absent in orangutans (Pongo spp.), reflecting phylogenetic variability within hominoids.^[21] In humans, the pyramidalis represents a vestigial structure, present bilaterally in approximately 80% of individuals but reduced in size and function compared to its primate ancestors, potentially linked to evolutionary shifts toward upright posture and altered abdominal mechanics.^[21] Its sporadic occurrence across mammals underscores a pattern of conservation in lineages with specialized abdominal demands, such as pouch support in marsupials or glandular functions in primates, while highlighting its diminishment in advanced eutherians.^[20]

Function

Role in abdominal mechanics

The pyramidalis muscle, a small triangular structure in the anterior abdominal wall, primarily functions to tense the linea alba, the fibrous raphe connecting the rectus abdominis muscles. This tensioning action is believed to enhance the stability of the midline abdominal wall by reinforcing the rectus sheath, though its mechanical contribution remains limited due to the muscle's diminutive size and force output. Studies indicate that the pyramidalis generates approximately 1 N of force per side, representing less than 1% of the force produced by the rectus abdominis, underscoring its subordinate role in overall abdominal tension.^[5] In coordination with larger abdominal muscles such as the rectus abdominis, external oblique, internal oblique, and transversus abdominis, the pyramidalis contributes to elevating intra-abdominal pressure during dynamic activities. This collective contraction supports physiological processes requiring increased abdominal compression, including forced expiration, coughing, defecation, micturition, and parturition. By subtly aiding in the compression of abdominal viscera, the pyramidalis helps maintain core stability and protect internal organs, although its absence in up to 20% of individuals suggests it is not essential for these mechanics.^[22]^[3] The muscle's fiber architecture, characterized by parallel fibers with a pennation angle of 0° and a mixed composition of approximately 54% slow-twitch fibers, aligns it more with postural and endurance functions rather than high-force exertions. This composition mirrors that of the rectus abdominis, implying a supportive rather than primary role in abdominal mechanics, potentially aiding in fine-tuning midline tension during prolonged upright posture or subtle movements.^[5]

Vestigial characteristics

The pyramidalis muscle is frequently classified as a vestigial structure in humans due to its small size, inconsistent presence, and minimal contribution to abdominal function. It is absent in approximately 8-20% of individuals, with bilateral presence observed in 72-83% of cases and unilateral in the remainder, indicating significant anatomical variability that does not impair overall abdominal mechanics.^[23]^[14] The muscle's contractile force is estimated at less than 1% of that generated by the rectus abdominis, suggesting it provides negligible tension to the linea alba, a role that larger abdominal muscles adequately fulfill.^[23] Absence of the pyramidalis does not result in any discernible functional deficit, further supporting its rudimentary status.^[14] Evolutionarily, the pyramidalis is considered a phylogenetic remnant derived from myotomes in lower thoracic segments that migrated ventrally during embryogenesis. In monotremes (e.g., platypus) and marsupials (e.g., kangaroo, koala), homologous muscles are more developed and associated with pouch contraction or milk expression from mammary glands.^[24]^[23] Its role likely diminished in primates as supernumerary nipples regressed, rendering the muscle vestigial in modern humans despite its presence in species like chimpanzees and gorillas but absence in orangutans.^[24]^[14] Some researchers propose it may have adapted with the evolution of erect posture to aid in linea alba stabilization, though this function remains debated and insufficient to negate its vestigial classification.^[14]

Clinical relevance

Surgical applications

The pyramidalis muscle serves as a reliable anatomical landmark during midline infraumbilical incisions, particularly in classical caesarean sections, where it helps surgeons identify the linea alba and maintain precise alignment to minimize complications such as hematoma or infection.^[3] This utility stems from its consistent bilateral insertion into the linea alba just above the pubic symphysis, providing a visible and palpable guide in procedures involving the lower abdominal wall.^[25] In reconstructive surgery, the pyramidalis muscle has been employed as a free flap for covering small, recalcitrant chronic wounds, especially in the foot and ankle region, due to its low donor site morbidity and inconspicuous scarring from Pfannenstiel incisions. A series of five cases demonstrated successful transfer in four instances, with pedicle lengths of 6–11 cm enabling microvascular anastomosis; one failure occurred due to venous thrombosis but was salvaged with an alternative flap, highlighting its viability for distal defects like osteomyelitis ulcers or chemical burns.^[26] The muscle's harvest preserves abdominal wall integrity, making it preferable over larger flaps for minor reconstructions.^[25] Additionally, the pyramidalis muscle has applications in urological surgery, including its use as a source of striated muscle stem cells following cryopreservation to treat post-prostatectomy stress urinary incontinence, offering a regenerative approach with potential for myoblast transplantation.^[3] It has also been incorporated in procedures to alleviate severe dysuria by fixing the prostate to the muscle combined with bladder pexy, providing mechanical support to improve urinary dynamics.^[3] These uses underscore its emerging role beyond mere anatomical reference in targeted therapeutic interventions.

Diagnostic and research uses

In regenerative medicine research, cryopreserved pyramidalis muscle specimens serve as a source of striated muscle stem cells (satellite cells) for treating post-prostatectomy stress urinary incontinence. A seminal study demonstrated that specimens stored at -80°C for 24-60 months yield high-purity (>99%) NCAM-positive satellite cells, which differentiate into myotubes and exhibit multipotency toward osteogenic and adipogenic lineages under BMP-7 and γ-linolenic acid induction, respectively.^[27] This approach leverages the muscle's accessibility during prostatectomy, minimizing donor morbidity while enabling autologous cell therapies. Diagnostically, the pyramidalis muscle is visualized on imaging modalities such as MRI, CT angiography, and ultrasound to evaluate lower abdominal and groin pathologies, particularly in athletes with chronic pain. In MRI protocols for athletic pubalgia, it is assessed for partial tears or avulsions (classified as type 5 or 6 injuries), often correlating with surgical findings and aiding differential diagnosis from adductor or rectus strains. Ultrasound imaging distinguishes its myofibrillar echotexture from potential masses at the rectus tendon level, preventing misdiagnosis in groin pain evaluations. CT angiography delineates its arterial supply, primarily from inferior epigastric branches, which informs preoperative planning and identifies rare duplications. These applications underscore its role as a landmark in non-invasive diagnostics for abdominal wall disorders. Recent cadaveric studies (as of 2024) on morphometric variability across populations further support its use in personalized surgical planning.^[11]